All ICDP Publications with Abstracts
From parent-sysfolder "Publications" + 2 folder-levels deep
669.
Estimating pore-space gas hydrate saturations from well log acoustic data
Geochemistry, Geophysics, Geosystems,
9
(7)
2008
ISSN: 15252027Abstract: ▾ Relating pore-space gas hydrate saturation to sonic velocity data is important for remotely estimating gas hydrate concentration in sediment. In the present study, sonic velocities of gas hydrate-bearing sands are modeled using a three-phase Biot-type theory in which sand, gas hydrate, and pore fluid form threehomogeneous, interwoven frameworks. This theory is developed using well log compressional and shear wave velocity data from the Mallik 5L-38 permafrost gas hydrate research well in Canada and applied to well log data from hydrate-bearing sands in the Alaskan permafrost, Gulf of Mexico, and northern Cascadia margin. Velocity-based gas hydrate saturation estimates are in good agreement with Nuclear Magneto Resonance and resistivity log estimates over the complete range of observed gas hydrate saturations. Copyright 2008 by the American Geophysical Union.
668.
Estimation of hydraulic permeability considering the micro morphology of rocks of the borehole YAXCOPOIL-1 (Impact crater Chicxulub, Mexico)
International Journal of Earth Sciences,
97
(2)
385-399
2008
Abstract: ▾ Internal surface, formation factor, Nuclear Magnetic Resonance (NMR)-T2 relaxation times and pore radius distributions were measured on representative core samples for the estimation of hydraulic permeability. Permeability is estimated using various versions of the classic Kozeny-Carman-equation (K-C) and a further development of K-C, the fractal PaRiS-model, taking into account the internal surface. In addition to grain and pore size distribution, directly connected to permeability, internal surface reflects the internal structure ("micro morphology"). Lithologies could be grouped with respect to differences in internal surface. Most melt rich impact breccia lithologies exhibit large internal surfaces, while Tertiary post-impact sediments and Cretaceous lithologies in displaced megablocks display smaller internal surfaces. Investigations with scanning electron microscopy confirm the correlation between internal surface and micro morphology. In addition to different versions of K-C, estimations by means of NMR, pore radius distributions and some gas permeability measurements serve for cross-checking and calibration. In general, the different estimations from the independent methods and the measurements are in satisfactory accordance. For Tertiary limestones and Suevites bulk with very high porosities (up to 35%) permeabilites between 10-14 and 10-16 m2 are found, whereas in lower Suevite, Cretaceous anhydrites and dolomites, bulk permeabilites are between 10-15 and 10-23m2. © Springer-Verlag 2007.
667.
Experimental volcanology on eruptive products of Unzen volcano
Journal of Volcanology and Geothermal Research,
175
(1-2)
110-119
2008
ISSN: 03770273Keywords:▾
Degassing; Experiments; Fracture fixation; Landforms; Volcanic rocks; Volcanoes, block-and-ash flow; Density distributions; elastic wave velocities; Eruption dynamics; Experimental investigations; Flexural strength; fragmentation behavior; Laboratory experiments; permeability; porosity; Pyroclastic flow deposits; Seismic velocities; Unzen; volcanology, Fracture toughness, ash flow; degassing; fracture toughness; permeability; porosity; seismic velocity; volcanic eruption; volcanology, Asia; Eurasia; Far East; Japan; Kyushu; Nagasaki [Kyushu]; Unzen Volcano
Abstract: ▾ Protracted dome-building eruptions may be profitably investigated using laboratory-based experiments. Density distribution studies on the pyroclastic flow deposits of Unzen 1990-1995 allow us to apply the results of experimental investigations on Unzen samples to the interpretation of the last eruption of Unzen. Here, primary laboratory experiments have focused on several aspects of the degassing (permeability) and the fragmentation behavior (threshold, speed, and efficiency). Those investigations have been flanked by analyses of flexural strength, fracture toughness, and seismic velocities, to provide new insights into eruption related processes. Here we present a review of these results and their application to the eruption dynamics of Unzen Volcano. We propose that efforts be made to incorporate routinely such comprehensive experimental analyses into the response to emerging volcanic crises in future. © 2008 Elsevier B.V. All rights reserved.
