All ICDP Publications with Abstracts
From parent-sysfolder "Publications" + 2 folder-levels deep
169.
Episodic thermal perturbations associated with groundwater flow: An example from Kilauea Volcano, Hawaii
Journal of Geophysical Research: Solid Earth,
107
(11)
ECV 13–1 – 13–10
2002
ISSN: 21699313
Publisher: Blackwell Publishing Ltd
Abstract: ▾ Temperature measurements in deep drill holes on volcano summits or upper flanks allow a quantitative analysis of groundwater induced heat transport within the edifice. We present a new temperature-depth profile from a deep well on the summit of Kilauea Volcano, Hawaii, and analyze it in conjunction with a temperature profile measured 26 years earlier. We propose two groundwater flow models to interpret the complex temperature profiles. The first is a modified confined lateral flow model (CLFM) with a continuous flux of hydrothermal fluid. In the second, transient flow model (TFM), slow conductive cooling follows a brief, advective heating event. We carry out numerical simulations to examine the timescales associated with each of the models. Results for both models are sensitive to the initial conditions, and with realistic initial conditions it takes between 750 and 1000 simulation years for either model to match the measured temperature profiles. With somewhat hotter initial conditions, results are consistent with onset of a hydrothermal plume ∼550 years ago, coincident with initiation of caldera subsidence. We show that the TFM is consistent with other data from hydrothermal systems and laboratory experiments and perhaps is more appropriate for this highly dynamic environment. The TFM implies that volcano-hydrothermal systems may be dominated by episodic events and that thermal perturbations may persist for several thousand years after hydrothermal flow has ceased.
168.
Origin and evolution of the western Snake River Plain: Implications from stratigraphy, faulting, and the geochemistry of basalts near Mountain Home, Idaho
Tectonic and Magmatic Evolution of the Snake River Plain Volcanic Province,
30343--361
2002
167.
Drilling program investigates permafrost gas hydrates
Eos,
83
(18)
193+198
2002
ISSN: 00963941
Publisher: American Geophysical Union
166.
Biot-Gassmann theory for velocities of gas hydrate-bearing sediments
Geophysics,
67
(6)
1711-1719
2002
ISSN: 00168033
Publisher: Society of Exploration Geophysicists
Keywords:▾
Concentration (process); Elasticity; Hydrates; Porosity; Sediments; Acoustic wave velocity; Gas hydrates; Gases; Hydration; Seismic waves; Seismology; Shear waves; Wave propagation, Elastic velocities; Compressional velocities; Effective medium theories; Empirical relationships; Gas hydrate bearing sediments; Gas hydrate concentrations; Shear wave velocity; Solid structures; Unconsolidated sediment, Geophysics; Shear flow, Biot theory; elastic wave; gas hydrate; P-wave; S-wave; velocity
Abstract: ▾ Elevated elastic velocities are a distinct physical property of gas hydrate-bearing sediments. A number of velocity models and equations (e.g., pore-filling model, cementation model, effective medium theories, weighted equations, and time-average equations) have been used to describe this effect. In particular, the weighted equation and effective medium theory predict reasonably well the elastic properties of unconsolidated gas hydrate-bearing sediments. A weakness of the weighted equation is its use of the empirical relationship of the time-average equation as one element of the equation. One drawback of the effective medium theory is its prediction of unreasonably higher shear-wave velocity at high porosities, so that the predicted velocity ratio does not agree well with the observed velocity ratio. To overcome these weaknesses, a method is proposed, based on Biot-Gassmann theories and assuming the formation velocity ratio (shear to compressional velocity) of an unconsolidated sediment is related to the velocity ratio of the matrix material of the formation and its porosity. Using the Biot coefficient calculated from either the weighted equation or from the effective medium theory, the proposed method accurately predicts the elastic properties of unconsolidated sediments with or without gas hydrate concentration. This method was applied to the observed velocities at the Mallik 2L-39 well, Mackenzie Delta, Canada.
165.
Occurrence of metastable rocks in deeply subducted continental crust from the Dabie Mountains, central China
Episodes,
25
(2)
84 – 89
2002
ISSN: 07053797
Publisher: International Union of Geological Sciences
Keywords:▾
China; Dabie Shan; continental crust; metamorphic facies; metamorphic rock; subduction
Abstract: ▾ A recent study reveals that metamorphic volcanic and sedimentary rocks have survived deep subduction of continental crust in the Dabie Mountains, central China. These low-grade (LG) rocks are intercalated within the ultrahigh-pressure (UHP) metamorphic terrane and well preserve primary volcanic breccia structure and sedimentary bedding. The presence of coherent field relations between LG and UHP rocks suggests that LG rocks are not of exotic origin and therefore suffered from UHP metamorphism. They, however, did not record the UHP event likely due to slow reaction kinetics, and only experienced epidote-amphibolite to greenschist facies metamorphism and deformation at the retrograde stage of UHP rocks. The protolith age of c. 790 Ma and metamorphic age of 232.2 Ma obtained from LG rocks, which agree well with those from UHP rocks, also suggest that both LG and UHP rocks shared the same evolutionary history since Late-Proterozoic. Except for the lack of deformation and fluid during UHP metamorphism, the rapid subduction, short stay at mantle depths and rapid exhumation are necessary for the preservation of primary assemblage and fabric in LG rocks within deeply subducted continental crust.
