First Radiocarbon Evidence of Poorly Ventilated Deep Central Indian Ocean during the Last Glaciation: Implication to Glacial Carbon Sequestration and Atmospheric CO2

The Indian Ocean accounts for over 20% of the global ocean volume, nearly on par with the Atlantic, and possesses unique hydrography linked to its total dependence on the subpolar ocean for deep ventilation, due to its northern land limit. Despite its hydrographic and volumetric importance, the role of the Indian Ocean in glacial-interglacial carbon cycle dynamics remains poorly constrained. Radiocarbon dates on foraminifera from two marine sediment cores from the Central Indian Ocean (CIO) basin deciphered past changes in the radiocarbon ventilation age of deep waters in the region. The last 37 ka timespan show coherent variations in both sediment cores, and indicate greatly enhanced ocean-atmosphere radiocarbon disequilibrium in the region during the last glaciation, with peak B-Atm age offsets of >5000 yrs occurring during Heinrich stadial-1 and Heinrich Stadial-2. Uniquely, as compared to the bulk of existing radiocarbon data for the last deglaciation, CIO radiocarbon ventilation ages only approach modern values during the Holocene, with B-Atm offsets remaining >3000 14C yrs during the Bølling-Allerød period ~15 ka BP. The more gradual rejuvenation of the CIO is supported by parallel stable isotope data and oxygenation indicators, as well as existing Nd isotope trends. Together, the data suggest that the CIO was isolated from well-ventilated North Atlantic sourced waters during the last glacial, and particularly during Heinrich stadials 2 and 1. These findings underline the important and under-appreciated role played by the Indian Ocean in deglacial carbon cycle change, particularly in the latter half of the last deglaciation.