Multiple carbon cycle mechanisms associated with the glaciation of Marine Isotope Stage 4

Here we use high-precision carbon isotope data (δ¹³C-CO₂) to show atmospheric CO₂ during Marine Isotope Stage 4 (MIS 4, ~70.5-59 ka) was controlled by a succession of millennial-scale processes. Enriched δ¹³C-CO₂ during peak glaciation suggests increased ocean carbon storage. Variations in δ¹³C-CO₂ in early MIS 4 suggest multiple processes were active during CO₂ drawdown, potentially including decreased land carbon and decreased Southern Ocean air-sea gas exchange superposed on increased ocean carbon storage. CO₂ remained low during MIS 4 while δ¹³C-CO₂ fluctuations suggest changes in Southern Ocean and North Atlantic air-sea gas exchange. A 7 ppm increase in CO₂ at the onset of Dansgaard-Oeschger event 19 (72.1 ka) and 27 ppm increase in CO₂ during late MIS 4 (Heinrich Stadial 6, ~63.5-60 ka) involved additions of isotopically light carbon to the atmosphere. The terrestrial biosphere and Southern Ocean air-sea gas exchange are possible sources, with the latter event also involving decreased ocean carbon storage.