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  • Articles  (35)
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  • 1
    Publication Date: 2017-06-19
    Description: Tectonically induced changes in oceanic seaways had profound effects on global and regional climate during the Late Neogene. The constriction of the Central American Seaway reached a critical threshold during the early Pliocene ~4.8–4 million years (Ma) ago. Model simulations indicate the strengthening of the Atlantic Meridional Overturning Circulation (AMOC) with a signature warming response in the Northern Hemisphere and cooling in the Southern Hemisphere. Subsequently, between ~4–3 Ma, the constriction of the Indonesian Seaway impacted regional climate and might have accelerated the Northern Hemisphere Glaciation. We here present Pliocene Atlantic interhemispheric sea surface temperature and salinity gradients (deduced from foraminiferal Mg/Ca and stable oxygen isotopes, δ18O) in combination with a recently published benthic stable carbon isotope (δ13C) record from the southernmost extent of North Atlantic Deep Water to reconstruct gateway-related changes in the AMOC mode. After an early reduction of the AMOC at ~5.3 Ma, we show in agreement with model simulations of the impacts of Central American Seaway closure a strengthened AMOC with a global climate signature. During ~3.8–3 Ma, we suggest a weakening of the AMOC in line with the global cooling trend, with possible contributions from the constriction of the Indonesian Seaway.
    Repository Name: EPIC Alfred Wegener Institut
    Type: Article , NonPeerReviewed
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  • 2
    Publication Date: 2017-01-07
    Description: Author Posting. © American Geophysical Union, 2013. This article is posted here by permission of American Geophysical Union for personal use, not for redistribution. The definitive version was published in Geochemistry, Geophysics, Geosystems 14 (2013): 3730–3750, doi:10.1002/ggge.20230.
    Description: The Sr/Ca ratio of coral aragonite is used to reconstruct past sea surface temperature (SST). Twenty-one laboratories took part in an interlaboratory study of coral Sr/Ca measurements. Results show interlaboratory bias can be significant, and in the extreme case could result in a range in SST estimates of 7°C. However, most of the data fall within a narrower range and the Porites coral reference material JCp-1 is now characterized well enough to have a certified Sr/Ca value of 8.838 mmol/mol with an expanded uncertainty of 0.089 mmol/mol following International Association of Geoanalysts (IAG) guidelines. This uncertainty, at the 95% confidence level, equates to 1.5°C for SST estimates using Porites, so is approaching fitness for purpose. The comparable median within laboratory error is 〈0.5°C. This difference in uncertainties illustrates the interlaboratory bias component that should be reduced through the use of reference materials like the JCp-1. There are many potential sources contributing to biases in comparative methods but traces of Sr in Ca standards and uncertainties in reference solution composition can account for half of the combined uncertainty. Consensus values that fulfil the requirements to be certified values were also obtained for Mg/Ca in JCp-1 and for Sr/Ca and Mg/Ca ratios in the JCt-1 giant clam reference material. Reference values with variable fitness for purpose have also been obtained for Li/Ca, B/Ca, Ba/Ca, and U/Ca in both reference materials. In future, studies reporting coral element/Ca data should also report the average value obtained for a reference material such as the JCp-1.
    Description: E.C.H. (MARUM Fellowship) and T.F. were supported by the DFG-Research Center/Excellence Cluster ‘‘The Ocean in the Earth System,’’ University of Bremen. HVM was supported by an AINSE Research Fellowship.
    Description: 2014-03-23
    Keywords: Coral Sr/Ca ratios
    Repository Name: Woods Hole Open Access Server
    Type: Article
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  • 3
    Publication Date: 2016-07-23
    Description: Author Posting. © American Geophysical Union, 2016. This article is posted here by permission of American Geophysical Union for personal use, not for redistribution. The definitive version was published in Paleoceanography 31 (2016): 185–202, doi:10.1002/2015PA002862.
    Description: The intertropical convergence zone and the African monsoon system are highly sensitive to climate forcing at orbital and millennial timescales. Both systems influence the strength and direction of the trade winds along northwest Africa and thus directly impact coastal upwelling. Sediment cores from the northwest African margin record upwelling-related changes in biological productivity connected to changes in regional and hemispheric climate. We present records of 230Th-normalized biogenic opal and Corg fluxes using a meridional transect of four cores from 19°N–31°N along the northwest African margin to examine changes in paleoproductivity since the last glacial maximum. We find large changes in biogenic fluxes synchronous with changes in eolian fluxes calculated using end-member modeling, suggesting that paleoproductivity and dust fluxes were strongly coupled, likely linked by changes in wind strength. Opal and Corg fluxes increase at all sites during Heinrich Stadial 1 and the Younger Dryas, consistent with an overall intensification of the trade winds, and changes in the meridional flux gradient indicate a southward wind shift at these times. Biogenic fluxes were lowest, and the meridional flux gradients were weakest during the African Humid Period when the monsoon was invigorated due to precessional changes, with greater rainfall and weaker trade winds over northwest Africa. These results expand the spatial coverage of previous paleoproxy studies showing similar changes, and they provide support for modeling studies showing changes in wind strength and direction consistent with increased upwelling during abrupt coolings and decreased upwelling during the African Humid Period.
