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

Here we use high-precision carbon isotope data (δ 13 C-CO 2 ) to show atmospheric CO 2 during Marine Isotope Stage 4 (MIS 4, ~70.5-59 ka) was controlled by a succession of millennial-scale processes. Enriched δ 13 C-CO 2 during peak glaciation suggests increased ocean carbon storage. Variations in δ 13 C-CO 2 in early MIS 4 suggest multiple processes were active during CO 2 drawdown, potentially including decreased land carbon and decreased Southern Ocean air-sea gas exchange superposed on increased ocean carbon storage. CO 2 remained low during MIS 4 while δ 13 C-CO 2 fluctuations suggest changes in Southern Ocean and North Atlantic air-sea gas exchange. A 7 ppm increase in CO 2 at the onset of Dansgaard-Oeschger event 19 (72.1 ka) and 27 ppm increase in CO 2 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. Summary for general audience: We used carbon stable isotope data from an Antarctic ice core to evaluate which mechanisms caused changes in atmospheric carbon dioxide 74-59 thousand years ago, including a ~40 ppm decrease at the beginning of the last ice age.