All ICDP Publications with Abstracts
From parent-sysfolder "Publications" + 2 folder-levels deep
578.
East African megadroughts between 135 and 75 thousand years ago and bearing on early-modern human origins
Proceedings of the National Academy of Sciences of the United States of America,
104
(42)
16416 – 16421
2007
ISSN: 10916490Keywords:▾
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
Abstract: ▾ 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.
577.
East African megadroughts between 135 and 75 thousand years ago and bearing on early-modern human origins
Proceedings of the National Academy of Sciences of the United States of America,
104
(42)
16416-16421
2007
ISSN: 00278424Keywords:▾
Africa; article; climate change; drought; environment; evolution; human; migration; nonhuman; Pleistocene; priority journal, Africa, Eastern; Animals; Evolution; Hominidae; Humans; Natural Disasters; Paleontology; Population; Tropical Climate, Homo sapiens
Abstract: ▾ 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.
576.
Ejecta range: A simulation study of terrestrial impacts
Planetary and Space Science,
55
(7-8)
900-914
2007
Abstract: ▾ In this work the meteorite and target mass partition into high-speed ejecta during the formation of terrestrial impact craters is investigated. Multi-material hydrocode calculations are carried out through the entire excavation phase, and the mass of each material moving upwards with velocities inside a range of intervals is obtained. Impact of a 10 km diameter stony asteroid with 20 km s- 1 into the continental crust is compared for the cases of a single layer of granite, taken to be representative for the crust, and of a two-layer crust with a 3 km thick sedimentary cover of limestone on top of granite basement, more appropriate for the Chicxulub crater. The proportion of meteorite and crustal material in high-speed ejecta is found as a function of velocity and time, and maximum distances to the crater can be estimated. The resulting distal (> 7000 km) ejecta mass for vertical impact is less than a percent of the impactor mass, assuming ballistic transport. Simulations of oceanic impact of a 1 km-sized stony asteroid into 5.5 km deep sea are also presented. Here, ejection of meteorite material initially is delayed, but finally it leaves the ocean in a cloud of steam and water. The velocities of meteorite material are much lower compared with the continental impact, insufficient to reach large distances on ballistic trajectories. © 2007 Elsevier Ltd. All rights reserved.
575.
Electron microscopy of clay minerals in mudrocks from the san andreas Fault observatory at depth (SAFOD)
Scientific Drilling
(1 SUPPL. 1)
68-70
2007
ISSN: 18168957574.
Estimation of velocity in the uppermost crust in a part of the western Gulf of Corinth, Greece, from the inversion of P and S arrival times using the neighbourhood algorithm
Journal of Seismology,
11
(2)
199 – 204
2007
ISSN: 13834649Keywords:▾
Gulf of Corinth; Ionian Sea; Mediterranean Sea; algorithm; arrival time; crustal structure; data inversion; P-wave; S-wave; seismic velocity; upper crust
Abstract: ▾ We determine the velocities in an upper crustal model, composed of three homogeneous layers, for one subregion of the western part of the Gulf of Corinth, NE of the town of Aigion, Greece. We have used local events that occurred there in the year 2001 and were recorded by the Corinth Rift Laboratory Network. Weighted P and S arrival time residuals are minimized using the Neighbourhood Algorithm of Sambridge (1999), combined with the grid search for source locations. The resolution of the inversion is tested by delete-one jackknifing. The model obtained is compared with some other models derived or applied to the subregion. A fast velocity increase between depths of 5 and 7 km is confirmed as the major structural element. © Springer Science+Business Media, Inc. 2007.
573.
