All ICDP Publications with Abstracts
From parent-sysfolder "Publications" + 2 folder-levels deep
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.
644.
Precipitation at Lake Qinghai, NE Qinghai-Tibet Plateau, and its relation to Asian summer monsoons on decadal/interdecadal scales during the past 500 years
Palaeogeography, Palaeoclimatology, Palaeoecology,
254
(3-4)
541 – 549
2007
ISSN: 00310182Keywords:▾
Asia; China; Eurasia; Far East; Qinghai; Qinghai Lake; climate variation; global climate; organic matter; precipitation assessment; stalagmite
Abstract: ▾ Knowledge of the variability of precipitation at Lake Qinghai and its relation to the Asian summer monsoons is helpful in constraining global climatic dynamics. Based on the high-resolution precipitation indicators of δ13C of the organic matter (δ13Corg), C/N atomic ratio, and the detrended total organic carbon content (TOCdetrended), we found that the trend of precipitation at Lake Qinghai is inversely correlated to that of the Indian Summer Monsoon (ISM) inferred from layer thickness of a stalagmite (S3) in southern Oman on decadal/interdecadal scales. The Chinese Drought/Flood (D/F) indices, which can indicate the dryness/wetness over large geographic areas, are also used to indicate the intensity of the monsoon rainfall. The D/F index of Xining near Lake Qinghai is synchronous with those of the regions in northern China where the East Asian Summer Monsoon (EASM) dominates; while it is anti-phase with those of southwestern China where ISM prevails. These materials suggest that, during the past 500 years, the source of moisture to Lake Qinghai on decadal/interdecadal scales is controlled mainly by the EASM, but not by the ISM. It is also suggested that the intensity of EASM is inversely related to that of the ISM on decadal/interdecadal scales. The decadal/interdecadal variability of ENSO may be responsible for the inverse relationship between the intensity of EASM and that of ISM. © 2007 Elsevier B.V. All rights reserved.
643.
Subsurface structure, fault zone characteristics, and, stress state in scientific drill holes of Taiwan Chelungpu fault drilling project
Scientific Drilling
(1 SUPPL. 1)
55-58
2007
ISSN: 18168957642.
Platinum group elements provide no indication of a meteoritic component in ICDP cores from the Bosumtwi crater, Ghana
Meteoritics and Planetary Science,
42
(4-5)
731-741
2007
Abstract: ▾ In an attempt to identify the type of projectile, 14 samples from the Bosumtwi crater in Ghana were analyzed for platinum group element (PGE) concentrations by nickel sulfide fire assay inductively coupled plasma-mass spectrometry (ICP-MS). The majority of the samples come from the impactite material recovered by cores LB-07A and LB-08A, which were drilled by the International Continental Scientific Drilling program (ICDP). One sample originates from the fallback material found at the contact between the impactite and the overlying lake sediment in core LB-05B. No clear signature of a meteoritic contamination was identified in the 13 impactite samples. The target rock apparently dominates the PGE contribution in the impactites. These results agree with the PGE concentrations reported for the suevites collected at the crater rim and in other parts of the Bosumtwi ICDP cores. However, based on Cr and Os isotopic signatures, a meteoritic component could be present in the sample of fallback material, supporting the reports of the existence of meteoritic material in the Ivory Coast tektites. Further analyses of the fallback material from the Bosumtwi drill cores should confirm (or not) this first result. © The Meteoritical Society, 2007. Printed in USA.
641.
Pliocene to Holocene geomorphic evolution and paleogeography of the El'gygytgyn Lake region, NE Russia
Journal of Paleolimnology,
37
(1)
37-47
2007
ISSN: 09212728Keywords:▾
chronology; crater lake; Holocene; lacustrine environment; lake evolution; lake level; paleogeography; Pleistocene; Pliocene; pollen; terrace, Chukchi; Eurasia; Lake El'gygytgyn; Russian Federation, Zanclea
Abstract: ▾ Geomorphic, lithologhic, and stratigraphic field studies as well as pollen data and mineralogical study have been used to propose Pliocene and Pleistocene paleogeographic reconstructions of the El'gygytgyn meteorite crater area. The moment of impact is recorded above the early Pliocene hill denudation plain as a "chaotic horizon" consisting of fragments of impactite rocks. This chaotic horizon lies between layers of late Pliocene alluvial sediments. During the second half of the late Pliocene, the region was tectonically active, when the Anadyr lowland was uplifted causing alluvial sediments to accumulate in the basins to the south of the crater. Regional climatic cooling, which supported the spread of tundra and the formation of permafrost is characteristically to late Pliocene. The 35-40 m high terrace that roughly follows the 530 m contour interval along the Enmyvaam River formed during the middle Pleistocene. This terrace represents the maximum lake level. Erosion and incision of the upper Enmyvaam River increased due to another wave of uplift. Additionally, El'gygytgyn Lake discharge increased causing lake level to begin to drop in the Middle Pleistocene. Cooling continued, which led to the development of herb-dominated arctic tundra. middle and late Pleistocene glaciations did not reach the El'gygytgyn lake region. The 9-11 m high lacustrine terrace was formed around the lake during the late Pleistocene and the 2-3 m high lacustrine terrace formed later during the Holocene. During the last 5000 years, the lake level has continued to drop as the modern coastline developed. © 2006 Springer Science+Business Media B.V.
