This file was created by the TYPO3 extension publications --- Timezone: CEST Creation date: 2026-04-05 Creation time: 18:08:49 --- Number of references 67 article Vinnepand2024 Situated within a 1.07 million-year-old meteorite crater, Lake Bosumtwi in Ghana stands as a pivotal location for comprehending climatic, ecological and environmental fluctuations within the sub-Saharan region of West Africa. The region's susceptibility to seasonal environmental shifts and climate oscillations is heightened by the annual movements of the tropical rain belt driven by atmospheric circulation. Yet, there is no satisfying age-depth model available for the entire sedimentary sequence strongly limiting our understanding of changes in this circulation pattern and associated (broad-scale) environmental responses during the last million years in the local to regional context of Lake Bosumtwi. To overcome this, we statistically examine the cyclicity in total natural gamma ray (NGR) data on a core from the lake's centre and create a cyclostratigraphic age-depth model. The calculated maximum age of 946 ka agrees well with the meteorite impact age (∼10 % offset). In order to refine this purely statistical approach, we also perform a correlative age-depth model using 33 tie points accounting for the complexity of climatic and environmental imprints to the NGR record that may exceed direct insolation related effects. Special attention is paid to the core's robustly dated (14C, OSL, U/Th) uppermost part covering the last 200 ka. Here, high NGR and co-varying K counts coincide with warm periods (except of the water-saturated and unconsolidated Holocene part) and the inverse for glacials and stadials. Based on this, we define tie points for correlating our NGR data to the age-depth model of a NE Atlantic SST record. Comparing our results to the correlation target, other global climate records and Sahara dust flux data reveals striking similarities and supports a proxy understanding with increased in wash of K-enriched terrigenous material from the crater rims in warm and moist periods (high NGR) and K-depleted dust input in stadials possibly contributing to low NGR values in addition to reduced input of K-enriched sediments from the crater rims. Our correlative age model results in precession amplitudes matching eccentricity well, providing further support especially because an over-tuning is unlikely with the used 33 tie points. Overall we provide crucial chronological context to numerous datasets along with environmental constrains that can be used to study the potential habitat availability of early anatomically modern humans in West Africa. © 2023 Article 2024 10.1016/j.quascirev.2023.108478 Quaternary Science Reviews 325 Ashanti; Ghana; Lake Bosumtwi; Sahara; Climate change; Climate models; Dust; Gamma rays; Meteorites; Rain; Sediments; Tropics; Climate dynamics; Depth models; Environmental change; ICDP; Lake bosumtwi; Natural gamma ray; Quaternary; Tie points; West Africa; Western africa; atmospheric circulation; climate variation; crater; environmental change; gamma radiation; potash; Quaternary; sedimentary sequence; Lakes https://www.scopus.com/inward/record.uri?eid=2-s2.0-85181056438&doi=10.1016%2fj.quascirev.2023.108478&partnerID=40&md5=40f1b179c6315358518740ce5462b8fe Cited by: 1 Mathias Vinnepand Christian Zeeden Thomas Wonik William Gosling Anders Noren Jochem Kück Simona Pierdominici Silke Voigt Mehrdad Sardar Abadi Arne Ulfers Sylvester Danour Kweku Afrifa Stefanie Kaboth-Bahr article Yirenkyi2024 Article 2024 10.1038/s41598-024-62959-3 Scientific Reports 14 1 https://www.scopus.com/inward/record.uri?eid=2-s2.0-85196481901&doi=10.1038%2fs41598-024-62959-3&partnerID=40&md5=0369e91c4999092a548b2360e503658d Cited by: 0; All Open Access, Gold Open Access Steven Yirenkyi Cyril D. Boateng Emmanuel Ahene Sylvester K. Danuor article RN96 2024 10.22498/pages.32.2.126 Past Global Changes Magazine 32 126-127 2 Christian Koeberl article Boateng2023 Review 2023 10.1016/j.sciaf.2023.e01893 Scientific African 21 https://www.scopus.com/inward/record.uri?eid=2-s2.0-85171385124&doi=10.1016%2fj.sciaf.2023.e01893&partnerID=40&md5=783073e1966dd8cbce25c22a7c967e11 Cited by: 2; All Open Access, Gold Open Access Cyril D. Boateng Christopher A. Akurugu David D. Wemegah Sylvester K. Danuor article Gosling2022 Article 2022 10.1126/science.abg4618 Science 376 6593 https://www.scopus.com/inward/record.uri?eid=2-s2.0-85129370139&doi=10.1126%2fscience.abg4618&partnerID=40&md5=849b73ea845ae6d71283eedf2c51e1e2 Cited by: 22; All Open Access, Bronze Open Access William D. Gosling Charlotte S. Miller Timothy M. Shanahan Philip B. Holden Jonathan T. Overpeck Frank Langevelde article Kuzmicheva2021795 Abstract—The formation of impact craters on the Earth’s surface is accompanied by the effect of shock waves on rocks. The shock wave compression results in rocks heating up to the point of melting and evaporation during unloading. The direct mechanical action of shock compression and residual heating change the magnetic properties of rocks. Geophysical modeling is used to determine the sources of the magnetic anomaly by interpreting the fields measured on the surface, but such modeling does not take into account the impact demagnetization of rocks. This work gives an example of analysis of the magnetic anomaly over the well-studied Bosumtwi crater (Ghana, 10.5 km diameter, 1 million years old), including the numerical modeling of the crater formation process and the construction of a magnetic anomaly model based on the simulated shock compression parameters and crater drilling data. It is shown that the morphological features of the crater— the crater rim and the central uplift—form positive magnetic anomalies around and inside the crater. © 2021, Pleiades Publishing, Ltd. Article 2021 English 10693513 10.1134/S1069351321050128 Izvestiya, Physics of the Solid Earth 57 Pleiades journals 795 – 804 5 https://www.scopus.com/inward/record.uri?eid=2-s2.0-85118729610&doi=10.1134%2fS1069351321050128&partnerID=40&md5=e74b9cd9f31164d62ed601ab671ad9d7 Cited by: 0 M. Yu. Kuzmicheva B.A. Ivanov article Wilke2016118 Sedimentary sequences in ancient or long-lived lakes can reach several thousands of meters in thickness and often provide an unrivalled perspective of the lake's regional climatic, environmental, and biological history. Over the last few years, deep-drilling projects in ancient lakes became increasingly multi- and interdisciplinary, as, among others, seismological, sedimentological, biogeochemical, climatic, environmental, paleontological, and evolutionary information can be obtained from sediment cores. However, these multi- and interdisciplinary projects pose several challenges. The scientists involved typically approach problems from different scientific perspectives and backgrounds, and setting up the program requires clear communication and the alignment of interests. One of the most challenging tasks, besides the actual drilling operation, is to link diverse datasets with varying resolution, data quality, and age uncertainties to answer interdisciplinary questions synthetically and coherently. These problems are especially relevant when secondary data, i.e., datasets obtained independently of the drilling operation, are incorporated in analyses. Nonetheless, the inclusion of secondary information, such as isotopic data from fossils found in outcrops or genetic data from extant species, may help to achieve synthetic answers. Recent technological and methodological advances in paleolimnology are likely to increase the possibilities of integrating secondary information. Some of the new approaches have started to revolutionize scientific drilling in ancient lakes, but at the same time, they also add a new layer of complexity to the generation and analysis of sediment-core data. The enhanced opportunities presented by new scientific approaches to study the paleolimnological history of these lakes, therefore, come at the expense of higher logistic, communication, and analytical efforts. Here we review types of data that can be obtained in ancient lake drilling projects and the analytical approaches that can be applied to empirically and statistically link diverse datasets to create an integrative perspective on geological and biological data. In doing so, we highlight strengths and potential weaknesses of new methods and analyses, and provide recommendations for future interdisciplinary deep-drilling projects. © 2016 Elsevier B.V. Review 2016 English 09218181 10.1016/j.gloplacha.2016.05.005 Global and Planetary Change 143 Elsevier B.V. 118 – 151 Biology; Geology; Paleolimnology; Analytical approach; Ancient lakes; Deep drilling; Evolutionary biology; Evolutionary information; Interdisciplinary project; Methodology; Sedimentary sequence; biological survey; core analysis; data quality; deep drilling; evolutionary biology; fossil record; geological theory; integrated approach; lacustrine environment; outcrop; paleolimnology; technological change; Lakes https://www.scopus.com/inward/record.uri?eid=2-s2.0-84975523752&doi=10.1016%2fj.gloplacha.2016.05.005&partnerID=40&md5=65b5fcf6cccc099d292cd51ee6749ff8 Cited by: 30; All Open Access, Bronze Open Access, Green Open Access Thomas Wilke Bernd Wagner Bert Van Bocxlaer Christian Albrecht Daniel Ariztegui Diana Delicado Alexander Francke Mathias Harzhauser Torsten Hauffe Jens Holtvoeth Janna Just Melanie J. Leng Zlatko Levkov Kirsty Penkman Laura Sadori Alister Skinner Björn Stelbrink Hendrik Vogel Frank Wesselingh Thomas Wonik article Losiak2014394 Rocks from drill cores LB-07A (crater fill) and LB-08A (central uplift) into the Bosumtwi impact crater, Ghana, were analyzed for the presence of the cosmogenic radionuclide 10Be. The aim of the study was to determine the extent to which target rocks of various depths were mixed during the formation of the crater-filling breccia, and also to detect meteoric water infiltration within the impactite layer. 10Be abundances above background were found in two (out of 24) samples from the LB-07A core, and in none of five samples from the LB-08A core. After excluding other possible explanations for an elevated 10Be signal, we conclude that it is most probably due to a preimpact origin of those clasts from target rocks close to the surface. Our results suggest that in-crater breccias were well mixed during the impact cratering process. In addition, the lack of a 10Be signal within the rocks located very close to the lake sediment-impactite boundary suggests that infiltration of meteoric water below the postimpact crater floor was limited. This may suggest that the infiltration of the meteoric water within the crater takes place not through the aerial pore-space, but rather through a localized system of fractures. © The Meteoritical Society, 2014. Article 2014 English 10869379 10.1111/maps.12256 Meteoritics and Planetary Science 49 University of Arkansas 394 – 411 3 https://www.scopus.com/inward/record.uri?eid=2-s2.0-84897022155&doi=10.1111%2fmaps.12256&partnerID=40&md5=e7312e686417090b078d22c67b6a7b34 Cited by: 3 Anna Losiak Eva Maria Wild Leonard Michlmayr Christian Koeberl article Miller20147 Terrestrial fossil pollen records are frequently used to reveal the response of vegetation to changes in both regional and global climate. Here we present a fossil pollen record from sediment cores extracted from Lake Bosumtwi (West Africa). This record covers the last c. 520 thousand years (ka) and represents the longest terrestrial pollen record from Africa published to date. The fossil pollen assemblages reveal dynamic vegetation change which can be broadly characterized as indicative of shifts between savannah and forest. Savannah formations are heavily dominated by grass (Poaceae) pollen (>55%) typically associated with Cyperaceae, Chenopodiaceae-Amaranthaceae and Caryophyllaceae. Forest formations are palynologically more diverse than the savannah, with the key taxa occurring in multiple forest zones being Moraceae, Celtis, Uapaca, Macaranga and Trema. The fossil pollen data indicate that over the last c. 520ka the vegetation of lowland tropical West Africa has mainly been savannah; however six periods of forest expansion are evident which most likely correspond to global interglacial periods. A comparison of the forest assemblage composition within each interglacial suggests that the Holocene (11-0ka) forest occurred under the wettest climate, while the forest which occurred at the time of Marine Isotope Stage 7 probably occurred under the driest climate. © 2013 The Authors. Review 2014 English 02773791 10.1016/j.quascirev.2013.10.027 Quaternary Science Reviews 84 Elsevier Ltd 7 – 25 Forestry; Isotopes; Lakes; Plants; Pollen; Ashanti; Georgia; Ghana; Lake Bosumtwi; Savannah; United States; West Africa; Amaranthaceae; Caryophyllaceae; Celtis; Cyperaceae; Macaranga; Moraceae; Poaceae; Trema; Uapaca; Isotopes; Lakes; Vegetation; Assemblage composition; Ghana; Interglacial periods; Marine Isotope Stage 7; Pollen; Quaternary; Terrestrial pollen records; West Africa; forest; fossil record; global climate; Holocene; interglacial; marine isotope stage; pollen; Quaternary; savanna; vegetation; Forestry https://www.scopus.com/inward/record.uri?eid=2-s2.0-84889046774&doi=10.1016%2fj.quascirev.2013.10.027&partnerID=40&md5=6cbdcaa0fc3f8cdbbddfb43253686a5e Cited by: 48; All Open Access, Green Open Access, Hybrid Gold Open Access Charlotte S. Miller William D. Gosling article Shanahan201396 The physical, inorganic and organic geochemical and stable isotopic characteristics of lacustrine sediments can provide valuable insights into past environmental changes, provided that the environmental controls on these characteristics are well understood. In the present study, a set of 155 modern vegetation, catchment soil, river sediments and lake surface sediment samples are used to characterize the spatial patterns of TOC, TN, C/N ratios, stable isotope, major element, and particle size distributions within Lake Bosumtwi, a meromictic crater lake in West Africa. Spatial variations in sediment characteristics are strongly correlated with depth and distance from shore, reflecting the dominant influence of lake level on the relative proportions of littoral and pelagic depositional systems and their impact on the physical and geochemical properties of lacustrine sediments. This is supported by a principal component analysis, which indicates that 65% of the variance in the dataset is explained by depth-related variability in the sedimentary components. Variations in sedimentary organic matter reflect the combined influences of productivity, preservation and the relative proportions of aquatic and terrestrial organic matter sources. Grain size and Si content are dominantly influenced by the delivery of clastic materials from the watershed to the lake, whereas Fe and Ca appear to be most strongly influenced by the delivery of reduced metals and nutrients from the anoxic hypolimnion to the surface. With the exception of grain size, all of the sediment parameters record synchronous century-scale variability over the past ~2.5ka, consistent with independent evidence for changes in lake level. However, the magnitude of changes in sediment characteristics differs, reflecting different sensitivities to water depth, particularly in the deepest part of the lake basin where the sediment cores were collected. However, even the parameters with the most linear and strongly significant relationships with depth (e.g., TOC, TN) significantly overestimate the magnitude of lake level changes, suggesting that these parameters may be best suited to qualitative paleolake level reconstructions. Over the last 2 centuries, changes in C/N and δ13C became decoupled from the other geochemical proxies and the inferred rise in lake level. We hypothesize that this reflects the influence of anthropogenic land use change on the composition of terrestrial organic matter sources contributing to the lake. This result highlights the potential difficulties in reconstructing past environmental changes from indirect proxy measurements when those proxies may be subject to multiple varying controls. © 2013 Elsevier B.V. Article 2013 English 00310182 10.1016/j.palaeo.2013.01.008 Palaeogeography, Palaeoclimatology, Palaeoecology 374 96 – 109 Ashanti; Ghana; Lake Bosumtwi; West Africa; correlation; data set; environmental change; geochemical survey; lake ecosystem; land use change; organic matter; paleoenvironment; parameterization; pelagic deposit; principal component analysis; spatiotemporal analysis; stable isotope https://www.scopus.com/inward/record.uri?eid=2-s2.0-84875080379&doi=10.1016%2fj.palaeo.2013.01.008&partnerID=40&md5=3d2392c062215985ec8dbc1389cdee90 Cited by: 20 Timothy M. Shanahan Nicholas McKay Jonathan T. Overpeck John A. Peck Christopher Scholz Clifford W. Heil John King article Gan2013345 Lake sediments commonly contain laminations and the occurrence and quantitative attributes of these microstrata contain signals of their depositional environment, limnological conditions, and past climate. However, the identification and measurement of laminae and their attributes remains a largely semi-manual process that is tedious, labor intensive, but subject to human error. Here, we present a method to automatically measure and accurately extract lamina properties from sediment core images. This method is comprised of four major components: (1) image enhancement that includes noise reduction and contrast enhancement to improve signal-to-background ratio and resolution of laminae; (2) identification of 1-D laminae for a user-chosen area in an image; (3) laminae connectivity analyses on the 1-D laminae to obtain a lamina stratigraphy; and (4) extraction and retrieval of the primary and derived lamination stratigraphic data. Sediment core images from Lake Hitchcock and Lake Bosumtwi were used for algorithm development and testing. Our experiments show a complete match between laminae produced by the software and manual process for images from Lake Hitchcock. Quantitative comparisons reveal an insignificant discrepancy in the number of laminae identified automatically by the software and manually by researchers, and in over 90% of the cases the position mismatch of individual laminae is less than one pixel between the software and the manual method for the experimental images from Lake Bosumtwi. © 2012 Elsevier Ltd. Article 2013 English 00983004 10.1016/j.cageo.2012.10.016 Computers and Geosciences 52 345 – 355 Ashanti; Ghana; Lake Bosumtwi; Algorithms; Image matching; Lakes; Laminating; Sedimentology; Sediments; Stratigraphy; 2-D laminae; Algorithm development; Best fit algorithm; Connectivity analysis; Contrast Enhancement; Depositional environment; Human errors; Image processing - methods; Labor intensive; Lake sediments; Lamina properties; Limnological conditions; Manual methods; Manual process; Paleoclimates; Quantitative attributes; Quantitative comparison; Sediment core; Signal-to-background ratio; Stratigraphic data; algorithm; identification method; image analysis; image processing; lacustrine deposit; paleoclimate; quantitative analysis; sediment core; Image processing https://www.scopus.com/inward/record.uri?eid=2-s2.0-84871527713&doi=10.1016%2fj.cageo.2012.10.016&partnerID=40&md5=e95ac178abf92a6081b439135c2de02c Cited by: 2 Stoney Q. Gan Christopher A. Scholz article Losiak2013297 The Bosumtwi crater is 10.5. km in diameter, 1.07. Ma old, well preserved impact structure located in Ghana (centered at 06°30'N, 01°25'W). It was excavated in rocks of the Early Proterozoic Birimian Supergroup, part of the West African craton. Here, we present a full and detailed characterization of the three granitoid complexes and one mafic dike in the vicinity of the Bosumtwi crater in terms of petrology, major and trace element geochemistry, geochronology, and isotopic composition. This allows us to characterize magmatic evolution of the West African Craton in this area and better understand the geological framework and target rocks of the impact.This study shows that the similar composition (strongly peraluminous muscovite granites and granodiorites) and age (between 2092. ±. 