Evolution of hot-spring travertine accumulation in Karlovy Vary/Carlsbad (Czech Republic) and its significance for the evolution of Teplá valley and Ohře/Eger rift
Zeitschrift fur Geomorphologie,
51
(4)
427 – 442
2007
ISSN: 03728854
Publisher: Schweizerbart Science Publishers
Keywords:▾
Central Europe; Czech Republic; Eurasia; Europe; Karlovarsky; Karlovy Vary; Tepla River; concentration (composition); dating method; Eemian; geochronology; Holocene; interglacial; landform evolution; thermal evolution; thermal spring; travertine
Abstract: ▾ The first set of U-series ages of the hot-spring travertine from the world-famous Karlovy Vary Spa indicates repeated growth of the travertine accumulation followed by its erosion by the Teplá River. The obtained data are important as an estimate of minimum duration of thermal water circulation in Karlovy Vary and in understanding of the river valley evolution during the youngest geological history. The oldest 230Th/ 234U age data (230 ± 14 ka BP and 155 ± 7 ka BP) have been obtained for travertine relicts located 22 m above the present-day bottom of the valley. The extensive travertine accumulation located directly on the valley bottom was formed by gradual growth, particularly during the first half of the Weichselian period, between 100 and 50 ka BP. These data show that the valley bottom was at approximately the same elevation as it is today already during the Eemian interglacial. At the end of the Weichselian the central part of the travertine accumulation was downcut by river erosion. During Holocene the new deposition of travertine was concentrated in this wide trench, being likely repeatedly modified by river activity. The travertine accumulation started to be strongly influenced by humans since the Middle Ages. A significant part of it was quarried out for lime production. The accumulation was later covered by buildings, streets and the reinforced course of the Teplá River. © 2007 Gebrüder Borntraeger.
572.
Continuous record of environmental changes in Chukotka during the last 350 thousand years
Russian Journal of Pacific Geology,
1
(6)
550-555
2007
ISSN: 18197140Abstract: ▾ The comprehensive study of the upper 1283 cm of sediment from Lake El'gygytgyn, which formed nearly 4 Ma ago following a meteorite impact in northern Chukotka, yielded the first continuous record of the extreme changes in the Beringian climate and vegetation from the middle Middle Pleistocene to recent time (equivalent of marine isotope stages of 1-7 and the upper part of isotope stage 8). During this period, the climate was warmer than at present between 8600 and 10 7000 14C years and during the Late Pleistocene (isotope substage 5e, 116-128 ka ago). In 2003, the German-Russian-USA expedition continued studying sediments of Lake El'gygytgyn to obtain new evidence of the change in the vegetation cover in the Middle Pleistocene and the first information on the Middle Pleistocene interglacial (isotope stage 9; 297-347 ka ago). Pollen spectra characterizing the Middle Pleistocene interglacial are similar to spectra of the early stage of the Early Pleistocene interglacial and the climatic optimum in the Pleistocene to Holocene transitional period. The climatic history of Lake El'gygytgyn is basic for stratigraphic interpretations and correlations in the eastern sector of the Arctic. These data also expand our understanding of climatic changes that are studied within the framework of the "Pole-Equator-Pole Paleoclimate," "Past Global Changes," and other international projects. © Pleiades Publishing, Ltd. 2007.
571.
Clast fabric examination of impact-generated breccias, borehole LB-07A, Bosumtwi, Ghana
Meteoritics and Planetary Science,
42
(4-5)
769 – 778
2007
ISSN: 10869379
Publisher: University of Arkansas
Abstract: ▾ 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.
570.
Gas hydrate property measurements in porous sediments with resonant ultrasound spectroscopy
Journal of Geophysical Research: Solid Earth,
112
(5)
2007
ISSN: 21699313
Publisher: Blackwell Publishing Ltd
Keywords:▾
core analysis; equilibrium; gas hydrate; laboratory method; measurement method; pore space; porewater; porous medium; resonance; saturation; spectroscopy
Abstract: ▾ Resonant ultrasound spectroscopy was used to characterize a natural geological core sample obtained from the Mallik 5L-38 gas hydrate research well at high pressure and subambient temperatures. Using deuterated methane gas to form gas hydrate in the core sample, it was discovered that resonance amplitudes are correlated with the fraction of the pore space occupied by the gas hydrate crystals. A pore water freezing model was developed that utilizes the known pore size distribution and pore water chemistry to predict gas hydrate saturation as a function of pressure and temperature. The model showed good agreement with the experimental measurements and demonstrated that pore water chemistry is the most important factor controlling equilibrium gas hydrate saturations in these sediments when gas hydrates are formed artificially in laboratory pressure vessels. With further development, the resonant ultrasound technique can provide a rapid, nondestructive, field portable means of measuring the equilibrium P-T properties and dissociation kinetics of gas hydrates in porous media, determining gas hydrate saturations, and may provide new insights into the nature of gas hydrate formation mechanisms in geologic materials. Copyright 2007 by the American Geophysical Union.
