All ICDP Publications with Abstracts
From parent-sysfolder "Publications" + 2 folder-levels deep
594.
Uppermost impact fallback layer in the Bosumtwi crater (Ghana): Mineralogy, geochemistry, and comparison with Ivory Coast tektites
Meteoritics and Planetary Science,
42
(4-5)
709 – 729
2007
ISSN: 10869379
Publisher: University of Arkansas
Abstract: ▾ 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.
593.
Using drill cutting separates to estimate the strength of narrow shear zones at SAFOD
Geophysical Research Letters,
34
(11)
2007
ISSN: 00948276Keywords:▾
Drilling; Friction; Shear strength, Friction coefficient; Frictional strength; Shear zone, Tectonics, deformation; estimation method; San Andreas Fault; serpentine; shear zone; strength, California; North America; Parkfield; United States
Abstract: ▾ A technique is presented for estimating frictional strength of narrow shear zones based on hand selection of drillhole cuttings separates. Tests were conducted on cuttings from the SAFOD scientific drillhole near Parkfield, California. Since cuttings are mixed with adjacent material as they travel up the drillhole, these fault-derived separates give a better representation of the frictional properties of narrow features than measurements from the bulk material alone. Cuttings from two shear zones (one an active trace of the San Andreas fault) contain a significant weight percent of clay-rich grains that exhibit deformation-induced slickensides. In addition, cuttings from the active SAF trace contain around 1% serpentine. Coefficients of friction for clay-rich and serpentine grains were 0.3-0.5 and 0.4-0.45, respectively. These values are around 0.12 lower than the friction coefficient of the corresponding bulk cuttings, providing an improved estimate of the frictional strength of the San Andreas fault. Copyright 2007 by the American Geophysical Union.
592.
Vibration characteristic analysis and information transmission test of bottom drilling tool (in Chinese with English abstract);[松科1井南孔钻井取心技术]
Oil Drilling & Production Technology,
5
(8-12)
117-118
2007
591.
Platinum group elements provide no indication of a meteoritic component in ICDP cores from the Bosumtwi crater, Ghana
Meteoritics and Planetary Science,
42
(4-5)
731 – 741
2007
ISSN: 10869379
Publisher: University of Arkansas
Abstract: ▾ 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.
590.
Three-dimensional shear-wave splitting tomography in the Parkfield, California, region
Geophysical Research Letters,
34
(24)
2007
ISSN: 00948276Keywords:▾
Anisotropy; Compressive stress; Mathematical models; Microcracks; Shear waves; Three dimensional; Tomography; Velocity, Shear wave splitting delay; Shear-wave splitting tomography; Spatial anisotropy distribution, Seismic waves, anisotropy; fault zone; imaging method; microcrack; polarization; ray tracing; S-wave; San Andreas Fault; seismic tomography; spatial variation; wave splitting, California; North America; Parkfield; United States
Abstract: ▾ We developed a three-dimensional (3D) shear-wave splitting tomography method to image the spatial anisotropy distribution by back projecting shear wave splitting delay times along ray paths derived from a 3D shear velocity model, assuming the delay times are accumulated along the ray paths. The local strength of the anisotropy is indicated by a parameter of anisotropy percentage, K. Using the shearwave splitting delay times for 575 earthquakes measured at PASO and HRSN stations, we imaged a detailed 3D anisotropy percentage model around the San Andreas Fault Observatory at Depth (SAFOD). The anisotropy percentage model shows strong heterogeneities, consistent with the strong spatial variations in both measured delay times and fast polarization directions. The San Andreas Fault (SAF) zone is highly anisotropic down to a depth of ∼4 km and then becomes less anisotropic at greater depths. Outside the fault zone, the highly anisotropic zone extends as deep as ∼7 km, consistent with the systematic depth dependence of the average time delays. To the southwest of the SAF, the Salinian granitic block shows relatively strong anisotropic: anomalies that are presumably caused by aligned microcracks consistent with the direction of the regional maximum compressive horizontal stress. To the northeast of the fault zone, a strong anisotropic anomaly between depths ∼2 and ∼4 km corresponds to a serpentinite body sandwiched between Franciscan rocks. Copyright 2007 by the American Geophysical Union.
589.
Physical property measurements: ICDP boreholes LB-07A and LB-08A, Lake Bosumtwi impact structure, Ghana
Meteoritics and Planetary Science,
42
(4-5)
801 – 809
2007
ISSN: 10869379
Publisher: University of Arkansas
Abstract: ▾ 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.
588.
Drill core LB-08A, Bosumtwi impact structure, Ghana: Geochemistry of fallback breccia and basement samples from the central uplift
Meteoritics and Planetary Science,
42
(4-5)
689 – 708
2007
ISSN: 10869379
Publisher: University of Arkansas
Abstract: ▾ 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.
587.