164.
Conjugate thrust faulting associated with the 1999 Chi-Chi, Taiwan, earthquake sequence
Geophysical Research Letters,
29
(8)
2002
ISSN: 00948276
Publisher: Blackwell Publishing Ltd
Keywords:▾
Earthquakes; Geophysics; Underground structures, Chi-Chi , Taiwan; Conjugate faults; Earthquake sequences; Focal mechanism; Geometrical structure; Seismicity pattern; Spatial patterns; Thrust faulting, Faulting
Abstract: ▾ The geometrical structure of the responsible faults of the 20 September 1999 Chi-Chi, Taiwan, earthquake (ML = 7.3, Mw = 7.6) and its aftershocks can be clearly depicted by well-located hypocenters and focal mechanisms of large aftershocks. The mainshock and two large aftershocks with ML = 6.8 were characterized by thrust faulting along a N-S striking fault plane dipping to the east. The underground structure of the Chelungpu fault, which is probably merging with the decollement beneath the Western Foothills, can be clearly associated with the seismicity pattern and the focal mechanisms of the three largest events. A group of deeper aftershocks including two moderate events (ML = 6.3 and 6.0, respectively) were located to the southeast of the mainshock along a fault plane dipping steeply to the west down to a depth of about 30 km. Our results suggest that the spatial pattern of the aftershocks in the southern part of the source area can be interpreted by a conjugate-fault system. This conjugate-fault system is comprised of the gently east-dipping Chelungpu fault and a steeply west-dipping deeper fault zone. Copyright 2002 by the American Geophysical Union.
163.
Chelungpu fault drilling could resolve seismological issues
Eos,
83
(23)
255
2002
ISSN: 00963941
Publisher: American Geophysical Union
162.
High-resolution seismic velocities and shallow structure of the San Andreas fault zone at Middle Mountain, Parkfield, California
Bulletin of the Seismological Society of America,
92
(6)
2493-2503
2002
ISSN: 00371106Keywords:▾
Granite; Imaging techniques; Landforms; Sandstone; Sediments; Surveying; Velocity measurement, Seismic imaging surveys, Seismology, crustal structure; fault zone; San Andreas Fault; seismic velocity; seismicity, United States
Abstract: ▾ A 5-km-long, high-resolution seismic imaging survey across the San Andreas fault (SAF) zone and the proposed San Andreas Fault Observatory at Depth (SAFOD) drill site near Parkfield, California, shows that velocities vary both laterally and vertically. Velocities range from <1.0 km/sec near the surface to as much as 4.8 km/sec at 750-m depth. The lowest velocities (<1.0 to ∼3.0 km/sec) correspond to unconsolidated sediment, mudstone, and sandstone in the near surface, and the higher velocities (>4.0 km/sec) probably correspond to granitic rock of the Salinian block, which is exposed a few kilometers southwest of the SAF. The depth to the top of probable granitic rock varies laterally along the seismic profile but is about 600 m below the surface at the proposed SAFOD site. We observe a prominent, lateral low-velocity zone (LVZ) beneath and southwest of the surface trace of the SAF. The LVZ is about 1.5 km wide at 300-m depth but tapers to about 600 m wide at 750-m depth. At the maximum depth of the velocity model (750 m), the LVZ is centered approximately 400 m southwest of the surface trace of the SAF. Similar velocities and velocity gradients are observed at comparable depths on both sides of the LVZ, suggesting that the LVZ is anomalous relative to rocks on either side of it. Velocities within the LVZ are lower than those of San Andreas fault gouge, and the LVZ is also anomalous with respect to gravity, magnetic, and resistivity measurements. Because of its proximity to the surface trace of the SAF, it is tempting to suggest that the LVZ represents a zone of fractured crystalline rocks at depth. However, the LVZ instead probably represents a tectonic sliver of sedimentary rock that now rests adjacent to or encompasses the SAF. Such a sliver of sedimentary rock implies fault strands on both sides and possibly within the sliver, suggesting a zone of fault strands at least 1.5 km wide at a depth of 300 m, tapering to about 600 m wide at 750-m depth. Fluids within the sedimentary sliver are probably responsible for observed low-resistivity values.
