All ICDP Publications with Abstracts
From parent-sysfolder "Publications" + 2 folder-levels deep
109.
11 - Paleoenvironmental changes in the Eurasian continent interior inferred from chemical elements in sediment cores (BDP96/1, BDP96/2) from Lake Baikal
In Koji Minoura, Editor
,
Lake Baikal
Page 127-135
Publisher
Elsevier Science B.V.
,
Amsterdam
2000
127-135108.
Mapping the Hawaiian plume conduit with converted seismic waves
Nature,
405
(6789)
938-941
2000
ISSN: 00280836DOI:10.1038/35016054
Keywords:▾
crustal structure; hot spot; mantle plume; seismic velocity, air temperature; article; atmosphere; geography; plume; priority journal; sea; shear rate; United States; volcano, Pacific Ocean
Abstract: ▾ The volcanic edifice of the Hawaiian islands and seamounts, as well as the surrounding area of shallow sea floor known as the Hawaiian swell, are believed to result from the passage of the oceanic lithosphere over a mantle hotspot. Although geochemical and gravity observations indicate the existence of a mantle thermal plume beneath Hawaii, no direct seismic evidence for such a plume in the upper mantle has yet been found. Here we present an analysis of compressional-to-shear (P-to-S) converted seismic phases, recorded on seismograph stations on the Hawaiian islands, that indicate a zone of very low shear-wave velocity (< 4 km s-1) starting at 130-140 km depth beneath the central part of the island of Hawaii and extending deeper into the upper mantle. We also find that the upper-mantle transition zone (410-660 km depth) appears to be thinned by up to 40-50 km to the south-southwest of the island of Hawaii. We interpret these observations as localized effects of the Hawaiian plume conduit in the asthenosphere and mantle transition zone with excess temperature of ~300°C. Large variations in the transition-zone thickness suggest a lower-mantle origin of the Hawaiian plume similar to the Iceland plume, but our results indicate a 100°C higher temperature for the Hawaiian plume.
107.
10 - Estimation of paleoenvironmental changes in the Eurasian continental interior during the past 5 million years inferred from organic components in the BDP96 1
In Koji Minoura, Editor
,
Lake Baikal
Page 119-126
Publisher
Elsevier Science B.V.
,
Amsterdam
2000
119-126106.
A new estimate of the Holocene lowstand level of Lake Titicaca, central Andes, and implications for tropical palaeohydrology
Holocene,
10
(1)
21 – 32
2000
Keywords:▾
Bolivia; Lake Titicaca; Holocene; lake level; lowstand; paleohydrology
Abstract: ▾ New evidence from piston cores and high-resolution seismic reflection data shows that water levels in Lake Titicaca were as much as 100 m below the present level during the early to mid-Holocene (between >6 and 3.8 14C kyr BP). Climatological and modelling studies indicate that Lake Titicaca rainfall depends on convective activity in upwind Amazonia; the lake-level data therefore suggest a drier Amazon Basin during this time. This view is bolstered by an excellent match between the Titicaca lake-level curve and decreased methane concentrations in Greenland ice, previously ascribed to drying of low-latitude wetlands (Blunier et al., 1995). The postglacial history of Lake Titicaca fits a global pattern of lake-level change in the tropics, characterized by opposite phasing between the Southern and Northern Hemispheres. This pattern is most likely the result of orbital controls over the intensity of summer insolation.
105.
A new record of Holocene climate change from the bottom sediments of Lake Baikal
Palaeogeography, Palaeoclimatology, Palaeoecology,
156
(3-4)
211 – 224
2000
104.
Clay minerals in bottom sediments of Lake Baikal as indicators of climate change
Geologiya i Geofizika,
41
(10)
1347 – 1359
2000
ISSN: 00167886Abstract: ▾ The mineral assemblage of Pliocene - Pleistocene bottom sediments of Lake Baikal, exposed by the deep-sea BDP-96-2 (Baikal Drilling Project) has been studied by the methods of XRD and IR spectroscopy. Complex XRD patterns of clay minerals have been modeled using an advanced algorithm based on calculation of interference function from one-dimensional disordered crystals of finite thickness. The modeling implies fitting of theoretical to observed patterns and optimization procedure by methods of nonlinear programming. It has been shown that major clay minerals in the sediments that formed for the past 2.5 Ma during cold stages are illite-illite-smectites with predominantly illite layers, and chlorite; the principal clay minerals in the sediments that formed during warm periods are smectite, illite-smectites with high contents of smectite layers, and chlorite-smectites. These data allow unravelling the climate history of Lake Baikal from biogenic silica record.