569.
An international and multidisciplinary drilling project into a young complex impact structure: The 2004 ICDP Bosumtwi Crater Drilling Project - An overview
Meteoritics and Planetary Science,
42
(4-5)
483 – 511
2007
ISSN: 10869379
Publisher: University of Arkansas
Abstract: ▾ 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.
568.
Petrophysical and paleomagnetic data of drill cores from the Bosumtwi impact structure, Ghana
Meteoritics and Planetary Science,
42
(4-5)
829 – 838
2007
ISSN: 10869379
Publisher: University of Arkansas
Abstract: ▾ 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.
567.
A helium isotope cross-section study through the San Andreas Fault at seismogenic depths
Geochemistry, Geophysics, Geosystems,
8
(1)
2007
ISSN: 15252027Abstract: ▾ We have analyzed noble gas isotopes in 19 mud gas samples from 116-3943 m borehole depth of the San Andreas Fault Observatory at Depth (SAFOD) Main Hole in the context of origin and spatial variability of fluids occurring at seismogenic depths. The measured 3He/4He ratios range between 0.40 and 1.02 Ra (Ra is the atmospheric 3He/4He ratio of 1.39 × 10-6), with 4He/20Ne values between 0.33 and 4.92, revealing a mixture of three components to the total helium: (1) atmospheric helium, (2) helium with a crustal signature, and (3) mantle-derived helium. The air-corrected 3He/4He ratios fall between 0.2 Ra and 0.9 Ra. Samples from the 2117-3196 m depth show a relatively constant helium isotope composition (0.35-0.46 Ra), indicating that ∼5% of the helium in this section the Pacific Plate is derived from the mantle. The contribution of mantle-derived helium increases slightly in the transition from the Pacific Plate to the North American Plate and reaches maximal values of ∼12% on the North American Plate (below ∼3500 m borehole depth). On the basis of our observations, we suggest that the San Andreas Fault plays a role for fluid flux from greater depths, but higher amounts of mantle-derived fluids rise up through other, more permeable faults, situated on the North American Plate of the San Andreas Fault Zone (SAFZ). Lateral fluid dispersion at shallow depths through permeable country rock of the North American Plate may explain the observed increase in 3He/ 4He ratios with increasing distance to the SAF. Copyright 2007 by the American Geophysical Union.
566.
