Subantarctic hydrography and intermediate-water carbon sequestration during MIS 11

Marine Isotope Stage 11 (MIS 11; ~424–374 ka) was an exceptionally long and warm interglacial, characterized by a ~30 kyr plateau in atmospheric CO₂ that remains poorly understood. Here we present new multiproxy evidence from the Pacific sector of the Southern Ocean indicating that MIS 11 operated under a distinct mode of carbon cycling dominated by intermediate-water processes. Stable isotope and Mg/Ca records from 54ºS and 45ºS reveal a pronounced δ¹³C enrichment in thermocline waters, contrasted by a muted surface expression at 54°S, indicating strong vertical decoupling in the transmission of ventilation signals. We interpret this pattern as consistent with enhanced ventilation and export of Antarctic Intermediate Water (AAIW), which efficiently transmitted high-δ¹³C signals into the Pacific thermocline, while anomalously warm and saline Subantarctic Mode Water (SAMW) at the surface limited the upward expression of these signals. Hydrographic reconstructions further indicate an unusual density structure during MIS 11, with buoyant surface waters overlying denser intermediate waters and enhanced advection within the Antarctic Circumpolar Current. Together, these findings support a circulation-driven mechanism in which intermediate waters facilitated subsurface carbon sequestration while muted surface feedbacks limited CO₂ drawdown, providing a coherent explanation for the prolonged stability of atmospheric CO₂ during MIS 11. This intermediate-water–dominated carbon-cycling mode highlights how prolonged warm interglacials may operate under ocean–carbon states distinct from shorter interglacials.