103.
Climate-induced fluctuations of 10Be concentration in Lake Baikal sediments
Nuclear Instruments and Methods in Physics Research, Section B: Beam Interactions with Materials and Atoms,
172
(1-4)
562 – 567
2000
ISSN: 0168583X
Publisher: Elsevier
Keywords:▾
Beryllium; Climate change; Erosion; Glacial geology; Lakes; Oxygen; Radioisotopes; Sediments; Stratigraphy; Watersheds; Weathering; Accelerated mass spectrometry; Marine oxygen isotope stages; Marine sediments; Mass spectrometry
Abstract: ▾ Sedimentary 10Be records covering the last 150 kyr were obtained from three cores collected at the Academician Ridge (BDP-96/hole2 core and VER96/st.3 core) and at the Buguldeika Saddle (BDP-93/hole2 core) in Lake Baikal. The 10Be concentrations of the three cores varied between 0.5 × 109 and 1.5 × 109 atoms/g, and coincidently dropped at the stratigraphic intervals of marine oxygen isotope stages (MIS) 2, 4, 5d and 6. The depositional fluxes of 10Be, on the other hand, generally rose in those stages having an increase in the dry bulk densities and sediment accumulation rates. These results are consistent with previous work (Horiuchi et al., 1999), suggesting that the dilution effects of low-10Beconcentration particles principally controlled the fluctuations of the 10Be concentrations of Lake Baikal sediments. Low-10Be-concentration particles have been intensively produced by mechanical weathering and physical erosion under the cold and dry climatic conditions during the peak glaciation period, and have been directly brought from the source areas into the lake as a result of the thin vegetative cover of the watershed. © 2000 Elsevier Science B.V. All rights reserved.
102.
Comparison of biogenic silica, Br and Nd distributions in the sediments of Lake Baikal as proxies of changing paleoclimates of the last 480 kyr
Nuclear Instruments and Methods in Physics Research, Section A: Accelerators, Spectrometers, Detectors and Associated Equipment,
448
(1)
400 – 406
2000
ISSN: 01689002
Publisher: Elsevier Science B.V.
Keywords:▾
Bromine; Geochemistry; Lakes; Neodymium; Organic compounds; Sediments; Silica; Spectrum analysis; Synchrotron radiation; Biogenic silica; Lake Baikal; Paleoclimate indicators; Sedimentary geochemistry; X ray fluorescence analysis; X ray analysis
Abstract: ▾ The paper presents a comparison of the profiles of biogenic silica (BiSi), Br and Nd in the first 20 m of the BDP-96-2 core of sediments of Lake Baikal obtained on top of the underwater Akademichesky Ridge. The concentrations of the two elements mentioned were measured by X-ray fluorescent analysis with synchrotron radiation (SR-XFA) at 10 cm intervals. Br correlates positively, and Nd negatively with BiSi. Spectral analysis of the profiles revealed the 100, 41, 23 and 19 kyr major external forcing periods. We studied the elemental composition of extant diatom algae of Lake Baikal in order to find out whether any elements are bound by them specifically. SR-XFA revealed, among others that Br is accumulated by extant diatom algae of Lake Baikal, presumably due to incorporation into their organic matter. Br can be regarded as a new proxy of the paleo-productivity of Lake Baikal. The paper also considers arguments suggesting that the first 2 m of sediment are missing in the BDP-96-2 core.
101.
Comparison of elastic velocity models for gas-hydrate-bearing sediments
Geophysical Monograph Series,
124179-187
2000
ISSN: 00658448
Publisher: Blackwell Publishing Ltd
Abstract: ▾ One of the distinct physical properties of gas-hydrate-bearing sediments is elevated seismic velocities. A number of velocity models and equations have been presented to describe the effect of gas hydrate on the seismic velocities; e.g., pore-filling model, cementation model, effective medium theory, a weighted equation, and time-average equation. The data set from Mallik 2L-38 gas hydrate research well drilled in northern Canada provided us a unique opportunity to test the velocity models for gas-hydrate-bearing sediments. Velocities predicted from an effective medium theory and those from a weighted equation are compared with observed well log velocities. In the case where there is no gas hydrate in the pore space, P-wave velocities predicted from the effective medium theory are lower than those from the weighted equation when porosity is less than about 30% and higher when porosity is higher than about 30%. For S-waves, effective medium theory predicts generally higher velocities than those from the weighted equation. Both theories predict similar increases in P- and S-wave velocities when gas hydrate occupies the pore space. Even though gas hydrate concentration in the pore space is not known accurately, analyses using both P- and S-wave velocities and their ratios enable us to test the validity of velocity models. Considering only P-wave velocities, there is not much difference between the effective medium and weighted equation. However, considering both P- and S-wave velocities and their ratios, the weighted equation is preferred to the effective medium theory in predicting elastic wave velocities for gas-hydrate-bearing sediments at the Mallik 2L-38 well. © 2001 by the American Geophysical Union.
