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  • Other Sources  (29)
  • Elsevier  (27)
  • American Chemical Society  (2)
  • American Chemical Society (ACS)
  • 1995-1999  (29)
  • 1
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    Secular changes of lead and neodymium in central Pacific seawater recorded by a Fe-Mn crust (1997)
    Elsevier
    In:  Geochimica et Cosmochimica Acta (61). pp. 3957-2974.
    Details
    Publication Date: 2016-05-25
    Description: Oceanic and atmospheric circulation patterns varied considerably during the Tertiary and Quaternary and influenced the geochemical cycles of elements in seawater. We report the first resolution lead and neodymium isotopic record of such changes at a high time resolution in two depths profiles from a hydrogenous FeMn crust. The crust, Va13-2, is located in the central Pacific (146°W, 9°25′N, 4830 m) and has previously been dated by 230Th and 10Be. The first profile was drilled with a sample time resolution of ∼3 kyr and allows evaluation of short-term changes to lead and neodymium sources to central Pacific seawater over the last 400 kyr (marine δ18O stages 2 to 11). Longer-term changes were monitored at lower time resolution in a second profile to an age of 10 Ma. Short-term variations in lead and neodymium isotope ratios are resolved in the high resolution profile (0 to 400 kyr). Superimposed on the short-term variations is a secular decrease in Full-size image (〈1 K) ratios beginning at ∼130 kyr in marine δ18O stage 5, implying a change in the lead sources to the central Pacific. Lead and neodymium isotopic compositions indicate an increased influence from Central American eolian sources to Pacific seawater at this time. Lead isotopes are found to be statistically more variable during interglacial than glacial periods. These observations are supported by the greater eolian dust fluxes found in sediment cores from the equatorial Pacific during interglacial stages. The most important paleoceanographic event of the last 10 Ma to affect Pacific seawater was the closure of the Panama gateway. Changes in lead and neodymium isotopes in Val3-2 during the last 10 Ma occurred along with gradual closure of the Panama straits. However, these changes did not occur in tandem: while neodymium isotope ratios increase between 10 and 8 Ma, lead isotope ratios remain constant. In contrast, the period 7 to 1 Ma is marked by a secular increase in lead isotope ratios but nearly constant neodymium. These changes are consistent with a source of radiogenic lead and neodymium conveyed by the Circumpolar Current into the Pacific, rather than by the Panama gateway, and involve 20 to 40% Southern Component Water (SCW) input of lead and neodymium. Modelling of lead and neodymium isotopic mixing between the different water masses involved in generating Pacific deep waters lead us to the following conclusions: (1) Small variations in the strength and composition of North Atlantic Deep Water (NADW) have a relatively minor effect on the amounts of lead and neodymium from SCW contributing to the Pacific and (2) an enhanced SCW flow with an open isthmus of Panama, as suggested by General Circulation Models (GCM), requires a corresponding reduction in NADW Pb and Nd contributions to SCW. The general agreement between the isotopic compositions of surface layers of Mn nodules, integrated over such long time intervals, and those of present-day bottom waters at their respective locations show that the present-day ocean circulation pattern has dominated through the Pleistocene. Our study of Mn crust Va13-2 shows that shorter-term changes in lead and neodymium isotope ratios can be resolved, provided that such crusts are sampled at an appropriate time resolution.
    Type: Article , PeerReviewed
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  • 2
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    Late Quaternary fluctuations of biogenic component fluxes on the continental slope of the Ross Sea, Antarctica (1998)
    Elsevier
    In:  Journal of Marine Systems, 17 . pp. 515-525.
    Details
    Publication Date: 2017-06-26
    Description: A sediment core, collected from the western part of the continental slope of the Ross Sea at 2380 m water depth, records events of the last two climatic cycles (250 kyr). A Full-size image (〈1 K)-based chronology was obtained and boundaries of the isotope stages were set assuming that biological productivity was enhanced during periods of less ice cover. Then, Full-size image (〈1 K), organic carbon, biogenic silica and biogenic Ba distributions were compared to the glacial–interglacial stage boundaries and corresponding ages of the Full-size image (〈1 K) record of Martinson et al. [Martinson, D.G., Pisias, N.G., Hays, J.D., Imbrie, J., Moore, T.C., Jr., Shackleton, N.J, 1987. Age dating and the orbital theory of the ice ages: development of a high-resolution 0 to 300,000-year chronostratigraphy. Quaternary Research, 27: 1–29]. Sediment accumulation rates ranged between 1.2 cm kyr−1 in the isotope stage 6 and 3.8 cm kyr−1 during the Holocene. Variations in the concentrations and fluxes of organic carbon, biogenic Ba, biogenic silica and Mn gave information on palaeoclimate changes. Processes of sediment redistribution in the Ross Sea margin were enlightened from a comparison of the measured and expected fluxes of Full-size image (〈1 K). Calculation of the focusing-corrected accumulation rates of biogenic Ba enabled us to evaluate the export palaeoproductivity. Corrected accumulation rates of biogenic components and calculated palaeoproductivities were low, compared to the Antarctic Polar Front in the Atlantic sector, throughout the last two climatic cycles. Glacial–interglacial changes of sea ice cover and ventilation of the Ross Sea were probably major causes of variations in biogenic particle flux and distribution of redox-sensitive elements within the sediment column.