161.
Initial science report of shallow drilling, penetrating into the Chelungpu fault zone, Taiwan
Terrestrial, Atmospheric and Oceanic Sciences,
13
(3)
227-251
2002
ISSN: 10170839
Publisher: Chinese Geoscience Union
Keywords:▾
breccia; Chi-Chi earthquake 1999; dip-slip fault; drilling; fault zone; microstructure; reverse fault, Chelungpu Fault; Taiwan
Abstract: ▾ The Chelungpu fault, a reverse fault with left lateral component dipping moderately to the east, was activated by the Chi-Chi earthquake (Mw = 7.6) in 21 September, 1999 with maximum vertical and lateral offsets of 5.6 m and 9.8 m. Characteristics of earthquake and related phenomena are contrasting between northern and southern regions along the Chelungpu fault. The northern region has (1) larger displacements (4 to 9 m), (2) low frequency seismic waves with higher velocity of slip surface, and (3) less disastrous except the most northern area compared to those in the southern region. Drilling into the Chelungpu fault was thus conducted at two locations, northern (Fengyuan) and southern (Nantou) sites, and successfully completed in March 2001. The project was motivated to explore the fundamental controlling factors of the mode of slip motion at northern and southern regions through analysis of intrafault materials. Meso- and microstructural examinations and measurements of static/ dynamic physical properties have been conducted for each drill core. The ongoing analyses have shown interesting results: (1) fault zone architecture is totally different between the northern and southern fault zones. The rocks are mainly composed of random fabric fault breccia with extremely thin fault gouge in the northern core, whereas the foliated fault breccia is dominantly associated with ultracataclasite and pseudotachylite in the southern core, (2) possible fault zones activated by the Chi-Chi earthquake can be listed up by combining geological, geophysical logging and reflection seismic data, which are 225 m and 330 fracture zones in the core from northern well and 177 m and 180 m fracture zones in the core from southern well, (3) water contents of the core of the 225 m rupture zone in the northern well attains up to 45 vol.%, and (4) some temperature rises were detected at 330 m fracture zone in the northern well and 180 m fracture zone in the southern well by temperature logging, which could be attributed to residual heat generated during the Chi-Chi earthquake or postseismic influx of hydrothermal fluid into the fault zones.
160.
Late Pliocene vegetation and climate of the Lake Baikal region, southern East Siberia, reconstructed from palynological data
Palaeogeography, Palaeoclimatology, Palaeoecology,
184
(1-2)
107 – 129
2002
ISSN: 00310182
Publisher: Elsevier B.V.
Keywords:▾
Lake Baikal; Russian Federation; Siberia; Bacteria (microorganisms); Corylus; Corylus; Dinophyceae; Gonyaulax; Gonyaulax; Malvaceae; Picea; Picea; Pinus; Quercus; Quercus; Selaginella; Selaginella selaginoides; Selaginella selaginoides; Tilia; Tilia; Tsuga; Tsuga; Ulmus; Ulmus; lacustrine deposit; paleoclimate; paleoenvironment; palynology; Pliocene; vegetation history
Abstract: ▾ Palynomorphs from a palaeomagnetically dated Late Pliocene sediment core (3.6-2.35 Ma) reflect highly variable climatic conditions and repeated expansion of steppe and boreo-alpine vegetation. Mixed coniferous forests (Picea, Tsuga, Pinus) with associated broadleaved taxa (Quercus, Corylus, Ulmus, Tilia) were affected by dry and cold intervals already between 3.48 and 3.39 Ma. Peak records of non-arboreal pollen types and spores, including subarctic-boreo-alpine Selaginella selaginoides, evidence advances of open vegetation, which can be correlated to glacial marine oxygen isotope stages. A cooling trend occurred from 3.15 Ma to 2.5 Ma, documented by cold-dry intervals and increased fluctuations of Picea and Pinus. Around 3-2.9 Ma the palaeoclimate variation pattern shifted towards cold-dry and warm-moist oscillations. Cooling strengthened after 2.89 Ma and severely restricted Tsuga development. Cool and oligotrophic limnic conditions are documented by abundant Gonyaulax dinoflagellate cysts in the interval 2.65-2.57 Ma. Due to decreasing precipitation extensive areas with open steppe and rock-steppe vegetation became permanently established after 2.62 Ma. Dry conditions became a dominant environmental factor in the Baikal region, coeval in time with the Red Clay-Loess shift in northern China. This climatic transition can be explained as a consequence of major intensification of Northern Hemisphere Glaciation around 2.75 Ma evidenced by marine records of ice-rafted debris. © 2002 Elsevier Science B.V. All rights reserved.