6. Ma and 2098. ±. 6. Ma) of all granitic intrusions in the proximity of the Bosumtwi crater suggest that they are co-genetic. Granitoids were probably formed as a result of anatexis of TTGs (or rocks derived from them) at relatively low pressure and temperature. We propose that the intrusions from the Bosumtwi area are genetically related to the Banso granite occurring to the east of the crater and can be classified as basin-type, late-stage granitoids. Also a mafic dike located to the NE of the Bosumtwi crater seems to be genetically related to those felsic intrusions. Based on those findings a revised version of the geological map of the Bosumtwi crater area is proposed. © 2013. Article 2013 English 00244937 10.1016/j.lithos.2013.06.002 Lithos 177 297 – 313 Ghana; geochronology; granite; granitoid; igneous geochemistry; igneous intrusion; impact structure; isotopic composition; magmatism; petrogenesis; petrology; Proterozoic; trace element https://www.scopus.com/inward/record.uri?eid=2-s2.0-84881500197&doi=10.1016%2fj.lithos.2013.06.002&partnerID=40&md5=147cdd6f1132cab881d301c7664c4b01 Cited by: 11 Anna Losiak Toni Schulz Robert Buchwaldt Christian Koeberl article Shanahan201347 The continuous 300-m long drill cores obtained from Lake Bosumtwi, Ghana represent one of the longest, continuous lacustrine sequences obtained from an extant lake, and contain an unprecedented record of late Quaternary climate change in West Africa. However, one of the main challenges associated with generating long paleoclimate time series from terrestrial records such as this is the development of accurate age-depth relationships because unlike marine records, lacustrine sequences cannot be tuned to global ice volume records via δ18O stratigraphy. The Lake Bosumtwi record thus offers an excellent case study for examining the potential and the challenges associated with different geochronological techniques in lacustrine systems. In the present study, we use a combination of radiocarbon, optically stimulated luminescence and U-series dating and paleomagnetic excursions to generate a chronology for the upper ca. 150 ka of sedimentation at Lake Bosumtwi and employ a Bayesian approach to generate a continuous age-depth relationship. The resultant chronology is then used to test the effectiveness of tuning of an environmental magnetic proxy for dust against a well-dated record of high latitude dust. Our approach highlights the advantages of using multiple dating approaches, and the dangers of relying on too few age constraints when dating long sedimentary sequences. However, the excellent agreement between the different approaches over most of the record suggest that well-constrained age-depth models for long sedimentary sequences can be produced using this combination of approaches. Furthermore, our data provide support for extending the chronology beyond the limit of radiocarbon, U-series and OSL in the future using paleomagnetic excursions/reversals and tuning against well-dated high latitude paleoclimate records. © 2012 Elsevier B.V. Article 2013 English 18711014 10.1016/j.quageo.2012.12.001 Quaternary Geochronology 15 47 – 60 Ashanti; Ghana; Lake Bosumtwi; Bayesian analysis; chronology; climate variation; lacustrine deposit; paleoclimate; paleomagnetism; radiocarbon dating; sedimentary sequence; sedimentation; time series; uranium series dating https://www.scopus.com/inward/record.uri?eid=2-s2.0-84875259409&doi=10.1016%2fj.quageo.2012.12.001&partnerID=40&md5=9106825168f72506e979d68b0be4cb9f Cited by: 29 Timothy M. Shanahan John A. Peck Nicholas McKay Clifford W. Heil John King Steven L. Forman Dirk L. Hoffmann David A. Richards Jonathan T. Overpeck Christopher Scholz article Dzirasah201295 Lake Bosumtwi in Ghana, has been the focus of international paleoclimate research during which continuous sediment cores were recovered for mutliproxy paleoclimate studies. Seventy-six (76) samples of the top 2-8 cm of the sediments were collected at different locations throughout the lake with Ekman dredge to determine indications of depositional environment and paleoclimate. The percentage (%) water and percentage (%) organic matter contents, and the magnetic mineralogy of the sediment samples were determined. The magnetic measurements carried out include isothermal remanent magnetization (IRM), anhysteric remanent magnetisation (ARM) and magnetic susceptibility. The results showed that the percentage (%) water and percentage (%) organic matter contents increased with increasing water depth. The magnetic susceptibilities of the samples increased with increasing water depth, whereas the percentage (%) frequency dependence of susceptibility has a gentle increase with water depth. Most parameters measured show an abrupt change at about 18 m water depth. Values for S-ratio averaged 0.93, indicating the presence of low coercivity magnetic minerals such as maghemite and magnetite. These low coercivity minerals in the surface sediments were deposited in the presently warm and wet interglacial climate with reduced Sahara/Sahel dust influx to Ghana. © EuroJournals Publishing, Inc. 2012. Article 2012 English 1450216X European Journal of Scientific Research 82 EuroJournals, Inc. 95 – 114 1 https://www.scopus.com/inward/record.uri?eid=2-s2.0-84864580139&partnerID=40&md5=ea2b94926a4b9f995fdd6aaff8e97ac7 Cited by: 1 W. Dzirasah S.K. Danuor A. Menyeh J.E. Peck article Shanahan201249 The Lake Bosumtwi sediment record represents one of the longest and highest-resolution terrestrial records of paleoclimate change available from sub-Saharan Africa. Here we report a new sediment age model framework for the last ~. 45. cal kyr of sedimentation using a combination of high-resolution radiocarbon dating, Bayesian age-depth modeling and lamination counting. Our results highlight the practical limits of these methods for reducing age model uncertainties and suggest that even with very high sampling densities, radiocarbon uncertainties of at least a few hundred years are unavoidable. Age model uncertainties are smallest during the Holocene (205. yr) and the glacial (360. yr) but are large at the base of the record (1660. yr), due to a combination of decreasing sample density, larger calibration uncertainties and increases in radiocarbon age scatter. For portions of the chronology older than ~. 35. cal kyr, additional considerations, such as the use of a low-blank graphitization system and more rigorous sample pretreatment were necessary to generate a reliable age depth model because of the incorporation of small amounts of younger carbon. A comparison of radiocarbon age model results and lamination counts over the time interval ~. 15-30. cal kyr agree with an overall discrepancy of ~. 10% and display similar changes in sedimentation rate, supporting the annual nature of sediment laminations in the early part of the record. Changes in sedimentation rates reconstructed from the age-depth model indicate that intervals of enhanced sediment delivery occurred at 16-19, 24 and 29-31. cal kyr, broadly synchronous with reconstructed drought episodes elsewhere in northern West Africa and potentially, with changes in Atlantic meridional heat transport during North Atlantic Heinrich events. These data suggest that millennial-scale drought events in the West African monsoon region were latitudinally extensive, reaching within several hundred kilometers of the Guinea coast. This is inconsistent with a simple southward shift in the mean position of the monsoon rainbelt, and requires changes in moisture convergence as a result of either a reduction in the moisture content of the tropical rainbelt, decreased convection, or both. © 2012 Elsevier B.V. Article 2012 English 00310182 10.1016/j.palaeo.2012.08.001 Palaeogeography, Palaeoclimatology, Palaeoecology 361-362 49 – 60 Ashanti; Ghana; Lake Bosumtwi; Sub-Saharan Africa; calibration; climate change; core analysis; drought; glacial-interglacial cycle; graphitization; heat transfer; Heinrich event; Holocene; laminar flow; moisture content; paleoclimate; radiocarbon dating; sedimentation rate; uncertainty analysis https://www.scopus.com/inward/record.uri?eid=2-s2.0-84866880919&doi=10.1016%2fj.palaeo.2012.08.001&partnerID=40&md5=bc3133df6b258859a109ff49092950ea Cited by: 30 Timothy M. Shanahan J. Warren Beck Jonathan T. Overpeck Nicholas P. McKay Jeffrey S. Pigati John A. Peck Christopher A. Scholz Clifford W. Heil John King article Poste2012340 Lake sediment cores have been used to reconstruct mercury deposition patterns in many parts of the world; however, no studies to date have used these methods in West Africa, nor are there any published measurements of mercury deposition to this region. We measured mercury in a 210Pb dated sediment core from a meromictic crater lake in West Africa (Lake Bosomtwe, Ghana). Lake Bosomtwe has a very small catchment area to lake area ratio (1.1) and the sediment mercury profile is expected to reflect past and present atmospheric mercury deposition to the lake. Mercury concentrations in sediments as well as mercury flux to the sediments increased from the mid-1800s to latter half of the 1900s, however there has been a sharp decline in mercury flux to Lake Bosomtwe in recent decades. The recent decline in mercury flux to Lake Bosomtwe's sediments does not appear to be consistent with trends in local, regional or global mercury emissions, and may instead reflect declining global atmospheric mercury concentrations or declining European emissions, highlighting the importance of long-range atmospheric transport of mercury. © 2012 Elsevier B.V. Article 2012 English 18791026 10.1016/j.scitotenv.2012.01.022 Science of the Total Environment 420 340 – 344 Air Movements; Air Pollution; Environmental Monitoring; Environmental Pollution; Geologic Sediments; Ghana; History, 19th Century; History, 20th Century; Lakes; Mercury; Ashanti; Ghana; Lake Bosumtwi; Anoxic sediments; Atmospheric chemistry; Atmospheric movements; Lakes; Lead; Mercury (metal); Sedimentology; cesium 137; lead 210; mercury; Africa; Atmospheric mercury; Atmospheric mercury deposition; Atmospheric transport; Crater lake; Global mercury emissions; Lake areas; Lake sediments; Long-range atmospheric transport; Mercury concentrations; Mercury depositions; Mercury fluxes; Past and present; Sediment core; Small catchment; West Africa; catchment; concentration (composition); crater lake; deposition; lacustrine deposit; mercury (element); meromictic lake; sediment core; article; catchment; chemical analysis; chemical composition; concentration (parameters); controlled study; geographic distribution; Ghana; isotope labeling; lake; lake sediment; pollution transport; priority journal; sedimentation rate; trend study; water pollution; Lake pollution https://www.scopus.com/inward/record.uri?eid=2-s2.0-84857644926&doi=10.1016%2fj.scitotenv.2012.01.022&partnerID=40&md5=523ac22cabe44b0f34a00d3378b763f7 Cited by: 4 Amanda E. Poste Derek C.G. Muir Megan K. Otu Roland I. Hall Robert E. Hecky article Ferrière2010443 Crater-fill impact breccia and basement rock samples from the 1.07 Ma Bosumtwi impact structure (Ghana) were recovered for the first time in 2004 during an International Continental Scientific Drilling Program (ICDP)-sponsored drilling project. Here, we present detailed results of major-and trace-element analyses of 119 samples from drill core LB-08A, together with the chemical compositions of melt particles from suevite. The meta-graywacke and phyllite/slate crater basement rocks can be easily distinguished from each other on the basis of their bulk chemical compositions. A comparison of the chemical compositions of crater-fill and fallout suevites, as well as between proximal and distal impactites, reveals that LB-08A suevites have higher MgO, CaO, and Na 2 O contents than fallout suevites and, similarly, that the CaO and Na 2 O contents are higher by a factor of approximately two in LB-08A suevites than in Ivory Coast tektites. Noticeable differences occur in Cr, Co, and Ni contents between the different impactites; higher abundances are observed for these elements in distal impactites. The observed differences in composition in the various impactites mainly reflect mixing of different proportions of the original target lithologies, as can be seen in the differences in the clast populations between crater-fill and fallout suevites. However, the original impactite compositions may have also been modifi ed by postimpact alteration, particularly in the proximal impactites. Melt particles in suevite show signifi cant differences in major-element compositions between the different samples investigated, but also within a given sample, indicating that they represent melts derived from different lithologies.© 2010 The Geological Society of America. All rights reserved. Article 2010 English 00721077 10.1130/2010.2465(22) Special Paper of the Geological Society of America 465 Geological Society of America 443 – 469 Ashanti; Ghana; Lake Bosumtwi; Chemical analysis; Core drilling; Fallout; Infill drilling; Magnesia; Rocks; Sodium compounds; Trace elements; Bulk chemical composition; Chemical compositions; Continental scientific drillings; Different proportions; Drilling projects; Impact structures; In compositions; Major and trace elements; basement rock; breccia; chemical composition; crater; geochemistry; impact structure; impactite; suevite; trace element; Meteor impacts https://www.scopus.com/inward/record.uri?eid=2-s2.0-78650961709&doi=10.1130%2f2010.2465%2822%29&partnerID=40&md5=ba1d3315b787f976aff62e4c29283d40 Cited by: 5 Ludovic Ferrière Christian Koeberl Franz Brandstätter Dieter Mader article Coney2010411 A petrographic and geochemical comparison of suevites from the LB-07A and LB-08A cores recovered during 2004 by the International Continental Scientific Drilling Program with suevites from outside of the crater rim of the Bosumtwi impact structure indicates contrasting mechanisms of formation for these respective impact breccias. The within-crater suevites form only a small part of the lithic impact breccia-dominated impactite crater fill, in contrast to the impactites from outside of the crater, which consist solely of suevite. The clasts of suevites from within the crater display relatively low levels of shock (for most material <45 GPa). The numbers of shocked quartz grains, as well as fragments of diaplectic glass of quartz and feldspar in suevites decrease with depth through the LB-07A core (maximum three sets of planar deformation features [PDFs]). In contrast, the out-of-crater suevites sampled north and south of the crater contain up to four PDF sets in quartz clasts, ballen cristobalite, and higher proportions of diaplectic glass than the within-crater suevites. In addition, the suevites from outside of the crater contain significantly more melt particles (18-37 vol%) than the within-crater suevites (<5 vol%). Melt fragment sizes in suevites from outside the crater are much larger than those from suevites within the crater (maximum 40 cm versus 1 cm). The currently known distribution of impactites outside of the crater would be consistent with a low-angle impact from the east. We propose that the within-crater suevites and polymict lithic breccias were emplaced either via slumping off the crater walls or lateral movement of some melted and much displaced target rock within the crater. Limited admixture of fallback material from the ejecta plume is evident in the uppermost impactite deposit encountered in core LB-05B. In contrast, the out-of-crater suevites formed by fallout from a laterally differentiated ejecta plume, which resulted in different clast populations to the north and south of the crater. © 2010 The Geological Society of America. All rights reserved. Article 2010 English 00721077 10.1130/2010.2465(21) Special Paper of the Geological Society of America 465 Geological Society of America 411 – 442 Ashanti; Ghana; Lake Bosumtwi; Feldspar; Glass; Quartz; Rocks; Silicate minerals; Continental scientific drillings; Crater formation; Impact structures; Lateral movement; Mechanisms of formation; Particle characteristics; Planar deformation; Shocked quartz; crater; geochemistry; impact structure; petrography; suevite; Meteor impacts