100.
Fluid history of UHP metamorphism in Dabie Shan, China: A fluid inclusion and oxygen isotope study on the coesite-bearing eclogite from Bixiling
Contributions to Mineralogy and Petrology,
139
(1)
1 – 16
2000
ISSN: 00107999
Publisher: Springer Verlag
Keywords:▾
Bixiling; China; Dabie Shan; coesite; eclogite; fluid composition; fluid inclusion; isotopic composition; oxygen; ultrahigh pressure metamorphism
Abstract: ▾ The coesite-bearing eclogites and associated ultramafic rocks of Bixiling form the largest metamorphic complex in the Dabie-Sulu ultrahigh-pressure (UHP) metamorphic belt. They mainly consist of 'fresh' eclogite, kyanite-rich eclogite, retrograded eclogite and garnet peridotite. Fluid inclusion and oxygen isotope studies have been carried out on the different types of eclogite and peridotite in order to identify the role of fluids during the metamorphic evolution culminating in UHP metamorphism. Five types of fluid inclusions were distinguished based on textural criteria and fluid compositions: (1) primary Ca-rich brines in quartz blebs in kyanite; (2) primary NaCl-dominated high-salinity inclusions in omphacite and kyanite; (3) primary NaCl-dominated medium- to high-salinity inclusions in matrix quartz; (4) carbonic inclusions in omphacite and matrix quartz; (5) secondary low-salinity aqueous (or pure water) inclusions in matrix quartz. The Ca-rich fluid inclusions in quartz blebs in kyanite represent the earliest recognizable fluids (prograde metamorphism) as they largely escaped late re-equilibration. Fluid inclusions in omphacite and kyanite may have been trapped during peak metamorphic conditions, whereas low-salinity aqueous inclusions in matrix quartz were trapped during the latest stage of uplift. UV-laser oxygen isotope measurements show that garnet and clinopyroxene from the 'fresh' eclogite, kyanite-rich eclogite and garnet peridotite have narrow δ18O values ranging from 3.0 to 3.9‰. In contrast, garnet and omphacite in retrograded eclogite have δ18O values of -1.8 to -1.2 and of -1.1 to -0.6‰, respectively. The difference in oxygen isotope composition is interpreted to result from partial oxygen isotope exchange between the UHP complex and retrograde fluids during late exhumation. Fluids derived from the surrounding gneiss were probably responsible for the low-salinity solutions found in secondary fluid inclusions and the lowering of the δ18O values of the retrograded eclogite.
99.
High precision lead isotope systematics of lavas from the Hawaiian Scientific Drilling Project
Chemical Geology,
169
(1-2)
187 – 209
2000
ISSN: 00092541Keywords:▾
United States; geochemistry; hot spot; lava; lead isotope
Abstract: ▾ We report Pb isotopic compositions for 35 samples of the volcanoes Mauna Loa and Mauna Kea from the Hawaiian Scientific Drilling Project (HSDP-1) core at Hilo. These data were obtained with an external precision of ~ 100 ppm (2σ(ext.)) on the ratios 206Pb/204Pb, 207Pb/204Pb and 208Pb/204Pb by using a Pb triple spike to correct for instrumental mass fractionation. The Pb isotopic compositions in the lower section (1200 to 280 m) of the core sample 200 to 400 ka-old Mauna Kea lavas, and display two well-defined linear arrays in 207Pb/204Pb-206Pb/204Pb and 208Pb/204Pb-206Pb/204Pb isotope spaces. There is a suggestion that Mauna Loa (0 to 280 m depth) also displays such linear array(s). However, analysis of the Mauna Loa samples is complicated by residual contamination and/or sample heterogeneity. While these latter data exhibit a satisfactory array in 208Pb/204Pb vs. 206Pb/204Pb, there still remains scatter in 207Pb/204Pb-206Pb/204Pb space, making it difficult to assess the true Pb isotope systematics of Mauna Loa. The presence of two linear Pb isotopic arrays in Mauna Kea can be interpreted as either reflecting two parallel isochrons or in terms of binary mixing. If interpreted as isochrons, the 207Pb/204Pb-206Pb/204Pb systematics correspond to an age of ~ 1.9 Ga. Comparison of measured Th/U ratios in the lavas and those inferred from Pb isotope systematics strongly suggest that the Pb isotopic arrays reflect