    Type: Article , PeerReviewed
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  • 3
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    New production in the Northeast Water Polynya: 1993 (1997)
    Elsevier
    In:  Journal of Marine Systems, 10 . pp. 199-209.
    Details
    Publication Date: 2017-06-26
    Description: The Northeast Water Polynya has been suggested as acting as a sink for carbon, especially during the spring and summer when phytoplankton growth is active. During 1993 the polynya was sampled for the entire growing period (late May through mid-August) in order to more accurately assess the magnitude, controls and patterns of new and total (ammonium, nitrate and urea) nitrogen production. This represents the first assessment of new production throughout an entire season in the Arctic. We found that, in 1993, new production, based on 15N-tracer techniques and integrated over the euphotic zone, was 0.141 mmol N m−2 h−1 (0.361 g C m−2 d−1 when converted using observed C/N ratios). Measured f ratios averaged 0.65 and demonstrate that the system, to a great extent, was using nitrate as a nitrogen source. In general f ratios were greatest early in the season and minimal in mid-summer. Urea uptake was highly variable and contributed slightly less than ammonium to phytoplankton nitrogen demand. Nitrate uptake at stations with low (〈 0.5μM) nitrate concentrations was significantly reduced, implying that nitrate concentrations limited phytoplankton growth late in the growing season. Long-term new production rates calculated from nutrient depletion patterns from the polynya as a whole as well as a time-series constructed from a single location were ca. 0.144-0.281 g C m−2 d−1. The relationship between new production as measured by incubations and nutrient depletion budgets suggests that phytoplankton growth is the dominant factor influencing the nitrogen budget of the polynya. The amount of material available for removal from the euphotic zone is limited and constrains the degree to which the polynya can act as a regional carbon sink.
    Type: Article , PeerReviewed
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  • 4
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    Linear-elastic crack models of jointing and faulting (1996)
    Elsevier
    In:  Professional Paper, Structural Geology and Personal Computers, New York, Elsevier, vol. 15, no. 16, pp. 359-388, (ISBN 1-86239-165-3, vi + 330 pp.)
    Details
    Publication Date: 1996
    Keywords: cracks and fractures (.NE. fracturing) ; Fracture ; Elasticity ; Rock mechanics ; Mathematica ; MATLAB ; MAPLE ; Modelling ; J ; w/out ; dot
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    BIBLIOGRAPHY SEISMOLOGY
  • 5
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    Seismic performance of bridges in California during the January 17, 1994, Northridge earthquake (1995)
    Elsevier
    In:  Bull., Open-File Rept., Urban Disaster Mitigation: The Role of Science and Technology, New York, Elsevier, vol. 5, no. 16, pp. 157-166, (ISBN 1-86239-165-3, vi + 330 pp.)
    Details
    Publication Date: 1995
    Keywords: Earthquake risk ; Earthquake engineering, engineering seismology ; Earthquake ; Strong motions ; USA
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    BIBLIOGRAPHY SEISMOLOGY
  • 6
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    Characteristic earthquakes and long-term earthquake forecasts: Implications of central California seismicity (1995)
    Elsevier
    In:  New York, Elsevier, vol. 138, no. 2, pp. 1-14, (ISBN 0-7923-5034-0)
    Details
    Publication Date: 1995
    Keywords: Seismicity ; Earthquake hazard ; Statistical investigations
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    BIBLIOGRAPHY SEISMOLOGY
  • 7
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    Taiwan strong motion instrumentation program (TSMIP) and preliminary analysis of site effects in Taipei basin from strong motion data (1995)
    Elsevier
    In:  Bull., Polar Proj. OP-O3A4, Urban Disaster Mitigation: The Role of Science and Technology, New York, Elsevier, vol. 66, no. 2, pp. 47-62, (ISBN 0080419208)
    Details
    Publication Date: 1995
    Keywords: Strong motions ; China ; Project report/description ; Site amplification
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    BIBLIOGRAPHY SEISMOLOGY
  • 8
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    Synthesis of polycrystalline methane hydrate, and its phase stability and mechanical properties at elevated pressure (1997)
    American Chemical Society
    In:  ACS Division of Fuel Chemistry Preprints, 42 (2). pp. 544-547.
