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  • 1
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    Publication Date: 2017-08-03
    Description: Author Posting. © The Author(s), 2016. This is the author's version of the work. It is posted here by permission of American Association for the Advancement of Science for personal use, not for redistribution. The definitive version was published in Science 353 (2016): 470-474, doi:10.1126/science.aaf5529.
    Description: The most recent ice age was characterized by rapid and hemispherically asynchronous climate oscillations, whose origin remains unresolved. Variations in oceanic meridional heat transport may contribute to these repeated climate changes, which were most pronounced during the glacial interval twenty-five to sixty thousand years ago known as marine isotope stage 3 (MIS3). Here we examine a sequence of climate and ocean circulation proxies throughout MIS3 at high resolution in a deep North Atlantic sediment core, combining the kinematic tracer Pa/Th with the most widely applied deep water-mass tracer, δ13CBF. These indicators reveal that Atlantic overturning circulation was reduced during every cool northern stadial, with the greatest reductions during episodic iceberg discharges from the Hudson Strait, and that sharp northern warming followed reinvigorated overturning. These results provide direct evidence for the ocean's persistent, central role in abrupt glacial climate change.
    Description: This research was supported in part by a NSF Graduate Research Fellowship to L.G.H, by awards from the Comer Science and Education Foundation and NSF ATM-0936496 to J.F.M., and an award from the LDEO Climate Center to L.G.H. and J.F.M. LDK and WBC were supported by ATM-0836472, and LDK was supported by AGS 1548160.
    Repository Name: Woods Hole Open Access Server
    Type: Preprint
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  • 3
    Publication Date: 2017-12-03
    Description: Author Posting. © American Geophysical Union, 2017. This article is posted here by permission of American Geophysical Union for personal use, not for redistribution. The definitive version was published in Paleoceanography 32 (2017): 512–530, doi:10.1002/2016PA003072.
    Description: The carbon isotope composition (δ13C) of seawater provides valuable insight on ocean circulation, air-sea exchange, the biological pump, and the global carbon cycle and is reflected by the δ13C of foraminifera tests. Here more than 1700 δ13C observations of the benthic foraminifera genus Cibicides from late Holocene sediments (δ13CCibnat) are compiled and compared with newly updated estimates of the natural (preindustrial) water column δ13C of dissolved inorganic carbon (δ13CDICnat) as part of the international Ocean Circulation and Carbon Cycling (OC3) project. Using selection criteria based on the spatial distance between samples, we find high correlation between δ13CCibnat and δ13CDICnat, confirming earlier work. Regression analyses indicate significant carbonate ion (−2.6 ± 0.4) × 10−3‰/(μmol kg−1) [CO32−] and pressure (−4.9 ± 1.7) × 10−5‰ m−1 (depth) effects, which we use to propose a new global calibration for predicting δ13CDICnat from δ13CCibnat. This calibration is shown to remove some systematic regional biases and decrease errors compared with the one-to-one relationship (δ13CDICnat = δ13CCibnat). However, these effects and the error reductions are relatively small, which suggests that most conclusions from previous studies using a one-to-one relationship remain robust. The remaining standard error of the regression is generally σ ≅ 0.25‰, with larger values found in the southeast Atlantic and Antarctic (σ ≅ 0.4‰) and for species other than Cibicides wuellerstorfi. Discussion of species effects and possible sources of the remaining errors may aid future attempts to improve the use of the benthic δ13C record.
    Description: U.S. National Science Foundation Grant Numbers: 1634719, 0926735, 1125181; Swiss National Science Foundation Grant Numbers: PP00P2_144811, 200021_163003; Canadian Institute for Advanced Research (CIFAR); Canadian Foundation for Innovation (CFI); Natural Sciences and Engineering Research Council (NSERC)
    Description: 2017-12-03
    Keywords: Carbon ; Isotopes ; Benthic ; Foraminifera ; Calibration
    Repository Name: Woods Hole Open Access Server
    Type: Article
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  • 4
    Publication Date: 2018-11-01
    Description: © The Author(s), 2018. This article is distributed under the terms of the Creative Commons Attribution License. The definitive version was published in Paleoceanography and Paleoclimatology 33 (2018): 1013-1034, doi:10.1029/2018PA003408.
    Description: The chemical composition of benthic foraminifera from marine sediment cores provides information on how glacial subsurface water properties differed from modern, but separating the influence of changes in the origin and end‐member properties of subsurface water from changes in flows and mixing is challenging. Spatial gaps in coverage of glacial data add to the uncertainty. Here we present new data from cores collected from the Demerara Rise in the western tropical North Atlantic, including cores from the modern tropical phosphate maximum at Antarctic Intermediate Water (AAIW) depths. The results suggest lower phosphate concentration and higher carbonate saturation state within the phosphate maximum than modern despite similar carbon isotope values, consistent with less accumulation of respired nutrients and carbon, and reduced air‐sea gas exchange in source waters to the region. An inversion of new and published glacial data confirms these inferences and further suggests that lower preformed nutrients in AAIW, and partial replacement of this still relatively high‐nutrient AAIW with nutrient‐depleted, carbonate‐rich waters sourced from the region of the modern‐day northern subtropics, also contributed to the observed changes. The results suggest that glacial preformed and remineralized phosphate were lower throughout the upper Atlantic, but deep phosphate concentration was higher. The inversion, which relies on the fidelity of the paleoceanographic data, suggests that the partial replacement of North Atlantic sourced deep water by Southern Ocean Water was largely responsible for the apparent deep North Atlantic phosphate increase, rather than greater remineralization.