A revised age model for core PG1351 from Lake El'gygytgyn, Chukotka, based on magnetic susceptibility variations tuned to northern hemisphere insolation variations
Journal of Paleolimnology,
37
(1)
65-76
2007
ISSN: 09212728Keywords:▾
anoxic conditions; climate variation; core analysis; lacustrine deposit; luminescence dating; magnetic susceptibility; Northern Hemisphere; opal; oxic conditions; total organic carbon, Chukchi; Eurasia; Lake El'gygytgyn; Russian Federation
Abstract: ▾ A combined analysis of magnetic susceptibility, total organic carbon (TOC), biogenic silica (opal), and TiO2 content of the 12.6 m long composite core PG1351 recovered from Lake El'gygytgyn, Chukotka Peninsula, indicate a clear response of the lacustrine sedimentary record to climate variations. The impact is not direct, but through variations in oxygenation of the bottom waters. Mixing of the water body is typical for warmer climates, whereas the development of a stratified water body associated with anoxic conditions at the lake floor appears during cold climates. Oxic conditions lead to a good magnetite preservation and thus to high magnetic susceptibilities, but also to a large-scale degradation of organic matter, as reflected by low TOC (total organic carbon) values. During anoxic conditions, magnetite is severely dissolved yielding very low susceptibility values, whereas organic matter is best preserved, reflected by high TOC values. Hence, in general, neither susceptibility reflects the lithogenic fraction, nor does TOC reflect bioproductivity in case of the studied El'gygytgyn sediments. Based on available infrared stimulated luminescence (IRSL) dating, the obtained susceptibility pattern of core PG1351 shows an obvious correlation to northern hemisphere insolation variations, with a dominating impact of the Earth's 18 and 23 kyr precessional cycles for the upper half of PG1351, that is, during the past 150 ka. Therefore, the whole susceptibility record, together with biogenic silica (as a proxy for bioproductivity), TOC (as an indicator for redox conditions), and TiO2 (as a proxy for lithogenic input), was systematically tuned to the northern hemisphere insolation yielding an age of about 250 ka for the base of the composite core. © 2006 Springer Science+Business Media B.V.
565.
Accurate relocation of İzmit earthquake (Mw = 7.4, 1999) aftershocks in Çιnarcιk basin using double difference method
Geophysical Research Letters,
34
(10)
2007
ISSN: 00948276Keywords:▾
Aftershocks; Double difference method; Double difference relocation algorithm; Seismic network; Seismicity, Algorithms; Correlation methods; Earthquake resistance; Mathematical models; Seismic response; Seismic waves; Waveform analysis, Earthquakes, aftershock; arrival time; Kocaeli earthquake 1999; seismicity; travel time; waveform analysis
Abstract: ▾ We relocated part of the aftershock activity in Çιnarcιk Basin and surrounding areas that are associated with the 1999 İzmit earthquake Mw 7.4. Double difference relocation algorithm is used to relocate the aftershocks. The data set was obtained from a temporary seismic network deployed 10 days after the main shock by cooperation between Boǧaziçi University Kandilli Observatory and Earthquake Research Institute, LGIT (Grenoble), and IPGP (Paris). For a better station coverage, additional data set was obtained from a network operated by TUBITAK Marmara Research Center. Differential travel times were calculated using both arrival time readings and waveform cross correlation method. We relocated 1145 of the aftershocks and interpreted the results with emphasis on the Yalova and Tuzla cluster located within the Marmara Sea. The results show better focused seismicity patterns for the Yalova cluster, providing clear evidence for the proposed models. We present a revised location of the 1963 Çιnarcιk earthquake which took place in area of Yalova cluster. Finally we propose that the activity of Tuzla cluster represents a parallel subsidiary fault of the Main Marmara Fault. Copyright 2007 by the American Geophysical Union.
564.
Amplitude loss of sonic waveform due to source coupling to the medium
Geophysical Research Letters,
34
(5)
2007
ISSN: 00948276Keywords:▾
Acoustic wave velocity; Boreholes; Gas hydrates; Sediments; Waveform analysis, Energy radiation; Sonic waveform, Petroleum prospecting, amplitude; pore space; seismic attenuation; sonic boom; wave velocity; waveform analysis, Canada; North America; Northwest Territories
Abstract: ▾ In contrast to hydrate-free sediments, sonic waveforms acquired in gas hydrate-bearing sediments indicate strong amplitude attenuation associated with a sonic velocity increase. The amplitude attenuation increase has been used to quantify pore-space hydrate content by attributing observed attenuation to the hydrate-bearing sediment's intrinsic attenuation. A second attenuation mechanism must be considered, however. Theoretically, energy radiation from sources inside fluid-filled boreholes strongly depends on the elastic parameters of materials surrounding the borehole. It is therefore plausible to interpret amplitude loss in terms of source coupling to the surrounding medium as well as to intrinsic attenuation. Analyses of sonic waveforms from the Mallik 5L-38 well, Northwest Territories, Canada, indicate a significant component of sonic waveform amplitude loss is due to source coupling. Accordingly, all sonic waveform amplitude analyses should include the effect of source coupling to accurately characterize a formation's intrinsic attenuation.
