Ninnemann Ulysses S.

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Ninnemann
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Ulysses S.
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  • Article
    Rapid switches in subpolar North Atlantic hydrography and climate during the Last Interglacial (MIS 5e)
    (American Geophysical Union, 2012-05-12) Irvalı, Nil ; Ninnemann, Ulysses S. ; Galaasen, Eirik V. ; Rosenthal, Yair ; Kroon, Dick ; Oppo, Delia W. ; Kleiven, Helga F. ; Darling, Kate F. ; Kissel, Catherine
    At the peak of the previous interglacial period, North Atlantic and subpolar climate shared many features in common with projections of our future climate, including warmer-than-present conditions and a diminished Greenland Ice Sheet (GIS). Here we portray changes in North Atlantic hydrography linked with Greenland climate during Marine Isotope Stage (MIS) 5e using (sub)centennially sampled records of planktonic foraminiferal isotopes and assemblage counts and ice-rafted debris counts, as well as modern analog technique and Mg/Ca-based paleothermometry. We use the core MD03-2664 recovered from a high accumulation rate site (∼34 cm/kyr) on the Eirik sediment drift (57°26.34′N, 48°36.35′W). The results indicate that surface waters off southern Greenland were ∼3–5°C warmer than today during early MIS 5e. These anomalously warm sea surface temperatures (SSTs) prevailed until the isotopic peak of MIS 5e when they were interrupted by a cooling event beginning at ∼126 kyr BP. This interglacial cooling event is followed by a gradual warming with SSTs subsequently plateauing just below early MIS 5e values. A planktonic δ18O minimum during the cooling event indicates that marked freshening of the surface waters accompanied the cooling. We suggest that switches in the subpolar gyre hydrography occurred during a warmer climate, involving regional changes in freshwater fluxes/balance and East Greenland Current influence in the study area. The nature of these hydrographic transitions suggests that they are most likely related to large-scale circulation dynamics, potentially amplified by GIS meltwater influences.
  • Article
    Meridional overturning circulation in the South Atlantic at the last glacial maximum
    (American Geophysical Union, 2006-10-03) Lynch-Stieglitz, Jean ; Curry, William B. ; Oppo, Delia W. ; Ninnemann, Ulysses S. ; Charles, Christopher D. ; Munson, Jenna
    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.