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  • Oxygen isotopes  (5)
  • Paleoceanography  (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
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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
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  • 3
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    Asynchronous warming and δ18O evolution of deep Atlantic water masses during the last deglaciation (2017)
    Details
    Publication Date: 2022-05-25
    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 Proceedings of the National Academy of Sciences of the United States of America 114 (2017): 11075-11080, doi: 10.1073/pnas.1704512114.
    Description: The large-scale reorganization of deep-ocean circulation in the Atlantic involving changes in North Atlantic Deep Water (NADW) and Antarctic Bottom Water (AABW) played a critical role in regulating hemispheric and global climate during the last deglaciation. However, changes in the relative contributions of NADW and AABW and their properties are poorly constrained by marine records, including δ18O of benthic foraminiferal calcite (δ18Oc). Here we use an isotope-enabled ocean general circulation model with realistic geometry and forcing conditions to simulate the deglacial water mass and δ18O evolution. Model results suggest that in response to North Atlantic freshwater forcing during the early phase of the last deglaciation, NADW nearly collapses while AABW mildly weakens. Rather than reflecting changes in NADW or AABW properties due to freshwater input as suggested previously, the observed phasing difference of deep δ18Oc likely reflects early warming of the deep northern North Atlantic by ~1.4°C while deep Southern Ocean temperature remains largely unchanged. We propose a thermodynamic mechanism to explain the early warming in the North Atlantic, featuring a strong mid-depth warming and enhanced downward heat flux via vertical mixing. Our results emphasize that the way ocean circulation affects heat, a dynamic tracer, is considerably different than how it affects passive tracers like δ18O, and call for caution when inferring water mass changes from δ18Oc records while assuming uniform changes in deep temperatures.
    Description: This work is supported by the U.S. NSF P2C2 projects (1401778 and 1401802) and OCE projects (1600080 and 1566432), China NSFC 41630527, and the Wisconsin Alumni Research Foundation
    Keywords: Atlantic water masses ; Last deglaciation ; Oxygen isotopes ; Deep ocean warming
    Repository Name: Woods Hole Open Access Server
    Type: Preprint
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  • 4
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    Reconstructing the thermal structure of the upper ocean : insights from planktic foraminifera shell chemistry and alkenones in modern sediments of the tropical eastern Indian Ocean (2011)
    American Geophysical Union
    Details
    Publication Date: 2022-05-25
    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 Paleoceanography 26 (2011): PA3219, doi:10.1029/2011PA002132.
    Description: Shell chemistry of planktic foraminifera and the alkenone unsaturation index in 69 surface sediment samples in the tropical eastern Indian Ocean off West and South Indonesia were studied. Results were compared to modern hydrographic data in order to assess how modern environmental conditions are preserved in sedimentary record, and to determine the best possible proxies to reconstruct seasonality, thermal gradient and upper water column characteristics in this part of the world ocean. Our results imply that alkenone-derived temperatures record annual mean temperatures in the study area. However, this finding might be an artifact due to the temperature limitation of this proxy above 28°C. Combined study of shell stable oxygen isotope and Mg/Ca ratio of planktic foraminifera suggests that Globigerinoides ruber sensu stricto (s.s.), G. ruber sensu lato (s.l.), and G. sacculifer calcify within the mixed-layer between 20 m and 50 m, whereas Globigerina bulloides records mixed-layer conditions at ∼50 m depth during boreal summer. Mean calcifications of Pulleniatina obliquiloculata, Neogloboquadrina dutertrei, and Globorotalia tumida occur at the top of the thermocline during boreal summer, at ∼75 m, 75–100 m, and 100 m, respectively. Shell Mg/Ca ratios of all species show a significant correlation with temperature at their apparent calcification depths and validate the application of previously published temperature calibrations, except for G. tumida that requires a regional Mg/Ca-temperature calibration (Mg/Ca = 0.41 exp (0.068*T)). We show that the difference in Mg/Ca-temperatures of the mixed-layer species and the thermocline species, particularly between G. ruber s.s. (or s.l.) and P. obliquiloculata, can be applied to track changes in the upper water column stratification. Our results provide critical tools for reconstructing past changes in the hydrography of the study area and their relation to monsoon, El Niño-Southern Oscillation, and the Indian Ocean Dipole Mode.
    Description: This project was funded by the German Ministry of Education and Research (BMBF project PABESIA) and the “Deutsche Forschungsgemeinschaft” (DFG project HE 3412/15–1).
    Keywords: Indian Ocean ; Mg/Ca ; Alkenone ; Oxygen isotopes ; Planktic foraminifera ; Thermal structure
    Repository Name: Woods Hole Open Access Server
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  • 5
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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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  • 6
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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
    Repository Name: Woods Hole Open Access Server
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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
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  • 8
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    Stable oxygen isotopes and Mg/Ca in planktic foraminifera from modern surface sediments of the Western Pacific Warm Pool : implications for thermocline reconstructions (2018)
    John Wiley & Sons
    Details
    Publication Date: 2022-05-26
    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): 1174–1194, doi:10.1002/2017PA003122.
    Description: Mg/Ca and stable oxygen isotope compositions (δ18O) of planktic foraminifera tests are commonly used as proxies to reconstruct past ocean conditions including variations in the vertical water column structure. Accurate proxy calibrations require thorough regional studies, since parameters such as calcification depth and temperature of planktic foraminifera depend on local environmental conditions. Here we present radiocarbon-dated, modern surface sediment samples and water column data (temperature, salinity, and seawater δ18O) from the Western Pacific Warm Pool. Seawater δ18O (δ18OSW) and salinity are used to calculate individual regressions for western Pacific surface and thermocline waters (δ18OSW = 0.37 × S-12.4 and δ18OSW = 0.33 × S-11.0). We combine shell δ18O and Mg/Ca with water column data to estimate calcification depths of several planktic foraminifera and establish regional Mg/Ca-temperature calibrations. Globigerinoides ruber, Globigerinoides elongatus, and Globigerinoides sacculifer reflect mixed layer conditions. Pulleniatina obliquiloculata and Neogloboquadrina dutertrei and Globorotalia tumida preserve upper and lower thermocline conditions, respectively. Our multispecies Mg/Ca-temperature calibration (Mg/Ca = 0.26exp0.097*T) matches published regressions. Assuming the same temperature sensitivity in all species, we propose species-specific calibrations that can be used to reconstruct upper water column temperatures. The Mg/Ca temperature dependencies of G. ruber, G. elongatus, and G. tumida are similar to published equations. However, our data imply that calcification temperatures of G. sacculifer, P. obliquiloculata, and N. dutertrei are exceptionally warm in the western tropical Pacific and thus underestimated by previously published calibrations. Regional Mg/Ca-temperature relations are best described by Mg/Ca = 0.24exp0.097*T for G. sacculifer and by Mg/Ca = 0.21exp0.097*T for P. obliquiloculata and N. dutertrei.
    Description: Bundesministerium für Bildung und Forschung (BMBF) Grant Number: 03G0228A; National Science Foundation (NSF) Grant Number: OCE1131371; DFG-Research Center/Cluster of Excellence “The Ocean in the Earth System”
    Description: 2018-05-09
    Keywords: Western Pacific Warm Pool ; Mg/Ca calibration ; Oxygen isotopes ; Planktic foraminifera ; Thermocline reconstruction
    Repository Name: Woods Hole Open Access Server
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