563.
An asteroid breakup 160 Myr ago as the probable source of the K/T impactor
Nature,
449
(7158)
48-53
2007
Abstract: ▾ The terrestrial and lunar cratering rate is often assumed to have been nearly constant over the past 3 Gyr. Different lines of evidence, however, suggest that the impact flux from kilometre-sized bodies increased by at least a factor of two over the long-term average during the past ∼100 Myr. Here we argue that this apparent surge was triggered by the catastrophic disruption of the parent body of the asteroid Baptistina, which we infer was a ∼170-km-diameter body (carbonaceous-chondrite-like) that broke up Myr ago in the inner main asteroid belt. Fragments produced by the collision were slowly delivered by dynamical processes to orbits where they could strike the terrestrial planets. We find that this asteroid shower is the most likely source (>90 per cent probability) of the Chicxulub impactor that produced the Cretaceous/Tertiary (K/T) mass extinction event 65 Myr ago. ©2007 Nature Publishing Group.
562.
Application of Coring with Formation Kept Techniques in SLCORE-I (in Chinese with English abstract);[保形取心技术及其在松科1井中的应用研究]
Drilling Engineering,
749-51
2007
561.
Chromium isotopic studies of terrestrial impact craters: Identification of meteoritic components at Bosumtwi, Clearwater East, Lappajärvi, and Rochechouart
Earth and Planetary Science Letters,
256
(3-4)
534 – 546
2007
ISSN: 0012821XKeywords:▾
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
Abstract: ▾ 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.
560.
Application of oriented Coring Techniques in the SLCORE-I (in Chinese with English abstract);[定向取心技术在松科1井中的应用]
Drilling Engineering,
1069-71
2007
559.
Applying SAR-IRSL methodology for dating fine-grained sediments from Lake El'gygytgyn, north-eastern Siberia
Quaternary Geochronology,
2
(1-4)
187-194
2007
ISSN: 18711014Keywords:▾
age determination; bedrock; climate change; crater; fine grained sediment; geochronology; insolation; lacustrine deposit; luminescence dating, Chukchi; Eurasia; Lake El'gygytgyn; Russian Federation; Siberia
Abstract: ▾ Lake El'gygytgyn is situated in a 3.6 Ma old impact crater in north-eastern Siberia and probably represents one of the most complete archives of Arctic climate change. Investigated here is the potential of infra-red stimulated luminescence (IRSL) using the single-aliquot regenerative-dose (SAR) approach for dating sediments from this lake. Independent age control is available from a published age model of a parallel core that is based on tuning sediment proxies with regional insolation and the results of previous multiple aliquot IRSL dating. Although the site is located within volcanic bedrock, anomalous fading seems to have little effect on the calculated ages. The modelled water content for the entire time of burial is seen as the most prominent uncertainty at this particular site. Despite these potential error sources, SAR-IRSL ages are in acceptable agreement with the given timeframe and clearly point to the possibility to establish independent chronologies at this site up to at least 400,000 years. © 2006.
558.