159.
Late-Quaternary lowstands of Lake Titicaca: Evidence from high-resolution seismic data
Palaeogeography, Palaeoclimatology, Palaeoecology,
179
(1-2)
97 – 111
2002
Keywords:▾
Bolivia; Peru; lake level; paleoclimate; Quaternary; seismic data; seismic reflection; seismic stratigraphy
Abstract: ▾ Approximately 600 km of high-resolution seismic reflection data were collected to investigate the late-Quaternary stratigraphic development of Lake Titicaca. The focus of this report is on two seismic sequence boundaries, which are interpreted as erosional surfaces formed at times of low lake level. The younger erosional surface occurs as much as 90 m below the present lake level and up to 8 m below the present sediment-water interface. This erosional surface is interpreted to be coeval with a well-documented early- to mid-Holocene lowstand, dated between ∼ 8000 and 3600 cal yr BP. An earlier and previously unknown erosional surface occurs at a sub-bottom depth of approximately 30 m, and as much as 240 m below the present lake level, which implies a major late-Pleistocene lowstand of Lake Titicaca. By extrapolation of sedimentation rates from the upper ∼ 14 m of sediment, we estimate the age of this older lowstand at > 90000 cal yr BP. Both lowstands of Lake Titicaca indicated by the seismic data are likely to have been a response to climatic change in the region. © 2002 Elsevier Science B.V. All rights reserved.
158.
Mapping the northern portion of the Chelungpu fault, Taiwan by shallow reflection seismics
Geophysical Research Letters,
29
(16)
37-1-37-3
2002
ISSN: 00948276
Publisher: American Geophysical Union
Keywords:▾
Cost effectiveness; Seismology; Structure (composition); Surveys, Fault slippages, Earthquakes, borehole; earthquake rupture; fault slip; seismic reflection; seismic survey, Chelungpu Fault; Taiwan
Abstract: ▾ The Chelungpu fault was activated by the 1999 Chi-Chi earthquake (Mw = 7.6), Taiwan. This fault exhibited extraordinarily large surface ruptures (up to 9.8 m) as well as underground fault slippages (up to 12 m) during the earthquake. These large displacements were concentrated along the northern portion of the fault, 40 km north of the epicenter. To prepare data for the future drilling of deep wells in this area, many shallow seismic reflection surveys were conducted to investigate the sites. An approximate 3D structure of the fault surface can be deduced by this cost-effective approach. Although the depth penetration may be limited (e.g., 3 km), the method still provides reliable information to study large ruptures, and to better plan future deep wells.
157.
Late Pliocene sedimentation in Lake Baikal: Implications for climatic and tectonic change in SE Siberia
Palaeogeography, Palaeoclimatology, Palaeoecology,
174
(4)
305 – 326
2001
ISSN: 00310182Keywords:▾
Russian Federation; Bacillariophyta; geochemistry; lacustrine deposit; paleoclimate; Pliocene; sedimentology; tectonic evolution; weathering
Abstract: ▾ Within the framework of the Baikal Drilling Project (BDP), a 192 m long sediment core (BDP-96-1) was recovered from the Academician Ridge, a submerged topographic high between the North and Central Basins of Lake Baikal. Sedimentological, clay mineralogical and geochemical investigations were carried out on the core interval between 90 and 124 m depth, corresponding to ca. 2.4-3.4 Ma. The aim was to reconstruct the climatic and tectonic history of the continental region during the intensification of Northern Hemisphere glaciation in Late Pliocene time. A major climate change occurred in the Lake Baikal area at about 2.65 Ma. Enhanced physical weathering in the catchment, mirrored in the illite to smectite ratio, and temporarily reduced bioproduction in the lake, reflected by the diatom abundance, evidence a change towards a colder and more arid climate, probably associated with an intensification of the Siberian High. In addition, the coincident onset of distinct fluctuations in these parameters and in the Zr/Al ratio suggests the beginning of the Late Cenozoic high amplitude climate cycles at about 2.65 Ma. Fluctuations in the Zr/Al ratio are traced back to changes in the aeolian input, with high values in warmer, more humid phases due to a weaker Siberian High. Assuming that the sand content in the sediment reflects tectonic pulses, the Lake Baikal area was tectonically active during the entire investigated period, but in particular around 2.65 Ma. Tectonic movements have likely led to a gradual catchment change since about 3.15 Ma from the western towards the eastern lake surroundings, as indicated in the geochemistry and clay mineralogy of the sediments. The strong coincidence between tectonic and climatic changes in the Baikal area hints at the Himalayan uplift being one of the triggers for the Northern Hemisphere Glaciation. © 2001 Elsevier Science B.V. All rights reserved.