666.
Fourier transform infrared spectroscopy, a new cost-effective tool for quantitative analysis of biogeochemical properties in long sediment records
Journal of Paleolimnology,
40
(2)
689 – 702
2008
ISSN: 09212728Keywords:▾
Chukchi; Elgygytgyn Lake; Eurasia; Europe; Lake Ohrid; Russian Federation; Southern Europe; biogenic deposit; biogeochemistry; cost-benefit analysis; FTIR spectroscopy; paleolimnology; quantitative analysis; sediment core
Abstract: ▾ Measurements of Fourier transform infrared spectroscopy (FTIRS) in the mid-infrared (MIR) region were conducted on sedimentary records from Lake El'gygytgyn, NE Siberia, and Lake Ohrid, Albania/Macedonia. Calibration models relating FTIR spectral information to biogeochemical property concentrations were established using partial least squares regression (PLSR). They showed good statistical performance for total organic carbon (TOC), total nitrogen (TN), and biogenic silica (opal) in the sediment record from Lake El'gygytgyn, and for TOC, total inorganic carbon (TIC), TN, and opal in sediments from Lake Ohrid. In both cases, the calibration models were successfully applied for down-core analysis. The results, in combination with the small amount of sample material needed, negligible sample pre-treatments, and low costs of analysis, demonstrate that FTIRS is a promising, cost-effective tool that allows high-resolution paleolimnological studies. © 2008 Springer Science+Business Media B.V.
665.
Fourier transform infrared spectroscopy, a new cost-effective tool for quantitative analysis of biogeochemical properties in long sediment records
Journal of Paleolimnology,
40
(2)
689-702
2008
ISSN: 09212728Keywords:▾
biogenic deposit; biogeochemistry; cost-benefit analysis; FTIR spectroscopy; paleolimnology; quantitative analysis; sediment core, Chukchi; Elgygytgyn Lake; Eurasia; Europe; Lake Ohrid; Russian Federation; Southern Europe
Abstract: ▾ Measurements of Fourier transform infrared spectroscopy (FTIRS) in the mid-infrared (MIR) region were conducted on sedimentary records from Lake El'gygytgyn, NE Siberia, and Lake Ohrid, Albania/Macedonia. Calibration models relating FTIR spectral information to biogeochemical property concentrations were established using partial least squares regression (PLSR). They showed good statistical performance for total organic carbon (TOC), total nitrogen (TN), and biogenic silica (opal) in the sediment record from Lake El'gygytgyn, and for TOC, total inorganic carbon (TIC), TN, and opal in sediments from Lake Ohrid. In both cases, the calibration models were successfully applied for down-core analysis. The results, in combination with the small amount of sample material needed, negligible sample pre-treatments, and low costs of analysis, demonstrate that FTIRS is a promising, cost-effective tool that allows high-resolution paleolimnological studies. © 2008 Springer Science+Business Media B.V.
664.