https://www.scopus.com/inward/record.uri?eid=2-s2.0-78650939968&doi=10.1130%2f2010.2465%2821%29&partnerID=40&md5=8d22d7c325bc4a147acc229b0b80bd6b Cited by: 12 Louise Coney Wolf Uwe Reimold Roger L. Gibson Christian Koeberl Paula Ogilvie article Giresse2010341 This study presents a synthetic approach based on the combined use of sediment and C org accumulation rates of well studied lake systems and oceanic margins. Importance of latitude is expressed by the advected flux of terrestrial sediment and, especially, of particulate organic carbon. This most important factor varies throughout the Quaternary, particularly with the intensification of fluxes during Holocene. These changes are linked to the strengthening of monsoon circulation.In various lake systems from Cameroon (Barombi Mbo, Ossa, Assom and Bambili), Gabon (Kamalete) and Congo (Kitina and Sinnda), the global sedimentation and the C org accumulation were slow during dry period and increased during wet period. This relationship is verified to the scale of the Gabon and Congo oceanic margins where the accumulation rates increase during extent of ombrophilous forest. However, the greatest fluxes of organic carbon during wet periods would be balanced by higher concentrations values during the dry period resulting in a nearly homogenous carbon accumulation. These carbon concentrations are generally explained by the input of coarse debris by abrupt floods and by a less degraded organic matter as a result of the cooling of the climate. But, according to specific morphology features or vegetation cover, some lake systems exhibit distinct trends of the sedimentary and C org accumulation rates: (1) Highest accumulation rates coincided with a forest retreat when the slope is too steep (Kamalete in Gabon and Bosumtwi in Ghana) or when river flood overrun (Nguene in Gabon) (2) Increase of the accumulation rate are registered without change of the vegetal cover and express only rainfall growth (Bambili and Assom). © 2010 Elsevier B.V. Article 2010 English 09218181 10.1016/j.gloplacha.2010.01.010 Global and Planetary Change 72 341 – 355 4 Africa; Atlantic Ocean; Gulf of Guinea; Anoxic sediments; Debris; Drought; Lakes; Sedimentology; Accumulation rates; Cameroon; Carbon accumulation; Carbon concentrations; Gulf of Guinea; Holocene; Holocenes; Nearly homogenous; Organic matter; Particulate organic carbon; Particulate organic carbon fluxes; Synthetic approach; Terrestrial sediments; Vegetal cover; Vegetation cover; Wet period; accumulation rate; carbon flux; coastal sediment; Holocene; lacustrine environment; particulate organic carbon; sediment chemistry; sedimentation; terrestrial deposit; Organic carbon https://www.scopus.com/inward/record.uri?eid=2-s2.0-77957277348&doi=10.1016%2fj.gloplacha.2010.01.010&partnerID=40&md5=2228ce4934cde97b788667e5a172e0f5 Cited by: 4 P. Giresse Makaya-Mvoubou article Ferrière2010254 The 1.07. Myr old Bosumtwi impact structure (Ghana), excavated in 2.1-2.2. Gyr old supracrustal rocks of the Birimian Supergroup, was drilled in 2004. Here, we present single crystal U-Pb zircon ages from a suevite and two meta-graywacke samples recovered from the central uplift (drill core LB-08A), which yield an upper Concordia intercept age of ca 2145 ± 82 Ma, in very good agreement with previous geochronological data for the West African Craton rocks in Ghana. Whole rock Rb-Sr and Sm-Nd isotope data of six suevites (five from inside the crater and one from outside the northern crater rim), three meta-graywacke, and two phyllite samples from core LB-08A are also presented, providing further insights into the timing of the metamorphism and a possibly related isotopic redistribution of the Bosumtwi crater rocks. Our Rb-Sr and Sm-Nd data show also that the suevites are mixtures of meta-greywacke and phyllite (and possibly a very low amount of granite). A comparison of our new isotopic data with literature data for the Ivory Coast tektites allows to better constrain the parent material of the Ivory Coast tektites (i.e., distal impactites), which is thought to consist of a mixture of metasedimentary rocks (and possibly granite), but with a higher proportion of phyllite (and shale) than the suevites (i.e., proximal impactites). When plotted in a Rb/Sr isochron diagram, the sample data points (n=29, including literature data) scatter along a regression line, whose slope corresponds to an age of 1846 ± 160 Ma, with an initial Sr isotope ratio of 0.703 ± 0.002. However, due to the extensive alteration of some of the investigated samples and the lithological diversity of the source material, this age, which is in close agreement with a possible "metamorphic age" of ~. 1.8-1.9. Ga tentatively derived from our U-Pb dating of zircons, is difficult to consider as a reliable metamorphic age. It may perhaps reflect a common ancient source whose Rb-Sr isotope systematics has not basically been reset on the whole rock scale during the Bosumtwi impact event, or even reflect another unknown geologic event. © 2010 Elsevier B.V. Article 2010 English 00092541 10.1016/j.chemgeo.2010.05.016 Chemical Geology 275 254 – 261 3-4 Cote d'Ivoire; Ghana; comparative study; country rock; geochronology; graywacke; impact structure; impactite; isotopic composition; isotopic ratio; lithology; metamorphism; neodymium isotope; phyllite; strontium isotope; tektite; uranium-lead dating; zircon https://www.scopus.com/inward/record.uri?eid=2-s2.0-77954533360&doi=10.1016%2fj.chemgeo.2010.05.016&partnerID=40&md5=c79ace79800cf950f45b8c7f4b47ce88 Cited by: 7; All Open Access, Green Open Access, Hybrid Gold Open Access Ludovic Ferrière Christian Koeberl Martin Thöni Chen Liang article Shanahan2009377 Although persistent drought in West Africa is well documented from the instrumental record and has been primarily attributed to changing Atlantic sea surface temperatures, little is known about the length, severity, and origin of drought before the 20th century. We combined geomorphic, isotopic, and geochemical evidence from the sediments of Lake Bosumtwi, Ghana, to reconstruct natural variability in the African monsoon over the past three millennia. We find that intervals of severe drought lasting for periods ranging from decades to centuries are characteristic of the monsoon and are linked to natural variations in Atlantic temperatures. Thus the severe drought of recent decades is not anomalous in the context of the past three millennia, indicating that the monsoon is capable of longer and more severe future droughts. Article 2009 English 10959203 10.1126/science.1166352 Science 324 377 – 380 5925 Africa; Ashanti; Atlantic Ocean; Ghana; Lake Bosumtwi; Sub-Saharan Africa; West Africa; rain; atmospheric forcing; drought; monsoon; reconstruction; sea surface temperature; twentieth century; Africa; article; Atlantic Ocean; controlled study; drought; environmental temperature; Ghana; hydrology; lake sediment; precipitation; priority journal; seasonal variation; spectroscopy https://www.scopus.com/inward/record.uri?eid=2-s2.0-65249152410&doi=10.1126%2fscience.1166352&partnerID=40&md5=5467b612752816295f744c8563fc4ab5 Cited by: 269 T.M. Shanahan J.T. Overpeck K.J. Anchukaitis J.W. Beck J.E. Cole D.L. Dettman J.A. Peck C.A. Scholz J.W. King article Escala20081777 Meteorite impacts are associated with locally profound effects for microorganisms living at the terrestrial surface and the subsurface of the impact zone. The Bosumtwi crater in Ghana (West Africa) is a relatively young (1.07 Myr) structure with a rim-to-rim diameter of about 10.5 km. In a preliminary study targeting the subsurface microbial life in the impact structure, seven samples of the impact breccia from the central uplift of the Bosumtwi crater were analyzed for the presence of typical archaeal membrane-lipids (GDGTs). These have been detected in four of the samples, at a maximum depth of 382 m below the lake surface, which is equivalent to 309 m below the surface sediment. The concentration of the GDGTs does not show a trend with depth, and their distribution is dominated by GDGT-O. Possible origins of these lipids could be related to the soils or rocks predating the impact event, the hydrothermal system generated after the impact, or due to more recent underground water transport. © The Meteoritical Society, 2008. Article 2008 English 10869379 10.1111/j.1945-5100.2008.tb00642.x Meteoritics and Planetary Science 43 University of Arkansas 1777 – 1782 11 https://www.scopus.com/inward/record.uri?eid=2-s2.0-61849165483&doi=10.1111%2fj.1945-5100.2008.tb00642.x&partnerID=40&md5=176bd66cafc5c8bc8f6b293e31640de5 Cited by: 2; All Open Access, Bronze Open Access Marina Escala Antoni Rosell-Melé Susanne Fietz Christian Koeberl article Shanahan2008339 The sediments from Lake Bosumtwi, Ghana contain a unique record of fine-scale (mm to sub-mm) laminations, which will provide a valuable annual chronometer for reconstructing paleoenvironmental changes in West Africa covering much of the last 1 Ma. Comparisons of laminae counts to independent 210Pb dates and the rise in anthropogenic "bomb" radiocarbon support the interpretation of the laminations in the uppermost sediments as registering annual events. Radiocarbon dates on in-situ fish-bone collagen are in agreement with varve counts, further supporting the annual nature of our varve chronology. Over the instrumental period (1925-1999), dark-varve thickness measurements are correlated with local rainfall (r = 0.54) and appear able to resolve decadal-scale changes in precipitation. The relationship between varve thickness and rainfall provides support for our interpretation that dark-colored varve thickness records catchment runoff during the rainy season rather than dust flux during the dry season. Dark laminae alternate with organic and carbonate-rich light laminae formed during the fall period of enhanced productivity. Downcore, varves undergo significant microstratigraphic and geochemical variations, but retain the same pattern of alternating clastic and organic-rich laminae, providing support that the laminae may represent annual time markers for reconstructions of the deeper part of the record. © 2007 Springer Science+Business Media B.V. Article 2008 English 09212728 10.1007/s10933-007-9164-4 Journal of Paleolimnology 40 339 – 355 1 Africa; Ashanti; Ghana; Lake Bosumtwi; Sub-Saharan Africa; West Africa; biogeochemistry; collagen; comparative study; crater lake; environmental indicator; lacustrine deposit; lamination; lead isotope; paleoenvironment; precipitation assessment; radiocarbon dating; reconstruction; stratigraphy; varve https://www.scopus.com/inward/record.uri?eid=2-s2.0-46249119211&doi=10.1007%2fs10933-007-9164-4&partnerID=40&md5=670f7c275cd5aeb934abd023913e385e Cited by: 36 Timothy M. Shanahan Jonathan T. Overpeck J. Warren Beck C. Winston Wheeler John A. Peck John W. King Christopher A. Scholz article Ferrière20081678 Shock wave attenuation rate and formation of central uplifts are not precisely constrained for moderately sized complex impact structures. The distribution of shock metamorphism in drilled basement rocks from the 10.5-kilometer-diameter Bosumtwi crater, and results of numerical modeling of inelastic rock deformation and modification processes during uplift, constrained with petrographic data, allowed reconstruction of the pre-impact position of the drilled rocks and revealed a shock attenuation by ∼5 gigapascals in the uppermost 200 meters of the central uplift. The proportion of shocked quartz grains and the average number of planar deformation feature sets per grain provide a sensitive indication of minor changes in shock pressure. The results further imply that for moderately sized craters the rise of the central uplift is dominated by brittle failure. Article 2008 English 10959203 10.1126/science.1166283 Science 322 1678 – 1681 5908 crater; deformation; numerical model; pressure; quartz; shock metamorphism; shock wave; uplift; article; calculation; Craterostigma; crystallography; priority journal; rock; scientific literature; shock wave https://www.scopus.com/inward/record.uri?eid=2-s2.0-58149236488&doi=10.1126%2fscience.1166283&partnerID=40&md5=decfce110c6880314dfe3592ce7c02a1 Cited by: 43 Ludovic Ferrière Christian Koeberl Boris A. Ivanov Wolf Uwe Reimold article McDonald2007743 An attempt was made to detect a meteoritic component in both crater-fill (fallback) impact breccias and fallout suevites (outside the crater rim) at the Bosumtwi impact structure in Ghana. Thus far, the only clear indication for an extraterrestrial component related to this structure has been the discovery of a meteoritic signature in Ivory Coast tektites, which formed during the Bosumtwi impact event. Earlier work at Bosumtwi indicated unusually high levels of elements that are commonly used for the identification of meteoritic contamination (i.e., siderophile elements, including the platinum group elements [PGE]) in both target rocks and impact breccias from surface exposures around the crater structure, which does not allow unambiguous verification of an extraterrestrial signature. The present work, involving PGE abundance determinations and Os isotope measurements on drill core samples from inside and outside the crater rim, arrives at the same conclusion. Despite the potential of the Os isotope system to detect even small amounts of extraterrestrial contribution, the wide range in PGE concentrations and Os isotope composition observed in the target rocks makes the interpretation of unradiogenic, high-concentration samples as an impact signature ambiguous. © The Meteoritical Society, 2007. Printed in USA. Article 2007 English 10869379 10.1111/j.1945-5100.2007.tb01071.x Meteoritics and Planetary Science 42 University of Arkansas 743 – 753 4-5 https://www.scopus.com/inward/record.uri?eid=2-s2.0-34250666645&doi=10.1111%2fj.1945-5100.2007.tb01071.x&partnerID=40&md5=0616ff4d98855dbd803b5bd9a7dec152 Cited by: 21; All Open Access, Bronze Open Access Iain McDonald Bernhard Peucker-Ehrenbrink Louise Coney Ludovic Ferrière Wolf Uwe Reimold Christian Koeberl article Morris2007801 Physical rock property measurements provide the primary constraints for any geological models hypothesized from geophysical observations. Previous geophysical models of the Bosumtwi impact structure hypothesized that a highly magnetic and dense impact-melt sheet might be the source of the observed magnetic anomalies. However, magnetic susceptibility and density measurements made on International Continental Scientific Drilling Program (ICDP) cores LB-07A and LB-08A from the interior of the Bosumtwi meteorite impact structure contain no evidence for that. Both density and magnetic susceptibility logs on both boreholes exhibit low-amplitude contrasts between the uppermost polymict lithic breccia and suevite, the intermediate monomict lithic breccia, and the lowermost bedrock. The depth extent of fracture-related density reduction is much greater at LB-08A than at LB-07A. A total magnetic intensity log from borehole LB-08A supports the suggestion that magnetic anomalies over Lake Bosumtwi are mainly sourced in undetected and/or covered bedrock intrusions, like the ones outcropping at the northeast and to the southwest of the lake. © The Meteoritical Society, 2007. Printed in USA. Article 2007 English 10869379 10.1111/j.1945-5100.2007.tb01076.x Meteoritics and Planetary Science 42 University of Arkansas 801 – 809 4-5 https://www.scopus.com/inward/record.uri?eid=2-s2.0-34250618300&doi=10.1111%2fj.1945-5100.2007.tb01076.x&partnerID=40&md5=9c64ab19c47759edb1ed8284b8e4ae0a Cited by: 9 William A. Morris Hernan Ugalde Christina Clark article Goderis2007731 In an attempt to identify the type of projectile, 14 samples from the Bosumtwi crater in Ghana were analyzed for platinum group element (PGE) concentrations by nickel sulfide fire assay inductively coupled plasma-mass spectrometry (ICP-MS). The majority of the samples come from the impactite material recovered by cores LB-07A and LB-08A, which were drilled by the International Continental Scientific Drilling program (ICDP). One sample originates from the fallback material found at the contact between the impactite and the overlying lake sediment in core LB-05B. No clear signature of a meteoritic contamination was identified in the 13 impactite samples. The target rock apparently dominates the PGE contribution in the impactites. These results agree with the PGE concentrations reported for the suevites collected at the crater rim and in other parts of the Bosumtwi ICDP cores. However, based on Cr and Os isotopic signatures, a meteoritic component could be present in the sample of fallback material, supporting the reports of the existence of meteoritic material in the Ivory Coast tektites. Further analyses of the fallback material from the Bosumtwi drill cores should confirm (or not) this first result. © The Meteoritical Society, 2007. Printed in USA. Article 2007 English 10869379 10.1111/j.1945-5100.2007.tb01070.x Meteoritics and Planetary Science 42 University of Arkansas 731 – 741 4-5 https://www.scopus.com/inward/record.uri?eid=2-s2.0-34250635058&doi=10.1111%2fj.1945-5100.2007.tb01070.x&partnerID=40&md5=457f83b4d36929ae843896d436d3eb39 Cited by: 17; All Open Access, Green Open Access S. Goderis R. Tagle R.T. Schmitt J. Erzinger Ph. Claeys article Artemieva2007883 Pre-drilling numerical modeling of the Bosumtwi impact event predicted a 200 m thick coherent melt layer, as well as abundant highly shocked target material within the central part of the crater structure. However, these predictions are in disagreement with data from drill core obtained in 2004-2005. Here I provide a brief overview of previous results and discuss possible reasons behind melt deficiency, such as specific impact scenarios (low impact velocity and/or low impact angle), and specific target properties (different composition, high porosity, high content of volatiles). I conclude that the most likely explanation is the dispersion of impactites due to the vaporization of pore water, which was not included in the original numerical model. © The Meteoritical Society, 2007. Printed in USA. Article 2007 English 10869379 10.1111/j.1945-5100.2007.tb01083.x Meteoritics and Planetary