Evolution of South Atlantic density and chemical stratification across the last deglaciation.

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Skinner, Luke C 
Peck, Victoria L 
Kender, Sev 

Explanations of the glacial-interglacial variations in atmospheric pCO2 invoke a significant role for the deep ocean in the storage of CO2. Deep-ocean density stratification has been proposed as a mechanism to promote the storage of CO2 in the deep ocean during glacial times. A wealth of proxy data supports the presence of a "chemical divide" between intermediate and deep water in the glacial Atlantic Ocean, which indirectly points to an increase in deep-ocean density stratification. However, direct observational evidence of changes in the primary controls of ocean density stratification, i.e., temperature and salinity, remain scarce. Here, we use Mg/Ca-derived seawater temperature and salinity estimates determined from temperature-corrected δ(18)O measurements on the benthic foraminifer Uvigerina spp. from deep and intermediate water-depth marine sediment cores to reconstruct the changes in density of sub-Antarctic South Atlantic water masses over the last deglaciation (i.e., 22-2 ka before present). We find that a major breakdown in the physical density stratification significantly lags the breakdown of the deep-intermediate chemical divide, as indicated by the chemical tracers of benthic foraminifer δ(13)C and foraminifer/coral (14)C. Our results indicate that chemical destratification likely resulted in the first rise in atmospheric pCO2, whereas the density destratification of the deep South Atlantic lags the second rise in atmospheric pCO2 during the late deglacial period. Our findings emphasize that the physical and chemical destratification of the ocean are not as tightly coupled as generally assumed.

South Atlantic, atmospheric CO2, density gradient, last deglaciation, ocean stratification
Journal Title
Proc Natl Acad Sci U S A
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Proceedings of the National Academy of Sciences
Natural Environment Research Council (NE/J010545/1)
European Research Council (267931)
We are grateful to I. Mather, J. Rolfe, F. Dewilde and G. Isguder for preparing and performing isotopic analyses, as well as C. Daunt, S. Souanef-Ureta and M. Greaves for technical assistance in performing trace element analysis. J.R. was funded jointly by the British Geological Survey/British Antarctic Survey (Natural Environment Research Council) and the University of Cambridge. J.G. was funded by the Gates Cambridge Trust. L.C.S. acknowledges support from the Royal Society and NERC grant NE/J010545/1. C.W. acknowledges support from the European Research Council grant ACCLIMATE/no 339108. This is LSCE contribution 5514. This work was funded (in part) by the European Research Council (ERC grant 2010-NEWLOG ADG-267931 HE). N.V.R. acknowledges support from EU RTN NICE (no. 36127). We thank the captain and crew of the RRS James Clark Ross for facilitating the collection of the marine sediment core GC528.