    Description: NSF Grant Numbers: OCE-1103262, OCE-1030784, OCE-0402348; Center for Climate and Life
    Description: 2016-07-23
    Keywords: Opal flux ; African margin ; African Humid Period ; Trade winds ; Abrupt change ; Deglaciation
    Repository Name: Woods Hole Open Access Server
    Type: Article
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  • 4
    Publication Date: 2017-01-07
    Description: Author Posting. © Elsevier B.V., 2007. This is the author's version of the work. It is posted here by permission of Elsevier B.V. for personal use, not for redistribution. The definitive version was published in Organic Geochemistry 38 (2007): 1607-1624, doi:10.1016/j.orggeochem.2007.06.008.
    Description: Integrated terrestrial and marine records of northeast African vegetation are needed to provide long, high resolution records of environmental variability with established links to specific terrestrial environments. In this study, we compare records of terrestrial vegetation preserved in marine sediments in the Gulf of Aden (DSDP Site 231) and an outcrop of lacustrine sediments in the Turkana Basin, Kenya, part of the East African Rift System. We analyzed higher plant biomarkers in sediments from both deposits of known equivalent age, corresponding to a ca. 50 – 100 ka humid interval prior to the β-Tulu Bor eruption ca. 3.40 Ma, when the Lokochot Lake occupied part of the Turkana Basin. Molecular abundance distributions indicate that long chain n-alkanoic acids in marine sediments are the most reliable proxy for terrestrial vegetation (Carbon Preference Index, CPI, = 4.5), with more cautious interpretation needed for n-alkanes and lacustrine archives. Marine sediments record carbon isotopic variability in terrestrial biomarkers of 2 – 3‰, roughly equivalent to 20% variability in the C3/C4 vegetation contribution. The proportion of C4 vegetation apparently increased at times of low terrigenous dust input. Terrestrial sediments reveal much larger (2 – 10‰) shifts in n-alkanoic acid δ13C values. However, molecular abundance and isotopic composition suggest that microbial sources may also contribute fatty acids, contaminating the lacustrine sedimentary record of terrestrial vegetation.
    Description: Funding was provided by the U.S. National Science Foundation HOMINID Grant 0218511.
    Repository Name: Woods Hole Open Access Server
    Type: Preprint
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  • 5
    Publication Date: 2017-02-15
    Description: © The Author(s), 2016. This article is distributed under the terms of the Creative Commons Attribution License. The definitive version was published in Science Advances 2 (2016): e1600445, doi:10.1126/sciadv.1600445.
    Description: Saharan mineral dust exported over the tropical North Atlantic is thought to have significant impacts on regional climate and ecosystems, but limited data exist documenting past changes in long-range dust transport. This data gap limits investigations of the role of Saharan dust in past climate change, in particular during the mid-Holocene, when climate models consistently underestimate the intensification of the West African monsoon documented by paleorecords. We present reconstructions of African dust deposition in sediments from the Bahamas and the tropical North Atlantic spanning the last 23,000 years. Both sites show early and mid-Holocene dust fluxes 40 to 50% lower than recent values and maximum dust fluxes during the deglaciation, demonstrating agreement with records from the northwest African margin. These quantitative estimates of trans-Atlantic dust transport offer important constraints on past changes in dust-related radiative and biogeochemical impacts. Using idealized climate model experiments to investigate the response to reductions in Saharan dust’s radiative forcing over the tropical North Atlantic, we find that small (0.15°C) dust-related increases in regional sea surface temperatures are sufficient to cause significant northward shifts in the Atlantic Intertropical Convergence Zone, increased precipitation in the western Sahel and Sahara, and reductions in easterly and northeasterly winds over dust source regions. Our results suggest that the amplifying feedback of dust on sea surface temperatures and regional climate may be significant and that accurate simulation of dust’s radiative effects is likely essential to improving model representations of past and future precipitation variations in North Africa.
    Description: This study was supported, in part, by NSF awards OCE-1030784 (to D.M. and P.B.d.) and OCE-09277247 (to P.B.d.); NASA grant NN14AP38G (to C. Heald, Massachusetts Institute of Technology), which supports D.A.R.; and the Columbia University Center for Climate and Life. A.F. is supported by the NSF grant AGS-1116885 and the National Oceanic and Atmospheric Administration (NOAA) grant NA14OAR4310277. S.H. is supported by the NASA Earth and Space Sciences Fellowship. We also acknowledge computational support from the NSF/NCAR Yellowstone Supercomputing Center and the Yale University High Performance Computing Center.