156.
Thermal properties of bottom sediments of Lake Baikal (according to data on temperature field recovery in underwater boreholes)
Geologiya i Geofizika,
42
(1-2)
298 – 307
2001
ISSN: 00167886Abstract: ▾ Thermal conductivity of rocks is a key parameter in heat flow determination. Thermal conductivity of the Baikal bottom sediments in cores from first underwater boreholes (BDP-93 and BDP-96) was measured by a needle probe and a thermal-conductivity comparator, and the two methods showed a difference of up to 30-50% in the values of both thermal conductivity and heat flow. In this study we used a new method implying evaluation of thermal conductivity of sediments through interpretation (inversion modeling) of temperature field recovery in boreholes after the cessation of drilling. The modeling parameters are steady-state temperature and thermal conductivity of bottom sediments at a depth of temperature monitoring. As a result, it was concluded that the needle probe measurements are reliable, and the thermal-conductivity comparator cannot be used to study unconsolidated sediments with water contents above 40%. The obtained estimates of steady-state temperature and geothermal gradient confirmed that the heat flow is relatively low (about 50 mW/m2) in the Bugul'deika saddle (BDP-93) and high (78 mW/m2, an updated value) in the axial part of the Akademichesky Ridge (BDP-96). The new method allowed us to estimate, for the first time, the thermal-diffusivity coefficient and the specific heat capacity of the Baikal sediments. Thus, the numerical analysis of temperature monitoring data permits estimation of all geothermal parameters of sediments.
155.
The sponge fauna of lake baikal in the late pliocene (according to studies of core samples from deep borehole BDP-96-1)
Geologiya i Geofizika,
42
(1-2)
130 – 137
2001
ISSN: 00167886Abstract: ▾ Spicules have been investigated in Late Pliocene core samples from the borehole BDP-96-1. The samples have been dated paleomagnetically. A total of 42 spicule types have been discovered, of which 15 are found in modern Baikal sponges and 27 are not. Analysis of the range of species and quantitative indices of the spicules in bottom sediments has revealed three stages of sponge-fauna development: 3.1-2.9, 2.9-2.5, and 2.5-2.1 Ma BP. The sponge fauna of the first stage is indicative of warm-water conditions. Most of thermophilic and fossil species disappeared from the second stage, which points to a dramatic cooling. At the third stage, regeneration of the sponge fauna began. However, it did not reach the efflorescence of the first stage. The results are in agreement with the palynological and diatom analysis of the core samples from the investigated depth range.
154.
The southwestern edge of the North Baikal basin: Geologic structure and correlation with cenozoic sections of Ol'khon and BDP-96 and BDP-98 cores
Geologiya i Geofizika,
42
(1-2)
373 – 383
2001
ISSN: 00167886Abstract: ▾ The paper presents new details of the structure of the basement and sedimentary cover of the southwestern edge of the North Baikal basin. The basement structure involves smaller-scale structures (Maloe More basin, Zama basin, and Zunduk-Zama horst), which, in turn, comprise third-order features. The presence of several en-echelon NE striking sub-basins within the Maloe More and Zama basins indicates that the tectonic framework of the region formed by listric and dextral faulting. Seismostratigraphy data and analysis of uppermost bottom sediments from BDP-96 and BDP-98 cores revealed three seismic layers in the sediment section, which are separated by discontinuities and can be correlated with deposits exposed on land in the neighboring Ol'khon region. Unit A' (X) correlates with the Lower-Middle Oligocene Ular'yar Formation, unit A, with the Lower-Middle Miocene Tagai Formation, and unit B, with the Upper Miocene-Pliocene Sasa Formation and with Quaternary deposits. The upper section of the Maloe More fill includes a separate seismic horizon correlatable with a layer of widely distributed fine-grained sand. Unit B, comprises three deposition centers within the limits of the Maloe More basin, i.e., large lakes existed in the region prior to the final stage of transgression from the North Baikal basin. Thus, the southwestern edge of the Baikal basin has an intricate structure and underwent a complex evolution.
153.
