All ICDP Publications with Abstracts
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1219.
Interpreting noble-gas concentrations as proxies for salinity and temperature in the world's largest soda lake (Lake Van, Turkey)
Journal of Asian Earth Sciences,
5999-107
2012
ISSN: 13679120Keywords:▾
chemical composition; concentration (composition); empirical analysis; noble gas; paleoclimate; paleosalinity; paleotemperature; porewater; salinity; temperature; water column, Lake Van; Turkey
Abstract: ▾ In this work we present noble-gas concentration profiles measured in the water column and in the sediment pore water of the Earth's largest soda lake: Lake Van (eastern Anatolia, Turkey). The concentrations of noble gases (in particular Ar, Kr, and Xe) in the water body deviate significantly from the expected equilibrium concentrations calculated from the in situ temperature and salinity using existing solubility functions for seawater. The specific chemical composition of the water of the soda lake seems to be responsible for the observed deviations. Our measurements allow the identification and quantification of salinity factors that can be applied to correctly calculate the noble-gas equilibrium concentrations for the lake. These salinity factors provide a solid and robust empirical basis for the interpretation of noble-gas concentration signals measured in the sediment pore water of Lake Van in terms of palaeosalinity and palaeotemperature. © 2012 Elsevier Ltd.
1218.
Inferred gas hydrate and permafrost stability history models linked to climate change in the Beaufort-Mackenzie Basin, Arctic Canada
Climate of the Past,
8
(2)
667-682
2012
ISSN: 18149324Keywords:▾
carbon dioxide; climate variation; gas hydrate; heat flux; Holocene; interglacial; lithology; methane; natural gas; paleoatmosphere; paleoclimate; permafrost; Phanerozoic; sea level change; surface temperature, Arctic Ocean; Beaufort Sea; Beaufort-Mackenzie Basin; Canada; Canadian Arctic, Calluna vulgaris
Abstract: ▾ Atmospheric methane from episodic gas hydrate (GH) destabilization, the "clathrate gun" hypothesis, is proposed to affect past climates, possibly since the Phanerozoic began or earlier. In the terrestrial Beaufort-Mackenzie Basin (BMB), GHs occur commonly below thick ice-bearing permafrost (IBP), but they are rare within it. Two end-member GH models, where gas is either trapped conventionally (Case 1) or where it is trapped dynamically by GH formation (Case 2), were simulated using profile (1-D) models and a 14 Myr ground surface temperature (GST) history based on marine isotopic data, adjusted to the study setting, constrained by deep heat flow, sedimentary succession conductivity, and observed IBP and Type I GH contacts in Mallik wells. Models consider latent heat effects throughout the IBP and GH intervals. Case 1 GHs formed at ∼0.9 km depth only ∼1 Myr ago by in situ transformation of conventionally trapped natural gas. Case 2 GHs begin to form at ∼290-300 m ∼6 Myr ago in the absence of lithological migration barriers. During glacial intervals Case 2 GH layers expand both downward and upward as the permafrost grows downward through and intercalated with GHs. The distinctive model results suggest that most BMB GHs resemble Case 1 models, based on the observed distinct and separate occurrences of GHs and IBP and the lack of observed GH intercalations in IBP. Case 2 GHs formed >255 m, below a persistent ice-filled permafrost layer that is as effective a seal to upward methane migration as are Case 1 lithological seals. All models respond to GST variations, but in a delayed and muted manner such that GH layers continue to grow even as the GST begins to increase. The models show that the GH stability zone history is buffered strongly by IBP during the interglacials. Thick IBP and GHs could have persisted since ∼1.0 Myr ago and ∼4.0 Myr ago for Cases 1 and 2, respectively. Offshore BMB IBP and GHs formed terrestrially during Pleistocene sea level low stands. Where IBP is sufficiently thick, both IBP and GHs persist even where inundated by a Holocene sea level rise and both are also expected to persist into the next glacial even if atmospheric CO2 doubles. We do not address the "clathrate gun" hypothesis directly, but our models show that sub-IBP GHs respond to, rather than cause GST changes, due to both how GST changes propagates with depth and latent heat effects. Models show that many thick GH accumulations are prevented from contributing methane to the atmosphere, because they are almost certainly trapped below either ice-filled IBP or lithological barriers. Where permafrost is sufficiently thick, combinations of geological structure, thermal processes and material properties make sub-IBP GHs unlikely sources for significant atmospheric methane fluxes. Our sub-IBP GH model histories suggest that similar models applied to other GH settings could improve the understanding of GHs and their potential to affect climate. © 2012 Author(s).
1217.
