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  • American Geophysical Union  (225,564)
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  • 1
    Journal cover
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    Wiley | American Geophysical Union
    Online: 1.2017 –
    Publisher: Wiley , American Geophysical Union
    Electronic ISSN: 2471-1403
    Topics: Geosciences , Medicine
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  • 2
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    American Geophysical Union
    In:  In: The Carbon Cycle and Atmospheric CO2: Natural Variations Archean to Present. , ed. by Sundquist, E. T. and Broecker, W. S. Geophysical Monograph, 32 . American Geophysical Union, Washington, D.C., pp. 504-529.
    Publication Date: 2018-09-04
    Description: The stratigraphic record from both deep‐sea and shallow‐water depositional environments indicates that during late Aptian through Cenomanian time (1) global climates were considerably warmer than at present; (2) latitudinal gradients of atmospheric and oceanic temperatures were considerably less than at present; (3) rates of accumulation of organic matter of both marine and terrestrial origin were as high as or higher than during any other interval in the Mesozoic or Cenozoic; (4) the rate and volume of accumulation of CaCO3 in the deep sea were reduced in response to a marked shoaling of the carbonate compensation depth; (5) seafloor spreading rates were somewhat more rapid than at any other time in the Cretaceous or Cenozoic; (6) off‐ridge volcanism was intense and widespread, particularly in the ancestral Pacific Ocean basin; and (7) sea level was relatively high, forming widespread areas of shallow shelf seas. A marked increase in the rate of CO2 outgassing due to volcanic activity between about 110 and 70 m.y. ago may have resulted in a buildup of atmospheric CO2. A significant fraction of this atmospheric CO2 may have been reduced by an increase in the production and burial of terrestrial organic carbon. Some excess CO2 may have been consumed by marine algal photosynthesis, but marine productivity apparently was low during the Aptian‐Albian relative to terrestrial productivity. Terrestrial productivity also may have been stimulated by increased rainfall that resulted from a warm global climate and increased marine transgression as well as by the higher CO2.
    Type: Book chapter , NonPeerReviewed
    Format: text
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  • 3
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    American Geophysical Union
    In:  Paleoceanography, 8 (1). pp. 7-21.
    Publication Date: 2019-02-22
    Description: Measurements of opal preservation in deep sea sediment cores have been presented in three ways: the opal concentration as a fraction of total dry weight (%opaltot), the opal concentration normalized to calcite‐free dry weight (%opalcalcite‐free), and me opal accumulation rate (opal MAR). It is tempting to interpret changes in these indices as indicators of rates of biological production in past oceans. Based on theoretical constraints, we argue that in typical tropical and subtropical sediments, both %Opalcalcite‐free and opal MAR reflect a significant artifact of dilution by other phases. Thus the band of high %Opalcalcite‐free in the equatorial Pacific appears to be caused in large part by the high %Calcite in that region, rather than by high opal productivity. The best candidate for a reliable paleoproductivity proxy appears to be %Opaltot. Unfortunately, present‐day %Opaltot data from tropical and subtropical regions show little or no systematic trend with the rain rate of opal. Pore water silica concentration data reveal that the apparent pore water opal solubility is not constant but correlates regionally with the rain rate of opal to the seafloor. A model that treats opal as a single homogeneous phase with a single well‐defined solubility product predicts a strong dependence of opal concentration on rain rate (in stark contrast to the data), and a constant asymptotic pore water Si. Two models representing opal as multiple heterogeneous phases with different solubilities are able to reproduce the observed asymptotic pore water Si/rain rate relationship, but not the lack of rain rate trend in the opal concentration data. Only by assuming a systematic trend in the quality of opal (i.e., the solubility) as a function of opal production, can we reproduce the observed pattern of opal preservation. The implication of this study is that changes in opal preservation in the geologic record cannot simply be interpreted in terms of changes in surface ocean productivity until our understanding of opal diagenesis can be improved.
    Type: Article , PeerReviewed
    Format: text
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  • 4
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    American Geophysical Union
    In:  Paleoceanography, 14 (2). pp. 97-102.