Groundmass crystallization in dacite dykes taken in Unzen Scientific Drilling Project (USDP-4)
Journal of Volcanology and Geothermal Research,
175
(1-2)
71-81
2008
ISSN: 03770273Keywords:▾
Crystallization; Crystallography; Drilling; Feeding; Feldspar; Metallic glass; Nanocrystalline alloys; Oxide minerals; Pyrites; Quartz; Silicon compounds; Textures, Alkali feldspar; annealing; Compositional analysis; conduit drilling; Crystallization processes; dacite dyke; Drilling depth; groundmass texture; hydrothermal alteration; Lower pressures; Microlite; Microlites; Unzen, Silicate minerals, crystallization; dacite; dike; drilling; hydrothermal alteration; lava dome; volcanic eruption, Asia; Eurasia; Far East; Japan; Kyushu; Nagasaki [Kyushu]; Unzen Volcano
Abstract: ▾ Groundmass textural and compositional analyses of the drilled dacite dykes of the Unzen Scientific Drilling Project (USDP-4) identify the feeder dyke of the 1990-1995 eruption and elucidate the crystallization process of dykes at depth. In the drilling depth range of 1582-1996 m ("conduit zone"), four dacite dykes were recognized. The groundmasses of all but one of these dykes have textures ranging from cryptocrystalline to microcrystalline aggregate of crystals < 10 μm across forming an equigranular mosaic of plagioclase, alkali feldspar, quartz, and pyrite. The samples include a small number of coarser-grained plagioclase microlites (20 μm to 0.3 mm long). The compositions of groundmass consisting only of grains < 10 μm plotted at the lower pressure (< 50 MPa) ternary minimum in the Qz'-Ab'-Or' system suggests that the crystallization of plagioclase, alkali feldspar, and quartz took place nearly simultaneously. The compositions of coarser plagioclase microlites and groundmass, the plagioclase microlite textures, and the phenocryst assemblages show significant differences from historical lavas exposed in the summit area. This implies the possibility that most of the dacite dykes are not feeder dykes for the lavas at the summit and remained beneath the surface, perhaps because of high viscosity associated with high SiO2. One sample C14-1-1 collected 1977 m, has a texture, composition, and phenocryst assemblage nearly identical to that of the dome lava of the 1990-1995 eruption, differing only in the presence of hydrothermal alternation. At this time we cannot definitely conclude that C14-1-1 was the feeder dyke for the 1990-1995 eruption until we can elucidate the time scale and the conditions governing hydrothermal alternation. © 2008 Elsevier B.V. All rights reserved.
663.
Impact effects and regional tectonic insights: Backstripping the Chesapeake Bay impact structure
Geology,
36
(4)
327-330
2008
ISSN: 00917613Keywords:▾
Backstripping; Eocene; Impact processes; Passive margin, Sedimentology; Structural geology; Subsidence, Tectonics, deposition; Eocene; impact structure; passive margin; sedimentation; tectonic setting; temperature anomaly, Chesapeake Bay; North America; United States
Abstract: ▾ The Chesapeake Bay impact structure is a ca. 35.4 Ma crater located on the eastern seaboard of North America. Deposition returned to normal shortly after impact, resulting in a unique record of both impact-related and subsequent passive margin sedimentation. We use backstripping to show that the impact strongly affected sedimentation for 7 m.y. through impact-derived crustal-scale tectonics, dominated by the effects of sediment compaction and the introduction and subsequent removal of a negative thermal anomaly instead of the expected positive thermal anomaly. After this, the area was dominated by passive margin thermal subsidence overprinted by periods of regional-scale vertical tectonic events, on the order of tens of meters. Loading due to prograding sediment bodies may have generated these events. © 2008 The Geological Society of America.
662.
Sedimentary geochemistry of core PG1351 from Lake El'gygytgyn-a sensitive record of climate variability in the East Siberian Arctic during the past three glacial-interglacial cycles
Journal of Paleolimnology,
37
(1)
89-104
2007
ISSN: 09212728Keywords:▾
climate variation; geochemistry; glacial-interglacial cycle; Holocene; insolation; marine isotope stage; paleoclimate; paleolimnology; sediment core, Chukchi; Eurasia; Lake El'gygytgyn; Russian Federation
Abstract: ▾ The ca. 13 m long sediment core PG1351, recovered in 1998 from the central part of Lake El'gygytgyn, NE Siberia, was investigated for lithostratigraphy, water content, dry bulk density (DBD), total organic carbon (TOC), total nitrogen (TN), total sulphur (TS) and biogenic silica (opal) contents, and for TOC stable isotope ratios (δ13CTOC). The event stratigraphy recorded in major differences in sediment composition match variations in regional summer insolation, thus confirming a new age model for this core, which suggests that it spans the last 250 ka BP. Four depositional units of contrasting lithological and biogeochemical composition have been distinguished, reflecting past environmental conditions associated with relatively warm, peak warm, cold and dry, and cold but more moist climate modes. A relatively warm climate, resulting in complete summer melt of the lake ice cover and seasonal mixing of the water column, prevailed during the Holocene and Marine Isotope Stages (MIS) 3, 5.1, 5.3, 6.1, 6.3, 6.5, 7.1-7.3, 7.5, 8.1 and 8.3. MIS 5.5 (Eemian) was characterized by significantly enhanced aquatic primary production and organic matter supply from the catchment, indicating peak warm conditions. During MIS 2, 5.2, 5.4, 6.2 and 6.4 the climate was cold and dry, leading to perennial lake ice cover, little regional snowfall, and a stagnant water body. A cold but more moist climate during MIS 4, 6.6, 7.4, 8.2 and 8.4 is thought to have produced more snow cover on the perennial ice, strongly reducing light penetration and biogenic primary production in the lake. While the cold-warm pattern during the past three glacial-interglacial cycles is probably controlled by changes in regional summer insolation, differences in the intensity of the warm phases and in the degree of aridity (changing snowfall) during cold phases likely were due to changes in atmospheric circulation patterns. © 2006 Springer Science+Business Media B.V.