Imaging the Mariánské Lázně Fault (Czech Republic) by 3-D ground-penetrating radar and electric resistivity tomography
Studia Geophysica et Geodaetica,
56
(4)
1019 – 1036
2012
ISSN: 00393169
Publisher: Kluwer Academic Publishers
Keywords:▾
Bohemia; Czech Republic; earthquake swarm; electrical resistivity; fault zone; geodynamics; ground penetrating radar; lithology; Quaternary; signal-to-noise ratio; three-dimensional modeling; tomography; trend analysis; two-dimensional modeling; volcanism
Abstract: ▾ Geodynamic activity in the area of West Bohemia is typified by the occurrence of earthquake swarms, Quaternary volcanism and high flux of mantle-derived CO2. The highest swarm activity occurs beneath the eastern edge of the Cheb basin, which is delineated by the NW-SE trending morphologically pronounced Mariánské Lázně Fault (MLF) controlling the formation of the basin. The previous trenching survey across the MLF zone has identified several fault strands with possible Quaternary activity. In this paper we present the results of the geophysical survey focused to trace the faults signatures in geophysical sections and to build an image of near surface tectonics. The method of electric resistivity tomography (ERT) along two profiles parallel to the trench identified a strong resistivity contrast between the bodies of sandy gravels in the middle and conductive clayey sands to the west and weathered crystalline basement to the east. The 2-D ground penetration radar (GPR) sections show direct correlation of reflections with lithological boundaries identified in the trench. As expected, the GPR signal amplitudes increase with the resistivities found in the ERT sections. Two of the four faults identified in the trench are indicated in the resistivity and GPR sections. A 3-D GPR measurement has identified a spot of high amplitudes elongated parallel to the MLF trend, which coincides with the high resistivity body. To improve the signal-to-noise ratio of the time slices we stacked the GPR time slices within vertically homogeneous blocks. This provided a contrast image of the sand-gravel body including its boundaries in three dimensions. The detailed analysis of the 3-D GPR cube revealed additional fault that limits the highly reflective sands and appears to be offset by another younger fault. Our results suggest a complex fault pattern in the studied area, which deserves a further study. © 2012 Institute of Geophysics of the ASCR, v.v.i.
1216.
Hotspot: The Snake River geothermal drilling project - Initial report
Transactions - Geothermal Resources Council,
36 2767 – 772
2012
ISSN: 01935933ISBN:
978-162276434-1
Abstract: ▾ The Snake River volcanic province (SRP) overlies a thermal anomaly that extends deep into the mantle; it represents one of the highest heat flow provinces in North America. The primary goal of this project is to evaluate geothermal potential in three distinct settings: (1) Kimama site: inferred high sub-aquifer geothermal gradient associated with the intrusion of mafic magmas, (2) Kimberly site: a valley-margin setting where surface heat flow may be driven by the up-flow of hot fluids along buried caldera ring-fault complexes, and (3) Mountain Home site: a more traditional fault-bounded basin with thick sedimentary cover. The Kimama hole, on the axial volcanic zone, penetrated 1912 m of basalt with minor intercalated sediment; no rhyolite basement was encountered. Temperatures are isothermal through the aquifer (to 960 m), then rise steeply on a super-conductive gradient to an estimated bottom hole temperature of ∼98°C. The Kimberly hole is on the inferred margin of a buried rhyolite eruptive center, penetrated rhyolite with intercalated basalt and sediment to a TD of 1958 m. Temperatures are isothermal at 55-60°C below 400 m, suggesting an immense passive geothermal resource. The Mountain Home hole is located above the margin of a buried gravity high in the western SRP. It penetrates a thick section of basalt and lacustrine sediment overlying altered basalt flows, hyaloclastites, and volcanic sediments, with a TD of 1821 m. Artesian flow of geothermal water from 1745 m depth documents a power-grade resource that is now being explored in more detail. In-depth studies continue at all three sites, complemented by high-resolution gravity, magnetic, and seismic surveys, and by downhole geophysical logging.
1215.
Iron deficiency in pyrrhotite of suevites from the Chesapeake Bay impact crater, USA-A consequence of shock metamorphism?
Meteoritics and Planetary Science,
47
(2)
277-295
2012
ISSN: 10869379Abstract: ▾ Pyrrhotite from suevite of the 35Ma Chesapeake Bay impact structure (CBIS) shows a shock metamorphism and we report on several mineralogical and magnetic features. Pyrrhotite shows strong brittle deformation with a high density of stacking faults, twinning parallel to the hexagonal (001) planes and average fault distances in the order of 10nm. Although the determination of a superstructure was not possible due to the lattice defects, the reflections of the NiAs subcell, which is typical of all pyrrhotite modifications, were clearly detected. This phase is ferrimagnetic with a Curie temperature (T C) between 350 and 365°C, and suevite with this phase does not show the 34K transition. The most peculiar feature is the low metal/sulfur ratio of 0.81, which indicates a distinctly higher vacancy concentration than for 4C pyrrhotite and a composition close to smythite (Fe 9S 11). This phase carries a stable natural remanent magnetization and is relatively hard magnetic. Steep inclinations of the natural remanent magnetization vector, however, suggest that this phase has been remagnetized by the drilling process. A possible explanation is the magnetic domain size of faultless areas of about 10nm in diameter, which is at the lower limit of the single domain size near the threshold, below which superparamagnetic behavior occurs. The low thermal stability of this phase excludes postshock heating above 300°C for the suevite of the CBIS. Our results imply that the iron-deficient pyrrhotite is produced by shock metamorphism, although an iron loss due to shock has never been reported before for pyrrhotite. © 2012 The Meteoritical Society.
1214.
