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  • Holocene  (4)
  • Benthic foraminifera  (3)
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  • 1
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    Meridional overturning circulation in the South Atlantic at the last glacial maximum (2006)
    American Geophysical Union
    Details
    Publication Date: 2022-05-25
    Description: Author Posting. © American Geophysical Union, 2006. 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 7 (2006): Q10N03, doi:10.1029/2005GC001226.
    Description: The geostrophic shear associated with the meridional overturning circulation is reflected in the difference in density between the eastern and western margins of the ocean basin. Here we examine how the density difference across 30°S in the upper 2 km of the Atlantic Ocean (and thus the magnitude of the shear associated with the overturning circulation) has changed between the last glacial maximum and the present. We use oxygen isotope measurements on benthic foraminifera to reconstruct density. Today, the density in upper and intermediate waters along the eastern margin in the South Atlantic is greater than along the western margin, reflecting the vertical shear associated with the northward flow of surface and intermediate waters and the southward flowing North Atlantic Deep Waters below. The greater density along the eastern margin is reflected in the higher δ 18O values for surface sediment benthic foraminifera than those found on the western margin for the upper 2 km. For the last glacial maximum the available data indicate that the eastern margin foraminifera had similar δ 18O to those on the western margin between 1 and 2 km and that the gradient was reversed relative to today with the higher δ 18O values in the western margin benthic foraminifera above 1 km. If this reversal in benthic foraminifera δ 18O gradient reflects a reversal in seawater density gradient, these data are not consistent with a vigorous but shallower overturning cell in which surface waters entering the Atlantic basin are balanced by the southward export of Glacial North Atlantic Intermediate Water.
    Description: This work was supported by NSF award OCE-9984989/OCE-0428803 to J.L.-S., NSF award OCE-9986748 to D.W.O. and W.B.C., NSF OCE-0222111 to C.D.C., and SEGRF fellowship at LLNL to J.M.
    Keywords: Last Glacial Maximum ; South Atlantic ; Meridional overturning circulation ; Oxygen isotopes ; Benthic foraminifera
    Repository Name: Woods Hole Open Access Server
    Type: Article
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  • 2
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    Last deglaciation in the Okinawa Trough : subtropical northwest Pacific link to Northern Hemisphere and tropical climate (2005)
    American Geophysical Union
    Details
    Publication Date: 2022-05-25
    Description: Author Posting. © American Geophysical Union, 2005. This article is posted here by permission of American Geophysical Union for personal use, not for redistribution. The definitive version was published in Paleoceanography 20 (2005): PA4005, doi:10.1029/2004PA001061.
    Description: Detailed deglacial and Holocene records of planktonic δ18O and Mg/Ca–based sea surface temperature (SST) from the Okinawa Trough suggest that at ∼18 to 17 thousand years before present (kyr B.P.), late spring/early summer SSTs were approximately 3°C cooler than today, while surface waters were up to 1 practical salinity unit saltier. These conditions are consistent with a weaker influence of the summer East Asian Monsoon (EAM) than today. The timing of suborbital SST oscillations suggests a close link with abrupt changes in the EAM and North Atlantic climate. A tropical influence, however, may have resulted in subtle decoupling between the North Atlantic and the Okinawa Trough/EAM during the deglaciation. Okinawa Trough surface water trends in the Holocene are consistent with model simulations of an inland shift of intense EAM precipitation during the middle Holocene. Millennial-scale alternations between relatively warm, salty conditions and relatively cold, fresh conditions suggest varying influence of the Kuroshio during the Holocene.
    Description: Funding for this research was provided by NSFC (grants 40106006 and 40206007), SKLLQG (grant LLQG0204), and the NSF (OCE-020776 to DWO). Y.S.'s visit to WHOI was supported via a NSF START Fellowship.