    Publication Date: 2019-05-10
    Description: A method for rapid determination of high‐precision Sr/Ca ratios in scleractinian corals is presented. Using an inductively coupled plasma atomic emission spectrophotometer, samples are corrected for instrument drift using a reference solution, similar to the approach used for analysis of stable isotopes using gas‐source mass spectrometry. Further correction for variation of the Sr/Ca ratio with Ca concentration is accomplished using internal standards. The precision, once all corrections have been made, is better than 0.1% (relative standard deviation, 1σ) for samples of similar Ca concentration and better than 0.2% for samples with variable Ca concentrations. This method increases the sample throughput by approximately a factor of 20 relative to thermal ionization mass spectrometry and significantly reduces instrument and per sample costs. Comparison of Sr/Ca data for a coral from the Galapagos Islands with an instrumental temperature record shows excellent agreement and demonstrates the potential for application of this technique to samples of modern and fossil scleractinian corals and other marine carbonates, including foraminifera.
    Type: Article , PeerReviewed
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  • 5
    Publication Date: 2014-08-03
    Description: The modern polar cryosphere reflects an extreme climate state with profound temperature gradients towards high-latitudes. It developed in association with stepwise Cenozoic cooling, beginning with ephemeral glaciations and the appearance of sea ice in the late middle Eocene. The polar ocean gateways played a pivotal role in changing the polar and global climate, along with declining greenhouse gas levels. The opening of the Drake Passage finalized the oceanographic isolation of Antarctica, some 40 Ma ago. The Arctic Ocean was an isolated basin until the early Miocene when rifting and subsequent sea-floor spreading started between Greenland and Svalbard, initiating the opening of the Fram Strait / Arctic-Atlantic Gateway (AAG). Although this gateway is known to be important in Earth’s past and modern climate, little is known about its Cenozoic development. However, the opening history and AAG’s consecutive widening and deepening must have had a strong impact on circulation and water mass exchange between the Arctic Ocean and the North Atlantic. To study the AAG’s complete history, ocean drilling at two primary sites and one alternate site located between 73°N and 78°N are proposed. These sites will provide unprecedented sedimentary records that will unveil (1) the history of shallow-water exchange between the Arctic Ocean and the North Atlantic, and (2) the development of the AAG to a deep-water connection and its influence on the global climate system. The specific overarching goals of our proposal are to study: • the influence of distinct tectonic events in the development of the AAG and the formation of deep water passage on the North Atlantic and Arctic paleoceanography, and • the role of the AAG in the climate transition from the Paleogene greenhouse to the Neogene icehouse for the long-term (~50 Ma) climate history of the northern North Atlantic.
    Repository Name: EPIC Alfred Wegener Institut
    Type: Conference , NonPeerReviewed
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  • 6
    Publication Date: 2015-06-26
    Description: Submarine permafrost degradation has been invoked as a cause for recent observations of methane emissions from the seabed to the water column and atmosphere of the East Siberian shelf. Sediment drilled 52 m down from the sea ice in Buor Khaya Bay, central Laptev Sea revealed unfrozen sediment overlying ice-bonded permafrost. Methane concentrations in the overlying unfrozen sediment were low (mean 20 µM) but higher in the underlying ice-bonded submarine permafrost (mean 380 µM). In contrast, sulfate concentrations were substantially higher in the unfrozen sediment (mean 2.5 mM) than in the underlying submarine permafrost (mean 0.1 mM). Using deduced permafrost degradation rates, we calculate potential mean methane efflux from degrading permafrost of 120 mg m−2 yr−1 at this site. However, a drop of methane concentrations from 190 µM to 19 µM and a concomitant increase of methane δ13C from −63‰ to −35‰ directly above the ice-bonded permafrost suggest that methane is effectively oxidized within the overlying unfrozen sediment before it reaches the water column. High rates of methane ebullition into the water column observed elsewhere are thus unlikely to have ice-bonded permafrost as their source.