High-temperature, low-H 2O silicic magmas of the yellowstone hotspot: An experimental study of rhyolite from the Bruneau-Jarbidge eruptive center, Central Snake River Plain, USA
Journal of Petrology,
53
(9)
1837-1866
2012
ISSN: 00223530Keywords:▾
concentration (composition); crystallization; experimental study; high temperature; hot spot; lava; magma; phase equilibrium; rhyolite; tectosilicate; volcanic eruption; water content, Idaho; Snake River Plain; United States
Abstract: ▾ The phase relations have been investigated experimentally at 200 and 500 MPa as a function of water activity for one of the least evolved (Indian Batt Rhyolite) and of a more evolved rhyolite composition (Cougar Point Tuff XV) from the 12·8-8·1 Ma Bruneau-Jarbidge eruptive center of the Yellowstone hotspot. Particular priority was given to accurate determination of the water content of the quenched glasses using infrared spectroscopic techniques. Comparison of the composition of natural and experimentally synthesized phases confirms that high temperatures (>900°C) and extremely low melt water contents (<1·5 wt % H. 2O) are required to reproduce the natural mineral assemblages. In melts containing ∼0·5-1·5 wt % H. 2O, the liquidus phase is clinopyroxene (excluding Fe-Ti oxides, which are strongly dependent on fO. 2), and the liquidus temperature of the more evolved Cougar Point Tuff sample (BJR; ∼940-1000°C) is at least 30°C lower than that of the Indian Batt Rhyolite lava sample (IBR2; 970-1030°C). For the composition BJR, the comparison of the compositions of the natural and experimental glasses indicates a pre-eruptive temperature of at least 900°C. The composition of clinopyroxene and pigeonite pairs can be reproduced only for water contents below 1·5 wt % H. 2O at 900°C, or lower water contents if the temperature is higher. For the composition IBR2, a minimum temperature of 920°C is necessary to reproduce the main phases at 200 and 500 MPa. At 200 MPa, the pre-eruptive water content of the melt is constrained in the range 0·7-1·3 wt % at 950°C and 0·3-1·0 wt % at 1000°C. At 500 MPa, the pre-eruptive temperatures are slightly higher (by ∼30-50°C) for the same ranges of water concentration. The experimental results are used to explore possible proxies to constrain the depth of magma storage. The crystallization sequence of tectosilicates is strongly dependent on pressure between 200 and 500 MPa. In addition, the normative Qtz-Ab-Or contents of glasses quenched from melts coexisting with quartz, sanidine and plagioclase depend on pressure and melt water content, assuming that the normative Qtz and Ab/Or content of such melts is mainly dependent on pressure and water activity, respectively. The combination of results from the phase equilibria and from the composition of glasses indicates that the depth of magma storage for the IBR2 and BJR compositions may be in the range 300-400 MPa (∼≤13 km) and 200-300 MPa (∼≤10 km), respectively. © The Author 2012. Published by Oxford University Press. All rights reserved.
1213.
Evidence for a bimaterial interface along the Mudurnu segment of the North Anatolian Fault Zone from polarization analysis of P waves
Earth and Planetary Science Letters,
327-32817-22
2012
ISSN: 0012821XKeywords:▾
Analysis techniques; Arrival time; Bi-material interfaces; Direct waves; Event location; Fault zone; Focal mechanism; Head waves; Imaging contrast; Moveout; Near-fault; North Anatolian Fault; North Anatolian Fault Zone; Northwestern Turkey; P waves; P-wave velocity; Polarization analysis; Seismic instrumentation; Seismic velocities; Wave forms, Polarization, Seismic waves, fault zone; focal mechanism; hazard assessment; imaging method; monitoring; normal fault; North Anatolian Fault; P-wave; seismic source; seismic velocity; seismicity; wave velocity; waveform analysis, Turkey
Abstract: ▾ We present results on imaging contrast of seismic velocities across the Mudurnu segment of the North Anatolian Fault Zone (NAFZ) in northwestern Turkey with polarization analysis of early P waveforms generated by near-fault seismicity and recorded by near-fault stations. The analysis uses changes in motion polarity from fault-normal to source-receiver directions to identify early-arriving fault zone head waves on the slow side of the fault, and measure the arrival times of the head and direct P waves. The moveout between the head and direct waves with increasing source-receiver distance along the fault provides an estimate of the average contrast of seismic velocities across the fault. The results indicate that the average contrast of P wave velocities across the Mudurnu segment of the NAFZ is at least 6%, with the south block being the faster side. The findings provide a basis for deriving improved event locations, focal mechanisms and estimated shaking hazard associated with earthquakes on the fault. The analysis technique can be used in other fault zones monitored with sparse seismic instrumentation. © 2012 Elsevier B.V..
1212.
Heating-induced changes in the anisotropy of magnetic susceptibility of impact breccias, Chicxulub Crater (Mexico)
Studia Geophysica et Geodaetica,
56
(3)
769-787
2012
Abstract: ▾ Initial results of a thermal treatment study on the anisotropy of magnetic susceptibility (AMS) of impact breccias from Chicxulub crater are used to investigate the nature of the magnetic fabrics. Chicxulub impact breccias are heterogeneous materials, with carbonate, basement and melt clasts within carbonate-rich or melt-rich matrix. Samples studied come from the carbonate-rich basal unit Lower Suevite in the Yaxcopoil-1 borehole impactite sequence (core depth interval: 885-895 m). The Lower Suevite is characterized by mixed prolate and oblate ellipsoids with shallow to steep principal susceptibility axes, which had been related to emplacement as an excavation flow with ground-surge components during the early cratering stages. Thermal treatment results in changes in the fabrics with a tendency to oblate fabrics. Stepwise thermal treatment up to 700°C reveals different behaviors for the oblate, neutral and prolate fabrics marked by changes in AMS parameters and principal susceptibility axis orientations. A sample with oblate fabrics and vertical minimum axes showed an increase of magnetic susceptibility at high temperatures, indicating formation of secondary magnetite and fabric enhancement. A sample with neutral ellipsoid showed heating-induced changes towards oblate fabrics and vertical minimum susceptibility axes. Samples characterized by prolate ellipsoids with horizontal maximum axes showed no directional changes. In a sample with apparent intermediate or inverse fabrics, vertical maximum axes showed changes to horizontal inclinations, with the intermediate and maximum axes switching positions. Changes induced by stepwise thermal treatment appear useful to characterize the fabrics of impact lithologies. Further investigation of heating-induced effects in mineralogy, grain size and textural changes is, however, required to relate the different behaviors observed after stepwise thermal treatment with the magnetic mineralogy and emplacement mode of the breccias. © 2012 Institute of Geophysics of the ASCR, v.v.i.