binary mixing, and this bears directly on the question of how many distinct components are present in the Hawaiian plume. Most of the new Mauna Kea data lie well outside the mixing-component triangle defined in the literature by the 'Kea', 'Loihi', and 'Koolau' components. On the basis of the relationships between Pb isotope ratios in 3D and a principal component analysis of the Mauna Kea Pb isotope dataset, we show here that a three-component mixing model can in principle explain both mixing lines. However, such an explanation requires a highly specific set of mixing conditions in order to produce parallel arrays in Pb isotope space (2D and 3D). Therefore, our preferred interpretation is that the two arrays reflect binary mixing, with four discrete source components involved in the generation of the Kea lavas. Comparison of the Pb isotope characteristics of these lavas with those of East Pacific Rise (EPR) MORB glasses further suggests that EPR-type Pacific lithosphere does not contribute to the source of Kea lavas. The position of samples along the mixing lines does not correlate with stratigraphic height in the core, and therefore the age of the lavas. Rather, it appears as though the relative proportions of the endmembers are controlled by the spatial configuration of these endmembers, and by melting and transport processes in the source itself. The stratigraphic fluctuations of Pb and Sr isotopes contrast with the monotonic decrease of ε(Nd) and ε(Hf) values as a function of age. This may in part be explained by differences in analytical precision of isotope measurements relative to the total range of values observed. This analytical resolution is far higher for Pb than for the other radiogenic isotopes. Alternatively, the observed fluctuation may be caused by the mobility of lead (as well as Rb and/or Sr) during the ancient differentiation process that created the differences in parent-daughter ratios. (C) 2000 Elsevier Science B.V. All rights reserved.
98.
High-Si phengite, mineral chemistry and P-T evolution of ultra-high-pressure eclogites and calc-silicates from the Dabie Shan, eastern China
Geological Journal,
35
(3-4)
185 – 207
2000
ISSN: 00721050
Publisher: John Wiley and Sons Ltd
DOI:10.1002/gj.863
Keywords:▾
China; Dabie Shan; eclogite; metamorphism; P-T conditions; ultrahigh pressure metamorphism
Abstract: ▾ A suite of coesite-eclogites and associated calc-silicate rocks from the ultra-high-pressure (UHP) belt in the Dabie Shan (eastern China) was investigated petrologically. Field relations and the presence of UHP minerals such as coesite, omphacite and high-Si phengite in the eclogites and the enclosing calc-silicates testify to a common metamorphic evolution for these two lithologies. Except for one sample, all bear phengite with unusually high silica contents (Si up to 3.7 per formula unit). Phengite occupies various textural positions indicating that different metamorphic stages are reflected by these white micas, which correlate with distinct mineral zonation patterns. Using the latest thermobarometric calibrations for eclogite-facies rocks, maximum pressure-temperature (P-T) conditions of 40-48 kbar at < 750°C were estimated for the peak-metamorphic mineral assemblages. These P-T conditions were calculated for both eclogitic garnet porphyroblasts with diffusion-controlled zoning as well as garnet porphyroblasts with prograde growth zonation patterns. Most samples were affected by a strong retrograde overprint mainly under eclogite- and amphibolite-facies conditions. Thermobarometry using mineral sets from different textural positions reveals cooling and decompression of the UHP rocks down to < 20 kbar at < 600°C for the bulk of the samples. Decompression and heating indicated by a few samples is interpreted to result from mineral chemical disequilibrium or late thermal influence. These new data show that subduction of continental crust in the Dabie Shan was deeper than previously thought, and also that some cooling and decompression took place at upper-mantle depths. Copyright © 2000 John Wiley and Sons, Ltd.
97.