Basic hydrology, limnology, and meteorology of modern Lake El'gygytgyn, Siberia
Journal of Paleolimnology,
37
(1)
17-35
2007
ISSN: 09212728Keywords:▾
crater lake; digital elevation model; hydrology; lacustrine deposit; lacustrine environment; lake dynamics; limnology; meteorology; sediment core, Chukchi; Eurasia; Lake El'gygytgyn; Russian Federation
Abstract: ▾ A survey of the modern physical setting of Lake El'gygytgyn, northeastern Siberia, is presented here to facilitate interpretation of a 250,000-year climate record derived from sediment cores from the lake bottom. The lake lies inside a meteorite impact crater that is approximately 18 km in diameter, with a total watershed area of 293 km2, 110 km2 of which is lake surface. The only surface water entering the lake comes from the approximately 50 streams draining from within the crater rim; a numbering system for these inlet streams is adopted to facilitate scientific discussion. We created a digital elevation model for the watershed and used it to create hypsometries, channel networks, and drainage area statistics for each of the inlet streams. Many of the streams enter shallow lagoons dammed by gravel berms at the lakeshore; these lagoons may play a significant role in the thermal and biological dynamics of the lake due to their higher water temperatures (>6°C). The lake itself is approximately 12 km wide and 175 m deep, with a volume of 14.1 km3. Water temperature within a column of water near the center of this oligotrophic, monomictic lake never exceeded 4°C over a 2.5 year record, though the shallow shelves (<10 m) surrounding the lake can reach 5°C in summer. Though thermally stratified in winter, the water appears completely mixed shortly after lake ice breakup in July. Mean annual air temperature measured about 200 m from the lake was -10.3°C in 2002, and an unshielded rain gage there recorded 70 mm of rain in summer of 2002. End of winter snow water equivalent on the lake was approximately 110 mm in May 2002. Analysis of NCEP reanalysis air temperatures (1948-2002) reveals that the 8 warmest years and 10 warmest winters have occurred since 1989, with the number of days below -30°C dropping from a pre-1989 mean of 35 to near 0 in recent years. The crater region is windy as well as cold, with hourly wind speeds exceeding 13.4 m s-1 (30 mph) typically at least once each month and 17.8 m s-1 (40 mph) in winter months, with only a few calm days per month; wind may also play an important role in controlling the modern shape of the lake. Numerous lines of evidence suggest that the physical hydrology and limnology of the lake has changed substantially over the past 3.6 million years, and some of the implications of these changes on paleoclimate reconstructions are discussed. © 2006 Springer Science+Business Media B.V.
557.
Characteristics of the lithology, fault-related rocks and fault zone structures in TCDP hole-A
Terrestrial, Atmospheric and Oceanic Sciences,
18
(2)
243-269
2007
ISSN: 10170839Keywords:▾
chemical property; fault zone; lithology; lithostratigraphy; mechanical property; Miocene; physical property; Pleistocene; Pliocene, Asia; Eurasia; Far East; Taiwan, Mollusca
Abstract: ▾ The main objective of the Taiwan Chelungpu-fault Drilling Project (TCDP) was to conduct an in-depth probe into a fault zone of recent major activity so as to gain a better understanding of and more insight into the physical, mechanical and chemical properties involved. By the end of 2004, with the completion of the drilling of Hole-A, cuttings from 0 to 431.34 m and cores from a 431.34- to 2003.26-m depth had been obtained. Stratigraphically, the Pliocene to Pleistocene Cholan Formation is found from the surface to a 1029-m depth and is predominantly composed of sandstone and sandstone-siltstone alternations with weak to intense bioturbation. The Pliocene Chinshui Formation is observed from a depth of 1029- to 1303-m and predominantly consists of siltstone with weak bioturbation. From 1303- to 1712-m down there is the late Miocene to early Pliocene Kueichulin Formation which is predominantly composed of massive sandstone with minor siltstone. Below 1712 m, the Formation again resembles the younger Cholan Formation with mollusca-rich, thick, layered shale and heavy bioturbated sandstone. Four types of fault-related rocks are identified in the cores. They are the fault breccia, gouges, foliated and non-foliated cataclasites and pseudotachylytes. At least six major fault zones are found in the cores: FZ1111, FZ1153, FZ1220, FZ1580, FZ1712, and FZ1812. Among these, FZ1111 most probably corresponds to the slip surface of the Chi-Chi earthquake, the Chelungpu fault, while FZ1712 very likely represents the Sanyi fault.
556.