1211.
Geothermal systems in the Snake River Plain, Idaho, characterizedby the Hotspot project
Transactions - Geothermal Resources Council,
36 1727-730
2012
ISSN: 01935933ISBN:
9781622764341
Abstract: ▾ The Hotspot project has sampled three different geothermal environments in the Snake River Plain (SRP) in Idaho. The project used slim-hole wire line coring in conjunction with a bottom hole temperature probe developed by DOSECC. The first hole at Kimama in the center of the eastern SRP was cored to a depth of 1,915 m. Temperature measurements showed the SRP fresh water aquifer extends to a depth of 965 m and masks the underlying temperature gradient of 74.5°C/Km. A core hole in the town of Kimberly reached a depth of 1,959 m and demonstrated a resource of >50°C from 800 m to the bottom of the hole. A core hole at Mountain Home AFB in the eastern SRP reached a depth of 1,821 m and documents an intermediate- to high-temperature resource.
1210.
Geological and geochemical characteristics of anoxic event bed in the Qingshankou Formation of Late Cretaceous in Songliao Basin
Geoscience,
26
(4)
741
2012
1209.
Drilling reveals fluid control on architecture and rupture of the Alpine fault, New Zealand
Geology,
40
(12)
1143-1146
2012
ISSN: 00917613DOI:10.1130/G33614.1
Keywords:▾
Alpine Faults; Alteration zones; Cataclasite; Damage zones; Deep fault; Drilling projects; Earthquake slip; Electrical resistivity; Fault core; Fault zone; Fluid control; Fluid migration; Fluid pressure gradient; Fluid pressures; Fluid-rock interaction; Hydraulic seals; Hydrogeological; Laboratory measurements; Low permeability; New zealand; Permeability measurements; Pressure relaxation; Regional barriers; Regional levels; Rock damage; Seismic properties; Slip zones; Slug test; Spontaneous potentials, Boring; Clay alteration; Earthquakes; Electric conductivity; Pressure gradient; Rock drilling, Faulting, cataclasite; coseismic process; drilling; earthquake rupture; electrical resistivity; fault slip; fault zone; fluid pressure; mineralization; mylonite; permeability; postseismic process; pressure gradient, Alpine Fault Zone; New Zealand; South Island
Abstract: ▾ Rock damage during earthquake slip affects fluid migration within the fault core and the surrounding damage zone, and consequently coseismic and postseismic strength evolution. Results from the first two boreholes (Deep Fault Drilling Project DFDP-1) drilled through the Alpine fault, New Zealand, which is late in its 200-400 yr earthquake cycle, reveal a >50-m-thick "alteration zone" formed by fluid-rock interaction and mineralization above background regional levels. The alteration zone comprises cemented low-permeability cataclasite and ultramylonite dissected by clay-filled fractures, and obscures the boundary between the damage zone and fault core. The fault core contains a <0.5-m-thick principal slip zone (PSZ) of low electrical resistivity and high spontaneous potential within a 2-m-thick layer of gouge and ultracataclasite. A 0.53 MPa step in fluid pressure measured across this zone confirms a hydraulic seal, and is consistent with laboratory permeability measurements on the order of 10-20 m2. Slug tests in the upper part of the boreholes yield a permeability within the distal damage zone of ~10-14 m2, implying a six-orders-of-magnitude reduction in permeability within the alteration zone. Low permeability within 20 m of the PSZ is confirmed by a subhydrostatic pressure gradient, pressure relaxation times, and laboratory measurements. The low-permeability rocks suggest that dynamic pressurization likely promotes earthquake slip, and motivates the hypothesis that fault zones may be regional barriers to fluid flow and sites of high fluid pressure gradient. We suggest that hydrogeological processes within the alteration zone modify the permeability, strength, and seismic properties of major faults throughout their earthquake cycles. © 2012 Geological Society of America.
1208.