Science 42 University of Arkansas 883 – 894 4-5 https://www.scopus.com/inward/record.uri?eid=2-s2.0-34250642321&doi=10.1111%2fj.1945-5100.2007.tb01083.x&partnerID=40&md5=f2e1cc99158466407479c5270ed72b8b Cited by: 23 N. Artemieva article Koeberl2007481 Editorial 2007 English 10869379 10.1111/j.1945-5100.2007.tb01056.x Meteoritics and Planetary Science 42 University of Arkansas 481 – 482 4-5 https://www.scopus.com/inward/record.uri?eid=2-s2.0-34250687520&doi=10.1111%2fj.1945-5100.2007.tb01056.x&partnerID=40&md5=f52a2ddfb24452838c4edac67edca218 Cited by: 1; All Open Access, Bronze Open Access Christian Koeberl Bernd Milkereit Wolf Uwe Reimold article Danour2007541 Gravity and magnetic measurements were carried out at the Bosumtwi crater to determine the geophysical signature of the crater. Land gravity data was acquired at 163 locations around the structure and on the shore of the lake. The separation between the gravity stations was 500 m for radial profiles, but 700-1000 m along roads and footpaths that ran parallel to the lake's shore. Additionally, a marine gravity survey was carried out along 14 north-south and 15 east-west profiles on the lake. Magnetic data was also acquired along 14 north-south profiles on the lake. In all marine surveys, the line spacing was 800 m, and navigation was provided by a Garmin 235 Echo Sounder/GPS. The gravity signature of the crater is characterized by a negative Bouguer anomaly with an amplitude of about -18 mgal. Using the seismic results as constraints, the gravity model obtained indicates the central uplift at a depth of 250 m. The negative anomaly is the contribution of the gravity deficiencies due to fractured and brecciated rocks in the rim area and below the crater floor, the impact breccias within the crater, and the sedimentary and water infilling of the lake. Magnetic modeling yielded a model for the causative body, which is located north of the central uplift: the model has a magnetic susceptibility of 0.03 S.I. and extends from a depth of 250 to 610 m. The causative bodies have been interpreted as impactites. © The Meteoritical Society, 2007. Printed in USA. Article 2007 English 10869379 10.1111/j.1945-5100.2007.tb01059.x Meteoritics and Planetary Science 42 University of Arkansas 541 – 547 4-5 https://www.scopus.com/inward/record.uri?eid=2-s2.0-34250631970&doi=10.1111%2fj.1945-5100.2007.tb01059.x&partnerID=40&md5=d51ea88805da42203e915923e76458b6 Cited by: 3 S.K. Danour A. Menyeh article Karikari2007513 Samples of the country rocks that likely constituted the target rocks at the 1.07 Myr old Bosumtwi impact structure in Ghana, West Africa, collected outside of the crater rim in the northern and southern parts of the structure, were studied for their petrographic characteristics and analyzed for their major- and trace-element compositions. The country rocks, mainly meta-graywacke, shale, and phyllite of the Early Proterozoic Birimian Supergroup and some granites of similar age, are characterized by two generations of alteration. A pre-impact hydrothermal alteration, often along shear zones, is characterized by new growth of secondary minerals, such as chlorite, sericite, sulfides, and quartz, or replacement of some primary minerals, such as plagioclase and biotite, by secondary sericite and chlorite. A late, argillic alteration, mostly associated with the suevites, is characterized by alteration of the melt/glass clasts in the groundmass of suevites to phyllosilicates. Suevite, which occurs in restricted locations to the north and to the south-southwest of the crater rim, contains melt fragments, diaplectic quartz glass, ballen quartz, and clasts derived from the full variety of target rocks. No planar deformation features (PDFs) in quartz were found in the country rock samples, and only a few quartz grains in the suevite samples show PDFs, and in rare cases two sets of PDFs. Based on a total alkali element-silica (TAS) plot, the Bosumtwi granites have tonalitic to quartz-dioritic compositions. The Nb versus Y and Ta versus Yb discrimination plots show that these granites are of volcanic-arc tectonic provenance. Provenance studies of the metasedimentary rocks at the Bosumtwi crater have also indicated that the metasediments are volcanic-arc related. Compared to the average siderophile element contents of the upper continental crust, both country rocks and impact breccias of the Bosumtwi structure show elevated siderophile element contents. This, however, does not indicate the presence of an extraterrestrial component in Bosumtwi suevite, because the Birimian country rocks also have elevated siderophile element contents, which is thought to result from regional hydrothermal alteration that is also related to widespread sulfide and gold mineralization. © The Meteoritical Society, 2007. Printed in USA. Article 2007 English 10869379 10.1111/j.1945-5100.2007.tb01058.x Meteoritics and Planetary Science 42 University of Arkansas 513 – 540 4-5 https://www.scopus.com/inward/record.uri?eid=2-s2.0-34250683440&doi=10.1111%2fj.1945-5100.2007.tb01058.x&partnerID=40&md5=13ea4c3df2f6edbbedca6660981650a4 Cited by: 24 Forson Karikari Ludovic Ferrière Christian Koeberl Wolf Uwe Reimold Dieter Mader article Shanahan20071678 Historical changes in the level of Lake Bosumtwi, Ghana, have been simulated using a catchment-scale hydrological model in order to assess the importance of changes in climate and land use on lake water balance on a monthly basis for the period 1939-2004. Several commonly used models for computing evaporation in data-sparse regions are compared, including the Penman, the energy budget, and the Priestley - Taylor methods. Based on a comparison with recorded lake level variations, the model with the energy-budget evaporation model subcomponent is most effective at reproducing observed lake level variations using regional climate records. A sensitivity analysis using this model indicates that Lake Bosumtwi is highly sensitive to changes in precipitation, cloudiness and temperature. However, the model is also sensitive to changes in runoff related to vegetation, and this factor needs to be considered in simulating lake level variations. Both interannual and longer-term changes in lake level over the last 65 years appear to have been caused primarily by changes in precipitation, though the model also suggests that the drop in lake level over the last few decades has been moderated by changes in cloudiness and temperature over that time. Based on its effectiveness at simulating the magnitude and rate of lake level response to changing climate over the historical record, this model offers a potential future opportunity to examine the palaeoclimatic factors causing past lake level fluctuations preserved in the geological record at Lake Bosumtwi. Copyright © 2006 John Wiley & Sons, Ltd. Article 2007 English 10991085 10.1002/hyp.6359 Hydrological Processes 21 1678 – 1691 13 Africa; Ashanti; Ghana; Lake Bosumtwi; Sub-Saharan Africa; West Africa; Atmospheric temperature; Catchments; Climate change; Computer simulation; Evaporation; Hydrology; Land use; Mathematical models; Precipitation (meteorology); Sensitivity analysis; Atmospheric temperature; Catchments; Climate change; Computer simulation; Evaporation; Land use; Mathematical models; Precipitation (meteorology); Sensitivity analysis; catchment; climate change; energy budget; hydrological modeling; lake level; lake water; land use; water budget; Catchment-scale hydrological model; Energy-budget evaporation model; Hydrology https://www.scopus.com/inward/record.uri?eid=2-s2.0-34447094371&doi=10.1002%2fhyp.6359&partnerID=40&md5=4c0a2923874a66bed668d24179c6c973 Cited by: 54 Timothy M. Shanahan Jonathan T. Overpeck W.E. Sharp Christopher A. Scholz Justice A. Arko article Morrow2007591 Standard and universal stage optical microscope and microRaman spectroscopic examination of quartz from the upper impactite interval of the International Continental Scientific Drilling Program (ICDP) Lake Bosumtwi crater drill core LB-07A demonstrates widespread but heterogeneous evidence of shock metamorphism. In the upper impactite, which comprises interbedded polymict lithic breccia and suevite from a drilling depth of 333.4-415.7 m, quartz occurs as a major component within metasedimentary lithic clasts and as abundant, isolated, single-crystal grains within matrix. The noted quartz shock-metamorphic features include phenomena related to a) deformation, such as abundant planar microstructures, grain mosaicism, and reduced birefringence; b) phase transformations, such as rare diaplectic quartz glass and very rare coesite; c) melting, such as isolated, colorless to dark, glassy and devitrified vesicular melt grains; and d) secondary, post-shock features such as abundant, variable decoration of planar microstructures and patchy grain toasting. Common to abundant planar deformation features (PDFs) in quartz are dominated by ω {1013}-equivalent crystallographic planes, although significant percentages of π{1012} and other higher index orientations also occur; notably, c(0001) planes are rare. Significantly, the quartz PDF orientations match most closely those reported elsewhere from strongly shocked, crystalline-target impactites. Barometry estimates based on quartz alteration in the upper impactite indicate that shock pressures in excess of 20 GPa were widely reached; pressures exceeding 40-45 GPa were more rare. The relatively high abundances of decorated planar microstructures and grain toasting in shocked quartz, together with the nature and distribution of melt within suevite, suggest a water- or volatile-rich target for the Bosumtwi impact event. © The Meteoritical Society, 2007. Printed in USA. Article 2007 English 10869379 10.1111/j.1945-5100.2007.tb01063.x Meteoritics and Planetary Science 42 University of Arkansas 591 – 609 4-5 https://www.scopus.com/inward/record.uri?eid=2-s2.0-34250642323&doi=10.1111%2fj.1945-5100.2007.tb01063.x&partnerID=40&md5=84376da44430d0bdc080c4f4168e0319 Cited by: 17; All Open Access, Bronze Open Access Jared R. Morrow article Scholz2007549 The Lake Bosumtwi impact structure in West Africa offers unparalleled opportunities for the study cratering processes, as the structure is young (1.1 Myr) and minimally eroded. Because the center part of the structure is covered by Lake Bosumtwi, which is 8 km in diameter and ∼70 m deep, it is possible to use marine-type seismic reflection techniques to obtain high-fidelity images of the lake subsurface, including key elements of the impact structure. Eight profiles of multichannel seismic reflection (MCS) data were acquired in a radial pattern across the basin, as well as two other high-resolution seismic reflection surveys. The MCS data show a well-defined central uplift near the north-central part of the lake. Observed within the annular moat surrounding the buried central uplift is a section of post-impact lacustrine sediments more than 300 m thick. The central uplift structure has a diameter of 1.9 km and a maximum height of 130 m above the annular moat. The central uplift has an overall irregular upper surface with a small graben structure. We observe a series of normal faults that extend as much as 120 m into the sedimentary section above the central uplift. We interpret the normal faults to be a consequence of ongoing compaction of the high-porosity materials that comprise the central uplift. The interpreted impact structure surface, defined using seismic reflection data, was combined with regional topographic data from outside the lake in the form of a digital elevation model, which provides a useful perspective of overall impact structure morphology. © The Meteoritical Society, 2007. Printed in USA. Article 2007 English 10869379 10.1111/j.1945-5100.2007.tb01060.x Meteoritics and Planetary Science 42 University of Arkansas 549 – 560 4-5 https://www.scopus.com/inward/record.uri?eid=2-s2.0-34250629649&doi=10.1111%2fj.1945-5100.2007.tb01060.x&partnerID=40&md5=fad922c8c746c98f69241617070f1cbf Cited by: 14; All Open Access, Bronze Open Access Christopher A. Scholz Tobias Karp Robert P. Lyons article Deutsch2007635 The 1.07 Myr old Lake Bosumtwi impact crater in Ghana was drilled within the framework of the International Continental Scientific Drilling Project (ICDP). Hole LB-08A, drilled into the outer flank of the central uplift and with a total depth of 451 m, yielded 215.71 m of impact-related rocks. This paper summarizes observations of the lithological logging on core LB-08A. Between a depth of 235.6 and ∼260 m, the section consists of a melt-bearing allochthonous, polymict, and mostly clast-supported impact breccia. Down to ∼418 m, the section comprises a rather uniform unit of meta-graywacke alternating with phyllite to slate (lower greenschist facies); few (par-) autochthonous impact breccia bodies and rare impact dike breccias are present. The lowermost part of the section contains several centimeter- to decimeter-thick melt-bearing breccia dikes in country rocks identical to those occurring above. Omnipresent fracturing was mapped in a qualitative manner. Most prominent shock effects in the uplifted target rocks comprise planar fractures and deformation elements in quartz and polysynthetic twinning in carbonate minerals; the maximum shock pressure as evidenced by quartz is below 26 GPa. The allochthonous breccias occasionally contain a few vol% of melt particles. Suevites occur outside the crater rim, carrying diaplectic crystals, coesite, and ballen quartz as well as true melt glasses and a variety of lithic clasts, among those spectacular staurolite-rich mica-schists. The recorded shock level in the uplifted target rocks is lower than expected and modeled. Shock recovery experiments with analogue carbonaceous graywackes at 34 and 39.5 GPa yielded nearly complete transformation of quartz into diaplectic glass. We therefore exclude a specific shock behavior of the soft, fluid-rich target material (carbonaceous graywackes, shales, slates) in core LB-08A as the prime or only reason for the melt deficit and the generally low shock levels recorded inside the Lake Bosumtwi impact crater. © The Meteoritical Society, 2007. Printed in USA. Article 2007 English 10869379 10.1111/j.1945-5100.2007.tb01065.x Meteoritics and Planetary Science 42 University of Arkansas 635 – 654 4-5 https://www.scopus.com/inward/record.uri?eid=2-s2.0-34250644945&doi=10.1111%2fj.1945-5100.2007.tb01065.x&partnerID=40&md5=011f7e936f22c35c6bac6fe3423e6a6c Cited by: 24; All Open Access, Bronze Open Access Alexander Deutsch Sabine Luetke Volker Heinrich article Boamah2007561 Lake Bosumtwi is a natural inland freshwater lake that originated from a meteorite impact. The lake is becoming a popular tourist attraction in Ghana and has the potential to be developed as an ecotourism site in the future. However, there have been some unregulated human activities and unplanned infrastructure development, and there are increased levels of pollutants in the lake water. In order to make ecotourism at Lake Bosumtwi successful in the long term, the Lake Bosumtwi Development Committee has been formed to ensure that local people are empowered to mobilize their own capacities. It has been realized that an important criterion required to develop ecotourism in a socially responsible, economically efficient, and environmentally viable way is to foster a constructive dialogue between the local people and tourists about the needs of the indigenous people. © The Meteoritical Society, 2007. Printed in USA. Article 2007 English 10869379 10.1111/j.1945-5100.2007.tb01061.x Meteoritics and Planetary Science 42 University of Arkansas 561 – 567 4-5 https://www.scopus.com/inward/record.uri?eid=2-s2.0-34250625648&doi=10.1111%2fj.1945-5100.2007.tb01061.x&partnerID=40&md5=9d6b9ca02224c27939e39c1e6644c108 Cited by: 17 Daniel Boamah Christian Koeberl article Ugalde2007793 The Bosumtwi impact structure in Ghana is the youngest and best-preserved medium-sized impact structure on Earth, and because of the vast amount of prior geophysical and geological data gathered in the area, it constitutes a great natural laboratory to try to develop new geophysical interpretation and modeling techniques. During the 2004 International Continental Scientific Drilling Program (ICDP) drilling campaign at Lake Bosumtwi, we made magnetic field observations at 162 stations around the lake. This study differs from all previous magnetic surveys at Bosumtwi, which only measured the scalar portion of the Earth's magnetic field, in that we measured the full magnetic vector at each station. Acquisition of the full magnetic vector was made possible by innovative use of a borehole deviation probe, which uses a magnetic sensor for absolute orientation reference. Estimates of the magnetic vector orientation and magnitude at each observation station were derived from a series of measurements collected at 50 cm spacing over a depth range of 25 m. In this study, we report a comparison between the scalar total field intensity derived from this new survey approach with the other two previously acquired marine and airborne magnetic data sets. The scalar total magnetic intensity (TMI) computed from the vector data set compares in close agreement with the other two data sets. Some discrepancies between the data sets can be explained by differences in the distances between the sensor and the magnetic sources for the various surveys. The highlight of this study is that we demonstrate that is possible to acquire at least partial vector data with readily available instrumentation. © The Meteoritical Society, 2007. Printed in USA. Article 2007 English 10869379 10.1111/j.1945-5100.2007.tb01075.x Meteoritics and Planetary Science 42 University of Arkansas 793 – 800 4-5 https://www.scopus.com/inward/record.uri?eid=2-s2.0-34250629233&doi=10.1111%2fj.1945-5100.2007.tb01075.x&partnerID=40&md5=8c0735e38c0622933d51dfc8b1b357ef Cited by: 5 Hernan Ugalde William A. Morris Christina Clark Brett Miles Bernd Milkereit article Ugalde2007867 The Bosumtwi impact structure (Ghana) is a young and well-preserved structure where a vast amount of information is available to constrain any geophysical model. Previous analysis of the airborne magnetic data and