661.
Seismology inside the fault zone: Applications to fault-zone properties and rupture dynamics
Scientific Drilling
(1 SUPPL. 1)
84-87
2007
ISSN: 18168957660.
Sedimentology, clay mineralogy and grain-size as indicators of 65 ka of climate change from El'gygytgyn Crater Lake, Northeastern Siberia
Journal of Paleolimnology,
37
(1)
105-122
2007
ISSN: 09212728Keywords:▾
bioturbation; clay mineral; climate change; crater lake; grain size; lacustrine deposit; paleoclimate; proxy climate record; sedimentology, Chukchi; Eurasia; Lake El'gygytgyn; Russian Federation
Abstract: ▾ El'gygytgyn Crater Lake, NE Siberia was investigated for sedimentological proxies for regional climate change with a focus on the past 65 ka. Sedimentological parameters assessed relative to magnetic susceptibility include stratigraphy, grain size, clay mineralogy and crystallinity. Earlier work suggests that intervals of high susceptibility in these sediments are coincident with warmer (interglacial-like) conditions and well-mixed oxygenated bottom waters. In contrast, low susceptibility intervals correlate with cold (glacial-like) conditions when perennial ice-cover resulted in anoxia and the dissolution of magnetic carrier minerals. The core stratigraphy contains both well-laminated to non-laminated sequences. Reduced oxygen and lack of water column mixing preserved laminated sequences in the core. A bioturbation index based upon these laminated and non- laminated sequences co-varies with total organic carbon (TOC) and magnetic susceptibility. Clay mineral assemblages include illite, highly inter-stratified illite/smectite, and chlorite. Under warm or hydrolyzing conditions on the landscape around the lake, chlorite weathers easily and illite/smectite abundance increase, which produces an inverse relationship in the relative abundance of these clays. Trends in relative abundance show distinct down-core changes that correlate with shifts in susceptibility. The mean grain-size (6.92 μm) is in the silt-size fraction, with few grains larger than 65 μm. Terrigenous input to the lake comes from over 50 streams that are filtered through storm berms, which limits clastic deposition into the lake system. The sedimentation rate and terrigenous input grain-size is reduced during glacial intervals. Measurements of particle-size distribution indicate that the magnetic susceptibility fluctuations are not related to grain size. Lake El'gygytgyn's magnetic susceptibility and clay mineralogy preserves regional shifts in climate including many globally recognized events like the Younger Dryas and Bolling/Allerod. The sedimentary deposits reflect the climatic transitions starting with MIS4 through the Holocene transition. This work represents the first extensive sedimentological study of limnic sediment proxies of this age from Chukotka (Fig. 1). © 2006 Springer Science+Business Media B.V.
659.
Search for a meteoritic component in drill cores from the Bosumtwi impact structure, Ghana: Platinum group element contents and osmium isotopic characteristics
Meteoritics and Planetary Science,
42
(4-5)
743 – 753
2007
ISSN: 10869379
Publisher: University of Arkansas
Abstract: ▾ 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.
