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  • Benthic foraminifera  (3)
  • Little Ice Age  (3)
  • Paleoceanography  (3)
  • Deglaciation
  • AAIW
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  • 1
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    Corals record persistent multidecadal SST variability in the Atlantic Warm Pool since 1775 AD (2012)
    American Geophysical Union
    Details
    Publication Date: 2022-05-25
    Description: Author Posting. © American Geophysical Union, 2012. This article is posted here by permission of American Geophysical Union for personal use, not for redistribution. The definitive version was published in Paleoceanography 27 (2012): PA3231, doi:10.1029/2012PA002313.
    Description: Accurate low-latitude sea surface temperature (SST) records that predate the instrumental era are needed to put recent warming in the context of natural climate variability and to evaluate the persistence of lower frequency climate variability prior to the instrumental era and the possible influence of anthropogenic climate change on this variability. Here we present a 235-year-long SST reconstruction based on annual growth rates (linear extension) of three colonies of the Atlantic coral Siderastrea siderea sampled at two sites on the northeastern Yucatan Peninsula, Mexico, located within the Atlantic Warm Pool (AWP). AWP SSTs vary in concert the Atlantic Multidecadal Oscillation (AMO), a basin-wide, quasiperiodic (∼60–80 years) oscillation of North Atlantic SSTs. We demonstrate that the annual linear growth rates of all three coral colonies are significantly inversely correlated with SST. We calibrate annual linear growth rates to SST between 1900 and 1960 AD. The linear correlation coefficient over the calibration period is r = −0.77 and −0.66 over the instrumental record (1860–2008 AD). We apply our calibration to annual linear growth rates to extend the SST record to 1775 AD and show that multidecadal SST variability has been a persistent feature of the AWP, and likely, of the North Atlantic over this time period. Our results imply that tropical Atlantic SSTs remained within 1°C of modern values during the past 225 years, consistent with a previous reconstruction based on coral growth rates and with most estimates based on the Mg/Ca of planktonic foraminifera from marine sediments.
    Description: Funding was provided by a scholarship to L.F.V.B. from ‘Consejo Nacional de Ciencia y Tecnología’ (CONACyT-Mexico), by CONACyT projects 104358 and 23749 to P.B., and by NSF OCE-0926986 to A.L.C. and D.W.O.
    Description: 2013-03-29
    Keywords: Atlantic Warm Pool ; Atlantic multidecadal variability ; Little Ice Age ; Sr/Ca ; Coral ; Sea surface temperature
    Repository Name: Woods Hole Open Access Server
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  • 2
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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
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  • 3
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    Tropical Atlantic climate response to low-latitude and extratropical sea-surface temperature : a Little Ice Age perspective (2010)
    American Geophysical Union
    Details
    Publication Date: 2022-05-25
    Description: Author Posting. © American Geophysical Union, 2009. 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 36 (2009): L11703, doi:10.1029/2009GL038677.
    Description: Proxy reconstructions and model simulations suggest that steeper interhemispheric sea surface temperature (SST) gradients lead to southerly Intertropical Convergence Zone (ITCZ) migrations during periods of North Atlantic cooling, the most recent of which was the Little Ice Age (LIA; ∼100–450 yBP). Evidence suggesting low-latitude Atlantic cooling during the LIA was relatively small (〈1°C) raises the possibility that the ITCZ may have responded to a hemispheric SST gradient originating in the extratropics. We use an atmospheric general circulation model (AGCM) to investigate the relative influence of low-latitude and extratropical SSTs on the meridional position of the ITCZ. Our results suggest that the ITCZ responds primarily to local, low-latitude SST anomalies and that small cool anomalies (〈0.5°C) can reproduce the LIA precipitation pattern suggested by paleoclimate proxies. Conversely, even large extratropical cooling does not significantly impact low-latitude hydrology in the absence of ocean-atmosphere interaction.
    Description: This work was supported by NSF grants OCE 0623364 and ATM 033746 as well as the student research fund of MIT’s Department of Earth, Atmospheric and Planetary Science.
    Keywords: Climate ; ITCZ ; Little Ice Age
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  • 4
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    Deglacial δ18O and hydrologic variability in the tropical Pacific and Indian Oceans (2014)
    Elsevier
    Details
    Publication Date: 2022-05-25
    Description: © The Author(s), 2013. This article is distributed under the terms of the Creative Commons Attribution License. The definitive version was published in Earth and Planetary Science Letters 387 (2014): 240–251, doi:10.1016/j.epsl.2013.11.032.