results of numerical simulation of impact predicted a strongly magnetic impact-melt body underneath the lake. Recent drilling through the structure did not penetrate such an expected impact-melt rock magnetic source. A new 3-D magnetic model for the structure was constructed based on a newly acquired higher-resolution marine magnetic data set, with consideration of the observed gravity data on the lake, previous seismic models, and the magnetic properties and lithology identified in the two International Continental Scientific Drilling Program (ICDP) deep boreholes. The new model contains highly magnetic bodies located in the northeast sector of the structure, not centered onto the drilling sites. As in previous models, higher magnetization than that measured in outcropping impactites had to be assigned to the unexposed source bodies. Integration of the new model with the borehole petrophysics and published geology indicates that these bodies likely correspond to an extension to the south of the Kumasi batholith, which outcrops to the northeast of the structure. The possibility that these source bodies are related to the seismically identified central uplift or to an unmapped impact-melt sheet predicted by previous models of the structure is not supported. Detailed magnetic scanning of the Kumasi batholith to the north, and the Bansu intrusion to the south, would provide a test for this interpretation. © The Meteoritical Society, 2007. Printed in USA. Article 2007 English 10869379 10.1111/j.1945-5100.2007.tb01082.x Meteoritics and Planetary Science 42 University of Arkansas 867 – 882 4-5 https://www.scopus.com/inward/record.uri?eid=2-s2.0-34250650095&doi=10.1111%2fj.1945-5100.2007.tb01082.x&partnerID=40&md5=7db204d1ebffd5cd39a09b98524a018b Cited by: 13; All Open Access, Bronze Open Access Hernan Ugalde William A. Morris Lauri J. Pesonen Sylvester K. Danuor article Koeberl2007709 In 2004, an International Continental Scientific Drilling Program (ICDP) drilling project at the Bosumtwi impact crater, Ghana (10.5 km in diameter, 1.07 Myr old), was performed to study the sediments that fill the lake as well as the underlying impactites. In one (LB-05) of 16 cores drilled into the lake sediments, the zone between the impact breccias and the post-impact sediments was penetrated, preserving the final, fine-grained impact fallback layer. This ∼30 cm thick layer contains in the top 10 cm. "accretionary" lapilli, microtektite-like glass spherules, and shocked quartz grains. Glass particles - mostly of splash form less than 1 mm size - make up the bulk of the grains (∼70-78% by number) in the coarser size fraction (> 125 μm) of the top of the fallback layer. About one-third of all quartz grains in the uppermost part of the layer are shocked, with planar deformation features (PDFs); almost half of these grains are highly shocked, with 3 or more sets of PDFs. K-feldspar grains also occur and some show shock deformation. The abundance of shocked quartz grains and the average shock level as indicated by the number of sets of PDFs, for both quartz and K-feldspar, decrease with depth into the layer. The well-preserved glass spherules and fragments are chemically rather homogeneous within each particle, and also show relatively small variations between the various particles. On average, the composition of the fallback spherules from core LB-5B is very similar to the composition of Ivory Coast tektites and microtektites, with the exception of CaO contents, which are about 1.5 to 2 times higher in the fallback spherules. This is a rare case in which the uppermost fallback layer and the transition to the post-impact sediments has been preserved in an impact structure; its presence indicates that the impactite sequence at Bosumtwi is complete and that Bosumtwi is a very well-preserved impact crater. © The Meteoritical Society, 2007. Printed in USA. Article 2007 English 10869379 10.1111/j.1945-5100.2007.tb01069.x Meteoritics and Planetary Science 42 University of Arkansas 709 – 729 4-5 https://www.scopus.com/inward/record.uri?eid=2-s2.0-34250612628&doi=10.1111%2fj.1945-5100.2007.tb01069.x&partnerID=40&md5=eca85547e63cf8f8277c8d7ee64c7d78 Cited by: 48 Christian Koeberl Franz Brandstätter Billy P. Glass Lutz Hecht Dieter Mader Wolf Uwe Reimold article Elbra2007829 Physical properties from rocks of the Bosumtwi impact structure, Ghana, Central Africa, are essential to understand the formation of the relatively young (1.07 Ma) and small (10.5 km) impact crater and to improve its geophysical modeling. Results of our petrophysical studies of deep drill cores LB-07A and LB-08A reveal distinct lithological patterns but no depth dependence. The most conspicuous difference between impactites and target lithologies are the lower bulk densities and significantly higher porosities of the suevite and lithic breccia units compared to meta-graywacke and metapelites of target lithologies. Magnetic susceptibility shows mostly paramagnetic values (200-500 × 10-6 SI) throughout the core, with an exception of a few metasediment samples, and correlates positively with natural remanent magnetization (NRM) and Q values. These data indicate that magnetic parameters are related to inhomogeneously distributed ferrimagnetic pyrrhotite. The paleomagnetic data reveals that the characteristic direction of NRM has shallow normal (in a few cases shallow reversed) polarity, which is in agreement with the Lower Jaramillo N-polarity chron direction, and is carried by ferrimagnetic pyrrhotite. However, our study has not revealed the expected high magnetization body required from previous magnetic modeling. Furthermore, the LB-07A and LB08-A drill cores did not show the predicted high content of melt in the rocks, requiring a new interpretation model for magnetic data. © The Meteoritical Society, 2007. Printed in USA. Article 2007 English 10869379 10.1111/j.1945-5100.2007.tb01078.x Meteoritics and Planetary Science 42 University of Arkansas 829 – 838 4-5 https://www.scopus.com/inward/record.uri?eid=2-s2.0-34250637525&doi=10.1111%2fj.1945-5100.2007.tb01078.x&partnerID=40&md5=d6e68467e6c2888bb8d7ee4a2c56e562 Cited by: 20; All Open Access, Bronze Open Access Tiiu Elbra Agnes Kontny Lauri J. Pesonen Norbert Schleifer Christina Schell article Petersen2007655 Drill-core samples from the Bosumtwi impact structure (1.07 Myr old and 10.5 km in diameter) in Ghana exhibit mineralogical evidence for post-impact hydrothermal alteration. Nine samples of drill core obtained through the 2004 International Continental Scientific Drilling Project (ICDP) were studied, including an uppermost fallback layer overlying impactite breccias, and partly deformed massive meta-graywacke bedrock. The petrographic study revealed alteration veins containing secondary sericitic muscovite (comparable to 2M1-muscovite) crosscutting original bedding in meta-graywacke and forming a matrix between clasts in impactite breccias. X-ray diffraction (XRD) shows that these impactite samples are rich in 2M1-muscovite, consistent with post-impact fluid deposition and alteration. Optical analysis indicates the presence of a pre-impact stratiform chlorite in meta-graywacke samples and a secondary alteration chlorite occurring in all samples. Secondary illite was detected in upper impactites of drill core LB-08A and samples containing accretionary lapilli. The lower temperature constraint for the hydrothermal event is given by 2M1-muscovite, secondary chlorite, and illite, all of which form at temperatures greater than 280 °C. An absence of recrystallization of quartz and feldspar indicates an upper temperature constraint below 900 °C. The presence of alteration materials associated with fractures and veins in the uppermost impactites of drill cores LB-07A and LB-08A indicates that a post-impact hydrothermal system was present in and adjacent to the central uplift portion of the Bosumtwi impact structure. A sample containing accretionary lapilli obtained from drill core LB-05A exhibits limited evidence that hydrothermal processes were more widespread within the impactites on the crater floor. © The Meteoritical Society, 2007. Printed in USA. Article 2007 English 10869379 10.1111/j.1945-5100.2007.tb01066.x Meteoritics and Planetary Science 42 University of Arkansas 655 – 666 4-5 https://www.scopus.com/inward/record.uri?eid=2-s2.0-34250666647&doi=10.1111%2fj.1945-5100.2007.tb01066.x&partnerID=40&md5=e3ee7ec17b4aa211974e2983ae696398 Cited by: 8; All Open Access, Bronze Open Access Michael T. Petersen Horton E. Newsom Melissa J. Nelson Duane M. Moore article Kontny2007811 Rock magnetic and magnetic mineralogy data are presented from the International Continental Scientific Drilling Program (ICDP) drill cores LB-07A and LB-08A of the Bosumtwi impact structure in order to understand the magnetic behavior of impact and target lithologies and their impact-related remagnetization mechanism. Basic data for the interpretation of the magnetic anomaly patterns and the magnetic borehole measurements as well as for new magnetic modeling are provided. Magnetic susceptibility (150-500 × 10-6 SI) and natural remanent magnetization (10-3-10-1 A/m) are generally weak, but locally higher values up to 10.6 × 10-3 SI and 43 A/m occur. Sixty-three percent of the investigated rock specimens show Q values above 1 indicating that remanence clearly dominates over induced magnetization, which is a typical feature of impact structures. Ferrimagnetic pyrrhotite is the main magnetite phase, which occurs besides minor magnetite and a magnetic phase with a Curie temperature between 330 and 350 °C, interpreted as anomalous pyrrhotite. Coercive forces are between 20 and 40 mT. Brecciation and fracturing of pyrrhotite is a common feature confirming its pre-impact origin. Grain sizes of pyrrhotite show a large variation but the numerous stress-induced nanostructures observable by transmission electron microscopy (TEM) are assumed to behave as single-domain grains. We suggest that the drilled rocks lost their pre-shock remanence memory during the shock event and acquired a new, stable remanence during shock-induced grain size reduction. The observed brittle microstructures indicate temperatures not higher than 250 °C, which is below the Curie temperature of ferrimagnetic pyrrhotite (310 °C). © The Meteoritical Society, 2007. Printed in USA. Article 2007 English 10869379 10.1111/j.1945-5100.2007.tb01077.x Meteoritics and Planetary Science 42 University of Arkansas 811 – 827 4-5 https://www.scopus.com/inward/record.uri?eid=2-s2.0-34250643137&doi=10.1111%2fj.1945-5100.2007.tb01077.x&partnerID=40&md5=fc936dee77ac0b53b2bc4806b17786a1 Cited by: 25; All Open Access, Bronze Open Access Agnes Kontny Tiiu Elbra Jana Just Lauri J. Pesonen Anja M. Schleicher Jochen Zolk article Ferrière2007611 During a recent drilling project sponsored by the International Continental Scientific Drilling Progam (ICDP), two boreholes (LB-07A and LB-08A) were drilled into the crater fill of the Bosumtwi impact structure and the underlying basement, into the deep crater moat and the outer flank of the central uplift, respectively. The Bosumtwi impact structure in Ghana (West Africa), which is 10.5 km in diameter and 1.07 Myr old, is largely filled by Lake Bosumtwi. Here we present the lithostratigraphy of drill core LB-08A (recovered between 235.6 and 451.33 m depth below lake level) as well as the first mineralogical and petrographic observations of samples from this core. This drill core consists of approximately 25 m of polymict, clast-supported lithic breccia intercalated with suevite, which overlies fractured/brecciated metasediment that displays a large variation in lithology and grain size. The lithologies present in the central uplift are metasediments composed dominantly of fine-grained to gritty meta-graywacke, phyllite, and slate, as well as suevite and polymict lithic impact breccia. The suevites, principally present between 235.6 and 240.5 m and between 257.6 and 262.2 m, display a fine-grained fragmental matrix (about 39 to 45 vol%) and a variety of lithic and mineral clasts that include meta-graywacke, phyllite, slate, quartzite, carbon-rich organic shale, and calcite, as well as melt particles, fractured quartz, unshocked quartz, unshocked feldspar, quartz with planar deformation features (PDFs), diaplectic quartz glass, mica, epidote, sphene, and opaque minerals). The crater-fill suevite contains calcite clasts but no granite clasts, in contrast to suevite from outside the northern crater rim. The presence of melt particles in suevite samples from the uppermost 25 meters of the core and in suevite dikelets in the basement is an indicator of shock pressures exceeding 45 GPa. Quartz grains present in suevite and polymict lithic impact breccia abundantly display 1 to (rarely) 4 sets of PDFs per grain. The shock pressures recorded by the PDFs in quartz grains in the polymict impact breccia range from 10 to ∼30 GPa. We also observed a decrease of the abundance of shocked quartz grains in the brecciated basement with increasing depth. Meta-graywacke samples from the basement are heterogeneously shocked, with shock pressures locally ranging up to 25-30 GPa. Suevites from this borehole show a lower proportion of melt particles and diaplectic quartz glass than suevites from outside the northern crater rim (fallback impact breccia), as well as a lack of ballen quartz, which is present in the external breccias. Similar variations of melt-particle abundance and shock-metamorphic grade between impact-breccia deposits within the crater and fallout impact breccia outside the crater have been observed at the Ries impact structure, Germany. © The Meteoritical Society, 2007. Printed in USA. Article 2007 English 10869379 10.1111/j.1945-5100.2007.tb01064.x Meteoritics and Planetary Science 42 University of Arkansas 611 – 633 4-5 https://www.scopus.com/inward/record.uri?eid=2-s2.0-34250667529&doi=10.1111%2fj.1945-5100.2007.tb01064.x&partnerID=40&md5=d66442439c12c7823abbf9878532c40b Cited by: 36 Ludovic Ferrière Christian Koeberl Wolf Uwe Reimold article Koeberl2007483 The Bosumtwi impact crater in Ghana, arguably the best-preserved complex young impact structure town on Earth, displays a pronounced rim and is almost completely filled by Lake Bosumtwi, a hydrologically closed basin. It is the source crater of the Ivory Coast tektites. The structure was excavated in 2.1-2.2 Gyr old metasediments and metavolcanics of the Birimian Supergroup. A drilling project was conceived that would combine two major scientific interests in this crater: 1) to obtain a complete paleoenvironmental record from the time of crater formation about one million years ago, at a near-equatorial location in Africa for which very few data are available so far, and 2) to obtain a complete record of impactites at the central uplift and in the crater moat, for ground truthing and comparison with other structures. Within the framework of an international and multidisciplinary drilling project led by the International Continental Scientific Drilling Program (ICDP), 16 drill cores were obtained from June to October 2004 at six locations within Lake Bosumtwi, which is 8.5 km in diameter. The 14 sediment cores are currently being investigated for paleoenvironmental indicators. The two impactite cores LB-07A and LB-08A were drilled into the deepest section of the annular moat (540 m) and the flank of the central uplift (450 m), respectively. They are the main subject of this special issue of Meteoritics & Planetary Science, which represents the first detailed presentations of results from the deep drilling into the Bosumtwi impactite sequence. Drilling progressed in both cases through the impact breccia layer into fractured bedrock. LB-07A comprises lithic (in the uppermost part) and suevitic impact breccias with appreciable amounts of impact melt fragments. The lithic clast content is dominated by graywacke, besides various metapelites, quartzite, and a carbonate target component. Shock deformation in the form of quartz grains with planar microdeformations is abundant. First chemical results indicate a number of suevite samples that are strongly enriched in siderophile elements and Mg, but the presence of a definite meteoritic component in these samples cannot be confirmed due to high indigenous values. Core LB-08A comprises suevitic breccia in the uppermost part, followed with depth by a thick sequence of graywacke-dominated metasediment with suevite and a few granitoid dike intercalations. It is assumed that the metasediment package represents bedrock intersected in the flank of the central uplift. Both 7A and 8A suevite intersections differ from suevites outside of the northern crater rim. Deep drilling results confirmed the gross structure of the crater as imaged by the pre-drilling seismic surveys. Borehole geophysical studies conducted in the two boreholes confirmed the low seismic velocities for the post-impact sediments (less than 1800 m/s) and the impactites (2600-3300 m/s). The impactites exhibit very high porosities (up to 30 vol%), which has important implications for mechanical rock stability. The statistical analysis of the velocities and densities reveals a seismically transparent impactite sequence (free of prominent internal reflections). Petrophysical core analyses provide no support for the presence of a homogeneous magnetic unit (= melt breccia) within the center of the structure. Borehole vector magnetic data point to a patchy distribution of highly magnetic rocks within the impactite sequence. The lack of a coherent melt sheet, or indeed of any significant amounts of melt rock in the crater fill, is in contrast to expectations from modeling and pre-drilling geophysics, and presents an interesting problem for comparative studies and requires re-evaluation of existing data from other terrestrial impact craters, as well as modeling parameters. © The Meteoritical Society, 2007. Printed in USA. Review 2007 English 10869379 10.1111/j.1945-5100.2007.tb01057.x Meteoritics and Planetary Science 42 University of Arkansas 483 – 511 4-5 https://www.scopus.com/inward/record.uri?eid=2-s2.0-34250687519&doi=10.1111%2fj.1945-5100.2007.tb01057.x&partnerID=40&md5=99cf93e5b9540d6fb93a7d4fd5110c08 