Geochemistry of eolian dust and its elemental contribution to Lake Qinghai sediment
Applied Geochemistry,
27
(8)
1546 – 1555
2012
ISSN: 08832927Keywords:▾
China; Qinghai; Qinghai Lake; Copper; Lakes; Lead; Manganese; Rubidium; Sedimentology; Sediments; Sodium; Strontium; Airborne dusts; Biogeochemical cycle; Chemical compositions; Dust deposition; Dust flux; Dust samples; Eolian dust; Lake Qinghai; Lithophile elements; Mobile elements; Seasonal variation; biogeochemical cycle; eolian deposit; lacustrine deposit; quantitative analysis; seasonal variation; sediment chemistry; Dust
Abstract: ▾ Located at the midpoint of the Asian " airborne dust corridor" , Lake Qinghai receives substantial dust annually, which may impact the biogeochemical cycles of the system. In order to determine quantitatively the flux and chemical contributions of dust to Lake Qinghai sediment, dust samples were collected monthly at two sites surrounding the lake from June 2009 to May 2011. The results demonstrate similar chemical compositions of dust samples to the local loess, implying strong representativeness of regional dust. The average dust deposition flux is 265.7±55.0g/m 2/a, constituting 56.6±11.7% of the modern sediment, approximating to previous estimates (~65%). Contributions of dust-derived elements in the sediment differ substantially, with a minimum of 16.7% for Sr and a maximum of 83.9% for Cu. Among these elements, the contribution of lithophile elements (Na, Al, K, Ti, Mn, Fe and Rb) is close to that of the bulk dust; the contributions of mobile elements (Mg, Ca and Sr) are low, only 16.7% (Sr)-26.1% (Mg), whereas potentially harmful metals (Cu, Zn and Pb) have high contributions (70.3-83.9%). Seasonal variations of elemental inputs indicate that springtime contributions dominate the annual dust fluxes for all elements into the sediment, in agreement with the high dust flux in spring. These observations not only quantify the contribution of dust to the sediment of Lake Qinghai, but also highlight the important role of dust in the accumulation of various elements in the sediment, especially for potentially harmful metals. © 2012 Elsevier Ltd.
1207.
Estimation of the stressed state of the rock mass in the area of the Outokumpu deep drill hole (OKU), Finland
Acta Geodynamica et Geomaterialia,
9
(1)
31 – 42
2012
ISSN: 12149705Abstract: ▾ The stressed state of the rock mass in the area of the Outokumpu deep drill hole, Finland, was investigated by the boundaryelement method in three mutually orthogonal planes with regard for the massif geological structure. It was found that subhorizontal components of the stress field are determined by tectonic forces acting on the boundaries of the European part of the Eurasian lithospheric plate. In the drill hole vertical section the widest variations of this field gradient occur at the depths of an ophiolite sequence in the range of 1.8-2.0 km.
1206.
Frictional properties and sliding stability of the San Andreas fault from deep drill core
Geology,
40
(8)
759-762
2012
ISSN: 00917613DOI:10.1130/G33007.1
Keywords:▾
California; Constitutive properties; Deep drilling; Drill core; Earthquake rupture; Fault strands; Friction coefficients; Friction values; Frictional behavior; Frictional properties; Frictional strength; Low rates; Repeating earthquake; San Andreas fault; Sharp increase; Sliding stability; Stress orientations; Tectonic faults, Core drilling; Creep; Earthquakes; Strike-slip faults, Friction, earthquake mechanism; earthquake recurrence; earthquake rupture; fault geometry; friction; rupture; San Andreas Fault; shear strength; slip rate; stability analysis; stress field; tectonic setting; wall rock, California; San Andreas; United States, Calluna vulgaris
Abstract: ▾ The strength of tectonic faults and the processes that control earthquake rupture remain central questions in fault mechanics and earthquake science. We report on the frictional strength and constitutive properties of intact samples across the main creeping strand of the San Andreas fault (SAF; California, United States) recovered by deep drilling. We find that the fault is extremely weak (friction coefficient, μ = ∼ 0.10), and exhibits both velocity strengthening frictional behavior and anomalously low rates of frictional healing, consistent with aseismic creep. In contrast, wall rock to the northeast shows velocity weakening frictional behavior and positive healing rates, consistent with observed repeating earthquakes on nearby fault strands. We also document a sharp increase in strength to values of μ > ∼0.40 over <1 m distance at the boundary between the fault and adjacent wall rock. The friction values for the SAF are sufficiently low to explain its apparent weakness as inferred from heat flow and stress orientation data. Our results may also indicate that the shear strength of the SAF should remain approximately constant at ∼10 MPa in the upper 5-8 km, rather than increasing linearly with depth, as is commonly assumed. Taken together, our data explain why the main strand of the SAF in central California is weak, extremely localized, and exhibits aseismic creep, while nearby fault strands host repeating earthquakes. © 2012 Geological Society of America.
1205.