    Keywords: Mineral dust ; North Africa ; Paleoclimate ; African Humid Period
    Repository Name: Woods Hole Open Access Server
    Type: Article
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  • 6
    Publication Date: 2017-01-04
    Description: Author Posting. © The Author(s), 2013. This is the author's version of the work. It is posted here by permission of Elsevier B.V. for personal use, not for redistribution. The definitive version was published in Geochimica et Cosmochimica Acta 121 (2013):196-213, doi:10.1016/j.gca.2013.07.028.
    Description: The Mg/Ca ratio in foraminiferal calcite is one of the principal proxies used for paleoceanographic temperature reconstructions, but recent core-top sediment observations suggest that salinity may exert a significant secondary control on planktic foraminifers. This study compiles new and published laboratory culture experiment data from the planktic foraminifers Orbulina universa, Globigerinoides sacculifer and Globigerinoides ruber, in which salinity was varied but temperature, pH and light were held constant. Combining new data with results from previous culture studies yields a Mg/Ca-sensitivity to salinity of 4.4±2.3%, 4.7±1.2%, and 3.3±1.7% per salinity unit (95% confidence), respectively, for the three foraminifer species studied here. Comparison of these sensitivities with core-top data suggests that the much larger sensitivity (27±4% per salinity unit) derived from Atlantic core-top sediments in previous studies is not a direct effect of salinity. Rather, we suggest that the dissolution correction often applied to Mg/Ca data can lead to significant overestimation of temperatures. We are able to reconcile culture calibrations with core-top observations by combining evidence for seasonal occurrence and latitude-specific habitat depth preferences with corresponding variations in physico-chemical environmental parameters. Although both Mg/Ca and δ18O yield temperature estimates that fall within the bounds of hydrographic observations, discrepancies between the two proxies highlight unresolved challenges with the use of paired Mg/Ca and δ18O analyses to reconstruct paleo-salinity patterns across ocean basins. The first step towards resolving these challenges requires a better spatially and seasonally resolved δ18Osw archive than is currently available. Nonetheless, site-specific reconstructions of salinity change through time may be valid.
    Description: This research was supported by NSF OCE 07-51764 (BH), OCE 05-50703 (HJS), ARC DP 8800010 (SE), OCE 07-52649 (PdeM), ERC 2010-NEWLOG ADG-267931 (HE) and a Columbia Climate Center Grant (KA and BH).
    Keywords: Sea surface temperatures ; Mg/Ca ; δ18O ; Planktic foraminifers ; Salinity
    Repository Name: Woods Hole Open Access Server
    Type: Preprint
    Format: application/pdf
    Format: application/vnd.ms-excel
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  • 7
    Publication Date: 2002-11-01
    Description: Interannual to decadal variations in Middle Eastern temperature, precipitationand streamflow reflect the far-field influence of the North Atlantic Oscillation (NAO), a dominant mode of Atlantic sector climate variability. Using a new sea surface temperature (SST) based index of the NAO and availablestreamflow data from five Middle Eastern rivers, we show that the first principal component of December through March streamflow variability reflects changes in the NAO. However, Middle East rivers have two primary flooding periods.The first is rainfall-driven runoff from December through March, regulated on interannual to decadal timescales by the NAO as reflected in local precipitation and temperature.The second period, from April through June, reflects spring snowmelt and contributes in excess of 50% of annual runoff.This period, known locally as the khamsin , displays no significantNAO connections and a less direct relationship with local climatic factors, suggesting that streamflow variability during this period reflects land-cover change, possibly related to agriculture and hydropower generation, and snowmelt. ©2002 Kluwer Academic Publishers
    Print ISSN: 0165-0009
    Electronic ISSN: 1573-1480
    Topics: Geosciences , Physics
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  • 8
    Publication Date: 2019-05-13
    Description: North Atlantic intermediate-water temperature variations based on ostracod Mg/Ca ratios from Ocean Drilling Program (ODP) Hole 1055B document a series of multi-centennial-scale abrupt warming events throughout the last deglaciation and Holocene (up to ∼3 °C). These events are coherent with abrupt climate reversals including Heinrich event 1, the Younger Dryas–Intra-Allerød cold period, and Holocene North Atlantic Deep Water (NADW) reduction periods. Deglacial–Holocene warm events were likely related to reduction in the strength of the upper NADW (Labrador Sea Water). We also found a long-term cooling trend in the ODP 1055 Mg/Ca record indicating continuous Labrador Sea Water strengthening throughout the Holocene. Our results help to better understand deglacial–Holocene upper NADW dynamics that remain poorly understood but can be important for regional and global climates.
    Print ISSN: 0091-7613
    Electronic ISSN: 1943-2682
    Topics: Geosciences
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  • 9
  • 10
    Publication Date: 2011-03-08
    Print ISSN: 0096-3941
    Electronic ISSN: 2324-9250
    Topics: Geosciences
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