    Description: National Science Foundation (NSF) Grant Numbers: OCE‐0750880, OCE‐1335191, OCE‐1558341, OCE‐1536380; Woods Hole Oceanographic Institution (WHOI) Grant Numbers: 27007592, 27000808
    Keywords: Glacial Atlantic circulation ; Preformed phosphate ; Remineralized phosphate ; Antarctic Intermediate Water ; Nutrient redistribution ; Tropical phosphate maximum
    Repository Name: Woods Hole Open Access Server
    Type: Article
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  • 5
    Publication Date: 2016-09-26
    Description: © The Author(s), 2015. This article is distributed under the terms of the Creative Commons Attribution License. The definitive version was published in Paleoceanography 30 (2015): 353–368, doi:10.1002/2014PA002667.
    Description: Approximately synchronous with the onset of Heinrich Stadial 1 (HS1), δ13C decreased throughout most of the upper (~1000–2500 m) Atlantic, and at some deeper North Atlantic sites. This early deglacial δ13C decrease has been alternatively attributed to a reduced fraction of high-δ13C North Atlantic Deep Water (NADW) or to a decrease in the NADW δ13C source value. Here we present new benthic δ18O and δ13C records from three relatively shallow (~1450–1650 m) subpolar Northeast Atlantic cores. With published data from other cores, these data form a depth transect (~1200–3900 m) in the subpolar Northeast Atlantic. We compare Last Glacial Maximum (LGM) and HS1 data from this transect with data from a depth transect of cores from the Brazil Margin. The largest LGM-to-HS1 decreases in both benthic δ13C and δ18O occurred in upper waters containing the highest NADW fraction during the LGM. We show that the δ13C decrease can be explained entirely by a lower NADW δ13C source value, entirely by a decrease in the proportion of NADW relative to Southern Ocean Water, or by a combination of these mechanisms. However, building on insights from model simulations, we hypothesize that reduced ventilation due to a weakened but still active Atlantic Meridional Overturning Circulation also contributed to the low δ13C values in the upper North Atlantic. We suggest that the benthic δ18O gradients above ~2300 m at both core transects indicate the depth to which heat and North Atlantic deglacial freshwater had mixed into the subsurface ocean by early HS1.
    Description: The work was supported by NSF grants OCE13-35191, OCE07-50880, and OCE05-84911 to the Woods Hole Oceanographic Institution.
    Keywords: Heinrich Stadial 1 ; Deglacial d13C minimum ; Atlantic Circulation ; Benthic d18O ; Benthic d13C
    Repository Name: Woods Hole Open Access Server
    Type: Article
    Format: application/pdf
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  • 6
    Publication Date: 2016-07-25
    Description: We have developd a computer controlled system to measure the calcium carbonate content of sediment samples. A menu driven program controls the analysis of each sample. The system first communicates with a Mettler digital balance to record the weights of the 40 samples which must be loaded into each run. The sample boats are next loaded into the sample carousel which is then sealed from the atmosphere. The system is first pumped down to a vacuum of 0.04 torr. The valve to the pump closes and the stepping motor turns the carousel, moving a sample boat over the delivery slot and dropping the sample into 80°C 100% phosphoric acid under vigorous spinning action. During the reaction, carbonate is evolved into H2O and CO2 and the resulting pressure change within the closed system is measured by a pressure transducer and recorded into memory next to the sample identification and sample weight. The system is pumped once again to 0.04 torr and the process continues until all 40 samples have been analyzed. The data can then be uploaded and converted to percent carbonate values using a regression line produced from multiple analyses of varying weights of a 100% carbonate standard. Precision of the system, based upon 120 replicate analysis ranges from 0.49% to 0.88%.
    Description: Funding was provided by the National Science Foundation through Grant Nos. OCE 85-11014 and OCE 88-13307.
    Keywords: Carbonate ; Automated ; Sediments
    Repository Name: Woods Hole Open Access Server
    Type: Technical Report
    Format: 1698741 bytes
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  • 7
    Publication Date: 2018-01-10
    Description: Author Posting. © The Author(s), 2017. This is the author's version of the work. It is posted here under a nonexclusive, irrevocable, paid-up, worldwide license granted to WHOI. It is made available for personal use, not for redistribution. The definitive version was published in Geology 45 (2017): 1047-1050, doi:10.1130/G39270.1.
    Description: The risk posed by intensification of North Atlantic hurricane activity remains controversial, in part due to a lack of available storm proxy records that extend beyond the relatively stable climates of the late Holocene. Here we present a record of storm-triggered turbidite deposition offshore the Dry Tortugas, south Florida, USA, that spans abrupt transitions in North Atlantic sea-surface temperature and Atlantic Meridional Overturning Circulation (AMOC) during the Younger Dryas (12.9–11.7 k.y. B.P.). Despite potentially hostile conditions for cyclogenesis in the tropical North Atlantic at this time, our record and numerical experiments suggest that strong hurricanes may have regularly impacted Florida. Less severe surface cooling at mid-latitudes (~20–40°N) than across much of the tropical North Atlantic (~10–20°N) in response to AMOC reduction may best explain strong hurricane activity during the Younger Dryas near the Dry Tortugas and, potentially, along the entire southeastern coast of the United States.
    Description: This work was supported by the U. S. Geological Survey Climate and Land Use Change Research and Development Program (Toomey), the Woods Hole Oceanographic Institution Ocean and Climate Change Institute (Toomey) and National Science Foundation grants (OCE-1356708 to Donnelly; 1356509 to van Hengstum).
    Repository Name: Woods Hole Open Access Server
    Type: Preprint
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  • 8
    Publication Date: 2018-09-01
    Print ISSN: 2572-4517
    Electronic ISSN: 2572-4525
    Topics: Geosciences
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  • 9
  • 10
    Publication Date: 2015-04-01
    Print ISSN: 2572-4517
    Electronic ISSN: 2572-4525
    Topics: Geosciences
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