Foraminiferal repopulation of the late eocene chesapeake bay impact crater
Micropaleontology,
58
(1-2)
1-206
2012
Abstract: ▾ The Chickahominy Formation is the initial postimpact deposit in the 85km-diameter Chesapeake Bay impact crater, which is centered under the town of Cape Charles, Virginia, USA. The formation comprises dominantly microfossil-rich, silty, marine clay, which accumulated during the final ~1.6myr of late Eocene time. At cored sites, the Chickahominy Formation is 16.8-93.7m thick, and fills a series of small troughs and subbasins, which subdivide the larger Chickahominy basin. Nine coreholes drilled through the Chickahominy Formation (five inside the crater, two near the cratermargin, and two ~3km outside the crater) record the stratigraphic and paleoecologic succession of 301 indigenous species of benthic foraminifera, as well as associated planktonic foraminifera and bolboformids. Two hundred twenty of these benthic species are described herein, and illustrated with scanning electron photomicrographs. The Chickahominy Formation can be categorized as a single benthic foraminiferal biozone (Cibicidoides pippeni Biozone), subdivided into five subzones, in stratigraphic order from bottom to top: Bulimina jacksonensis Subzone; Lagenoglandulina virginiana Subzone; Uvigerina dumblei Subzone; Bolivina tectiformis Subzone, and; Siphonina jacksonensis Subzone. Two planktonic datums and four benthic datums provide a biochronostratigraphic framework in which to estimate the duration and temporal distribution patterns of discrete microfossil assemblages. Apaleoseral succession from pioneer to equilibrium paleocommunities reflects the temporal and spatial evolution from early unstable benthic paleoenvironments to later stable benthic paleoenvironments. Initial reoccupation of the newly formed crater basin is marked by a dramatic immigration of 32 indigenous species, which replaced the sparse, entirely reworked (allochthonous) foraminiferal assemblages of a preceding inhospitable dead zone. At all nine core sites, attainment of benthic paleoenvironmental equilibrium (29-190kyr postimpact) is signaled by a notable reduction in the number of new immigrant species arriving in the Chickahominy basin. In addition, at five sites inside the crater, early unstable benthic paleoenvironments can be differentiated from later stable benthic paleoenvironments by the presence of an agglutinated Psammosiphonella biofacies in basalChickahominy strata and a shift from short-term to long-term benthic foraminiferal generic dominance facies. Restriction of the dead zone and Psammosiphonella biofacies to intracrater sites indicates unusual benthic paleoenvironmental conditions (warm, saline bottomwater and porewater) derived from the impact, which lasted as long as ~350kyr postimpact at one site. Absence of key planktonic foraminiferal and Bolboforma species in early Chickahominy sediments indicates that detrimental effects of the impact also disturbed the upper oceanic water column for at least 80-100kyr postimpact. Nine genera (Bolivina, Uvigerina, Gyroidinoides, Globocassidulina, Angulogerina, Nuttallides, Cibicidina, Caucasina, Epistominella) and two generic groups (buliminids, stilostomellids) are the most abundant taxa among 17 generic dominance facies that characterize Chickahominy core sites. Most dominant taxa were epifaunal or shallow infaunal opportunists, which thrived under conditions of oxygen depletion (dysoxia) and high organic flux rates. After an average of ~73kyr of stressed, rapidly fluctuating paleoenvironments, which were destabilized by after-effects of the impact, most of the cored Chickahominy subbasins maintained stable, nutrient-rich, low-oxygen bottom waters and interstitial microhabitats for the remaining ~1.3myr of late Eocene time.
1204.
Fault zone Q values derived from Taiwan Chelungpu Fault borehole seismometers (TCDPBHS)
Tectonophysics,
57876-86
2012
ISSN: 00401951Keywords:▾
Chelungpu Fault; Drilling projects; Fault-zone attenuation; Qp; Qs; Seismic attenuation, Seismographs; Strike-slip faults, Seismology, borehole; Chi-Chi earthquake 1999; earthquake rupture; fault zone; footwall; San Andreas Fault; sedimentary rock; seismic attenuation; seismograph; slip, Chelungpu Fault Zone; Taiwan
Abstract: ▾ The attenuation factor, Q, at a fault zone is an important parameter for understanding the physical properties. In this study, we investigated the Q value of the Chelungpu Fault, the main rupture of the Mw 7.6 Chi-Chi earthquake, using the 7-level TCDP borehole seismometer array (TCDPBHS). The TCDPBHS was deployed at depths from 945 to 1270m throughout the 1999 ruptured slip zone at 1111m. Three borehole seismometers (BHS1-BHS3) were placed in the hanging wall, and the remaining three (BHS5-BHS7) were placed in the foot wall, with BHS4 near the slip zone. The configuration allowed us to estimate the Q-structure of the recent ruptured fault zone. In this study, we estimated Q values between BHS1 and BHS4, Qs 1 (Qp 1) at the fault zone and between BHS4 to 2km in depth, Qs 4 (Qp 4) beneath the fault zone. We utilized two independent methods, the spectral ratio and spectral fitting analyses, for calculating the Q value of Qs 1 (Qp 1) in order to provide a reliability check. After analyzing 26 micro-events for Qs and 17 micro-events for Qp, we obtained consistent Q values from the two independent methods. The values of Qs 1 and Qp 1 were 21-22 and 27-35, respectively. The investigation for the value of Qs 4 was close to 45, and Qp 4 was 85. These Qp and Qs values are quiet consistent with observations obtained for the San Andreas Fault at the corresponding depth. A low Qs 1 value for the recent Chelungpu Fault zone suggests that this fault zone has been highly fractured. Qs values within the Chelungpu Fault, similar to those within the San Andreas Fault, suggest that the Q structure within the fault zone is sedimentary rock independent. However, the possible existence of fluids, fractures, and cracks dominates the attenuation feature in the fault zone. © 2011 Elsevier B.V.
1203.