    Repository Name: EPIC Alfred Wegener Institut
    Type: Article , NonPeerReviewed
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  • 7
    Publication Date: 2015-01-08
    Description: The last transition from full glacial to current interglacial conditions was accompanied by distinct short-term climate fluctuations caused by changes in the global ocean circulation system. Most palaeoceanographic studies focus on the documentation of the behaviour of the Atlantic Meridional Overturning Circulation (AMOC) during the last deglaciation in response to freshwater forcing events. In this respect, the role of Arctic sea ice remained relatively unconsidered - primarily because of the difficulty of its reconstruction. Here we present new proxy data on late glacial (including the Last Glacial Maximum; LGM) and deglacial sea ice variability in the Arctic Ocean and its main gateway - the Fram Strait - and how these changes in sea ice coverage contributed to AMOC perturbations observed during Heinrich Event 1 and the Younger Dryas. Recurrent short-term advances and retreats of sea ice in Fram Strait, prior and during the LGM, are in line with a variable (or intermittent) North Atlantic heat flow along the eastern corridor of the Nordic Seas. Possibly in direct response to the initial freshwater discharge from melting continental ice-sheets, a permanent sea ice cover established only at about 19 ka BP (i.e. post-LGM) and lasted until 17.6 ka BP, when an abrupt break-up of this thick ice cover and a sudden discharge of huge amounts of sea ice and icebergs through Fram Strait coincided with the weakening of the AMOC during Heinrich Event 1. Similarly, another sea ice maximum at about 12.8 ka BP is associated with the slowdown of the AMOC during the Younger Dryas. The new data sets clearly highlight the important role of Arctic sea ice for the re-organisation of the oceanographic setting in the North Atlantic during the last deglaciation. Further studies and sensitivity experiments to identify crucial driving (and feedback) mechanisms within the High Latitude ice-ocean-atmosphere system will contribute the understanding of rapid climate changes.
    Repository Name: EPIC Alfred Wegener Institut
    Type: Conference , NonPeerReviewed
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  • 8
    Publication Date: 2015-01-08
    Description: Since the Pliocene, global climate history is distinguished by the transition into a colder world, dominated by the onset and intensification of major Northern Hemisphere glaciations which have also changed in their duration and intensity. Potential drivers for these events include falling atmospheric CO2, progressive sub-glacial erosion, tectonic uplift, and associated feedbacks. At present, isolating climate as the driver of evolving continental ice volume since the Pliocene is hindered by the limited long term data sets which directly link climate changes to evidence for ice-sheet advance/retreat, erosion, and tectonic evolution over million year timescales. IODP Expedition 341 drilled a cross-margin transect in the Gulf of Alaska from ice-proximal sites on the continental shelf to distal sites in the deep Pacific. This study focuses on the distal site (Site U1417, c.4190 m water depth) which contains variable biogenic and terrigenous contributions, and evidence for deposition through pelagic, mass movement and glacial processes. Our aim is to investigate links between north-east Pacific paleoceanography and the history of the north-west Cordilleran ice sheet, neither of which are fully understood given limited data pre-dating the Last Glacial Maximum. We reconstruct SSTs during the mid-Pliocene, Plio-Pleistocene Transition (PPT) and mid-Pleistocene transition (MPT) using the UK37’ index. We consider the interaction between SSTs and primary production by examining the absolute and relative abundances of plankton biomarkers (e.g. for haptophytes, diatoms and dinoflagellates), carbon/nitrogen ratios, stable isotopes (δ13C, δ15N) and diatom assemblages. Links between these climatic events and the north-west Cordilleran ice-sheet advance/retreat history are initially made using shipboard stratigraphy; emerging data sets on ice-rafting from members of the Expedition 341 Scientific Party will refine these relationships.