    Keywords: Okinawa Trough ; Deglaciation ; Holocene ; Kuroshio Current ; East Asian monsoon ; Mg/Ca ; Oxygen isotopes ; Foraminifera
    Repository Name: Woods Hole Open Access Server
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  • 3
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    Temperature and carbonate ion effects on Mg/Ca and Sr/Ca ratios in benthic foraminifera : aragonitic species Hoeglundina elegans (2010)
    American Geophysical Union
    Details
    Publication Date: 2022-05-25
    Description: Author Posting. © American Geophysical Union, 2006. This article is posted here by permission of American Geophysical Union for personal use, not for redistribution. The definitive version was published in Paleoceanography 21 (2006): PA1007, doi:10.1029/2005PA001158.
    Description: Core top samples from Atlantic (Little Bahama Banks (LBB)) and Pacific (Hawaii and Indonesia) depth transects have been analyzed in order to assess the influence of bottom water temperature (BWT) and aragonite saturation levels on Mg/Ca and Sr/Ca ratios in the aragonitic benthic foraminifer Hoeglundina elegans. Both the Mg/Ca and Sr/Ca ratios in H. elegans tests show a general decrease with increasing water depth. Although at each site the decreasing trends are consistent with the in situ temperature profile, Mg/Ca and Sr/Ca ratios in LBB are substantially higher than in Indonesia and Hawaii at comparable water depths with a greater difference observed with increasing water depth. Because we find no significant difference between results obtained on “live” and “dead” specimens, we propose that these differences are due to primary effects on the metal uptake during test formation. Evaluation of the water column properties at each site suggests that in situ CO3 ion concentrations play an important role in determining the H. elegans Mg/Ca and Sr/Ca ratios. The CO3 ion effect is limited, however, only to aragonite saturation levels ([ΔCO3]aragonite) below 15 μmol kg−1. Above this level, temperature exerts a dominant effect. Accordingly, we propose that Mg/Ca and Sr/Ca in H. elegans tests can be used to reconstruct thermocline temperatures only in waters oversaturated with respect to the mineral aragonite using the following relationships: Mg/Ca = (0.034 ± 0.002)BWT + (0.96 ± 0.03) and Sr/Ca = (0.060 ± 0.002)BWT + (1.53 ± 0.03) (for [ΔCO3]aragonite 〉 15 μmol kg−1). The standard error associated with these equations is about ±1.1°C. Reconstruction of deeper water temperatures is complicated because in undersaturated waters, changes in Mg/Ca and Sr/Ca ratios reflect a combination of changes in [CO3] and BWT. Overall, we find that Sr/Ca, rather than Mg/Ca, in H. elegans may be a more accurate proxy for reconstructing paleotemperatures.
    Description: Yair Rosenthal acknowledges the support of Amtzia Genin and the Hebrew University, Forchheimer Fellowship, during his sabbatical in the Inter-University Institute in Eilat, Israel. This project has been funded by NSF Awards OCE 0220922 to Y.R. and OCE 0220776 to D.W.O. and B.K.L.
    Keywords: Benthic foraminifera ; Paleothermometry ; Magnesium
    Repository Name: Woods Hole Open Access Server
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  • 4
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    Paleoenvironmental change in the middle Okinawa Trough since the last deglaciation : evidence from the sedimentation rate and planktonic foraminiferal record (2007)
    Details
    Publication Date: 2022-05-26
    Description: Author Posting. © The Authors, 2006. 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 Palaeogeography, Palaeoclimatology, Palaeoecology 243 (2007): 378-393, doi:10.1016/j.palaeo.2006.08.016.