Exploration and resource assessment at Mountain Home Air Force Base, Idaho, using an integrated team approach
Transactions - Geothermal Resources Council,
36 1615-619
2012
ISSN: 01935933ISBN:
9781622764341
Abstract: ▾ The U.S. Air Force is facing a number of challenges as it moves into the future, one of the biggest being how to provide safe and secure energy to support base operations. A team of scientists and engineers met at Mountain Home Air Force Base near Boise, Idaho, to discuss the possibility of exploring for geothermal resources under the base. The team identified that there was a reasonable potential for geothermal resources based on data from an existing well. In addition, a regional gravity map helped identify several possible locations for drilling a new well. The team identified several possible sources of funding for this well - the most logical being to use U.S. Department of Energy funds to drill the upper half of the well and U.S. Air Force funds to drill the bottom half of the well. The well was designed as a slimhole well in accordance with State of Idaho Department of Water Resources rules and regulations. Drilling operations commenced at the Mountain Home site in July of 2011 and were completed in January of 2012. Temperatures increased gradually, especially below a depth of 2000 ft. Temperatures increased more rapidly below a depth of 5500 ft. The bottom of the well is at 5976 ft, where a temperature of about 140°C was recorded. The well flowed artesian from a depth below 5600 ft, until it was plugged off with drilling mud. Core samples were collected from the well and are being analyzed to help understand permeability at depth. Additional tests using a televiewer system will be run to evaluate orientation and directions at fractures, especially in the production zone. A final report on the well exploitation will be forthcoming later this year. The Air Force will use it to evaluate the geothermal resource potential for future private development options at Mountain Home AFB.
1202.
Evolution of the Lake Titicaca basin and its diatom flora over the last ~370,000 years
Palaeogeography, Palaeoclimatology, Palaeoecology,
317-31893 – 103
2012
Keywords:▾
Andes; Lake Titicaca; Bacillariophyta; Cyclostephanos; deep drilling; diatom; glacial-interglacial cycle; lacustrine deposit; morphology; paleoclimate; paleoecology; paleoenvironment; paleogeography; paleolimnology; plankton; Quaternary; speciation (biology)
Abstract: ▾ In recent years, deep drilling undertaken as part of the International Continental Drilling Program has generated multiple long lacustrine sedimentary records to reconstruct continental paleoclimate. In many cases, the tectonic and geomorphic history of these basins is under-constrained and poorly known, which affects the interpretation of climate history from geophysical, geochemical, and paleobiotic proxies in the sedimentary record. In addition, non-analog biotic assemblages that reflect evolutionary processes may constrain the reconstruction of past environments. In the drill-core record of Lake Titicaca, spanning the last ~. 370. ka, the diatom stratigraphy reflects both the influence of climate and the long-term evolution of the lake basin and its biota. In the upper part of the drill-core sequence, glacial intervals were deep and dominated by freshwater planktic taxa, and peak interglacial intervals were shallow and dominated by benthic species, some with saline affinities. In the basal sections of the drill-core record, benthic diatoms are dominant in both glacial and interglacial units, with freshwater taxa dominating the glacial strata. This suggests that the ancient lake basin was shallower during intervals of both wet and dry climate, and that the modern deep lake may result from a progressive subsidence and deepening of the basin over time. In addition, morphological evolution in one of the major lineages of planktic diatoms, Cyclostephanos, indicates substantial change in the limnological environment that affected species morphology and may have driven speciation. © 2011 Elsevier B.V.
1201.
Fifteen new diatom (Bacillariophyta) species from Lake Ohrid, Macedonia
Phytotaxa,
301-41
October2011
1200.
Salinity control on long-chain alkenone distributions in lake surface waters and sediments of the northern Qinghai-Tibetan Plateau, China
Geochimica et Cosmochimica Acta,
75
(7)
1693 – 1703
2011
ISSN: 00167037
Publisher: Elsevier Ltd
Keywords:▾
China; Qaidam Basin; Qinghai; Qinghai-Xizang Plateau; alkenone; biomarker; concentration (composition); isotopic ratio; lacustrine deposit; lake water; paleoenvironment; salinity; suspended particulate matter; water temperature
Abstract: ▾ Long-chain alkenones in lacustrine settings are potentially excellent biomarkers for the reconstruction of past terrestrial environmental conditions, and have been found in many different types of lakes around the globe. A wider range of factors influence the occurrence and distribution of alkenones in lake sediments and waters when compared to marine systems. Lake environmental conditions, such as temperature (in particular) and salinity, are among the key factors controlling alkenone distributions in lacustrine settings. Here we investigated alkenone distribution patterns in lakes of the northern Qinghai-Tibetan Plateau, China, and their possible relationship with environmental conditions, by analyzing paired samples of suspended particulate matter in surface waters and surface sediments. Salinity of investigated lake waters ranges from almost 0 to ∼100g/L, while temperature variation among the lakes is minimal, effectively eliminating temperature effects on the alkenone distribution patterns observed here. We show that (1) alkenone concentrations vary substantially between the lakes, yet controlling mechanisms remain elusive; (2) C37/C38 ratios are substantially lower in the lakes of the Qaidam Basin than in the Lake Qinghai region, probably indicating different alkenone producers in the two regions; and (3) large variations in %C37:4 (the percentage of the C37:4 alkenone), determined from both surface waters and sediments, are negatively correlated with salinity. We suggest that the %C37:4 index could be used as a salinity indicator at least on a regional scale, with careful considerations of other potentially complicating factors. However, potential reasons for why salinity could significantly affect %C37:4 values need further investigation. © 2011 Elsevier Ltd.
1199.
