Mechanisms of atmospheric CO2 drawdown during Marine Isotope Stage 4 based on Atlantic deep-water temperature and bottom-water oxygenation reconstructions

Understanding the evolution of deep ocean circulation and chemistry over the last glacial cycle is key to elucidating the ocean’s role in modulating atmospheric CO₂ changes on millennial and orbital timescales. MIS 4 is a key paleoclimatic interval of the last glacial inception for assessing the role of the deep-ocean carbon storage in driving atmospheric CO₂ levels, because it is characterized by a large decrease of air temperature and a rapid atmospheric CO₂ drop of ~40 ppmv, and includes several millennial climatic events, for example Heinrich Stadial 6. Although various paleo proxy records suggest a weakened Atlantic overturning during MIS 4, and particularly HS 6, changes in AMOC strength and the geometric extent of NADW shoaling remain poorly understood. Here, we present deep-water temperature reconstructions based on infaunal benthic foraminiferal Mg/Ca ratios and bottom water oxygen concentration reconstructions using redox-sensitive foraminiferal U/Ca, from the deep North (~2.65km) and South (~3.8km) Atlantic to assess the changes in deep water hydrography and by extension circulation.

Our reconstructed deep-water temperature changes from the Iberian Margin (~2.65 km water depth) suggest a stronger influence of colder southern sourced waters during MIS 4 and particularly during HS 6; and a clear subsurface warming during MIS 5a stadials. Meanwhile, changes in deep-water temperatures in the Atlantic Sector of the Southern Ocean (SO) closely follow variations in Antarctic temperature, atmospheric CO₂ and the mean ocean temperature, likely mediated by buoyancy forcing in the SO, which is in turn likely linked to sea-ice expansion at the MIS 5a/4 transition. Together with (arguably smaller) contributions from reduced air-sea gas exchange efficiency in the SO, these combined changes would have lowered atmospheric CO₂through more efficient carbon sequestration in an expanded deep Atlantic reservoir during MIS 4, through their impact on the solubility- and soft tissue “pumps” (i.e. the ocean’s disequilibrium and respired carbon budgets). Indeed, bottom water oxygenation reconstructions from the South Atlantic support the conclusion that the Southern Ocean appears to have represented a significant reservoir for sequestering CO₂ away from the atmosphere during MIS 4.