The Southern Ocean’s role in the global carbon cycle over the last 800 kyr constrained using reconstructions of the CO2 system

The critical role of the Southern Ocean in controlling the Pleistocene atmospheric CO2 oscillations is widely acknowledged. However, owing to sampling difficulties surrounding Antarctica, the underlying mechanism and associated pathway of ocean-atmosphere CO2 exchange in the Antarctic Zone (AZ) of the Southern Ocean remains mysterious. Here, we present a new planktonic δ11B record from sediment core PS1506 (68.73°S, 5.85°W) that tracks the pH and surface pCO2 of the AZ over the last 8 glacial cycles. These data are complemented by benthic B/Ca and carbonate preservation indices; due to the location of this core on the continental margin of the eastern Weddell Sea, these data allow us to track the source CO2 chemistry of the dense Antarctic waters that feed the ocean’s lower overturing cell. We find coherent CO2 change between surface and deep waters, indicating persistent formation of AABW that transfers Antarctic surface water signals to depth. Critically, we discover abrupt AZ CO2 decline during glacial onset conditions, coinciding with initial atmospheric CO2 drawdown, highlighting the AZ’s key control on glacial-interglacial CO2 change. After assessing proposed drivers, our findings implicate shifts in Southern Ocean circulation linked to changes in sea ice and/or the Southern Hemisphere westerlies in this glacial onset CO2 change, while productivity, solubility, and sea ice 'lid' effects appear insignificant or counterproductive in this region and time interval. Overall, these reconstructed CO2 system dynamics provide critical insights into Southern Ocean carbon cycling and the associated influence on the atmosphere.