Cited by: 79; All Open Access, Green Open Access Christian Koeberl Bernd Milkereit Jonathan T. Overpeck Christopher A. Scholz Philip Y.O. Amoako Daniel Boamah Sylvester K. Danuor Tobias Karp Jochem Kueck Robert E. Hecky John W. King John A. Peack article Schell2007839 Petrophysical data are commonly used for the discrimination of different lithologies, as the variation in mineralogy, texture, and porosity is accompanied by varying physical properties. A special field of investigation is the analysis of the directional dependence (anisotropy) of the petrophysical properties, which can provide further information on the characteristics of the lithologies, due to the fact that this parameter is different in the various rock-forming and rock-changing processes, e.g., deformation or sedimentation. To characterize the rocks in drill cores LB-07A and LB-08A, which were drilled into the deep crater moat and central uplift of the Bosumtwi impact structure, Ghana, samples were taken for the study of petrophysical properties. In the present work the magnetic properties of these samples were determined in the laboratory. The results are discussed in relation to the various lithologies represented by this sample suite. The shape and degree of magnetic anisotropy, in combination with the magnetic susceptibility, proved useful in distinguishing between the different lithologies present in the drill cores (polymict lithic breccia, suevite, shale component, and meta-graywacke). It was possible to correlate layers of high (shale component), intermediate (graywacke, polymict lithic breccia), and low (suevite) anisotropy degree with the lithostratigraphic sequences determined for cores LB-07A and LB-08A. The shape of the anisotropy showed that foliation is most dominant within the shale component, whereas lineation is more pronounced in the meta-graywacke and polymict lithic breccia. An overall increase of the anisotropy degree was observed from core LB-07A towards core LB-08A. Thus magnetic anisotropy data provide a useful contribution towards an improved petrophysical characterization of the lithostratigraphic sequences in drillcores from the Bosumtwi impact structure. © The Meteoritical Society, 2007. Printed in USA. Article 2007 English 10869379 10.1111/j.1945-5100.2007.tb01079.x Meteoritics and Planetary Science 42 University of Arkansas 839 – 847 4-5 https://www.scopus.com/inward/record.uri?eid=2-s2.0-34250685349&doi=10.1111%2fj.1945-5100.2007.tb01079.x&partnerID=40&md5=0839c7c8f57443ad53c58ee0ae0ef49c Cited by: 3 Christina Schell Norbert Schleifer Tiiu Elbra article Coney2007569 Lithostratigraphic and petrographic studies of drill core samples from the 545.08 m deep International Continental Scientific Drilling Program (ICDP) borehole LB-07A in the Bosumtwi impact structure revealed two sequences of impactites below the post-impact crater sediments and above coherent basement rock. The upper impactites (333.38-415.67 m depth) comprise an alternating sequence of suevite and lithic impact breccias. The lower impactite sequence (415.67-470.55 m depth) consists essentially of monomict impact breccia formed from meta-graywacke with minor shale, as well as two narrow injections of suevite, which differ from the suevites of the upper impactites in color and intensity of shock metamorphism of the clasts. The basement rock (470.55-545.08 m depth) is composed of lower greenschist-facies metapelites (shale, schist and minor phyllite), meta-graywacke, and minor meta-sandstone, as well as interlaminated quartzite and calcite layers. The basement also contains a number of suevite dikelets that are interpreted as injection veins, as well as a single occurrence of granophyric-textured rock, tentatively interpreted as a hydrothermally altered granitic intrusion likely related to the regional pre-impact granitoid complexes. Impact melt fragments are not as prevalent in LB-07A suevite as in the fallout suevite facies around the northern crater rim; on average, 3.6 vol% of melt fragments is seen in the upper suevites and up to 18 vol% in the lower suevite occurrences. Shock deformation features observed in the suevites and polymict lithic breccias include planar deformation features in quartz (1 to 3 sets), rare diaplectic quartz glass, and very rare diaplectic feldspar glass. Notably, no ballen quartz, which is abundant in the fallout suevites, has been found in the within-crater impact breccias. An overall slight increase in the degree of shock metamorphism occurs with depth in the impactites, but considerably lower shock degrees are seen in the suevites of the basement rocks, which show similar features to each other. The bulk of the suevite in LB-07A appears to have been derived from the <35 GPa shock zone of the transient crater. © The Meteoritical Society, 2007. Printed in USA. Article 2007 English 10869379 10.1111/j.1945-5100.2007.tb01062.x Meteoritics and Planetary Science 42 University of Arkansas 569 – 589 4-5 https://www.scopus.com/inward/record.uri?eid=2-s2.0-34250686622&doi=10.1111%2fj.1945-5100.2007.tb01062.x&partnerID=40&md5=50a1bdbdd2acf2952bad0f4684cbfd0a Cited by: 29; All Open Access, Bronze Open Access Louise Coney Roger L. Gibson Wolf Uwe Reimold Christian Koeberl article Morris2007769 An impact event always creates a cloud of ejecta generated through excavation of the target. Subsequent in-filling of the void by crater-fill deposits provides a record of post-impact processes. Full-core digital photographic scans of core segments from borehole LB-07 in the Bosumtwi impact crater provide a complete record of the in-fill process. The shape, orientation, and size of clasts within the impact breccia were measured using a best-fit ellipsoid approach. Clast size and variance, together with clast orientation data, suggest the impact breccias at Bosumtwi can be divided into a simple two-fold subdivision that loosely agrees with the lithological zonation of a lower monomict breccia overlain by a polymict breccia. The lower unit is characterized by a uniform and finer-grained clast size together with a uniform flat-lying clast orientation. The boundary between the two zones is defined by a sharp increase in clast size. The upper zone shows an average increase in clast size with decreasing depth, but full grain size spectrum together with increased grain size deviation suggest that this is a result of mixing between two populations with different grain size distribution. The main population of clasts shows an incremental decrease of clast size with decreasing depth. The upper zone also contains weakly defined shallowly dipping clast fabrics, which may be suggestive of horizontal transport or deposition onto an inclined surface. © The Meteoritical Society, 2007. Printed in USA. Article 2007 English 10869379 10.1111/j.1945-5100.2007.tb01073.x Meteoritics and Planetary Science 42 University of Arkansas 769 – 778 4-5 https://www.scopus.com/inward/record.uri?eid=2-s2.0-34250619597&doi=10.1111%2fj.1945-5100.2007.tb01073.x&partnerID=40&md5=586462d9c7133e8b2e8c6c368031ac20 Cited by: 5; All Open Access, Bronze Open Access William A. Morris Hernan Ugalde Christina Clark Brett Miles book Koeberl200795 Currently about 170 impact craters are known on Earth; about one third of those structures are not exposed on the surface and can only be studied by geophysics or drilling. The impact origin of geological structures can only be confirmed by petrographic and geochemical studies; thus, it is of crucial importance to obtain samples of subsurface structures. In addition, structures that have surface exposures commonly require drilling and drill cores to obtain information of the subsurface structure, to provide ground-truth for geophysical studies, and to obtain samples of rock types not exposed at the surface. For many years, drilling of impact craters was rarely done in dedicated projects, mainly due to the high cost involved. Structures were most often drilled for reasons unrelated to their impact origin. In the former Soviet Union a number of impact structures were drilled for scientific reasons, but in most of these cases the curation and proper care of the cores was not guaranteed. More recently the International Continental Scientific Drilling Program (ICDP) has supported projects to study impact craters. The first ICDPsupported study of an impact structure was the drilling into the 200-kmdiameter, K-T boundary age, subsurface Chicxulub impact crater, Mexico, which occurred between December 2001 and February 2002. The core retrieved from the borehole Yaxcopoil-1, 60 km SSW from the center of the structure, reached a depth of 1511 m and intersected 100 m of impact melt breccia and suevite, which has been studied by an international team. From June to October 2004, the 10.5 km Bosumtwi crater, Ghana, was drilled within the framework of an ICDP project, to obtain a complete 1 million year paleoenvironmental record in an area for which only limited data exist, and to study the subsurface structure and crater fill of one of the best preserved large, young impact structures. From September to December 2005, the main part of another ICDP-funded drilling project was conducted, at the 85-km-diameter Chesapeake Bay impact structure, eastern USA, which involved drilling to a depth of 1.8 km. In 2008, it is likely that the El'ygytgyn structure (Arctic Russia) will be drilled as well. So far only few craters have been drilled - not enough to gain a broad understanding of impact crater formation processes and consequences. In this chapter we summarize the current status of scientific drilling at impact craters, and provide some guidance and suggestions about future drilling projects that are relevant for impact research. Points we cover include: what is the importance of studying impact craters and processes, why is it important to drill impact craters or impact crater lakes, which important questions can be answered by drilling, which craters would be good targets and why; is there anything about the impact process, or of impact relevance, that can be learned by drilling outside any craters; what goals should be set for the future; how important is collaboration between different scientific fields? In the following report, we first briefly discuss the importance of impact cratering, then summarize experience from past drilling projects (ICDP and others), and finally we try to look into the future of scientific drilling of impact structures. © 2007 Springer-Verlag Berlin Heidelberg. Book chapter 2007 English 978-354068777-1 10.1007/978-3-540-68778-8_3 Continental Scientific Drilling: A Decade of Progress, and Challenges for the Future Springer Berlin Heidelberg 95 – 161 https://www.scopus.com/inward/record.uri?eid=2-s2.0-62849087451&doi=10.1007%2f978-3-540-68778-8_3&partnerID=40&md5=9355d072f6daf494807ad2d6383dc4f9 Cited by: 11; All Open Access, Bronze Open Access Christian Koeberl Bernd Milkereit article Ferrière2007689 The 1.07 Myr old Bosumtwi impact structure in Ghana (West Africa), which measures 10.5 km in diameter and is largely filled by Lake Bosumtwi, is associated with one of four currently known tektite strewn fields. Two boreholes were drilled to acquire hard-rock samples of the deep crater moat and from the flank of the central uplift (LB-07A and LB-08A, respectively) during a recent ICDP-sponsored drilling project. Here we present results of major and trace element analysis of 112 samples from drill core LB-08A. This core, which was recovered between 235.6 and 451.33 m depth below lake level, contains polymict lithic breccia intercalated with suevite, which overlies fractured/brecciated metasediment. The basement is dominated by meta-graywacke (from fine-grained to gritty), but also includes some phyllite and slate, as well as suevite dikelets and a few units of a distinct light greenish gray, medium-grained meta-graywacke. Most of the variations of the major and trace element abundances in the different lithologies result from the initial compositional variations of the various target rock types, as well as from aqueous alteration processes, which have undeniably affected the different rocks. Suevite from core LB-08A (fallback suevite) and fallout suevite samples (from outside the northern crater rim) display some differences in major (mainly in MgO, CaO, and Na2O contents) and minor (mainly Cr and Ni) element abundances that could be related to the higher degree of alteration of fallback suevites, but also result from differences in the clast populations of the two suevite populations. For example, granite clasts are present in fallout suevite but not in fallback breccia, and calcite clasts are present in fallback breccia and not in fallout suevite. Chondrite-normalized rare earth element abundance patterns for polymict impact breccia and basement samples are very similar to each other. Siderophile element contents in the impact breccias are not significantly different from those of the metasediments, or compared to target rocks from outside the crater rim. So far, no evidence for a meteoritic component has been detected in polymict impact breccias during this study, in agreement with previous work. © The Meteoritical Society, 2007. Printed in USA. Article 2007 English 10869379 10.1111/j.1945-5100.2007.tb01068.x Meteoritics and Planetary Science 42 University of Arkansas 689 – 708 4-5 https://www.scopus.com/inward/record.uri?eid=2-s2.0-34250674455&doi=10.1111%2fj.1945-5100.2007.tb01068.x&partnerID=40&md5=4e21c73a5fbedae128a60e706e2415b6 Cited by: 16 Ludovic Ferrière Christian Koeberl Wolf Uwe Reimold Dieter Mader article Koeberl2007534 Chromium isotopic compositions and elemental abundances in impact melt rock and impact glass samples from four terrestrial impact craters were measured to verify the presence of an extraterrestrial component and to identify the meteorite type of the impactor. All meteorite classes have Cr isotopic signatures that are different from those of terrestrial rocks; thus, precise measurements of Cr isotopic abundances can unequivocally distinguish terrestrial from extraterrestrial materials. For all four studied craters - Bosumtwi (Ghana), Clearwater East (Canada), Lappajärvi (Finland), and Rochechouart (France) we found positive 53Cr excesses that eliminate carbonaceous chondrite projectiles (because those would show apparent negative excesses) and enstatite chondrites (because of the magnitude of the excess). In all four cases, ordinary chondrites have been identified as the best fit for the data; in the case of Lappajärvi interelement correlations together with the Cr isotope data make an H-chondrite the most likely projectile, whereas in the case of Clearwater East both L or H chondrites are possible. For Bosumtwi and Rochechouart the high indigenous contents of the siderophile elements, and disturbances of the elemental abundances by weathering and hydrothermal alteration, respectively, do not allow further constraints to be placed on the type of ordinary chondrite involved in the impact. © 2007 Elsevier B.V. All rights reserved. Article 2007 English 0012821X 10.1016/j.epsl.2007.02.008 Earth and Planetary Science Letters 256 534 – 546 3-4 Africa; Canada; Eurasia; Europe; Finland; France; Ghana; North America; Northern Europe; Scandinavia; Sub-Saharan Africa; West Africa; Western Europe; Chromium; Landforms; Meteorites; Rocks; Stratigraphy; Chondrites; Crater; Meteoritic component; chromium; crater; hydrothermal alteration; isotopic composition; meteorite; weathering; Geochemistry https://www.scopus.com/inward/record.uri?eid=2-s2.0-34047109673&doi=10.1016%2fj.epsl.2007.02.008&partnerID=40&md5=f90628e7bbe93952cb885ce670d9a9ba Cited by: 55 Christian Koeberl Alex Shukolyukov Günter W. Lugmair article Scholz200716416 The environmental backdrop to the evolution and spread of early Homo sapiens in East Africa is known mainly from isolated outcrops and distant marine sediment cores. Here we present results from new scientific drill cores from Lake Malawi, the first long and continuous, high-fidelity records of tropical climate change from the continent itself. Our record shows periods of severe aridity between 135 and 75 thousand years (kyr) ago, when the lake's water volume was reduced by at least 95%. Surprisingly, these intervals of pronounced tropical African aridity in the early late-Pleistocene were much more severe than the Last Glacial Maximum (LGM), the period previously recognized as one of the most arid of the Quaternary. From these cores and from records from Lakes Tanganyika (East Africa) and Bosumtwi (West Africa), we document a major rise in water levels and a shift to more humid conditions over much of tropical Africa after ≈70 kyr ago. This transition to wetter, more stable conditions coincides with diminished orbital eccentricity, and a reduction in precession-dominated climatic extremes. The observed climate mode switch to decreased environmental variability is consistent with terrestrial and marine records from in and around tropical Africa, but our records provide evidence for dramatically wetter conditions after 70 kyr ago. Such climate change may have stimulated the expansion and migrations of early modern human populations. © 2007 by The National Academy of Sciences of the USA. Article 2007 English 10916490 10.1073/pnas.0703874104 Proceedings of the National Academy of Sciences of the United States of America 104 16416 – 16421 42 Africa, Eastern; Animals; Evolution; Hominidae; Humans; Natural Disasters; Paleontology; Population; Tropical Climate; Homo sapiens; Africa; article; climate change; drought; environment; evolution; human; migration; nonhuman; Pleistocene; priority journal https://www.scopus.com/inward/record.uri?eid=2-s2.0-36749081537&doi=10.1073%2fpnas.0703874104&partnerID=40&md5=f40e36c3f7331068c1f45b8461f35257 Cited by: 332; All Open Access, Green Open Access Christopher A. Scholz Thomas C. Johnson Andrew S. Cohen John W. King John A. Peck Jonathan T. Overpeck Michael R. Talbot Erik T. Brown Leonard Kalindekafe Philip Y. O. Amoako Robert P. Lyons Timothy M. Shanahan Isla S. Castañeda Clifford W. Heil Steven L. Forman Lanny R. McHargue Kristina R. Beuning Jeanette Gomez James Pierson article Coney2007667 In 2004, a drilling