    Details
    Publication Date: 2018-04-11
    Description: Test specimens of methane hydrate were grown under static conditions by combining cold, pressurized CH4 gas with H2O ice grains, then warming the system to promote the reaction CH4 (g) + 6H2O (s???l) ??? CH4??6H2O. Hydrate formation evidently occurs at the nascent ice/liquid water interface, and complete reaction was achieved by warming the system above 271.5 K and up to 289 K, at 25-30 MPa, for approximately 8 hours. The resulting material is pure methane hydrate with controlled grain size and random texture. Fabrication conditions placed the H2O ice well above its melting temperature before reaction completed, yet samples and run records showed no evidence for bulk melting of the ice grains. Control experiments using Ne, a non-hydrate-forming gas, verified that under otherwise identical conditions, the pressure reduction and latent heat associated with ice melting is easily detectable in our fabrication apparatus. These results suggest that under hydrate-forming conditions, H2O ice can persist metastably at temperatures well above its melting point. Methane hydrate samples were then tested in constant-strain-rate deformation experiments at T= 140-200 K, Pc= 50-100 MPa, and ????= 10-4-10-6 s-1. Measurements in both the brittle and ductile fields showed that methane hydrate has measurably different strength than H2O ice, and work hardens to a higher degree compared to other ices as well as to most metals and ceramics at high homologous temperatures. This work hardening may be related to a changing stoichiometry under pressure during plastic deformation; x-ray analyses showed that methane hydrate undergoes a process of solid-state disproportionation or exsolution during deformation at conditions well within its conventional stability field.
    Type: Article , NonPeerReviewed
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  • 9
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    Late Eocene-Oligocene paleoceanography in the southern Indian Ocean (ODP Site 744) (1996)
    Elsevier
    In:  Marine Geology, 130 (1-2). pp. 99-119.
    Details
    Publication Date: 2016-02-12
    Description: The late Eocene through Oligocene changes in the paleoceanography of the southernmost Indian Ocean have been reconstructed by means of a coarse fraction analysis of closely spaced samples (20 cm = about 20 kyr) from ODP Site 744, Kerguelen Plateau. Surface water productivity, reconstructed from accumulation rates of opal skeletons and benthic foraminifers is low in the early late Eocene, increases at about 36 Ma and shows a sharp increase to maximum values in the earliest Oligocene. In the early late Oligocene it decreases gradually to a minimum and increases again to a maximum in the latest Oligocene. Beside this general trend productivity varies in short-term cycles of a duration of about 400 kyr (340 kyr in the latest Oligocene) with maxima in productivity in warmer Oligocene periods. These productivity variations are reflected by strong variations in carbonate dissolution. Changes in bottom water mass chemistry have been deduced from the degree of carbonate dissolution and it's relation to productivity proxies. Following Kennett and Stott (1990), it is suggested that a proto-antarctic bottom water (proto-AABW) and an overlying warm, saline deep water (WSDW) from low latitudes shifted vertically and latitudinally with time and exerted their influence on the sediments of Site 744. Proto-AABW is detected by means of strong carbonate dissolution when productivity is low. WSDW is detected by means of an excellent carbonate preservation despite high productivity. Terrigenous material occurs as very coarse ice-rafted detritus (IRD) in the late Eocene (167–168.5 mbsf) and after a main shift in oxygen isotopes in the early Oligocene. Very low amounts of 40–125 μm sized mica and very few quartz grains occur only in high productivity periods and at the transition from low to high productivity periods. These occurrences are attributed to wind/current supply during warmer Oligocene intervals.
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  • 10
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    Late Eocene-Oligocene paleoceanography in the southern Indian Ocean (ODP Site 744) (1996)
    Elsevier
    In:  Marine Geology, 130 (1-2). pp. 99-119.
    Details
    Publication Date: 2016-02-15
    Description: The late Eocene through Oligocene changes in the paleoceanography of the southernmost Indian Ocean have been reconstructed by means of a coarse fraction analysis of closely spaced samples (20 cm = about 20 kyr) from ODP Site 744, Kerguelen Plateau. Surface water productivity, reconstructed from accumulation rates of opal skeletons and benthic foraminifers is low in the early late Eocene, increases at about 36 Ma and shows a sharp increase to maximum values in the earliest Oligocene. In the early late Oligocene it decreases gradually to a minimum and increases again to a maximum in the latest Oligocene. Beside this general trend productivity varies in short-term cycles of a duration of about 400 kyr (340 kyr in the latest Oligocene) with maxima in productivity in warmer Oligocene periods. These productivity variations are reflected by strong variations in carbonate dissolution. Changes in bottom water mass chemistry have been deduced from the degree of carbonate dissolution and it's relation to productivity proxies. Following Kennett and Stott (1990), it is suggested that a proto-antarctic bottom water (proto-AABW) and an overlying warm, saline deep water (WSDW) from low latitudes shifted vertically and latitudinally with time and exerted their influence on the sediments of Site 744. Proto-AABW is detected by means of strong carbonate dissolution when productivity is low. WSDW is detected by means of an excellent carbonate preservation despite high productivity. Terrigenous material occurs as very coarse ice-rafted detritus (IRD) in the late Eocene (167–168.5 mbsf) and after a main shift in oxygen isotopes in the early Oligocene. Very low amounts of 40–125 μm sized mica and very few quartz grains occur only in high productivity periods and at the transition from low to high productivity periods. These occurrences are attributed to wind/current supply during warmer Oligocene intervals.
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