JAPEX/JNOC/GSC Mallik 2L-38 gas hydrate research well, Mackenzie Delta, N.W.T.: overview of field operations
SPE Proceedings - Gas Technology Symposium
733-742
2000
DOI:10.2523/59795-ms
Keywords:▾
Additives; Gas hydrates; Heat exchangers; Mud logging; Natural gas well drilling; Oil field development; Organic polymers; Petroleum industry; Petroleum prospecting; Potassium compounds; Sediments; Societies and institutions, Chemical mud additive; Coring; Coring gas hydrate bearing strata; Polymer drilling mud; Potassium chloride drilling mud, Natural gas fields
ISBN:
9781555633493
Abstract: ▾ The Mallik 2L-38 well was drilled in February and March, 1998 to a depth of 1150m at a site located in the Mackenzie Delta, N. W. T., Canada. Undertaken as a collaborative agreement between the Japan National Oil Corporation and the Geological Survey of Canada, the well was conducted as a research and development project with engineering goals to evaluate various technologies for drilling and coring gas-hydrate-bearing strata. The Mallik site was chosen as it had favorable logistics and was though to contain a thick interval of gas hydrates between 897 and 1110 m depth. Drilling operation included a surface hole (with 8 coring runs) to 687m for installation of a 340mm surface casing, and a main hole (with 16 coring runs) to the target depth of 1150m. The drilling system utilized a KCl/polymer drilling mud that was cooled to 2°C using a plate type heat exchanger. Drilltreat, a chemical mud additive was used in the main hole to stabilize the hydrate within drill cuttings and formation sediments. Drilling operations were conducted without any serious hole problems, accidents, or mishaps. However, delays were caused by adverse weather and mechanical problems, causing adjustments in the overall program. Coring in the main hole was particularly successful allowing evaluation of four different core barrels. Gas-hydrate-bearing core was collected in a variety of sediments between 886 and 952m. The excellent condition of the core samples, controlled gas hydrate dissociation within the mud column, and near-gauge hole, confirmed that the combination of chilled mud with Drilltreat performed extremity well.
96.
Dissociation of natural gas hydrates observed by X-ray CT scanner
Annals of the New York Academy of Sciences,
9121011-1020
2000
ISSN: 00778923
Publisher: New York Academy of Sciences
Keywords:▾
methane; natural gas, computer assisted tomography; conference paper; dissociation; gas; gas flow; pressure; sand; temperature; X ray analysis
Abstract: ▾ Core samples containing pore-spare gas hydrate within granular sands were collected from 913.76 m of the research well named JAPEX/JNOC/GSC Mallik 2L-38. X-ray CT images of the core were acquired while warming from -18 to 4°C, and subsequently during stepped decreases of 0.1 MPa in the chamber pressure below the methane hydrate equilibrium pressure. Discharged gas flows and sample temperatures were monitored continuously. Changes in CT values indicated that gas hydrate dissociated simultaneously both on the exposed surfaces and within the pore spaces of the sample in response to pressure changes. This suggested that pressure reductions were effectively transmitted through the sample most likely because the samples contained some amount of fluids. The result of gas flow measurements indicated that a larger pressure drawdown caused a higher dissociation rate.
95.
Multi-wavelength synchrotron radiation XRF determination of U and Th in sedimentary cores from Lake Baikal
Geostandards Newsletter,
24
(2)
217 – 226
2000
ISSN: 01505505Abstract: ▾ A new technique for the determination of uranium and thorium in lacustrine sediments is based on non-destructive synchrotron radiation X-ray fluorescence analysis (SR-XRF) of sediment samples using monochromatic beams of different energies as excitation sources, with the recording and subsequent iterative processing of the X-ray fluorescence spectra. The technique has a multielement capability and enables fast, simultaneous analysis of a few tens of elements. The proposed SR-XRF technique was tested against INAA and ICP-MS methods and showed a number of advantages with a generally good correlation of results by the three methods. Uranium and Th profiles have been measured at a time resolution of 2 kyr in a drill core (BDP-96) from Lake Baikal bottom sediments deposited between 780 and 40 kyr BP. During this time span, peaks in the U and U/Th concentration marked "warm" climates. Oscillations of U and U/Th in Baikal bottom sediments during the Brunhes chron reflect climate-induced global change in the volume of polar ice, controlled by long-term cyclicity of the Earth's orbital parameters. The response of these "warm" periods is similar to that observed earlier in shorter cores that span the two last interglacials (220-0 kyr BP).
94.