640.
Possible reasons of shock melt deficiency in the Bosumtwi drill cores
Meteoritics and Planetary Science,
42
(4-5)
883 – 894
2007
ISSN: 10869379
Publisher: University of Arkansas
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.
639.
Post-drilling destabilization of temperature profile in borehole Yaxcopoil-1, Mexico
Hydrogeology Journal,
15
(2)
423-428
2007
Abstract: ▾ As part of the International Continental Scientific Drilling Program (ICDP), the 1.5-km-deep borehole Yaxcopoil-1, located in the Chixculub meteor impact structure in Mexico, has undergone further study after drilling operations ceased. Temperature logs were repeated ten times at intervals 0.3-0.8, 15, 24 and 34 months after borehole shut-in. The logs bear a distinct signature of transient heat transfer by groundwater flow manifested by a gradual distortion of the linear temperature profile when acoldwaveof0.8-1.6°C amplitude was detected propagating downward from 145 to 312 m at a rate of 4-6 m/month. To understand the nature of this moving anomaly, a 20-day monitoring of the cold wave was carried out at a depth of 307 m that showed further cooling of 0.6°C during the first 16 days of the passage followed by temperature stabilisation. As an explanation of this unusual phenomenon, a theory is proposed, whereby the drilling mud has accumulated within the overlying and cooler highly porous and permeable karstic rocks during the drilling and migrates downward. The observed migration rate suggests a permeability higher than 10-11 m2. This indicates a high vulnerability to contamination of the only freshwater aquifer in the Yucatan region. © Springer-Verlag 2006.
638.
Post-impact structural crater modification due to sediment loading: An overlooked process
Meteoritics and Planetary Science,
42
(11)
2013-2029
2007
Abstract: ▾ Post-impact crater morphology and structure modifications due to sediment loading are analyzed in detail and exemplified in five well-preserved impact craters: Mjølnir, Chesapeake Bay, Chicxulub, Montagnais, and Bosumtwi. The analysis demonstrates that the geometry and the structural and stratigraphic relations of post-impact strata provide information about the amplitude, the spatial distribution, and the mode of post-impact deformation. Reconstruction of the original morphology and structure for the Mjølnir, Chicxulub, and Bosumtwi craters demonstrates the long-term subsidence and differential compaction that takes place between the crater and the outside platform region, and laterally within the crater structure. At Mjølnir, the central high developed as a prominent feature during post-impact burial, the height of the peak ring was enhanced, and the cumulative throw on the rim faults was increased. The original Chicxulub crater exhibited considerably less prominent peak-ring and inner-ring/crater-rim features than the present crater. The original relief of the peak ring was on the order of 420-570 m (currently 535-575 m); the relief on the inner ring/ crater rim was 300-450 m (currently ∼700 m). The original Bosumtwi crater exhibited a central uplift/high whose structural relief increased duringburial (current height 101-110 m, in contrast to the original height of 85-110 m), whereas the surrounding western part of the annular trough was subdued more that the eastern part, exhibiting original depths of 43-68 m (currently 46 m) and 49-55 m (currently 50 m), respectively. Furthermore, a quantitative model for the porosity change caused by the Chesapeake Bay impact was developed utilizing the modeled density distribution. The model shows that, compared with the surrounding platform, the porosity increased immediately after impact up to 8.5% in the collapsed and brecciated crater center (currently +6% due to post-impact compaction). In contrast, porosity decreased by 2-3% (currently -3 to -4.5% due to post-impact compaction) in the peak-ring region. The lateral variations in porosity at Chesapeake Bay crater are compatible with similar porosity variations at Mjølnir crater, and are considered to be responsible for the moderate Chesapeake Bay gravity signature (annular low of -8 mGal instead of -15 mGal). The analysis shows that the reconstructions and the long-term alterations due to post-impact burial are closely related to the impact-disturbed target-rock volume and a brecciated region of laterally varying thickness and depth-varying physical properties. The study further shows that several crater morphological and structural parameters are prone to post-impact burial modification and are either exaggerated or subdued during post-impact burial. Preliminary correction factors are established based on the integrated reconstruction and post-impact deformation analysis. The crater morphological and structural parameters, corrected from post-impact loading and modification effects, can be used to better constrain cratering scaling law estimates and impact-related consequences. © The Meteoritical Society, 2007.
