Radiocarbon evidence for early deglacial changes in deep ocean upwelling near the Antarctic Divergence Zone in the Atlantic sector of the Southern Ocean

Antarctic ice core evidence indicates that atmospheric CO₂ levels increased during Heinrich Stadial (HS) 1 and the Younger Dryas (YD) during the last deglaciation. A substantial fraction of this carbon is believed to have stemmed from the ocean interior, released, in part, through enhanced wind-driven upwelling and air-sea CO₂ exchange in the Southern Ocean. This was highlighted by two deglacial opal flux peaks identified in sediment core TN057-13-PC4 (53.17 °S, 5.13 °E, 2818 m water depth) from the Atlantic Southern Ocean south of the Polar Front, proximal to the Antarctic Divergence Zone (Anderson et al., 2009). However, there is limited information on changes in deep-ocean ¹⁴C ventilation and surface ocean hydrography in the Atlantic Antarctic Divergence, and their role in atmospheric CO₂ variations during these two periods of deglacial CO₂ rise. Here, we provide a new set of 12 mixed-benthic and 63 planktonic foraminiferal (i.e., Neogloboquadrina pachyderma) ¹⁴C ages obtained with a MIni-CArbon-DAting-System (MICADAS) in sediment core TN057-13-PC4, along with high-resolution multi-proxy (sub-)sea surface temperature reconstructions for the same site (N. pachyderma Mg/Ca ratios, TEX₈₆, diatom assemblages). Our data help better constrain the nature, timing, and impacts of deep-ocean upwelling on surface ocean hydrography and on atmospheric CO₂ exchange near the Antarctic Divergence of the Southern Ocean. Our data show strong (sub-)surface warming in the Antarctic Divergence during HS1 and YD that is accompanied by a rapid decline in benthic-minus-planktic ¹⁴C ages towards mean Holocene values at the onset of the deglaciation. We also observe millennial-scale increases in seawater d¹⁸O (paired N. pachyderma Mg/Ca-d¹⁸O analyses), hence local surface salinity and marked variations in ¹⁴C surface ocean reservoir ages that parallel changes in Antarctic sea ice extent. This corroborates previous evidence indicating increased upwelling of Circumpolar Deep Water in the Atlantic Antarctic Divergence during HS1 and YD, yet suggests an onset of strong Southern Ocean ventilation earlier than what is expected from increases in opal fluxes alone. Our data support a fundamental role of upwelling and CO₂ outgassing in the Antarctic Divergence of the Southern Ocean in the two-step atmospheric CO₂ rise during the last deglaciation, and further suggest that possible variations in CO₂ solubility and sea-ice retreat amplified the effects of physical circulation changes on Southern Ocean air-sea CO₂ exchange.

References: Anderson, R.F., Ali, S., Bradtmiller, L.I., Nielsen, S.H.H., Fleisher, M.Q., Anderson, B., Burckle, L.H., 2009. Wind-driven upwelling in the Southern Ocean and the deglacial rise in atmospheric CO₂. Science 323, 1443–1448. doi: 10.1126/science.1167441