The new BDP-98 600-m drill core from Lake Baikal: A key late Cenozoic sedimentary section in continental Asia
Quaternary International,
80-8119 – 36
2001
ISSN: 10406182Keywords:▾
Asia; Russian Federation; Cenozoic; lacustrine deposit; sediment core
Abstract: ▾ The new 600-m drill core BDP-98 from the Academician Ridge of Lake Baikal recovered a continuous sedimentary record of the past 10 Ma. The entire section is represented by lacustrine sediments, which gradually change from distal deltaic facies at the bottom of the section to fine undisturbed hemipelagic sediments of the upper 300-m interval. The entire 10-Ma lacustrine section contains abundant diatoms, thus allowing extension of Plio-Pleistocene diatom and biogenic silica records into the Miocene. Above the Matuyama/Gauss paleomagnetic reversal boundary, the BDP-98 record contains clearly delineated glacial/interglacial lithologic cycles. Below this boundary the diatom signal is quite different: average diatom contents are higher and variations are of lower amplitude. Although most likely paleoclimatic in origin, these variation presumably reflect past changes in the moisture regime of southeast Siberia under conditions of warm subtropical climate during the Miocene and Early-Middle Pliocene. The continuous BDP-98 drill core, which covers the hiati present in the composite continental sections of the Baikal region, is a key section for reconstructing the Neogene-Quaternary climatic evolution of continental Asia. The BDP-98 section also places several important time constraints on the rifting history of Lake Baikal by providing reliable correlation of lithological and physical properties of the drill core sediments with calculated positions of the acoustic reflection boundaries interpreted from multichannel seismic studies. The lithologic composition indicates that, on the stable block of Academician Ridge where the BDP-96 and BDP-98 drill sites are located, acoustic reflection boundaries are not associated with major erosional events, but instead result from changes in sediment density and composition. Several lithologic indices further suggest that significant changes have occurred in the physics and chemistry of Lake Baikal waters, affecting the carbonate equilibrium and oxygen regime of Baikal. © 2001 Published by Elsevier Science Ltd.
152.
The link between tectonic and paleoclimatic events at 2.8-2.5 Ma BP in the Lake Baikal region
Quaternary International,
80-8137 – 46
2001
ISSN: 10406182Keywords:▾
Russian Federation; lacustrine deposit; neotectonics; paleoclimate; paleomagnetism; sediment core
Abstract: ▾ The records of diatom abundance, clay fraction content and magnetic susceptibility from Lake Baikal drill core BDP-96-1 reflect the climate-driven Plio-Pleistocene evolution of the Lake Baikal sedimentation system. In addition to the dramatic variations in the proxy records in response to the Northern Hemisphere ice ages, the Baikal record also indicates climatic deterioration from 2.8 to 2.5 Ma BP with evidence for an early Siberian glaciation around the Matuyama/Gauss paleomagnetic reversal boundary. The drill core data also allow correlation of this early glacial interval with the basin-wide seismic sequence boundary B10, which marks the unconformity produced by the active neotectonic phase in the Baikal rift zone. At the BDP-96 drill site, however, the strong B10 acoustic reflection was produced not by an erosional boundary, but instead by deposition of glacial clay layers. The paleomagnetic age scale of BDP-96-1 constrains the upper age of the Neobaikalian uplift/subsidence phase in the Baikal-Sayan region at ca. 2.5 Ma BP. The coincident timing of the paleoclimatic and tectonic events recorded in Lake Baikal sediments suggests a close causal link between regional tectonics and climate and indicate that the late Pliocene uplift contributed to the initiation of the early glaciation in southeast Siberia between 2.8 and 2.6 Ma BP by creating an elevated terrain with lower snowline, favorable for alpine glaciation, and by changing the heat balance of the region. © 2001 Elsevier Science Ltd and INQUA. All rights reserved.
151.
Paleoclimate record in bottom sediments of lake Baikal, from magnetic susceptibility data
Geologiya i Geofizika,
42
(1-2)
87 – 97
2001
ISSN: 00167886Abstract: ▾ Two boreholes on the Akademichesky Ridge, 100 and 200 m deep (BDP-96-1 and BDP-96-2), were drilled and cored continuously as part of the Baikal Drilling Project. Results of low-frequency magnetic susceptibility measurements of the two cores were correlated with the SPECMAP oceanic oxygen isotope curve, and a composite section was compiled. The SPECMAP curve and the magnetic susceptibility of the BDP-96 cores showed a good fit. Frequency analysis of time variations in magnetic susceptibility showed a periodicity of the paleoclimate signal with intervals of 23, 35, 41, 52, 68, 85, 106, and 164 ka. The periods of 23, 41, 52, and 106 ka correspond to cycles of precession, inclination of the Earth's axis, an eccentricity harmonic overlapped with inclination, and the Earth's orbit eccentricity itself, respectively. The periods of 35, 68, and 164 ka, distinguished for the first time in magnetic susceptibility studies but noted earlier in biogenic silica analysis, reflect different eccentricity harmonics, and the period of 68 ka bears an effect of precession. The period of 85 ka has never been revealed before in the climate record and is most likely related to regional periodicity rather than to any astronomic cycles.
150.
