Evolution of South Atlantic Density and Chemical Stratification across the Last Deglaciation

Abstract:

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 exists supporting the idea 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.

In this study, we present proxy reconstructions showing the evolution of the physical density gradient in high-latitude South Atlantic over the last deglaciation. Mg/Ca-derived seawater temperature estimates combined with salinity estimates determined from temperature-corrected δ¹⁸O measurements on the benthic foraminifer Uvigerina spp. from deep and intermediate water-depth marine sediment cores are used to reconstruct the changes in density of sub-Antarctic South Atlantic water masses over the last deglaciation. Comparison is made between the evolution of the density gradient and benthic δ¹³C and ¹⁴C records from the two sites in order to assess the hypothesis of a causal link between the physical and chemical properties of the deglacial ocean.

Our records suggest that the major breakdown in the physical density stratification significantly lags the breakdown of the deep-intermediate chemical divide. The main chemical destratification event coincided with the early deglacial increase in atmospheric pCO₂, whereas the density destratification of the deep South Atlantic occurred at the onset of the Holocene. Our findings emphasise that the physical and chemical destratification of the ocean are not as tightly coupled as generally assumed. We argue that at least in the South Atlantic, physical ocean density destratification was not a major driver of atmospheric CO₂ change during the last glacial termination.