Meridional overturning circulation in the South Atlantic at the last glacial maximum

Lynch-Stieglitz, J, Curry WB, Oppo DW, Ninneman US, Charles CD, Munson J.  2006.  Meridional overturning circulation in the South Atlantic at the last glacial maximum. Geochemistry Geophysics Geosystems. 7

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abyssal ocean, benthic foraminifera, circulation, deep-water circulation, global ocean, heat-transport, isotope composition, last glacial maximum, meridional overturning, north-atlantic, ocean circulation, oceanography :, oxygen isotopes, paleoceanography : glacial, paleoceanography : thermohaline, physical : general circulation, South Atlantic, thermohaline circulation, variability


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 degrees 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 delta(18)O 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 delta(18)O to those on the western margin between 1 and 2 km and that the gradient was reversed relative to today with the higher delta(18)O values in the western margin benthic foraminifera above 1 km. If this reversal in benthic foraminifera delta(18)O 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.






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