Multiwave XRF-SR determination of U and Th in bottom sediments of Lake Baikal: Brunhes paleoclimatic chronology
Geologiya i Geofizika,
42
(1-2)
186 – 193
2001
ISSN: 00167886Abstract: ▾ The paper presents a new method of U and Th determination in bottom sediments implying synchrotron radiation with monochromatic beams at different energies, registration, and iterative joint processing of the resultant XRF (X-ray fluorescent) spectra series. The validity of the new method was proved by comparison of U and Th determinations by XRF-SR in different sediment samples with earlier INAA and ICP-MS results for the same samples. Profiles of U and Th were measured at the sampling rate of 2 ka in a drilling core (BDP-96-2) of bottom sediments from Lake Baikal deposited 40 to 780 ka BP. Oscillations of U contents and U/Th ratios record global climate change throughout the Brunhes epoch (780 ka BP), and the response of these "warm" proxies is similar to that observed earlier in shorter cores spanning the last two interglacials (220-0 ka BP).
149.
Mineral inclusions in zircons of para- and orthogneiss from pre-pilot drillhole CCSD-PP1, Chinese Continental Scientific Drilling Project
Lithos,
59
(4)
199 – 215
2001
ISSN: 00244937Keywords:▾
China; drilling; mineralogy; orthogneiss; paragenesis; zircon
Abstract: ▾ The pre-pilot drillhole CCSD-PPI, Chinese Continental Scientific Drilling Project (CCSD), with depth of 432 m, is located in the Donghai area in the southwestern Sulu terrane. The core samples are mainly comprised of paragneiss, orthogneiss and ultramafic rock with minor intercalated layers of eclogite and phengite-bearing kyanite quartzite. All analyzed paragneiss and orthogneiss samples were overprinted on amphibolite facies retrograde metamorphism. Coesite and coesite-bearing ultrahighpressure (UHP) mineral assemblages were identified by Raman spectroscopy and electron microprobe analysis as inclusions in zircons separated from paragneiss, eclogite and phengite-bearing kyanite quatzite samples. In the paragneiss samples, UHP mineral inclusion assemblages mainly consist of Coe + Omp+ Grt+ Phe, Coe + Jd + Phe + Ap preserved in the mantles (M) and rims (R) of zircons. These UHP mineral inclusion assemblages yield temperatures of 814-852 °C and pressures of ≥28 kbar, presenting the P-T condition of UHP peak metamorphism of these country rocks. According to the mineral inclusions and cathodoluminescence images of zircons, the orthogneisses can be divided into two types: UHP (OGI) and non-UHP (OG2). In OGI orthogneisses, low-pressure mineral inclusion assemblage, mainly consisting of Qtz + Phe +Ab +Ksp + Ap, were identified in zircon cores (C), while coesite or coesite-bearing UHP mineral inclusions were identified in the mantles (M) and rims (R) of the same zircons. These features suggest that the OG1 orthogneisses, together with the paragneisses, phengitebearing kyanite quartzite and eclogite experienced widespread UHP metamorphism in the Sulu terrane. However, in the zircons of OG2 orthogneiss samples, no UHP mineral inclusions were found. Inclusions mainly comprised Qtz + Phe + Ap and were identified in cores (C), mantles (M) and rims (R) of OG2 zircons; the cathdoluminescence images of all analyzed zircons showed clear zonings from cores to rims. These features indicate that the OG2 orthogneisses in pre-pilot drillhole CCSD-PP1 did not experience UHP metamorphism. Therefore, we should not rule out the possibility that some orthogneisses in Sulu terrane might represent relatively low-pressure granitic intrusives emplaced after the UHP event. © 2001 Elsevier Science B.V. All rights reserved.
148.
Late quaternary climate and hydrology of tropical South America inferred from an isotopic and chemical model of Lake Titicaca, Bolivia and Peru
Quaternary Research,
56
(1)
1 – 9
2001
Keywords:▾
Bolivia; Lake Titicaca; Peru; geochemistry; lake level; paleoclimate; paleohydrology; paleolimnology; Quaternary
Abstract: ▾ A simple mass balance model provides insight into the hydrologic, isotopic, and chemical responses of Lake Titicaca to past climatic changes. Latest Pleistocene climate of the Altiplano is assumed to have been 20% wetter and 5°C colder than today, based on previous modeling. Our simulation of lacustrine change since 15,000 cal yr B.P. is forced by these modeled climate changes. The latest Pleistocene Lake Titicaca was deep, fresh, and overflowing. The latest Pleistocene riverine discharge from the lake was about 8 times greater than the modern average, sufficient to allow the expansion of the great paleolake Tauca on the central Altiplano. The lake δ18O value averaged about - 13‰ SMOW (the modern value is about -4.2‰). The early Holocene decrease in precipitation caused Lake Titicaca to fall below its outlet and contributed to a rapid desiccation of paleolake Tauca. Continued evaporation caused the 100-m drop in lake level, but only a slight (1-2‰) increase (relative to modern) in δ18O of early Holocene lake waters. This Holocene lowstand level of nearly 100 m was most likely produced by a precipitation decrease, relative to modern, of about 40%. The lake was saline as recently as 2000 cal yr B.P. The timing of these hydrologic changes is in general agreement with calculated changes of insolation forcing of the South American summer monsoon. © 2001 University of Washington.