project by the International Continental Scientific Drilling Program (ICDP) at the Bosumtwi impact crater, Ghana (1.07 Myr old and 10.5 km in diameter), obtained drill core LB-07A, which sampled impactites and underlying metasediments in the crater moat surrounding the small central uplift of the structure. The LB-07A core consists of three sequences: 82.29 m of an upper impactite sequence of alternating polymict lithic and suevitic impact breccias overlying 54.88 m of so-called lower impactite of monomict impact breccia with several suevite intercalations, and 74.53 m of meta-graywacke and altered shale of the basement, also containing a number of suevite intercalations. Major- and trace-element characteristics of all three sequences have been determined to investigate breccia formation and the role of the respective basement lithologies therein. Compositions of polymict impact breccias of the crater fill revealed by core LB-07A are compared with the compositions of the Ivory Coast tektites and the fallout suevites. The impactites of the LB-07A borehole appear well homogenized with respect to the silicate component, and little change in the ranges of many major- and trace-element differences is seen along the length of the borehole (except for Fe2O3, MgO, and CaO contents). Much scatter is observed for a number of elements, and in many cases this increases with depth. It is proposed that any variability in composition is likely the function of clast population differences (i.e., also of relatively small sample sizes). No systematic compositional difference between polymict lithic and suevitic impact breccias is evident. An indication of carbonate enrichment due to hydrothermal alteration is observed in samples from all lithologies. The impactites of the borehole generally show intermediate compositions to previously defined target rocks. The fallout suevites have comparable major element abundances, except for relatively lower MgO contents. The Ivory Coast tektites are generally similar in composition to the LB-07A suevites, but broader ranges in MgO and CaO contents are observed for the LB-07A suevites. © The Meteoritical Society, 2007. Printed in USA. Article 2007 English 10869379 10.1111/j.1945-5100.2007.tb01067.x Meteoritics and Planetary Science 42 University of Arkansas 667 – 688 4-5 https://www.scopus.com/inward/record.uri?eid=2-s2.0-34250612631&doi=10.1111%2fj.1945-5100.2007.tb01067.x&partnerID=40&md5=d88eb0e070deddf53b50a3f762008eb4 Cited by: 17 Louise Coney Wolf Uwe Reimold Roger L. Gibson Christian Koeberl article LHeureux2007849 The recent drilling of the Bosumtwi impact structure, Ghana, has provided a unique opportunity to study the petrophysical properties of a young, well-preserved impact crater. The damage induced by impact results in extensive fracturing and mixing of target materials. We discuss here a means of using sonic velocity and density logs from two boreholes through the Bosumtwi crater fill and basement to estimate the degree of heterogeneity and fracturing within the impacted target, in order to understand the discrepancy between the large impedances derived from the log data and the nonreflective zone of impactites observed in seismic sections. Based on an analysis of the stochastic fluctuations in the log data, the Bosumtwi impactites are characterized by vertical scale lengths of 2-3 m. From the resolution of the seismic data over the crater, horizontal scale lengths are estimated at <12 m. The impactites therefore fall within the quasi-homogeneous scattering regime, i.e., seismic energy will propagate through the medium with little disruption. Scale lengths as small as these are observed in the fractured basement rocks of impact structures, whereas non-impact related crystalline environments are characterized by scale lengths an order of magnitude larger. Assuming that the high-frequency fluctuations observed in the log data are more sensitive to fracture distribution than petrology, this suggests that the small scale lengths observed within impact structures are characteristic of impact-induced damage, and could be used to estimate the extent of fracturing undergone by the rocks at any depth below an impact structure. © The Meteoritical Society, 2007. Printed in USA. Article 2007 English 10869379 10.1111/j.1945-5100.2007.tb01080.x Meteoritics and Planetary Science 42 University of Arkansas 849 – 858 4-5 https://www.scopus.com/inward/record.uri?eid=2-s2.0-34250625239&doi=10.1111%2fj.1945-5100.2007.tb01080.x&partnerID=40&md5=c14ff69edf2abd6e684a31abf7bb1c82 Cited by: 5; All Open Access, Bronze Open Access Elizabeth L'Heureux Bernd Milkereit article Schmitt2007755 In order to assist in the interpretation of previous seismic refraction and reflection surveys, a vertical seismic profile was acquired in the Lake Bosumtwi (Ghana) hard-rock core hole LB-08A. No seismic reflections are seen in the up-going wave field obtained, and this observation is consistent with the lack of reflectivity observed in the corresponding 2-D surface seismic profile obtained in earlier studies. Direct down-going P-waves were found both in the cased sediment column from a depth of 73 m to 239 m below the lake surface and in the open-hole "hard rock" section from a depth of 239 m to 451 m of LB-08A. Analysis of the observed travel times indicates a nearly constant P-wave velocity of 1520 m/s through the soft lacustrine sediments. In the hard-rock, however, the P-wave velocity rapidly increases by nearly 30% from 2600 m/s to 3340 m/s. These values are in good agreement with the gross velocity structure obtained in the earlier joint inversion of seismic reflection and refraction data. These values are low relative to those expected for the metasedimentary protoliths, an observation that has been made at other young impact structures of similar size. The low velocities, together with the fact that they increase so rapidly, is suggestive of a decreasing density of fractures and microcracks with depth. Consequently, the seismic velocity trend may provide a proxy measure of damage, and hence, the decay of the shock pressure from the impact point. Validation of this requires additional detailed studies of the porosity structure in the core. © The Meteoritical Society, 2007. Printed in USA. Article 2007 English 10869379 10.1111/j.1945-5100.2007.tb01072.x Meteoritics and Planetary Science 42 University of Arkansas 755 – 768 4-5 https://www.scopus.com/inward/record.uri?eid=2-s2.0-34250680541&doi=10.1111%2fj.1945-5100.2007.tb01072.x&partnerID=40&md5=098dc7541b8156cecf70bf34dbd26631 Cited by: 18 Doug R. Schmitt Bernd Milkereit Tobias Karp Christopher Scholz Sylvester Danuor D. Meillieux M. Welz article Ugalde2007859 The Bosumtwi impact structure of central Ghana was drilled in 2004 as part of the International Continental Scientific Drilling Program (ICDP). A vast amount of geoscience data is available from the pre-site surveys and the actual drilling phase. A 3-D gravity model was constructed and calibrated with the available data from the two ICDP boreholes, LB-07A and LB-08A. The 3-D gravity model results agree well with both the sediment thickness and size of the central uplift revealed by previously collected seismic data, and with the petrophysical data from the LB-08A and LB-07A core materials and the two borehole logs. Furthermore, the model exhibits lateral density variations across the structure and refines the results from previous 2.5-D modeling. An important new element of the 3-D model is that the thickness of the intervals comprising polymict lithic impact breccia and suevite, monomict lithic breccia and fractured basement is much smaller than that predicted by numerical modeling. © The Meteoritical Society, 2007. Printed in USA. Article 2007 English 10869379 10.1111/j.1945-5100.2007.tb01081.x Meteoritics and Planetary Science 42 University of Arkansas 859 – 866 4-5 https://www.scopus.com/inward/record.uri?eid=2-s2.0-34250661060&doi=10.1111%2fj.1945-5100.2007.tb01081.x&partnerID=40&md5=d34db7d32a5c05d17e87f405c35979c0 Cited by: 9; All Open Access, Bronze Open Access Hernan Ugalde Sylvester K. Danuor Bernd Milkereit conference Meillieux2007447 We present here an integrated case study of the Lake Bosumtwi impact structure, Ghana, consisting of VSP, geophysical logging, and laboratory measurements on core samples. Bosumtwi crater is 1.07Ma old and has a diameter of 11km, the target rocks being metasediments of 2Ga. The seismic measurements show lower P and S-waves velocities than what is expected in those kinds of rocks. The data are altered by high energy reverberations originating from the air bubble in the water and the steel casing in the sediments. However we obtained velocities between 220 and 350m, going from 2500 to 3000 m/s for the P-wave and from 1000 to 1100 m/s for the S-wave. This indicates a high degree of fracturation and comminution in the hard rock. He-porosity values show a bimodal distribution, with porosities ranging from 1% to 9%, and from 15% to almost 40%. There is a strong linear relationship between porosity and envelope density, but nothing apparent between porosity and grain density or depth. First results with Hg-porosimetry show a bimodal distribution of pore sizes. More samples need to be analyzed in order to draw conclusions on pore structures, but first hypotheses suggest different sizes of crack or the presence of interstitial material depending on rock types and position within the crater. © 2007 Society of Exploration Geophysicists. All rights reserved. Conference paper 2007 English 978-160423897-6 10523812 10.1190/1.2792460 Society of Exploration Geophysicists - 77th SEG International Exposition and Annual Meeting, SEG 2007 26 Society of Exploration Geophysicists 447 – 451 1 Lakes; Pore size; Rocks; Seismic waves; Seismology; Shear waves; Bimodal distribution; Different sizes; Geophysical logging; Impact structures; Interstitial materials; Laboratory measurements; Linear relationships; Seismic measurements; Seismic prospecting https://www.scopus.com/inward/record.uri?eid=2-s2.0-85055678596&partnerID=40&md5=e03580ff15d8a2236a623af145f29089 Cited by: 0 Damien Meillieux Douglas Schmitt Bernd Milkereit Sylvester Danour Kwame Nkrumah article Hunze2007779 Bosumtwi is a very well-preserved 1.07 Myr old, complex terrestrial impact crater located in south-central Ghana, West Africa. The impact structure has a diameter of about 10.5 km and was formed in 2.1-2.2 Gyr Precambrian metasedimentary and metavolcanic rocks. Drilling and logging was carried out during the Lake Bosumtwi Drilling Project (BCDP) which was supported by the International Continental Scientific Drilling Program (ICDP). One of the aims of this project is to achieve detailed information on the subsurface structure and crater fill of one of the best preserved large young impact structures. We interpreted the wireline logs and televiewer images. The physical properties including shallow resistivity, p-wave velocity, magnetic susceptibility, and borehole diameter of the breccia differ significantly from those of the meta-graywackes and slate/phyllites. Fractures observed in the televiewer images are interpreted to determine their characteristic structural features. The fracture dip angles are steep (50-70°) and the two main dip directions are southeast and southwest. Most fractures observed in the borehole are open. The indicated main stress direction is north-south. © The Meteoritical Society, 2007. Printed in USA. Article 2007 English 10869379 10.1111/j.1945-5100.2007.tb01074.x Meteoritics and Planetary Science 42 University of Arkansas 779 – 792 4-5 https://www.scopus.com/inward/record.uri?eid=2-s2.0-34250673565&doi=10.1111%2fj.1945-5100.2007.tb01074.x&partnerID=40&md5=44cf4765b47b9ae95c3ac2bee78a6c3c Cited by: 9; All Open Access, Bronze Open Access Sabine Hunze Thomas Wonik article Boamah20061761 Field studies and a shallow drilling program carried out in 1999 provided information about thickness and distribution of suevite to the north of the Bosumtwi crater rim. Suevite occurrence there is known from an ∼1.5 km2 area; its thickness is ≤15 m. The present suevite distribution is likely the result of differential erosion and does not reflect the initial areal extent of continuous Bosumtwi ejecta deposits. Here we discuss the petrographic characteristics of drill core samples of melt-rich suevite. Macroscopic constituents of the suevites are melt bodies and crystalline and metasedimentary rock (granite, graywacke, phyllite, shale, schist, and possibly slate) clasts up to about 40 cm in size. Shock metamorphic effects in the clasts include multiple sets of planar deformation features (PDFs), diaplectic quartz and feldspar glasses, lechatelierite, and ballen quartz, besides biotite with kink bands. Basement rock clasts in the suevite represent all stages of shock metamorphism, ranging from samples without shock effects to completely shock-melted material that is indicative of shock pressures up to ∼60 GPa. © The Meteoritical Society, 2006. Article 2006 English 10869379 10.1111/j.1945-5100.2006.tb00450.x Meteoritics and Planetary Science 41 University of Arkansas 1761 – 1774 11 Africa; Ghana; Sub-Saharan Africa; West Africa; crater; crystalline rock; ejecta; fallout; impact structure; metasedimentary rock; suevite https://www.scopus.com/inward/record.uri?eid=2-s2.0-33751354610&doi=10.1111%2fj.1945-5100.2006.tb00450.x&partnerID=40&md5=81fff0917c487cd736818e9b033394a4 Cited by: 25 Daniel Boamah Christian Koeberl article Shanahan2006287 A detailed investigation of geomorphological evidence of paleoshorelines and exposed stratigraphic sections of lake deposits, combined with a chronology based on radiocarbon dated charcoal and in-situ 14C dating of wave polished bedrock, provide important new constraints on lake level changes of Lake Bosumtwi, Ghana. Thick sequences of laminated silts, alternating with transgressive sands and deltaic gravels, attest to a long history of climatically controlled lake level variations. The post-glacial rise in lake level began sometime after 16.3 ka, reached stable levels first at 14.5 ± 0.6 ka and then rose again after ca. 14.3 ka. A significant lake level regression spanned the interval from 12.6 ± 0.3 to 11.6 ± 0.5 ka, synchronous with the Younger Dryas. Deep lake conditions were reestablished after ca. 11 ka, at which time the lake overtopped the crater. Overflow continued until 8.8 ± 0.5 ka, when another significant but short-lived regression occurred. Deep, but probably not overflowing conditions were again reestablished by > 7.2 ± 0.3 ka and continued until around 3.2 ± 0.1 ka, when lake level dropped precipitously. Multicentury late Holocene highstands occurred at 2.2 ± 0.1 and 1.7 ± 0.2 ka, although these were significantly lower than those registered in the late glacial and early Holocene. The timing of late glacial events is similar to those recorded elsewhere in Africa and the higher latitudes, and likely reflects the dominant control of high latitude northern hemisphere conditions on the African tropics during the times of large northern hemisphere ice sheets. Mid- to late-Holocene variations appear to be less coupled with changes across Africa and elsewhere, suggesting that regional forcing may be more important during warmer periods. © 2006. Article 2006 English 00310182 10.1016/j.palaeo.2006.06.007 Palaeogeography, Palaeoclimatology, Palaeoecology 242 287 – 302 3-4 Africa; Ashanti; Ghana; Lake Bosumtwi; Sub-Saharan Africa; West Africa; climate variation; crater lake; highstand; Holocene; ice sheet; lacustrine deposit; lake level; Last Glacial; monsoon; paleoclimate; Pleistocene; radiocarbon dating; Younger Dryas https://www.scopus.com/inward/record.uri?eid=2-s2.0-33750980556&doi=10.1016%2fj.palaeo.2006.06.007&partnerID=40&md5=f4fffa937d0dd36e93a9853708b82033 Cited by: 127 Timothy M. Shanahan Jonathan T. Overpeck C. Winston Wheeler J. Warren Beck Jeffrey S. Pigati Michael R. Talbot Christopher A. Scholz John Peck John W. King article Dai20051493 Major and trace element data, including platinum group element abundances, of representative impactites and target rocks from the crater rim and environs of the Bosumtwi impact structure, Ghana, have been investigated for the possible presence of a meteoritic component in impact-related rocks. A comparison of chemical data for Bosumtwi target rocks and impactites with those for Ivory Coast tektites and microtektites supports the interpretation that the Bosumtwi structure and Ivory Coast tektites formed during the same impact event. High siderophile element contents (compared to average upper crustal abundances) were determined for target rocks as well as for impactites. Chondrite-normalized (and iron meteorite-normalized) abundances for target rocks and impactites are similar. They do not, however, allow the unambiguous detection of the presence, or identification of the type, of a meteoritic component in the impactites. The indigenous siderophile element contents are high and possibly related to regional gold mineralization, although mineralized samples from the general region show somewhat different platinum-group element abundance patterns compared to the rocks at Bosumtwi. The present data underline the necessity of extensive target rock analyses at Bosumtwi, and at impact structures in general, before making any conclusions regarding the presence of a meteoritic component in impactites. © The Meteoritical Society, 2005. Article 2005 English 10869379 10.1111/j.1945-5100.2005.tb00414.x Meteoritics and Planetary Science 40 University of Arkansas 1493 – 1511 9-10 crater; impact structure; impactite; meteorite; shock metamorphism https://www.scopus.com/inward/record.uri?eid=2-s2.0-29244471693&doi=10.1111%2fj.1945-5100.2005.tb00414.x&partnerID=40&md5=8fe12ac1d0c08a40c6160782d7760e8e Cited by: 19; All Open Access, Bronze Open Access Xiongxin Dai Daniel Boamah Christian Koeberl Wolf Uwe Reimold Gordon Irvine Iain McDonald article Brooks2005235 Results from the first high-resolution, single-channel seismic-reflection survey of tropical Lake Bosumtwi, Ghana, and sedimentological data from a 14C-dated sediment piston core were used to revise and extend the basin's late-Quaternary lake level