The application of Raman spectroscopy to the study of gas hydrates
Annals of the New York Academy of Sciences,
912859-872
2000
ISSN: 00778923
Publisher: New York Academy of Sciences
Keywords:▾
methane; water, aqueous solution; conference paper; crystallization; gas; gas analysis; Raman spectrometry
Abstract: ▾ Raman spectroscopy is reviewed with particular emphasis placed on its application to gas hydrates. Experimental examples discussed include studies of the totally symmetric C-H stretching vibration v1 (A1) of methane in both synthetic and natural hydrate samples (from the JAPEX/JNOC/GSC Mallik 2L-38 research well); a comparison of the coupled O-H vibrations of water in the host lattice of CH4 hydrate and ice I(h) at low temperature; and local structural details of the relaxation of the hydrogen-bonded water on crystallization to structure II hydrate of amorphous tetrahydrofuran (THF) aqueous solutions. This paper is intended to be an introduction to Raman spectroscopy with specific examples from research at the National Research Council of Canada, and is aimed at those who wish to apply the technique as a tool to investigate gas hydrates.
93.
1 - Baikal drilling project
In Koji Minoura, Editor
,
Lake Baikal
Page 1-14
Publisher
Elsevier Science B.V.
,
Amsterdam
2000
1-1492.
Numerical experiments for seismic tomography using the Singular Value Decomposition (SVD) technique
Science Reports of the Kyushu University, Department of Earth and Planetary Sciences,
21
(1)
9-18
2000
ISSN: 13480545Abstract: ▾ The purpose of this study is to learn how to consider setting the borehole and arrangement of the measuring instruments for seismic tomography. For this purpose we made an underground structure model of Unzen Volcano based on the results of previous studies, and simulated the seismic tomography identifying this model with the true structure because one of some plans in "Scientific drilling project at Unzen Volcano" is seismic tomography by using a borehole approaching conduit" We located sources and receivers along the borehole and at the surface, and calculated synthetic seismograms at the receiver positions by a 2-D finite difference method. We measured the first P arrival time in the synthetic seismograms, and constructed the observational equation for seismic tomography. We investigated the solution of this equation by the Singular Value Decomposition (SVD) technique. We found we can consider borehole location and arrangement of sources and receivers through such a simulation by Singular values and eigenvalue that got in this way.
91.
UNAM Scientific shallow-drilling program of the Chicxulub impact crater
International Geology Review,
42
(10)
928-940
2000
Abstract: ▾ A scientific drilling program is being carried out by the National Autonomous University of Mexico (UNAM) at the southern sector of the Chicxulub impact crater in the Yucatan Peninsula, Mexico. Eight boreholes, ranging in depth from 60 m to 702 m, with a total of 2.62 km of continuos core, were recovered. A high recovery rate of up to 99% (overall average recovery rate for the eight boreholes is 87%) allows us to investigate in detail the stratigraphy of the impact lithologies and the Tertiary carbonate sequence. Three of the boreholes (UNAM-5, UNAM-6, and UNAM-7, with core recovery rates from 89 to 99%) sampled impact breccias that were classified in two units-an upper breccia sequence rich in basement clasts, impact glass, and fragments of melt (suevite-like breccia) and a lower breccia sequence rich in limestone, dolomite, and evaporite clasts (bunte-like breccia). Depths of contact between the Tertiary carbonate sequence and the impact breccias are 332.0 m in UNAM-5, 222.2 m in UNAM-7, and 282.8 m in UNAM-6, giving the depth to the K/T boundary. In UNAM-7, the contact between the upper and the lower breccias is at 348.4 m, which yields a thickness of 126.2 m for the suevite-like breccia. The rest of the boreholes sampled part of the Tertiary carbonate sequence (∼200 m thick), composed mainly of limestones, dolomitized carbonates, and calcarenite, with some fossiliferous horizons.
90.
