Synergetic impact of Southern Ocean ventilation and -export production on atmospheric CO2 during the last deglacial and glacial periods

Abstract:

Millennial climate changes during the last deglacial and glacial periods were accompanied by rapid changes in atmospheric CO₂ (CO_2,atm) that still remain unexplained. While the role of the Southern Ocean as major control of ocean-atmosphere CO₂ exchange has often been emphasized, it has been debated whether millennial-scale rises in CO_2,atm are linked with variations in biological export productivity in the sub-Antarctic region, possibly driven by fluctuations in airborne dust supply, or with variations in vertical mixing in the Antarctic region, possibly driven by changes in westerly wind stress or density stratification of the water column. We present high-resolution multi-proxy bottom water [O₂]- (closely linked to dissolved organic carbon concentrations in the ocean), biological export production- and ¹⁴C ventilation age reconstructions in the sub-Antarctic Atlantic sediment core MD07-3076Q (14°13.7’W, 44°9.2’S, 3770 m water depth), to provide new insights into the role of the Southern Ocean in the global carbon cycle from a deep-sea perspective. Our new data show that millennial-scale changes in CO_2,atm during the last 70,000 years were paralleled by decreases in deep-ocean carbon storage, which were linked with increases in deep-ocean ventilation of the Southern Ocean carbon pool (via southern-sourced water masses) and decreases in the export of carbon to the deep sub-Antarctic Atlantic. Reconstructed bottom water [O₂] changes indicate a deep-ocean carbon loss of 509±68 Gt C during the early deglaciation and 292±111 Gt C during mid-glacial events, when extrapolated to the global deep ocean below 3 km, which may fully account for the observed CO_2,atm changes. We propose that the Southern Ocean's 'organic carbon pump' has exerted a tight control on CO_2,atm and global climate specifically via a synergy of both physical (e.g. ocean dynamics, and air-sea CO₂ exchange) and biological processes (e.g. export productivity).