Circulation patterns of the glacial Southern Ocean

Better insight into the glacial whereabouts of CO2 is crucial to achieve a comprehensive overview of processes driving the global carbon cycle. Opposing patterns of increasing atmospheric CO2 and decreasing Δ14C values point toward the release of ancient 14C-depleted CO2 during Heinrich Stadial 1 (HS1) and the Younger Dryas (YD). Today, the deep ocean below ~2000 m, stores up to 60-times more carbon than the atmosphere and is therefore considered to be a major driver of the atmospheric CO2 pattern, storing CO2 during glacials, releasing it during deglacial transitions. Throughout the last deglaciation, two pronounced phases show a collapse of the Atlantic Meridional Overturning Circulation (AMOC) that coincided with the HS1 and YD rise in atmospheric CO2. These collapses are thought to have reduced the efficiency of the biological pump and might have therefore contributed to rising CO2-levels. Yet, these AMOC-induced changes in the biological pump fail to explain the significant drop in atmospheric Δ14C. In this respect, several studies point to the presence of very old, 14C-depleted deep-waters in the deep glacial Southern Ocean, which rejuvenated during the last termination. However, to allow for the aging of water masses and thus the accumulation of CO2 in the deep Southern Ocean, circulation patterns must have been significantly different from today’s layout. Here we present a multiproxy approach to reconstruct circulation patterns of glacial Southern Ocean intermediate- and deep-waters and their evolution throughout the deglacial period. Our data point to a significant slowdown of Southern Ocean overturning throughout the glacial, which might have culminated in the accumulation of CO2 in Circumpolar Deep Waters. Parallel to rising atmospheric CO2-levels, the deep water masses pick up their pace, ultimately transporting the 14C-depleted deep-waters to the surface, where their load of ancient CO2 could be released back to the atmosphere