The Parkfield, California earthquake experiment: An update in 2000
Current Science,
79
(9)
1226-1236
2000
ISSN: 00113891Abstract: ▾ The US Geological Survey, in cooperation with other institutions, continues to monitor the San Andreas Fault (SAF) near Parkfield, California, hoping to capture high resolution records of continuous deformation before, during and after a magnitude 6 earthquake, as well as the details of its rupture initiation and strong ground motion. Despite the failure of the prediction that the next M 6 Parkfield earthquake would occur before 1993, Parkfield still has a higher known probability (1 to 10% per year) than anywhere else in the US of a M 6 or greater earthquake. Parkfield instrumentation is still largely in place, although there have been losses due to attrition as well as improvements made possible by new technology. Most Parkfield data sets are now available via the Internet, and all others may be obtained upon request from individual investigators. Detailed seismic monitoring has shown that events with identical seismograms, recurring in exactly the same locations, account for a high proportion of the background seismicity at Parkfield. Geophysical studies have revealed that fault zone seismic and electrical properties are consistent with high fluid content. The rate of interseismic slip on the SAF changed significantly in late 1992 or early 1993, during a period of relatively high seismic activity. The strain-rate change, measured by borehole tensor strainmeters and the two-colour electronic distance-measuring network, was also manifested as shortened recurrence intervals of repeating microearthquakes. Whether or not the accelerated deformation turns out to be an intermediate-term precursor to the next M 6 Parkfield earthquake, documenting the variation of interseismic strain rates with time has important implications for fault dynamics and seismic hazard estimation. Two possible instances of pre-earthquake signals have been recorded at Parkfield: water-level and strain changes over a period of three days prior to the nearby 1985 Mw 6.1 Kettleman Hills, California, earthquake and anomalous electromagnetic signals prior to the M 5 earthquake near Parkfield on 20 December 1994. Future work planned at Parkfield includes a National Science Foundation proposal to construct an SAF Observatory at Depth (SAFOD), as part of the Earthscope initiative. The Observatory will consist of a 4-km-deep borehole to penetrate the SAF and a shallow microearthquake cluster on Middle Mountain, directly above the hypocenter of the 1966 Parkfield earthquake.
89.
Sedimentation processes and new age constraints on rifting stages in Lake Baikal: Results of deep-water drilling
International Journal of Earth Sciences,
89
(2)
183 – 192
2000
ISSN: 14373254Keywords:▾
Russian Federation; depositional environment; paleoclimate; paleoenvironment; Pliocene; Quaternary; rifting; sedimentation rate
Abstract: ▾ With this paper we present a first attempt to combine the direct results on lithology, composition and age dating in the boreholes BDP-93, BDP-96 and BDP-97 with geological and seismic data from the areas where those sections were drilled. The sedimentary environments represented by the BDP boreholes are markedly different and possess characteristic lithological features. The results of the deep drilling provide the essential means for testing numerous age models used in geological reconstructions of the Lake Baikal rifting dynamics. Neither the basin-wide unconformity interpreted from seismic data, nor the interpreted change from shallow-water to deep-water facies at the boundary of the seismic stratigraphic complexes were found in the BDP-96 boreholes on Academician Ridge. Also, lithology does not support the proposed reconstructions of intense lake level fluctuations and transgressions during the Pliocene at Academician Ridge. The continuous deep-water hemipelagic sedimentation at Academician Ridge has existed for the past 5 Ma. The beginning of an intense rifting phase of the Neobaikalian sub-stage and related drastic changes in sedimentation processes were interpreted on seismic sections as the basin-wide unconformity B10. Different age estimates for this boundary ranged from Late Pliocene (3.5 Ma) to Plio-Pleistocene boundary. As shown by BDP-96 borehole, B10 is associated with a lithological change from diatomaceous ooze to dense silty clay and not with an erosional contact. The new age for this boundary in BDP-96 is approximately 2.5 Ma. This new age constraint suggests that the upper sedimentary strata of Northern Baikal (1.5-1.7 km thick) have formed during the past 2.5 Ma with average sedimentation rates of 60-70 cm/ka. The BDP-93 boreholes at Buguldeika suggest that uplift in Primorsky Range took place prior to 1.07-1.31 Ma, a date which exceeds the age of previous geological models.
88.
Results from a TEM survey for gas hydrate delineation in permafrost
SEG Technical Program Expanded Abstracts,
19
(1)
1347-1350
2000
ISSN: 10523812
Publisher: Society of Exploration Geophysicists
Keywords:▾
Electric logging; Gases; Hydration; Permafrost; Petroleum prospecting; Surveys, Electrically conductive; Hydrate zones; Induced polarization; Resistive zones; TEM surveys; Time domain electromagnetics, Gas hydrates
Abstract: ▾ In conjunction with the 1998 Mallik 2L-38 gas-hydrate research well program (Dallimore et al, 1999), two time-domain electromagnetic sounding profiles were surveyed in an attempt to delineate a known gas-hydrate zone at depth. The research well is located on Richard's Island in the Mackenzie Delta, Northwest Territories. Electrical resistivity logging at the Mallik site in 1972 and 1998 defined a resistive zone from 900 m to 1100 m coincident with a gas-hydrate zone lying within unfrozen and electrically conductive sediments. A resistive layer, interpreted as the gas-hydrate zone was detected by the TEM survey at all soundings except for those overlain by thin permafrost and a thick unfrozen zone. This thin permafrost/thick unfrozen zone assemblage resulted in greater than expected conductivity-thickness product and limited the depth investigation of the survey. In addition, late-time negative transients interpreted as induced polarization effects, obscured the clear detection of gas hydrates. © 2000 Society of Exploration Geophysicists.