658.
Seismic investigation of the El'gygytgyn impact crater lake (Central Chukotka, NE Siberia): Preliminary results
Journal of Paleolimnology,
37
(1)
49-63
2007
ISSN: 09212728Keywords:▾
crater lake; impact structure; lacustrine deposit; paleoclimate; seismic data; seismic survey, Chukchi; Eurasia; Lake El'gygytgyn; Russian Federation
Abstract: ▾ The 12 km wide and about 175 m deep El'gygytgyn crater lake in Central Chukotka, NE Siberia, is of special interest for investigation as it could provide the first undisturbed 3.6 Ma terrestrial record from the Arctic realm, reaching back a million years before the first major glaciation of the Northern Hemisphere. A single-channel seismic survey was carried out on an expedition to the lake in 2000, in which both high resolution and deep penetration data were acquired. Seismic data suggest an impact crater structure in Cretaceous volcanic bedrock, indicated by velocities of >5000 m s-1, whose upper 500-600 m is brecciated. The lake is filled with two units of sediments, the upper one well stratified and the lower one massive. In the center of the lake, the combined thickness of the two sedimentary units is estimated to be 320-350 m. The upper unit is draped over the location of an interpreted central peak and is locally intercalated with debris flows, mainly in the western part of the lake and at the lake margins. Most of the lower unit is obscured by multiples as a result of high reflection coefficients in the upper unit. As at least the upper unit appears to be undisturbed by glaciation, the lake should yields unique information on the paleoclimatic development of the East Siberian Arctic. © 2006 Springer Science+Business Media B.V.
657.
Seismically induced changes of the fluid signature detected by a multi-isotope approach (He, CO2, CH4, N2) at the Wettinquelle, Bad Brambach (central Europe)
Journal of Geophysical Research: Solid Earth,
112
(4)
2007
ISSN: 21699313
Publisher: Blackwell Publishing Ltd
Keywords:▾
Central Europe; Eger; Eurasia; Europe; Heves; Hungary; coseismic process; fluid composition; geodynamics; hydrothermal system; isotopic composition; thermal spring
Abstract: ▾ We present a study considering the systematics of gas and isotope compositions (3He/4He, δ13 CCO2, δ13 CCH4, δ 15N) of a permanent magmatic CO2 flux in the hydrothermal system of the spring "Wettinquelle" in Bad Brambach and their relation to the seismic activity beneath the western Eger rift. The gas and isotope compositions were monitored for more than 3 years. The time series includes periods before, during, and after a 4-month-long seismically active period from the end of August to the end of December 2000. Shifts due to admixture of crustal components were found for all monitored isotope ratios during and after the seismically active period. In case of helium and nitrogen, isotopic anomalies occurred already about 6 weeks before the beginning of the seismically active period, contemporaneous with a water level anomaly of the well VL4 near the Wettinquelle. On the one hand, preseismic deformations may be responsible for the observed isotope anomalies; on the other hand, coseismic fracturing processes in the surroundings of the hypocenters may play a role. Both effects produce greater permeability and result in the release of crustal fluids. The migration and admixture of these crustal components to the "permanent" upper mantle-derived fluid flux result in geochemical anomalies that persist for more than 2 years. The results of the detailed isotope monitoring have proven to be an important contribution to understand the geodynamic processes that may presently be going on in the region Vogtland/NW Bohemia. Copyright 2007 by the American Geophysical Union.
656.
Seismology inside the fault zone: Applications to fault-zone properties and rupture dynamics
Scientific Drilling
(1 SUPPL. 1)
84-87
2007
ISSN: 18168957655.