    Description: Well-dated, high-resolution records of planktonic foraminifera and oxygen isotopes from two sediment cores, A7 and E017, in the middle Okinawa Trough reveal strong and rapid millennial-scale climate changes since ~18 to 17 thousand years before present (kyr B.P.). Sedimentation rate shows a sudden drop at ~11.2 cal. kyr B.P. due to a rapid rise of sea-level after the Younger Dryas (YD) and consequently submergence of the large continental shelf on the East China Sea (ECS) and the retreat of the estuary providing sediment to the basin. During the last deglaciation, the relative abundance of warm and cold species of planktonic foraminifera fluctuates strongly, consistent with the timing of sea surface temperature (SST) variations determined from Mg/Ca measurements of planktonic foraminifera from one of the two cores. These fluctuations are coeval with climate variation recorded in the Greenland ice cores and North Atlantic sediments, namely Heinrich event 1 (H1), Bølling-Allerød (B/A) and YD events. At about 9.4 kyr B.P., a sudden change in the relative abundance of shallow to deep planktonic species probably indicates a sudden strengthening of the Kuroshio Current in the Okinawa Trough, which was synchronous with a rapid sea-level rise at 9.5-9.2 kyr B.P. in the ECS, Yellow Sea (YS) and South China Sea (SCS). The abundance of planktonic foraminiferal species, together with Mg/Ca based SST, exhibits millennial-scale oscillations during the Holocene, with 7 cold events (at about 1.7, 2.3-4.6, 6.2, 7.3, 8.2, 9.6, 10.6 cal. kyr BP) superimposed on a Holocene warming trend. This Holocene trend, together with centennial-scale SST variations superimposed on the last deglacial trend, suggests that both high and low latitude influences affected the climatology of the Okinawa Trough.
    Description: This study was supported by the National Natural Science Foundation of China (Grant Nos. 40206007, 40106006, 90211022 and 40506027), the Chinese Academy of Sciences innovation program (KZCX3-SW-220), and the NSF (OCE05-29600 to DWO).
    Keywords: Okinawa Trough ; Last deglaciation ; Holocene ; Planktonic foraminifera ; Sedimentation rate ; Kuroshio Current ; Millennial- scale climate changes ; Oxygen isotope
    Repository Name: Woods Hole Open Access Server
    Type: Preprint
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  • 5
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    Temperature calibration of Mg/Ca ratios in the intermediate water benthic foraminifer Hyalinea balthica (2011)
    American Geophysical Union and the Geochemical Society
    Details
    Publication Date: 2022-05-26
    Description: Author Posting. © American Geophysical Union, 2011. 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 12 (2011): Q04003, doi:10.1029/2010GC003333.
    Description: Core top samples from Indonesian and northeast Atlantic depth transects were used to calibrate Mg/Ca and δ18O in tests of the calcitic benthic foraminifer Hyalinea balthica to bottom water temperature between 4°C and 13°C. This shallow infaunal species is primarily abundant in neritic to upper bathyal sediments (〈600 m). Both linear and exponential calibrations suggest a temperature sensitivity of ~12% per °C that is ~4 times higher than observed in other species of deep-sea benthic foraminifera. Culture experiments support the core top calibration. We find no discernible effect of salinity and saturation on Mg/Ca. Comparison between the measured benthic foraminiferal δ18O and predicted equilibrium values suggests that on average H. balthica δ18O is 0.64‰ ± 0.13‰ lower than predicted from the equilibrium composition. To test the reliability of using paired H. balthica Mg/Ca and δ18O measurements for reconstructing seawater δ18Osw and salinity, we apply this calibration to another depth transect from Cape Ghir off NW Africa, which was not included in the calibration. Based on error analysis of the calibration data and this validation test, we show that the uncertainty of reconstructing bottom water temperature and salinity from paired Mg/Ca and δ18O measurements of H. balthica is better than ±0.7°C and ±0.69 practical salinity scale, respectively. The small uncertainties allow for the reconstruction of seawater density to better than 0.3σθ units, which is precise enough for the identification of specific water masses and reconstruction of changes in their properties. We propose that the relatively high Mg content and temperature sensitivity of H. balthica might be due to minor, biologically mediated contribution of high-Mg calcite to the primarily low Mg calcite test, which is influenced by the ambient temperature. This hypothesis, if correct, suggests that benthic species with relatively high Mg/Ca may be better suited for deepwater temperature reconstructions than species that have thus far been more commonly used.
