Stepwise Glacial Inception and Prolonged Southern Ocean CO ₂ Outgassing Linked With Marine Isotope Stage 11

Abstract The climatic optimum of Marine Isotope Stage (MIS) 11, MIS 11c, was a remarkably long warmer‐than‐present climate interval with prolonged elevated atmospheric CO 2 levels. While the Pacific Southern Ocean has a strong leverage on atmospheric CO 2 through oceanic CO 2 release, past changes in carbon cycle dynamics in this region and their influence on atmospheric CO 2 during MIS 11 and the glacial inception to MIS 10 remain poorly constrained. Here, we assess changes in vertical mixing (planktic vs. benthic foraminiferal stable carbon isotope gradients) and respired carbon storage ( Cibicides kullenbergi B/Ca) in the central Pacific Southern Ocean at International Ocean Discovery Program Site U1540 for MIS 11 and the onset of glacial MIS 10, complemented with previous reconstructions of deep ocean oxygenation (foraminiferal U/Mn), ocean temperature ( Uvigerina spp. Mg/Ca, Neogloboquadrina pachyderma abundances) and ocean circulation (foraminiferal/fish debris Nd isotopes). Our data reveal a temporal offset between ocean cooling/global ice sheet growth and Southern Ocean carbon storage changes after peak MIS 11c warmth. This coincides with minimum obliquity, which favors southern‐hemisphere westerly wind (SWW)‐driven upwelling of CO 2 ‐rich waters in the Southern Ocean and CO 2 evasion (via obliquity control on meridional pressure gradients and SWW position/strength), explaining high sustained atmospheric CO 2 levels during MIS 11c. The glacial inception following peak MIS 11c thus shows opposing effects of low obliquity favoring ice sheet growth through polar cooling and poleward moisture supply, yet maintaining high atmospheric CO 2 levels through continued Southern Ocean CO 2 outgassing that likely decelerated global ice sheet growth.