Stress orientations of Taiwan Chelungpu-Fault Drilling Project (TCDP) hole-A as observed from geophysical logs
Geophysical Research Letters,
34
(1)
2007
ISSN: 00948276Keywords:▾
Boreholes; Earthquakes; Image sensors; Mechanics; Structural geology; Tectonics, Dipole Sonic Imager (DSI); Formation Micro Imager (FMI); Geophysical logs, Geophysical prospecting, bedding plane; borehole; Chi-Chi earthquake 1999; earthquake rupture; Eurasian plate; Philippine Sea plate; shear zone; stress field; well logging, Asia; Eurasia; Far East; Taiwan
Abstract: ▾ The Taiwan Chelungpu-fault Drilling Project (TCDP) drilled a 2-km-deep research borehole to investigate the structure and mechanics of the Chelungpu Fault that ruptured in the 1999 Mw 7.6 Chi-Chi earthquake. Geophysical logs of the TCDP were carried out over depths of 500-1900 in, including Dipole Sonic Imager (DSI) logs and Formation Micro Imager (FMI) logs in order to identify bedding planes, fractures and shear zones. From the continuous core obtained from the borehole, a shear zone at a depth of 1110 meters is interpreted to be the Chelungpu fault, located within the Chinshui Shale, which extends from 1013 to 1300 meters depth. Stress-induced borehole breakouts were observed over nearly the entire length of the wellbore. These data show an overall stress direction (∼N115°E) that is essentially parallel to the regional stress field and parallel to the convergence direction of the Philippine Sea plate with respect to the Eurasian plate. Variability in the average stress direction is seen at various depths. In particular there is a major stress orientation anomaly in the vicinity of the Chelungpu fault. Abrupt stress rotations at depths of 1000 in and 1310 in are close to the Chinshui Shale's upper and lower boundaries, suggesting the possibility that bedding plane slip occurred during the Chi-Chi earthquake. Copyright 2007 by the American Geophysical Union.
654.
Shock-metamorphic petrography and microRaman spectroscopy of quartz in upper impactite interval, ICDP drill core LB-07A, Bosumtwi impact crater, Ghana
Meteoritics and Planetary Science,
42
(4-5)
591 – 609
2007
ISSN: 10869379
Publisher: University of Arkansas
Abstract: ▾ 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.
653.
Simulating the response of a closed-basin lake to recent climate changes in tropical West Africa (Lake Bosumtwi, Ghana)
Hydrological Processes,
21
(13)
1678 – 1691
2007
ISSN: 10991085DOI:10.1002/hyp.6359
Keywords:▾
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
Abstract: ▾ 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.
652.
Stratigraphy of the Hawai'i Scientific Drilling Project core (HSDP2): Anatomy of a Hawaiian shield volcano
Geochemistry, Geophysics, Geosystems,
8
(2)
2007
ISSN: 15252027Abstract: ▾ The Hawai'i Scientific Drilling Project (HSDP2) successfully drilled ∼3.1 km into the island of Hawai'i. Drilling started on Mauna Loa volcano, drilling 247mof subaerial lavas before encountering 832m of subaerial Mauna Kea lavas, followed by 2019 m of submarine Mauna Kea volcanic and sedimentary units. The 2.85 km stratigraphic record of Mauna Kea volcano spans back to ∼650 ka. Mauna Kea subaerial lavas have high average olivine contents (13 vol.%) and low average vesicle abundances (10 vol.%). Most subaerial Mauna Kea flows are 'a'ā (∼63%), whereas the Mauna Loa section contains nearly equal amounts of pāhoehoe and 'a'ā (like its current surface). The submarine Mauna Kea section contains an upper, ∼900 m thick, hyaloclastite-rich section and a lower, ∼1100 m thick, pillow-lavadominated section. These results support a model that Hawaiian volcanoes are built on a pedestal of pillow lavas capped by rapidly quenched, fragmented lava debris. The HSDP2 section is compared here to a 1.7 km deep hole (SOH1) on Kilauea's lower east rift zone. Differences in the sections reflect the proximity to source vents and the lower magma supply to Kilauea's rift zone. Both drill core sections are cut by intrusions, but the higher abundance of intrusions in SOH1 reflects its location within a rift zone, causing more extensive alteration in the SOH1 core. The HSDP2 site recovered a relatively unaltered core well suited for geochemical analyses of the single deepest and most complete borehole ever drilled through a Hawaiian or any other oceanic island volcano. Copyright 2007 by the American Geophysical Union.