    Description: This project was funded by NSF Awards OCE 02‐20922 and 09‐02977 to YR, OCE 09‐28607 to MK, OCE02‐20776 to DWO, and DFG priority program INTERDYNAMIK to AM.
    Keywords: Mg/Ca ; Benthic foraminifera ; Temperature calibration ; Isotope
    Repository Name: Woods Hole Open Access Server
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  • 6
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    Seawater cadmium in the Florida Straits over the Holocene and implications for Upper AMOC variability (2022)
    American Geophysical Union
    Details
    Publication Date: 2022-08-19
    Description: Author Posting. © American Geophysical Union, 2022. This article is posted here by permission of American Geophysical Union for personal use, not for redistribution. The definitive version was published in Paleoceanography and Paleoclimatology 37, (2022): e2021PA004379, https://doi.org/10.1029/2021pa004379.
    Description: Atlantic Meridional Overturning Circulation (AMOC) plays a central role in the global redistribution of heat and precipitation during both abrupt and longer-term climate shifts. Over the next century, AMOC is projected to weaken due to greenhouse gas warming, though projecting its future behavior is dependent on a better understanding of how AMOC changes are forced. Seeking to resolve an apparent contradiction of AMOC trends from paleorecords of the more recent past, we reconstruct seawater cadmium, a nutrient-like tracer, in the Florida Straits over the last ∼8,000 years, with emphasis on the last millennium. The gradual reduction in seawater Cd over the last 8,000 years could be due to a reduction in AMOC, consistent with cooling Northern Hemisphere temperatures and a southward shift of the Intertropical Convergence Zone. However, it is difficult to reconcile this finding with evidence for an increase in geostrophic flow through the Florida Straits over the same time period. We combine data from intermediate water depth sediment cores to extend this record into the Common Era at sufficient resolution to address the broad scale changes of this time period. There is a small decline in the Cd concentration in the Late Little Ice Age relative to the Medieval Climate Anomaly, but this change was much smaller than the changes observed over the Holocene and on the deglaciation. This suggests that any trend in the strength of AMOC over the last millennium must have been very subtle.
    Description: This work was funded by the NSF Graduate Research Fellowship DGE-1148903 (SV) and NSF grant OCE-1459563 and OCE-1851900 (JLS).
    Keywords: AMOC ; seawater cadmium ; Florida Straits ; Holocene ; Little Ice Age
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  • 7
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    Seawater isotope constraints on tropical hydrology during the Holocene (2010)
    American Geophysical Union
    Details
    Publication Date: 2022-05-26
    Description: Author Posting. © American Geophysical Union, 2007. This article is posted here by permission of American Geophysical Union for personal use, not for redistribution. The definitive version was published in Geophysical Research Letters 34 (2007): L13701, doi:10.1029/2007GL030017.
    Description: Paleoceanographic data from the low latitude Pacific Ocean provides evidence of changes in the freshwater budget and redistribution of freshwater within the basin during the Holocene. Reconstructed Holocene seawater δ 18O changes compare favorably to differences predicted between climate simulations for the middle Holocene (MH) and for the pre-Industrial late Holocene (LH). The model simulations demonstrate that changes in the tropical hydrologic cycle affect the relationship between δ 18Osw and surface salinity, and allow, for the first time, quantitative estimates of western Pacific salinity change during the Holocene. The simulations suggest that during the MH, the mean salinity of the Pacific was higher because less water vapor was transported from the Atlantic Ocean and more was transported to the Indian Ocean. The salinity of the western Pacific was enhanced further due both to the greater advection of salt to the region by ocean currents and to an increase in continental precipitation at the expense of maritime precipitation, the latter a consequence of the stronger Asian summer monsoon.
    Description: This work was supported by NSF grants ATM-0501241, ATM-0501351, and WHOI’s Ocean and Climate Change Institute.
    Keywords: Holocene ; Tropical Pacific ; Hydrology ; Paleoceanography ; Geochemical tracers ; Insolation forcing
    Repository Name: Woods Hole Open Access Server
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