637.
Preface
Meteoritics and Planetary Science,
42
(4-5)
481 – 482
2007
ISSN: 10869379
Publisher: University of Arkansas
636.
Reconstruction of the Chicxulub ejecta plume from its deposits in drill core Yaxcopoil-1
Bulletin of the Geological Society of America,
119
(9-10)
1151-1167
2007
DOI:10.1130/B26116.1
Abstract: ▾ Formation conditions of suevite-like impactites from an ∼100 m thick drill core sequence through the Cretaceous-Tertiary Chicxulub crater were reconstructed from empirical data obtained by petrologic and image analytical methods. The temporal evolution of the cratering process from the initial stage of excavation to the collapse of the ejecta plume is evidenced by the petrographic characteristics and modal composition of the suevitic rocks, including the size distribution and shape parameters of melt particles. Emplacement of the lowermost suevitic deposits likely started in the first minute after the impact by the passing ejecta curtain that interacted with the expanding ejecta plume. These ejecta deposits were capped by a tongue of coherent impact melt that was transported outward from the crater center during the collapse of the central uplift ∼5 min after impact. On top of this brecciated impact melt rock, the collapsing ejecta plume deposited air-fall suevites. The basal air-fall unit, Middle Suevite, may have been deposited due to a density current-like clumping of hot debris. With progressive cooling, regions of the ejecta plume were entrained in its collapse that produced vapor condensates, accretionary rims, and different oxygen fugacities. After cooling progressed, atmospheric conditions began to reestablish over the crater and turbulence decreased, supposedly after the first 10 min of initial ejecta plume collapse. This led to a winnowing out of fine matrix material and distinct sorting. However, due to aquatic reworking, only material that was deposited until ∼1 h after cessation of turbulent atmospheric conditions was retained. © 2007 Geological Society of America.
635.
Preface to the special issue on Taiwan Chelungpu-Fault Drilling Project (TCDP): Site characteristics and on-site measurements
Terrestrial, Atmospheric and Oceanic Sciences,
18
(2)
I-VI
2007
ISSN: 10170839634.
Preliminary results of stress measurement using drill cores of TCDP Hole-A: An application of anelastic strain recovery method to three-dimensional in-situ stress determination
Terrestrial, Atmospheric and Oceanic Sciences,
18
(2)
379-393
2007
ISSN: 10170839Keywords:▾
anelasticity; borehole; Chi-Chi earthquake 1999; core analysis; drilling; fault zone; in situ measurement; siltstone; strain; stress measurement; three-dimensional modeling
Abstract: ▾ In order to understand the feature of rock stress change at different depths above, within, and beneath the Chelungpu fault after the Chi-Chi earthquake, we employed a core-based stress measurement method, anelastic strain recovery (ASR) technique to determine both the orientations and magnitudes of present three-dimensional principal rock stresses using drill core samples retrieved from Taiwan Chelungpu-fault Drilling Project (TCDP) main Hole-A. The core samples used were taken from three depths; and their lithology were sandstone at depths of 592 and 1755 m and siltstone at 1112 m. The anelastic strains of the specimens in nine directions, including six independent directions, were measured after its in-situ stress was released. Acquired anelastic strains were of high quality and reached several hundred microstrains, which is sufficiently high for the accuracy of the measurement system used. Thus, the strain data could be used for three-dimensional analysis resulting in the determination of orientations and the estimation of magnitudes of the principal in-situ stresses. Preliminary stress measurement results showed that the orientations of principal stresses changed between the shallower depth above the fault and the deeper depth beneath it, that is, the present stress distribution in the TCDP hole might be influenced by the Chelungpu fault rupture. At the same time, anelastic strain recovery measurement is well suited for the task of directly determining the orientations of principal in-situ stresses and to estimate the magnitude of stresses at large/great depth.
633.
Progress in the Research on Major Geological Events in the Earth Surface System during the Cretaceous and Greenhouse Climate Change (in Chinese with English abstract);[白垩纪地球表层系统重大地质事件与温室气候变化研究进展]
China Basic Science,
622-26
2007
632.