651.
Stress orientation evaluated from strain localisation analysis in Aigion Fault
Tectonophysics,
442
(1-4)
3 – 13
2007
ISSN: 00401951Keywords:▾
Gulf of Corinth; Ionian Sea; Mediterranean Sea; active fault; constitutive equation; elastoplasticity; fault zone; shear band; strain analysis; stress analysis; triaxial test
Abstract: ▾ Within the frame of the 'CRL' (Corinth Rift Laboratory project) [Cornet, F., Bernard, P., Moretti, I., (2004a): The Corinth Rift Laboratory, Comptes Rendus Geosciences, 336, 4-5, 235-242.] centered on the south western sector of the Gulf of Corinth (http://www.corinth-rift-lab.org), fault zone cores from the active Aigion Fault have been collected continuously from depths between 708 and 782 m. Inside this clayey core, a clear shearing surface with marked slip lines is visible on a plane that makes a 68° angle with respect to the core axis. This failure surface was not induced by the decompression process but is indeed a slip plane as clear striation is observed at the interface. On the basis of an elasto-plastic constitutive model calibrated on triaxial tests on the clayey gouge, it is shown that shear band formation inside the clayey core is possible. The solution for the orientation of the shear band is compared to the orientation of an existing slip surface inside the clayey gouge and this result is used to deduce the orientation of the principal stresses. It is shown that as commonly observed in weak fault zones, the orientation of the principal stresses is locally almost parallel and perpendicular to the fault axis. © 2007 Elsevier B.V. All rights reserved.
650.
Structure and morphology of the Bosumtwi impact structure from seismic reflection data
Meteoritics and Planetary Science,
42
(4-5)
549 – 560
2007
ISSN: 10869379
Publisher: University of Arkansas
Abstract: ▾ 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.
649.
Results of pre-drilling potential field measurements at the Bosumtwi crater
Meteoritics and Planetary Science,
42
(4-5)
541 – 547
2007
ISSN: 10869379
Publisher: University of Arkansas
Abstract: ▾ 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.
648.
Structure and properties of the San Andreas fault in central California: Recent results from the SAFOD experiment
Scientific Drilling
(1 SUPPL. 1)
29-32
2007
ISSN: 18168957647.
San Andreas fault zone mineralogy, geochemistry, and physical properties from SAFOD cuttings and core
Scientific Drilling
(1 SUPPL. 1)
64-67
2007
ISSN: 18168957646.
Possible reasons of shock melt deficiency in the Bosumtwi drill cores
Meteoritics and Planetary Science,
42
(4-5)
883-894
2007
Abstract: ▾ 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.
645.
Repeating earthquake finite source models: Strong asperities revealed on the San Andreas fault
Geophysical Research Letters,
34
(23)
2007
ISSN: 00948276Keywords:▾
Mathematical models; Seismic response; Seismographs; Seismology; Tectonics, Rupture process; Rupture speed; Seismic moment rate; Tectonic loading, Earthquakes, deconvolution; earthquake magnitude; earthquake rupture; Green function; San Andreas Fault; seismic moment; spatial distribution
Abstract: ▾ We investigate the rupture process of a sequence of repeating Mw 2.1 earthquakes on the San Andreas Fault in Parkfield spanning the occurrence of the September 28, 2004 mainshock by inverting seismic moment rate functions obtained from empirical Green's function deconvolution. The results show that these events have extremely concentrated slip patches with radii on the order of 10-20 m, with peak slip between 8.4 and 11.4 cm. The rupture speed and rise time are consistent with values of larger earthquakes. The spatial distribution of stress drop for the events shows low average values 2.5-5.6 MPa and very large peak values of 66.7-93.9 MPa. The results show that strong asperities can exist at small scales on an otherwise weak fault, and helps reconcile differences between traditional spectra-based and tectonic loading methods for determining the stress drop of small repeating earthquakes. Copyright 2007 by the American Geophysical Union.