    Repository Name: EPIC Alfred Wegener Institut
    Type: Conference , NonPeerReviewed
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  • 9
    Publication Date: 2015-01-08
    Description: The Mid Pleistocene Transition (MPT) constitutes a fundamental shift in Earth's climate system from a 41 ka to a 100 ka periodicity in glacial oscillations. The exact timing and mechanism(s) that caused this change from a low- to high-amplitude glacial variability are still under debate and only recently Pena & Goldstein (2014) suggested that a disruption of the thermohaline circulation at about 900 ka BP and a subsequent change in ocean circulation might have acted as a trigger for the onset of 100 ka glacial-interglacial cycles. Most studies targeting the MPT are based on Atlantic sediment records whereas only few data sets are available from the North Pacific (see e.g. Clark et al., 2006 and McClymont et al., 2013 for reviews). IODP Expedition 341 distal deep-water site U1417 in the Gulf of Alaska (subpolar NE Pacific) now provided a continuous sediment record for reconstructing Miocene to Late Pleistocene changes in the sea surface conditions and how these relate to orbital and millennial scale climate variability. Here we present organic geochemical biomarker data covering the 1.5 Ma to 0.1 Ma time interval with special focus on the MPT. Alkenone, sterol, n-alkane and C25 highly branched isoprenoid data are used to reconstruct sea surface temperatures, primary productivity and terrigenous organic matter input (via sea ice, icebergs, meltwater discharge or aeolian transport). In addition, the diatom concentration and the species composition of the diatom assemblage deliver information on changes in palaeoproductivity and nutrient (silicate) availability. A major change in the environmental setting between 1.2 and 0.8 Ma is recorded by the biomarkers. This shift seems to be associated with a significant cooling of the surface waters in the Gulf of Alaska. Matching this shift, a significant change in the main components of the diatom community occurred between 1.2 and 0.8 Ma. References Clark, P.U., Archer, D., Pollard, D., Blum, J.D., Rial, J.A., Brovkin, V., Mix, A.C., Pisias, N.G., Roy, M., 2006. Quaternary Science Reviews, 25, (23–24), 3150-3184. McClymont, E.L., Sosdian, S.M., Rosell-Melé, A., Rosenthal, Y., 2013. Earth-Science Reviews, 123, 173-193. Pena, L.D. and Goldstein, S.L., 2014. Science, 345, 318-322.
    Repository Name: EPIC Alfred Wegener Institut
    Type: Conference , NonPeerReviewed
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  • 10
    Publication Date: 2015-01-08
    Description: Reconstructing the timing and nature of past changes in aquatic productivity in the Gulf of Alaska (GoA) can shed light on the primary processes driving biogeochemical cycling over geologic timescales. Here, we present sedimentologic, physical property, stable isotope, and biogenic opal concentration data from IODP Expedition 341 Sites U1417 and U1419 and identify intervals where diatom ooze lithofacies and geochemical evidence for increased algal productivity are prevalent during the Pleistocene. Sites U1417 and U1419 are located in the center and the margin of the Fe-limited GoA, respectively, and they offer the potential to characterize past changes in biogeochemical cycling during different Pleistocene time intervals. Site U1419 cores were collected from a small slope basin at the edge of the continental shelf. Sediment cores reveal two prominent ~6-m-thick intervals of diatomaceous ooze. Between these intervals are numerous 20-cm-thick sections of biogenic-rich sediment, interbedded with gray mud that commonly contains lonestones. Based on preliminary age models, the two diatom ooze intervals likely correspond to the Holocene and MIS 3, while the intervening interbedded glacigenic and biogenic sediment can broadly be ascribed to MIS 2. Diatomaceous ooze and diatom-rich sediments are generally characterized by lower magnetic susceptibility, natural gamma ray, bulk density, and higher b* color reflectance. Initial C & N concentration and stable isotopic data show elevated concentrations and more positive stable isotope values during the Holocene and MIS 3, which approximate the isotopic signature of modern phytoplankton measured in the GoA. Within the glacial period, the biogenic-rich intervals are also characterized by more positive C and N isotopic values. When combined with the shipboard physical property data, the stable isotopic results are indicative of millennial-scale variations in productivity and/or changes in glacial ice extent in the GoA during the last glacial period. We will discuss these results in the context of an improved isotope stratigraphy and ongoing work examining multiple interglacial productivity variations at Site U1417.
    Repository Name: EPIC Alfred Wegener Institut
    Type: Conference , NonPeerReviewed
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