Progress report on the Iceland Deep Drilling Project (IDDP)
Scientific Drilling
(4)
26-29
2007
ISSN: 18168957631.
Putting it all together: Exhumation histories from a formal combination of heat flow and a suite of thermochronometers
Journal of Geophysical Research: Solid Earth,
112
(8)
2007
ISSN: 21699313
Publisher: Blackwell Publishing Ltd
Keywords:▾
burial (geology); exhumation; genetic algorithm; heat flow; numerical model; parameterization; thermochronology, California; North America; San Andreas; United States, Calluna vulgaris
Abstract: ▾ A suite of new techniques in thermochronometry allow analysis of the thermal history of a sample over a broad range of temperature sensitivities. New analysis tools must be developed that fully and formally integrate these techniques, allowing a single geologic interpretation of the rate and timing of exhumation and burial events consistent with all data. We integrate a thermal model of burial and exhumation, (U-Th)/He age modeling, and fission track age and length modeling. We then use a genetic algorithm to efficiently explore possible time-exhumation histories of a vertical sample profile (such as a borehole), simultaneously solving for exhumation and burial rates as well as changes in background heat flow. We formally combine all data in a rigorous statistical fashion. By parameterizing the model in terms of exhumation rather than time-temperature paths (as traditionally done in fission track modeling), we can ensure that exhumation histories result in a sedimentary basin whose thickness is consistent with the observed basin, a physically based constraint that eliminates otherwise acceptable thermal histories. We apply the technique to heat flow and thermochronometry data from the 2.1 -km-deep San Andreas Fault Observatory at Depth pilot hole near the San Andreas fault, California. We find that the site experienced <1 km of exhumation or burial since the onset of San Andreas fault activity ∼30 Ma.
630.
Quantification of soil erosion rates related to ancient Maya deforestation
Geology,
35
(10)
915-918
2007
ISSN: 00917613Keywords:▾
Lake sediments; Seismic stratigraphy; Soil erosion rate, Erosion; Lakes; Sediments; Seismology; Stratigraphy, Structural geology, anthropogenic effect; Classic period; core analysis; deforestation; deposition; erosion rate; gyttja; lacustrine deposit; land use; seismic data; seismic stratigraphy; soil erosion, Central America; Guatemala [Central America]
Abstract: ▾ We used seismic and sediment core data to quantify soil erosion rates for the past ∼6000 yr in the closed catchment of Lake Salpetén, in the tropical lowlands of northern Guatemala. The region was affected by ancient Maya land use from before ca. 1000 B.C. to A.D. 900. This period of human impact coincided with deposition in the lake of a detrital unit (Maya Clay) as much as 7 m thick that contrasts sharply with the relatively organic-rich gyttja deposited both before and after Maya occupation of the watershed. The greatest soil loss, with mean sustained values of ∼1000 t/kM2 yr-1, occurred in the Middle and Late Preclassic Periods (700 B.C. to A.D. 250), associated with relatively low Maya population densities. Soil erosion slowed during the period of maximum population density in the Late Classic Period (A.D. 550-830), indicating a decoupling between human population density and soil erosion rate. The most rapid soil loss occurred early during initial land clearance, suggesting that even low numbers of people can have profound impacts on lowland tropical karst landscapes. © 2007 The Geological Society of America.
629.
Quaternary glaciation and hydrologic variation in the South American tropics as reconstructed from the Lake Titicaca drilling project
Quaternary Research,
68
(3)
410 – 420
2007
Keywords:▾
Lake Titicaca; South America; Bacillariophyta; glaciation; ice sheet; interglacial; paleoclimate; paleohydrology; Quaternary; reconstruction; sediment core; tropical region; water budget
Abstract: ▾ A 136-m-long drill core of sediments was recovered from tropical high-altitude Lake Titicaca, Bolivia-Peru, enabling a reconstruction of past climate that spans four cycles of regional glacial advance and retreat and that is estimated to extend continuously over the last 370,000 yr. Within the errors of the age model, the periods of regional glacial advance and retreat are concordant respectively with global glacial and interglacial stages. Periods of ice advance in the southern tropical Andes generally were periods of positive water balance, as evidenced by deeper and fresher conditions in Lake Titicaca. Conversely, reduced glaciation occurred during periods of negative water balance and shallow closed-basin conditions in the lake. The apparent coincidence of positive water balance of Lake Titicaca and glacial growth in the adjacent Andes with Northern Hemisphere ice sheet expansion implies that regional water balance and glacial mass balance are strongly influenced by global-scale temperature changes, as well as by precessional forcing of the South American summer monsoon. © 2007 University of Washington.