history. We report four seismic sequence boundaries and an exposure surface from a sediment core, which are interpreted as erosional surfaces formed at times of drastic low lake level. The youngest erosional surface occurs as much as 31 m below present lake level (bpll) and up to 0.7 m below the present sediment-water interface. This most recent unconformity observed in the seismic data is interpreted to be coeval with the basin-wide late-Holocene dry period between 0.5 and 1 cal ky BP (calendar years before present). Another exposure surface observed in a sediment core is based on an abrupt contact separating low density, wet, clay rich sediments from underlying high density, compact, silt-rich and rootlet-rich sediments, and is interpreted to have developed prior to 16.8 cal ky BP when the lake was ∼60 m bpll. Three older, erosional surfaces occur at depths of ∼92±3, 102±3, and 107±4 m bpll, suggesting numerous lowstands in Lake Bosumtwi during the late-Pleistocene. By extrapolation of average sedimentation rates (0.41 m/ky) from the upper ∼10.5 m of sediment, we estimate the ages of these older lowstands to be ∼65, ∼86, ∼108 cal ky BP. The lowstands of Lake Bosumtwi evidenced from the seismic and sediment core data are interpreted as a response to increased aridity in this part of the equatorial tropics and may correlate to other observed continent-wide shifts in African climate over the past 100 ky, and possibly to rapid climatic shifts observed at high latitudes. Determining the precise timing of these lowstands will ultimately reveal much about the drought dynamics of tropical and subtropical Africa. © 2004 Elsevier B.V. All rights reserved. Article 2005 English 00310182 10.1016/j.palaeo.2004.10.005 Palaeogeography, Palaeoclimatology, Palaeoecology 216 Elsevier B.V. 235 – 249 3-4 Africa; Ashanti; Eastern Hemisphere; Ghana; Lake Bosumtwi; Sub-Saharan Africa; West Africa; World; lake level; lowstand; Quaternary; seismic survey https://www.scopus.com/inward/record.uri?eid=2-s2.0-11844278535&doi=10.1016%2fj.palaeo.2004.10.005&partnerID=40&md5=a7e39859c8c7c70baebac4d016b7b68c Cited by: 29 Keely Brooks Christopher A. Scholz John W. King John Peck Jonathan T. Overpeck James M. Russell Philip Y.O. Amoako article Koeberl200523 The 10.5 -km-diameter, 1.07-Ma Bosumtwi impact crater was the subject of a multi-disciplinary and international drilling effort of the International Continental Scientific Drilling Program (ICDP) from July to October 2004. Sixteen different holes were drilled at six locations within the lake, to a maximum depth of 540 m. A total of about 2.2 km of core material was obtained. Despite some technical and logistical challenges, the project has been very successful and it is anticipated that the first scientific results will be available in late 2005. © 2005 Copernicus GmbH. Article 2005 English 18168957 10.2204/iodp.sd.1.04.2005 Scientific Drilling 1 Copernicus GmbH 23 – 27 Coremaking; Lakes; Continental scientific drillings; Core material; Drilling projects; Impact craters; Scientific results; Infill drilling https://www.scopus.com/inward/record.uri?eid=2-s2.0-84871521923&doi=10.2204%2fiodp.sd.1.04.2005&partnerID=40&md5=308aa0fdc62e3243bdbf725b0795463a Cited by: 10; All Open Access, Gold Open Access, Green Open Access Christian Koeberl John Peck John King Bernd Milkereit Jonathan Overpeck Christopher Scholz article Peck200437 We report magnetic hysteresis results from sediment cores obtained from Lake Bosumtwi, Ghana. As a hydrologically closed basin, the water budget of Lake Bosumtwi is extremely sensitive to changes in the precipitation/ evapotranspiration balance. Lake Bosumtwi lies in the path of the seasonal migration of the intertropical convergence zone (ITCZ); hence, the lake is ideally situated to study monsoon variability in West Africa. Five distinctive magnetic mineral zones (A-E) were identified in the 11-m-long sediment cores that span the last 26,000 calendar years. Prior to 12 calendar (cal) ka, low concentrations of multidomain, high-coercivity magnetic minerals are present. Three prominent shifts towards very high concentrations of high-coercivity iron sulfide (greigite) magnetic minerals are centered at 12,470, 17,290, and 22,600 calendar years during the last glacial period (magnetic zones D1-3). Between 12 and 3.2 cal ka, there is an abrupt shift to moderately high concentrations of mixed multidomain and single-domain, low-coercivity minerals and an organic-rich sapropel lithology. Since 3.2 cal ka, the magnetic mineral parameters reveal a shift to increased amounts of high-coercivity magnetic minerals. These magnetic mineral zones document tropical climate variability on a variety of temporal scales. Glacial age sediments have a high-coercivity magnetic mineralogy due to increased aeolian dust transport from the Sahel to Lake Bosumtwi as well as postdepositional reductive diagenesis. During the last glacial period, the increased strength of Harmattan and North African continental trade winds, the southward depression of the ITCZ, and weakened summer monsoon strength resulted in increased regional aridity and greater dust flux out of Sahel source regions. The greigite-bearing D magnetic zones correspond to brief lowstands in the level of Lake Bosumtwi and likely represent periods of intensified aridity in West Africa. The D magnetic zones closely resemble the timing and duration of Heinrich events and suggest a hemispheric-scale climatic coupling between the tropics and poles. The well-documented African humid period (AHP) is characterized by abrupt shifts in magnetic parameters between 12 and 3.2 cal ka. Dust flux to Lake Bosumtwi is inferred to be very low during this humid interval due to the strengthening of the summer monsoon. Since 3.2 cal ka, the magnetic mineral parameters suggest increased aridity as compared to the AHP. This work demonstrates that the magnetic properties of Lake Bosumtwi sediment are a sensitive recorder of abrupt climate change of global significance. © 2004 Elsevier B.V. All rights reserved. Article 2004 English 00310182 10.1016/S0031-0182(04)00438-9 Palaeogeography, Palaeoclimatology, Palaeoecology 215 Elsevier B.V. 37 – 57 1-2 Africa; Ashanti; Eastern Hemisphere; Ghana; Lake Bosumtwi; Sub-Saharan Africa; West Africa; World; climate change; dust; greigite; magnetic mineral; monsoon; paleoclimate; paleolimnology; Quaternary; sedimentation; water budget https://www.scopus.com/inward/record.uri?eid=2-s2.0-8444234767&doi=10.1016%2fS0031-0182%2804%2900438-9&partnerID=40&md5=a909ad7ba355f1387c2b65115d316b3c Cited by: 95 John A. Peck Ryan R. Green Tim Shanahan John W. King Jonathan T. Overpeck Christopher A. Scholz article Glass20043971 We have determined the major and trace element compositions of 176 individual microtektites/spherules from the Australasian, Ivory Coast, and North American microtektite and clinopyroxene-bearing (cpx) spherule layers. Trace element contents for up to 30 trace elements were determined by instrumental neutron activation analysis (INAA), and major element compositions were determined using energy dispersive X-ray (EDX) analysis in combination with a scanning electron microscope (SEM). In addition, petrographic data were obtained for the cpx spherules using the SEM and EDX. This is the first trace element study of individual Australasian microtektites, and the data revealed the presence of a previously unrecognized group of Australasian microtektites with high contents of Ni (up to 471 ppm). In previous studies the high-Mg (HMg) Australasian microtektites were thought to be related to the HMg Australasian tektites, but our trace element data suggest that the high-Ni (HNi) Australasian microtektites, rather than the high-Mg microtektites, are related to the high-Mg Australasian tektites. We find that Cenozoic microtektites/spherules from a given layer can be distinguished from microtektites/spherules from other layers as a group, but it is not always possible to determine which layer an individual microtektite/spherule came from based only on trace element compositions. The cpx spherules and most of the microtektites have Cr, Co, and Ni contents that are higher than the average contents of these elements in the upper continental crust, suggesting the presence of a meteoritic component. The highest Cr, Co, and Ni contents are found in the cpx spherules (and low-Si cpx-related microtektites). Unetched to slightly etched cpx spherules have Ni/Cr and Ni/Co ratios that generally lie along mixing curves between the average upper continental crust and chondrites. The best fit appears to be with an LL chondrite. The moderately to heavily etched cpx spherules have values that lie off the mixing curves in a direction that suggests Ni loss, probably as a result of solution of a Ni-rich phase (olivine?). The Ni-rich Australasian microtektites also have Ni values that lie close to mixing curves between the average upper continental crust and chondrites. However, both the cpx spherules and HNi Australasian microtektites appear to have Ir (and to a lesser extent Au) contents that are much too low to have Ni/Ir ratios similar to chondritic values. We have no explanation for the low-Ir and -Au contents except to speculate that they may be the result of a complex fractionation process. The Ivory Coast and North American microtektites do not have high enough siderophile element contents to reach any firm conclusions regarding the presence of, or nature of, a meteoritic component in them. Trace element compositions are consistent with derivation of the Cenozoic microtektite/spherule layers from upper continental crust. The normal Australasian microtektites appear to have been derived from a graywacke or lithic arenite with a range in clay and quartz content. The source rock for the high-Mg Australasian microtektites is not known, but the HMg microtektites do not appear to be normal Australasian microtektites that were simply contaminated by meteorites or ultramafic rocks. The average Ivory Coast microtektite composition can be matched with a mixture of target rocks at the Bosumtwi crater. The average composition of the North American microtektites suggests an arkosic source rock, but with graywacke and quartz-rich end members. However, we could not match the composition of the North American microtektites with lithologies in impact breccias recovered from the Chesapeake Bay impact structure that is believed to be the source crater. Likewise, we could not match the composition of the cpx spherules with mixtures of basement rocks and overlying sedimentary deposits (for which compositional data are available) at the Popigai impact crater that may be the source crater for the cpx spherules. This may be because the cpx spherules were derived, in large part, from clastic surface rocks (sandstones and shales) for which no compositional data are available. Copyright © 2004 Elsevier Ltd. Article 2004 English 00167037 10.1016/j.gca.2004.02.026 Geochimica et Cosmochimica Acta 68 3971 – 4006 19 Cenozoic; clinopyroxene; geochemistry; spherule; tektite https://www.scopus.com/inward/record.uri?eid=2-s2.0-9144271217&doi=10.1016%2fj.gca.2004.02.026&partnerID=40&md5=d3172b3624d7f936143b793133aa6054 Cited by: 63 Billy P. Glass Heinz Huber Christian Koeberl article Boamah20031137 The 1.07 Ma well-preserved Bosumtwi impact structure in Ghana (10.5 km in diameter) formed in 2 Ga-old metamorphosed and crystalline rocks of the Birimian system. The interior of the structure is largely filled by the 8 km diameter Lake Bosumtwi, and the crater rim and region in the environs of the crater is covered by tropical rainforest, making geological studies rather difficult and restricted to road cuts and streams. In early 1999, we undertook a shallow drilling program to the north of the crater rim to determine the extent of the ejecta blanket around the crater and to obtain subsurface core samples for mineralogical, petrological, and geochemical studies of ejecta of the Bosumtwi impact structure. A variety of impactite lithologies are present, consisting of impact glass-rich suevite and several types of breccia: lithic breccia of single rock type, often grading into unbrecciated rock, with the rocks being shattered more or less in situ without much relative displacement (autochthonous?), and lithic polymict breccia that apparently do not contain any glassy material (allochtonous?). The suevite cores show that melt inclusions are present throughout the whole length of the cores in the form of vesicular glasses with no significant change of abundance with depth. Twenty samples from the 7 drill cores and 4 samples from recent road cuts in the structure were studied for their geochemical characteristics to accumulate a database for impact lithologies and their erosion products present at the Bosumtwi crater. Major and trace element analyses yielded compositions similar to those of the target rocks in the area (graywacke-phyllite, shale, and granite). Graywacke-phyllite and granite dikes seem to be important contributors to the compositions of the suevite and the road cut samples (fragmentary matrix), with a minor contribution of Pepiakese granite. The results also provide information about the thickness of the fallout suevite in the northern part of the Bosumtwi structure, which was determined to be ≤15 m and to occupy an area of ∼1.5 km2. Present suevite distribution is likely to be caused by differential erosion and does not reflect the initial areal extent of the continuous Bosumtwi ejecta deposits. Our studies allow a comparison with the extent of the suevite at the Ries, another well-preserved impact structure. Article 2003 English 10869379 10.1111/j.1945-5100.2003.tb00304.x Meteoritics and Planetary Science 38 University of Arkansas 1137 – 1159 8 Bosumtwi Impact Structure; Ghana; breccia; ejecta; geochemistry; geological structure; impact structure; tektite https://www.scopus.com/inward/record.uri?eid=2-s2.0-0346963758&doi=10.1111%2fj.1945-5100.2003.tb00304.x&partnerID=40&md5=b2feacda66e69800674021b5a82a8302 Cited by: 31 Daniel Boamah Christian Koeberl article Russell2003133 Lake Bosumtwi is one of the most widely studied palaeoclimate archives in West Africa. Results from numerous AMS 14C dates of samples from four piston cores from Lake Bosumtwi show that an abrupt sedimentary transition from a mid-Holocene sapropel to calcareous laminated muds occurred at about 3200 cal yr B.P. High-resolution analyses of the nitrogen isotopic composition of organic matter across this transition confirm its abrupt nature, and suggest that the change may signal a step toward increased aridity and intensified surface winds that affected western equatorial Africa from Ghana to the Congo basin. Northern and Eastern Africa experienced a similar abrupt shift toward aridity during the late Holocene, but at about 5000 cal yr B.P., a difference in timing that illustrates the regional nature of climate changes during the Holocene and the importance of feedback mechanisms in regulating Holocene climate variability. Furthermore, an abrupt change at about 3000 cal yr B.P. occurs at several sites adjacent to the tropical and subtropical Atlantic, which may hint at major changes in the surface temperatures of the tropical Atlantic and/or Pacific at this time. © 2003 University of Washington. Published by Elsevier Inc. All rights reserved. Article 2003 English 00335894 10.1016/S0033-5894(03)00065-6 Quaternary Research 60 Academic Press Inc. 133 – 141 2 Ghana; Lake Bosumtwi; aridity; Holocene; lacustrine deposit; nitrogen isotope; paleoclimate; paleolimnology https://www.scopus.com/inward/record.uri?eid=2-s2.0-0141683529&doi=10.1016%2fS0033-5894%2803%2900065-6&partnerID=40&md5=2ffb4e2d8cf77b587a364f5c0a809cba Cited by: 63 James Russell Michael R. Talbot Brian J. Haskell article Karp2002735 The Lake Bosumtwi impact crater in Ghana, West Africa, has a diameter of 10.5 km and is one of the youngest (1.07 Ma) well-preserved large craters on Earth. It has a total dynamic range of topography of more than 400 m, and it is the source crater of tektites and microtektites of the Ivory Coast strewn field. The crater was excavated in early Proterozoic rocks. According to its size, the Bosumtwi impact crater should be a complex impact structure, with a central peak. Multichannel seismic (MCS) reflection and wide angle data, using Ocean-Bottom-Hydrophones (OBHs), were acquired in order to investigate the structure's subsurface, image the presumed central uplift and determine the thickness of impact-related formations and the post-impact sediments. An integrated interpretation of the seismic data sets, and modelling and inversion of the OBH data yield an initial 2D velocity-depth model, which shows indications for a central peak feature. Due to the relatively low seismic velocity (3.0 kms-1) of the corresponding layer, the top of the uplifted structure is interpreted to consist of allochthonous breccia. The central peak has a width of ∼1.8 km and a maximum height of 120 m above the top of the breccia away from the center. Fracturing may be responsible for the relatively low velocity of 3.8 kms-1 in the crater floor. The post-impact sediments covering the crater structure are 180-300 m thick. The apparent crater depth, defined as the difference between the original target surface and the top of the breccia layer, is ∼550 m and thereby slightly deeper than some other larger complex impact structures on Earth. The results indicate that the Lake Bosumtwi impact structure provides an interesting setting for scientific drilling of a young large impact crater and will be supplemented by complimentary recent geophysical (potential field) and possibly future drilling studies. © 2002 Elsevier Science Ltd. All rights reserved. Article 2002 English 00320633 10.1016/S0032-0633(02)00049-1 Planetary and Space Science 50 735 – 743 7-8 Drilling; Earth (planet); Geophysics; Hydrophones; Rocks; Topography; Seismic prospecting https://www.scopus.com/inward/record.uri?eid=2-s2.0-0036621801&doi=10.1016%2fS0032-0633%2802%2900049-1&partnerID=40&md5=668ad55a8144f89c4bceb93f3b01d75e Cited by: 40 Tobias Karp Bernd Milkereit Peter Janle Sylvester K. Danuor Jean Pohl Hans Berckhemer Christopher A. Scholz