87.
Relation between gas hydrate and physical properties at the Mallik 2L-38 research well in the Mackenzie delta
Annals of the New York Academy of Sciences,
91294-100
2000
ISSN: 00778923
Publisher: New York Academy of Sciences
Keywords:▾
methane; natural gas, Arctic; Canada; conference paper; gas; gas analysis; porosity; river; salinity; sand; sediment; temperature; water content
Abstract: ▾ As part of an interdisciplinary field program, a 1150-m deep well was drilled in the Canadian Arctic to determine, among other goals, the location, characteristics, and properties of gas hydrate. Numerous physical properties of the host sediment were measured in the laboratory and are presented in relation to the lithology and quantity of in situ gas hydrate. Profiles of measured and derived properties presented from that investigation include: sediment wet bulk density, water content, porosity, grain density, salinity, gas hydrate content (percent occupancy of non-sediment grain void space), grain size, porosity, and post-recovery core temperature. The greatest concentration of gas hydrate is located within sand and gravel deposits between 897 and 922 m. Silty sediment between 926 and 952 m contained substantially less, or no, gas hydrate perhaps because of smaller pore size.
86.
Recycled oceanic crust observed in 'ghost plagioclase' within the source of Mauna Loa lavas
Nature,
404
(6781)
986-990
2000
ISSN: 00280836DOI:10.1038/35010098
Keywords:▾
basalt; mantle plume; melt inclusion; oceanic crust; petrogenesis; petrology; recycling; strontium, article; geography; geology; natural science; oceanic regions; priority journal; United States; volcano, United States
Abstract: ▾ The hypothesis that mantle plumes contain recycled oceanic crust is now widely accepted. Some specific source components of the Hawaiian plume have been inferred to represent recycled oceanic basalts, pelagic sediments or oceanic gabbros. Bulk lava compositions, however, retain the specific trace- element fingerprint of the original crustal component in only a highly attenuated form. Here we report the discovery of exotic, strontium-enriched melt inclusions in Mauna Loa olivines. Their complete trace-element patterns strongly resemble those of layered gabbros found in ophiolites, which are characterized by cumulus plagioclase with very high strontium abundances. The major-element compositions of these melts indicate that their composition cannot be the result of the assimilation of present-day oceanic crust through which the melts have travelled. Instead, the gabbro has been transformed into a (high-pressure) eclogite by subduction and recycling, and this eclogite has then been incorporated into the Hawaiian mantle plume. The trace-element signature of the original plagioclase is present only as a 'ghost' signature, which permits specific identification of the recycled rock type. The 'ghost plagioclase' trace-element signature demonstrates that the former gabbro can retain much of its original chemical identity through the convective cycle without completely mixing with other portions of the former oceanic crust.
85.
Overlapping volcanoes: The origin of Hilo Ridge, Hawaii
Geology,
28
(6)
547 – 550
2000
ISSN: 19432682
Publisher: Geological Society of America
Keywords:▾
Hawaii [(ISL) Hawaiian Islands]; Hawaii [United States]; Hawaiian Islands; Hilo; Kohala; Mauna Kea; United States; Isotopes; Hilo Ridge; Isotope ratios; Kohala; Mauna Kea; Scientific drilling; isotopic ratio; shoreline; submarine; terrace; volcanism; volcano; Volcanoes
Abstract: ▾ The submarine Hilo Ridge has been interpreted as a part of Mauna Kea volcano, but is crossed at ~1100 m depth by a submerged shoreline terrace composed of basalts that are isotopically distinct from those of Mauna Kea and similar to those of Kohala volcano. This terrace evidently is a product of Kohala instead of Mauna Kea. Almost all of Hilo Ridge below the terrace therefore must predate the principal growth of Mauna Kea, which has superficially isolated the ridge from its Kohala source by overlapping its proximal segment. The Mauna Kea section penetrated by the Hawaii Scientific Drilling Project is predicted to be thinner than expected previously, owing to the overlap. Similar overlaps are suspected among other volcanoes and may cause significant changes in the understanding of Hawaiian volcanism.