    Description: Evidence from geologic archives suggests that there were large changes in the tropical hydrologic cycle associated with the two prominent northern hemisphere deglacial cooling events, Heinrich Stadial 1 (HS1; ∼19 to 15 kyr BP; kyr BP = 1000 yr before present) and the Younger Dryas (∼12.9 to 11.7 kyr BP). These hydrologic shifts have been alternatively attributed to high and low latitude origin. Here, we present a new record of hydrologic variability based on planktic foraminifera-derived δ18O of seawater (δ18Osw) estimates from a sediment core from the tropical Eastern Indian Ocean, and using 12 additional δ18Osw records, construct a single record of the dominant mode of tropical Eastern Equatorial Pacific and Indo-Pacific Warm Pool (IPWP) hydrologic variability. We show that deglacial hydrologic shifts parallel variations in the reconstructed interhemispheric temperature gradient, suggesting a strong response to variations in the Atlantic Meridional Overturning Circulation and the attendant heat redistribution. A transient model simulation of the last deglaciation suggests that hydrologic changes, including a southward shift in the Intertropical Convergence Zone (ITCZ) which likely occurred during these northern hemisphere cold events, coupled with oceanic advection and mixing, resulted in increased salinity in the Indonesian region of the IPWP and the eastern tropical Pacific, which is recorded by the δ18Osw proxy. Based on our observations and modeling results we suggest the interhemispheric temperature gradient directly controls the tropical hydrologic cycle on these time scales, which in turn mediates poleward atmospheric heat transport.
    Description: ThisworkwasfundedbytheNationalScienceFoundation;theOceanandClimateChangeInstituteandtheAcademicProgramsOfficeatWoodsHoleOceano-graphicInstitution;BMBF(PABESIA);andDFG(He3412/15-1)
    Keywords: Indo-Pacific ; Eastern Equatorial Pacific ; δ18O of seawater ; Deglaciation ; Heat transport
    Repository Name: Woods Hole Open Access Server
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  • 5
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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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  • 6
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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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  • 7
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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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  • 8
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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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  • 9
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    Comparison of equatorial Pacific sea surface temperature variability and trends with Sr/Ca records from multiple corals (2016)
    John Wiley & Sons
    Details
    Publication Date: 2022-05-26
    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): 252–265, doi:10.1002/2015PA002897.
    Description: Coral Sr/Ca is widely used to reconstruct past ocean temperatures. However, some studies report different Sr/Ca-temperature relationships for conspecifics on the same reef, with profound implications for interpretation of reconstructed temperatures. We assess whether these differences are attributable to small-scale oceanographic variability or “vital effects” associated with coral calcification and quantify the effect of intercolony differences on temperature estimates and uncertainties. Sr/Ca records from four massive Porites colonies growing on the east and west sides of Jarvis Island, central equatorial Pacific, were compared with in situ logger temperatures spanning 2002–2012. In general, Sr/Ca captured the occurrence of interannual sea surface temperature events but their amplitude was not consistently recorded by any of the corals. No long-term trend was identified in the instrumental data, yet Sr/Ca of one coral implied a statistically significant cooling trend while that of its neighbor implied a warming trend. Slopes of Sr/Ca-temperature regressions from the four different colonies were within error, but offsets in mean Sr/Ca rendered the regressions statistically distinct. Assuming that these relationships represent the full range of Sr/Ca-temperature calibrations in Jarvis Porites, we assessed how well Sr/Ca of a nonliving coral with an unknown Sr/Ca-temperature relationship can constrain past temperatures. Our results indicate that standard error of prediction methods underestimate the actual error as we could not reliably reconstruct the amplitude or frequency of El Niño–Southern Oscillation events as large as ± 2°C. Our results underscore the importance of characterizing the full range of temperature-Sr/Ca relationships at each study site to estimate true error.
    Description: This study was supported by an NSF Graduate Research Fellowship to A.A. and by NSF-OCE-0926986 and NSF-OCE-1031971.
    Description: 2016-08-06
    Keywords: Corals ; Paleoceanography ; Proxies
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  • 10
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    Stability of North Atlantic water masses in face of pronounced climate variability during the Pleistocene (2010)
    American Geophysical Union
    Details
    Publication Date: 2022-05-26
    Description: Author Posting. © American Geophysical Union, 2004. This article is posted here by permission of American Geophysical Union for personal use, not for redistribution. The definitive version was published in Paleoceanography 19 (2004): PA2008, doi:10.1029/2003PA000921.
    Description: Geochemical profiles from the North Atlantic Ocean suggest that the vertical δ13C structure of the water column at intermediate depths did not change significantly between glacial and interglacial time over much of the Pleistocene, despite large changes in ice volume and iceberg delivery from nearby landmasses. The most anomalous δ13C profiles are from the extreme interglaciations of the late Pleistocene. This compilation of data suggests that, unlike today (an extreme interglaciation), the two primary sources of northern deep water, Norwegian-Greenland Sea and Labrador Sea/subpolar North Atlantic, had different characteristic δ13C values over most of the Pleistocene. We speculate that the current open sea ice conditions in the Norwegian-Greenland Sea are a relatively rare occurrence and that the high-δ13C deep water that forms in this region today is geologically unusual. If northern source deep waters can have highly variable δ13C, then this likelihood must be considered when inferring past circulation changes from benthic δ13C records.
    Description: National Science Foundation grants OCE-0118005 and OCE-0118001, which supported MER and DWO.
    Keywords: Paleoceanography ; North Atlantic Deep Water ; Pleistocene
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