Description: Decades ago, the analysis of ancient air – trapped deep inside of Antarctic glaciers – revealed an astonishing pattern of atmospheric CO2. Ever since we’ve first laid our eyes on these intriguing signals– alternating between glacial lows and interglacial highs – an overall question emerged: Where was the CO2 stored during the glacials and how was it released back to the atmosphere during deglacial transitions? In general, several carbon reservoirs like the terrestrial biosphere or permafrost soils might interact with, and drive the atmosphere on these glacial-interglacial timescales. By far the largest influence however, might come from the deep ocean. Today, this reservoir stores up to 60-times the carbon, of which is stored in the entire atmosphere. Hence, tiny changes in the oceanic C-cycle might have severe ramifications for atmospheric CO2-levels. Parallel to global CO2 atm, Antarctic temperatures rose, while the expanded ice shelves suffered from a massive deglacial collapse. The timing and succession of events pointed to the climatic role of the Southern Hemisphere in general and the Southern Ocean in particular and raised a second question: What was the physical process, which connected these deglacial events? Until today, the international community compiled numerous studies from terrestrial and marine (distal and proximal) archives to shed light onto this dynamic system. These studies revealed a closely connected system between Antarctic sea ice and ice shelves, deep-water and atmospheric circulation, oceanic stratification, the biological pump and also bipolar teleconnections. Here, we want to discuss the current scientific knowledge and present new isotopic data – measured on planktic and benthic foraminifers as well as bulk sediments – that show how Southern Ocean overturning circulation changed on glacial-interglacial timescales and influenced the residence times of circumpolar deep-waters as well as their transport onto the continental shelf regions. In combination with other parameters, the deglacial increase in Southern Ocean overturning represents a plausible link that might explain the parallel evolution of several deglacial climate parameters.

Description: 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.