Role of the deep mantle in generating the compositional asymmetry of the Hawaiian mantle plume
Nature Geoscience,
4
(12)
831-838
2011
ISSN: 17520894DOI:10.1038/ngeo1328
Keywords:▾
asymmetry; data set; geochemical survey; isotopic analysis; mantle plume; Pacific plate; sampling; seismic velocity; volcanic island, Atlantic Ocean; French Southern Territories; Hawaiian Islands; Indian Ocean; Kerguelen; Kerguelen Islands
Abstract: ▾ Linear chains of volcanic ocean islands are one of the most distinctive features on our planet. The longest, the Hawaiiang-Emperor Chain, has been active for more than 80 million years, and is thought to have formed as the Pacific Plate moved across the Hawaiian mantle plume, the hottest and most productive of Earth's plumes. Volcanoes fed by the plume today form two adjacent trends, including Mauna Kea and Mauna Loa, that exhibit strikingly different geochemical characteristics. An extensive data set of isotopic analyses shows that lavas with these distinct characteristics have erupted in parallel along the Kea and Loa trends for at least 5 million years. Seismological data suggest that the Hawaiian mantle plume, when projected into the deep mantle, overlies the boundary between typical Pacific lower mantle and a sharply defined layer of apparently different material. This layer exhibits low seismic shear velocities and occurs on the Loa side of the plume. We conclude that the geochemical differences between the Kea and Loa trends reflect preferential sampling of these two distinct sources of deep mantle material. Similar indications of preferential sampling at the limit of a large anomalous low-velocity zone are found in Kerguelen and Tristan da Cunha basalts in the Indian and Atlantic oceans, respectively. We infer that the anomalous low-velocity zones at the core-mantle boundary are storing geochemical anomalies that are enriched in recycled material and sampled by strong mantle plumes. © 2011 Macmillan Publishers Limited. All rights reserved.
1198.
Paleosalinity significance of occurrence and distribution of methyltrimethyltridecyl chromans in the Upper Cretaceous Nenjiang Formation, Songliao Basin, China
Organic Geochemistry,
42
(11)
1411-1419
2011
ISSN: 01466380Keywords:▾
Bottom water; Depositional environment; Low salinity; Occurrence and distribution; Redox condition; Reducing conditions; Songliao basin; Source rocks; Stratified waters; Upper Cretaceous; Water layers, Anoxic sediments; Saline water, Salinity measurement, biomarker; borehole; Cretaceous; depositional environment; lacustrine deposit; paleosalinity; redox conditions; relative abundance; sampling; source rock; Tertiary; water column, China; Songliao Basin
Abstract: ▾ A group of methyltrimethyltridecyl chromans (MTTCs) was found in core samples of Member 1 (K2n1) and Member 2 (K2n2) of the Nenjiang Formation (Upper Cretaceous) from the SK-l southern borehole (Songliao Basin, China). They are assigned for the first time in sediments older than the Tertiary in China. Their composition and distribution are indicative mainly of the redox and salinity conditions in the depositional environment. The δ-MTTC isomer is in relatively higher abundance in samples from enhanced salinity and reducing conditions, but was not detected in samples from aerobic and low salinity environments, while α-MTTC appears to be present in samples from environments with a broad range of salinity and redox conditions. The α-MTTC/γ-MTTC (α/γ) ratio has a similar indication as that of the α/δ ratio and could be used as a corroborative ratio of paleosalinity. A combination of biomarker parameters suggests that the massive lacustrine petroleum-prone source rocks associated with the K2n1 interval were likely developed under a stratified water column with enhanced salinity and an anoxic bottom water layer, associated with a much less saline (fresh to brackish) upper water layer. © 2011 Elsevier Ltd.
1197.
Reduced interannual rainfall variability in East Africa during the last ice age
Science,
333
(6043)
743-747
2011
ISSN: 00368075Keywords:▾
rain, annual variation; climate modeling; El Nino-Southern Oscillation; flooding; food security; lacustrine deposit; paleoclimate; Pleistocene; precipitation intensity, Africa; article; climate; El Nino; food security; glacial period; lake sediment; priority journal; seasonal variation, Kenya
Abstract: ▾ Interannual rainfall variations in equatorial East Africa are tightly linked to the El Niño Southern Oscillation (ENSO), with more rain and flooding during El Niño and droughts in La Niña years, both having severe impacts on human habitation and food security. Here we report evidence from an annually laminated lake sediment record from southeastern Kenya for interannual to centennial-scale changes in ENSO-related rainfall variability during the last three millennia and for reductions in both the mean rate and the variability of rainfall in East Africa during the Last Glacial period. Climate model simulations support forward extrapolation from these lake sediment data that future warming will intensify the interannual variability of East Africa's rainfall.
1196.
Project Hotspot: Preliminary analysis of secondary mineralization in basaltic core, Central Snake River Plain
Transactions - Geothermal Resources Council,
35 2987-989
2011
ISSN: 01935933ISBN:
9781618394828
Abstract: ▾ Project Hotspot offers further understanding of the history and evolution of the Snake River Plain (SRP) through drilling wells in the SRP. The first well at Kimama encountered almost entirely basalt. Kimama drilled through a gradual transition from fresh to altered basalt with depth. The evolution of mineralization with depth at Kimama was initially calcite and some quartz followed by smectite clay minerals (nontronite and saponite) and zeolites. Smectite clay minerals are a great indicator of paleo-temperature of geothermal fluids. Description of the Kimama core, mineral habits, and forthcoming chemical analyses will be discussed in this paper.
1195.
Preface "evolutionary and geological history of the Balkan lakes Ohrid and Prespa"
Biogeosciences,
8
(4)
995 – 998
2011
ISSN: 17264189Keywords:▾
Balkans; Lake Mikri Prespa; Lake Ohrid; geomorphological mapping; historical record; history of geology; lake evolution