Core description and characteristics of fault zones from Hole-A of the Taiwan Chelungpu-Fault Drilling Project
Terrestrial, Atmospheric and Oceanic Sciences,
18
(2)
327-357
2007
ISSN: 10170839Keywords:▾
Chi-Chi earthquake 1999; core analysis; deformation mechanism; drilling; fault zone; lithology; normal fault; rupture; tectonic structure, Asia; Eurasia; Far East; Taichung; Taiwan
Abstract: ▾ Taiwan Chelungpu-fault Drilling Project was conducted in drill site Dakeng, Taichung City of central western Taiwan during 2004 - 2005 principally to investigate the rupture mechanism in the northern segment of the Chi-Chi earthquake of 21 September 1999, and also to examine regional stratigraphy and tectonics. Core examination (500 - 1800 m) of Hole-A gave profound results aiding in illustrating the lithologic column, deformation structure, and architectural pattern of fault zones along the borehole. Lithology column of Hole-A was identified downward as the Cholan Formation (500 - 1027 m), Chinshui Shale (1027 - 1268 m), Kueichulin Formation (1268 - 1712 m), and back to the Cholan Formation (1712 - 2003 m) again. A dramatic change is observed regarding sedimentation age and deformation structure around 1712 m. Along the core, most bedding dips 30° toward N105°. Around 1785 m, bedding dip jumps up to 70° until the bottom of borehole. Five structure groups of different orientations (dip direction/dip) are observed throughout the core. Based on the orientation and sense of shear, they are categorized as thrust (105/30), left-lateral fault (015/30 - 80), right-lateral fault (195/30 - 80), normal fault (105/5 - 10), and backthrust (285/40 - 50). Ten fault zones have been recognized between 500 and 2003 m. We interpret the fault zone located at around 1111 m as being the most likely candidate for rupture deformation during Chi-Chi earthquake. The fault zone seated around 1712 m is recognized as the Sanyi fault zone which is 600 m beneath the Chelungpu fault zone. Ten fault zones including thrust faults, strike-slip faults and backthrust are classified as the Chelungpu Fault System (<1250 m) and the Sanyi Fault System (>1500 m). According to the deformation textures within fault zones, the fault zones can be categorized as three types of deformation: distinct fracture deformation, clayey-gouge deformation, and soft-rock deformation. Fracture deformation is dominant within the Chelungpu Fault System and abother two architectures prevail in the Sanyi Fault System. The fracture deformation pattern is asymmetric, which depended the shear sense of fault zone. From the core examination of TCDP Hole-A, the lithology plays an important role in controlling the location and deformation of fault zones.
586.
Core slabbing and nannofossil analysis on the Chelungpu fault zone, Taichung, Taiwan
Terrestrial, Atmospheric and Oceanic Sciences,
18
(2)
295-325
2007
ISSN: 10170839Keywords:▾
biostratigraphy; core analysis; lithology; mudstone; nanofossil; Pliocene-Pleistocene boundary; shear zone; slab, Asia; Eurasia; Far East; Taichung; Taiwan
Abstract: ▾ The results of this nannofossil analysis supply essential information for determining the formation boundaries in the upper Pliocene to Pleistocene. These results also verify the existence of a repetition fossil zone. The TCDP well-A was sunk through the soft fine-grain muddy sandstone and mudstone dominated formations of the Pliocene and Pleistocene in the Taichung area. This study determines methods for providing core preservation in wells at fault zones and establishes a nannofossil biostratigraphy for the integrated Taiwan Chelungpu-fault Drilling Project (TCDP). Good core fabrics are useful for core description and sampling. In this present study, over 400 meters of subsurface cores were covered in resin and slabbed. Digitized images were created for all the core fabrics. More than 150 rock samples were analyzed for nannofossils to give a detailed appraisal of the biostratigraphic column of TCDP well-A. A fossil zone at a depth interval of 431 - 869 m is a NN16 - 18 biozone. This zone is within the Cholan Formation, a lithologic stratigraphy in northern and central Taiwan. The depth interval 883-1226 m is NN15, and is within the Chinshui Shale. The Chelungpu fault is composed of five major shear zones. These are all found at depth within the marine Chinshui Shale. At a depth interval of 1293.37 - 1710 m is a NN12 - 14 biozone; this interval is within the Kueichulin Formation. Interestingly, both the interval beneath 1714 m and the nannofossil zone near the well bottom are NN16 - 18 (Cholan Formation), indicating a repeat of the Cholan Formation. The lowest fossil zone is also abundant in secondary reworked fossils in its assemblages. Hence, the repetition of the younger fossil zone, NN16 - 18, at the bottom of the well verifies the subsurface position of the Sanyi Fault and indicates that TCDP well-A must have passed through it.
585.