147.
Biostratigraphic significance of new fossil species of the diatom genera Stephanodiscuts and Cyclotella from Upper Cenozoic deposits of Lake Baikal, Siberia
Micropaleontology,
47
(1)
47 – 71
2001
ISSN: 00262803DOI:10.2113/47.1.47
Keywords:▾
Russian Federation; biostratigraphy; Cenozoic; diatom; lacustrine deposit; new species; paleoclimate
Abstract: ▾ Three new extinct taxa of the genus Stephanodiscus Ehrenberg (S. williamsii sp., nov., S. princeps sp. nov., S. yukonensis var. antiquus var. nov.) and four new extinct species and two new extinct varieties of the genus Cyclotella (Kützing) Brébisson (C. iris var. insueta var. nov., C. tempereiformica sp. nov., C. distincta sp. nov., C. comtaeformica sp. nov., C. comtaeformica var. spinata var. nov. and C. praeminuta sp. nov.) are described from Upper Cenozoic lacustrine sediments of Lake Baikal (boreholes BDP-96-1 and BDP-96-2). The narrow biostratigraphic ranges of the new taxa warrant their use as index-fossil species for defining a diatom zonation, for accurate correlation and age control of Lake Baikal sediments. The comparative morphological analysis of the newly described Cyclotella taxa allowed us to trace the evolution of morphological features in these diatoms through time. The short time intervals during which the Stephanodiscus taxa existed in ancient Baikal imply that these species were unable to adapt to rapidly changing paleoecological and paleolimnological conditions caused by abrupt Pleistocene climatic fluctuations.
146.
Lake Titicaca: An archive of South American paleoclimate
Geotimes,
46
(12)
20 – 21
2001
145.
A high-resolution record of east siberian paleoclimates in the early and middle pleistocene by palynological studies of baikal sediments from the deep borehole BDP-96-1
Geologiya i Geofizika,
42
(1-2)
98 – 107
2001
ISSN: 00167886Abstract: ▾ Two boreholes, BDP-96-1 (200 m thick) and BDP-96-2 (100 m thick), drilled at the top of the underwater Akademichesky Ridge at 53°41′48″N and 108°21′06″, gave two parallel cores which were dated paleomagnetically. The correlation of the paleomagnetic data with the global magnetic scale has shown the age of the sediments exposed at a depth of 200 m to be 5 Ma. The average sedimentation rate was constant, about 4 cm/ka. Earlier palynological studies of core samples from BDP-96-1 were performed at 2 m intervals, which corresponds to the period of ∼45-50 ka. In this paper, we present results of a detailed palynological analysis of the upper 30 m of the core performed at 20 cm intervals (4-5 ka). The age of the investigated core from BDP-96-1 covers most of the Brunhes epoch. Taking into account the lost upper 630 cm, it matches the time range from 170 to 780 ka BP. Palynological analysis of the sediments from the range under discussion revealed 13 epochs in the development of the regional flora structure: seven epochs of predominance of forest plant formations and six epochs of its significant degradation. The epochs are correlated with the stages of changes in the volume of global ice, recorded in the marine oxygen isotope scale. It is difficult to determine the extents of the coolings, because spores and pollen are scarce in the sediments of these epochs. However, it is apparent that forests did not disappear from East Siberia during the coolings. All main arboreal species now growing there persisted during Pleistocene glaciations. The structure of the vegetative cover underwent profound changes. The areas occupied by forests decreased. Larch and spruce north-taiga forests were predominant under the humid cold conditions of the beginning of interglacials and the end of glaciations; they were then replaced by fir and cedar pine forests. Larch-pine and cedar pine middle-taiga forests expanded under the moderately warm and relatively dry climate of the mid-interglacials. Obviously, this was accompanied by a wide spread of steppe vegetation. A comparison of the frequency and habit of changes in vegetation type in the study region 170-780 ka BP with those in various regions of the Northern Hemisphere demonstrates that the East Siberian vegetation responded to global climatic changes synchronously with the vegetations of other regions.