Radiocarbon constraints on North Indian deep ocean ventilation and deglacial carbon dioxide release

Abstract

Late Quaternary Indian Ocean circulation evolution and its role in carbon cycle dynamics remain poorly constrained. Here, we reconstructed the radiocarbon ventilation history of the North Indian Ocean over the past 30 kyr BP, using core I106 at the Ninetyeast Ridge. The results show higher ¹⁴C ventilation ages (Benthic-Atmospheric ¹⁴C offsets reaching ∼3000 yrs in average) during the last glacial period than nowadays, indicating that deep-water ventilation of the glacial North Indian Ocean was reduced and Indian Deep Waters (IDWs) were part of the glacial aged carbon pool. They also present a rapid rise in ventilation of IDWs during late Heinrich Stadial 1 and the Younger Dryas event, consistent with existing stable isotope data and Nd isotope trends that imply an enhanced inflow of well-ventilated, low-CO₂ North Atlantic Deep Water (NADW) into the North Indian Ocean. The increase in ventilation of IDWs and NADW incursion would have caused a shift of the deep-ocean carbon pool with evidence of degassing from the North Indian Ocean. Together with ¹⁴C ventilation records in the Indian sector of the Southern Ocean, it can be inferred that portions of deep Indian Ocean carbon pool were likely transferred via the IDW transport southward and subsequently released to the atmosphere from the Southern Ocean. These findings highlight that part of ¹⁴C-depleted, CO₂-rich IDWs contributed to the rapid rise in deglacial atmospheric CO₂ via a Southern Ocean link.