Crustal structure due to collisional and escape tectonics in the Eastern Alps region based on profiles Alp01 and Alp02 from the ALP 2002 seismic experiment
Journal of Geophysical Research: Solid Earth,
112
(6)
2007
ISSN: 21699313
Publisher: Blackwell Publishing Ltd
Keywords:▾
Alps; Eastern Alps; Eurasia; Europe; Pannonian Basin; crustal structure; forward modeling; Moho; ray tracing; seismic reflection; seismic refraction; seismic velocity; seismotectonics
Abstract: ▾ Alp01 and Alp02 are the longest profiles recorded during ALP 2002, a large international seismic refraction and wide-angle reflection experiment undertaken in the Eastern Alps in 2002. Alp01 crosses the Alpine orogen from north to south, thus providing a cross section mainly affected by the collision between Europe and the Adriatic microplate. Alp02 extends from the Eastern Alps to the Pannonian basin, supplying evidence on the relation between Alpine crustal structure and tectonic escape to the Pannonian basin. During this experiment, 363 single-channel recorders were deployed along these profiles with an average spacing of 3.2 km. Recordings from 20 inline shots were used in this study. Two-dimensional forward modeling using interactive ray-tracing techniques produced detailed P wave velocity models that contain many features of tectonic significance. Along Alp01, the European Moho dips generally to the south and reaches a maximum depth of 47 km below the transition from the Eastern to the Southern Alps. The Adriatic Moho continues further south at a significantly shallower depth. Moho topography and a prominent south-dipping mantle reflector in the Alpine area support the idea of southward subduction of the European lithosphere below the Adriatic microplate. The most prominent tectonic feature on the Alp02 profile is a vertical step of the Moho at the transition between the Alpine and Pannonian domains, suggesting the existence of a separate Pannonian plate fragment. The development of the Pannonian fragment is interpreted to be a consequence of crustal thinning due to tectonic escape from the Alpine collision area to the Pannonian basin. Copyright 2007 by the American Geophysical Union.
584.
Diatom stratigraphy of the last 250 ka at Lake El'gygytgyn, northeast Siberia
Journal of Paleolimnology,
37
(1)
155-162
2007
ISSN: 09212728Keywords:▾
climate conditions; core analysis; diatom; paleoclimate; periphyton; plankton; Quaternary; stratigraphy, Chukchi; Eurasia; Lake El'gygytgyn; Russian Federation, Bacillariophyta; Cyclotella ocellata
Abstract: ▾ Diatom species counts were conducted on 171 sediment samples from the 13-m-long core PG1351 from Lake El'gygytgyn, northeast Siberia. The planktonic Cyclotella ocellata-complex dominates the diatom assemblage through most of the core record, persisting through a variety of climate conditions. Periphytic diatoms, although less abundant, have greater diversity and greater down-core assemblage variation. During warm climate modes, longer summer ice-free conditions may have allowed more complex diatom communities to develop in shallow-water habitats, and enhanced circulation may have increased transport of these diatoms to deeper parts of the lake. Zones of low overall diatom abundance further support inferred intervals of low lake productivity during times of extended lake ice and snow cover. More data on the modern spatial and temporal distribution of diatom species in the Lake El'gygytgyn system will improve inferences from core records. © 2006 Springer Science+Business Media B.V.
583.
A dynamic study of frictional and viscous effects on earthquake rupture: A case study of the 1999 Chi-Chi, Taiwan, earthquake
Bulletin of the Seismological Society of America,
97
(4)
1233-1244
2007
ISSN: 00371106Keywords:▾
Approximation theory; Computer simulation; Dynamics; Friction; Mathematical models; Physical properties; Viscosity, Earthquake rupture; Fault-striking direction; Transpressive fault, Earthquakes, Chi-Chi earthquake 1999; displacement; earthquake magnitude; earthquake rupture; friction; simulation; strike-slip fault; transpression; viscosity, Asia; Eurasia; Far East; Taiwan
Abstract: ▾ Friction is commonly considered an important factor in controlling earthquake rupture. In this work, it is assumed that viscosity is also a significant factor. A strike-slip-type, two-body spring-slider model in the presence of both friction and viscosity is applied to approximate the rupture processes of an earthquake along the fault-striking direction. Results show that in addition to friction, viscosity is also an important factor in controlling rupture. The Ms 7.6 Chi-Chi earthquake which struck central Taiwan on 20 September 1999, ruptured a 100-km-long east-dipping transpressive fault (the Chelungpu fault). Measured and inferred results show that there are differences in physical properties between the northern and southern segments of the fault. Simulation results from a two-body model can explain the differences in displacement, velocity, acceleration, and predominant period between the two fault segments.
582.
Directional Coring Technology in Well Songke-1 (in Chinese with English abstract);[松科1井定向取心技术]
Petroleum Drilling Techniques,
622-26
2007
581.
Drill core LB-08A, Bosumtwi impact structure, Ghana: Petrographic and shock metamorphic studies of material from the central uplift
Meteoritics and Planetary Science,
42
(4-5)
611 – 633
2007
ISSN: 10869379
Publisher: University of Arkansas
Abstract: ▾ 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.
580.
CONTINENT sampling positions and related projects
579.
Drilling of the chelungpu fault after the 1999 Chi-Chi, Taiwan earthquake (Mw7.6): Understanding physics of faulting
Scientific Drilling
(1 SUPPL. 1)
33-34
2007
ISSN: 18168957
Publisher: Integrated Ocean Drilling Program
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.