Characterization of aftershock-fault plane orientations of the 1999 İzmit (Turkey) earthquake using high-resolution aftershock locations
Geophysical Research Letters,
34
(20)
2007
ISSN: 00948276Keywords:▾
Earthquake effects; Mathematical models; Seismic waves; Waveform analysis, Hypocenters; Seismically active regions; Turkey, Seismology, aftershock; earthquake event; earthquake hypocenter; fault plane; focal mechanism; seismic velocity, Eurasia; Izmit; Kocaeli [Turkey]; Turkey
Abstract: ▾ Joint inversion for hypocentral parameters and the velocity field is nowadays a state of the art tool to obtain high-resolution images of seismically active regions. In this study, we focus on the location accuracy of aftershocks of the 1999 Mw = 7.4 İzmit (NW Turkey) earthquake. We obtained a new velocity model for the region, and depicted its improvement on absolute locations in terms of uncertainty and misfit. Two well-developed aftershock clusters located in the Akyazi area and Karadere-Düzce region, were analyzed in detail based on a waveform cross-correlation approach that allowed improving the location accuracy by a factor of 6. Relocation results reveal that hypocenters form narrow planes of activity that can be correlated with focal mechanisms of the larger aftershocks as well as nearby clouds of activity with no internal structure down to the resolved scale of ∼300 m. Copyright 2007 by the American Geophysical Union.
555.
Characterization of the log lithology of cores LB-07A and LB-08A of the Bosumtwi impact structure by using the anisotropy of magnetic susceptibility
Meteoritics and Planetary Science,
42
(4-5)
839 – 847
2007
ISSN: 10869379
Publisher: University of Arkansas
Abstract: ▾ 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.
554.
Chicxulub impact predates K-T boundary: New evidence from Brazos, Texas
Earth and Planetary Science Letters,
255
(3-4)
339-356
2007
Abstract: ▾ Multidisciplinary studies, including stratigraphy, sedimentology, mineralogy and geochemistry, of the new core Mullinax-1 and outcrops along the Brazos River and Cottonmouth Creek, Falls County, Texas, reveal the complex history of the Chicxulub impact, the event deposit and the K-T boundary event. The K-T boundary, as identified by the negative δ13C shift, first occurrence of Danian planktic foraminifera and palynomorphs occurs 80 cm above the event deposit in core Mullinax-1. The underlying 80 cm interval was deposited in a shallow low oxygen environment during the latest Maastrichtian, as indicated by high stress microfossil assemblages, small shells and burrows infilled with framboidal pyrite. The underlying event deposit, commonly interpreted as K-T impact tsunami, consists of a basal conglomerate with clasts containing Chicxulub impact spherules, repeated upward fining units of spherule-rich sands, followed by hummocky cross-bedded and laminated sands, which are burrowed by Thalassinoides, Planolites and Ophiomorpha and truncated by erosion. This suggests a series of temporally separated storm events with re-colonization of the ocean floor by invertebrates between storms, rather than a series of waning tsunami-generated waves. The lithified clasts with impact spherules at the base of the event deposit provide strong evidence that the Chicxulub impact ejecta layer predates the event deposit, but was eroded and re-deposited during the latest Maastrichtian sea level lowstand. The original Chicxulub ejecta layer was discovered in a 3 cm thick yellow clay layer interbedded in undisturbed late Maastrichtian clay- and mudstones 40 cm below the base of the event deposit and near the base of planktic foraminiferal zone CF1, which spans the last 300 kyr of the Maastrichtian. The yellow clay consists of cheto smectite derived from alteration of impact glass, as indicated by rare altered glass spherules with similar chemical compositions as reworked spherules from the event deposit and Chicxulub impact spherules from NE Mexico and Haiti. The Brazos sections thus provide strong evidence that the Chicxulub impact predates the K-T boundary by about 300 kyr, consistent with earlier observations in NE Mexico and the Chicxulub crater core Yaxcopoil-1. © 2007 Elsevier B.V. All rights reserved.