Radiocarbon evidence for the last glacial-interglacial ventilation changes in the northern Indian Ocean

The efficiency of the ocean to store or release gases, such as carbon, is mainly governed by overturning circulation and air-sea gas exchanges, thereby it regulates the carbon dioxide (CO₂) sequestration in the ocean interior and its subsequent outgassing. Changes in the ocean circulation are considered as one of the primary drivers of atmospheric CO₂ fluctuations during the last glacial-interglacial cycle. Although Indian Ocean role plays an important role in the global ocean circulation, its role in carbon cycle during the last glacial termination remains scantily studied. In this study, the ventilation records from the northern Indian Ocean over the last 25 kyr has been compiled and examined, where the ventilation ages are calculated as the difference between the radiocarbon ages of coexisting benthic and planktic foraminifera.  The most notable feature from our result is the stratification between the intermediate and deep water of the northern Indian Ocean during the Last Glacial Maximum (LGM). During this period, the water mass at a depth of ~2000 m below was poorly ventilated, characterized by low-¹⁴C, enrich in CO₂ and high ventilation ages exceeding 2000 ¹⁴C years. In contrast, the reported ventilation ages of water mass above ~2000 m depth were low (~1400 ¹⁴C years) indicating relatively better ventilated water. This strong vertical stratification between the water masses implies a reduced renewal of deep water in the northern Indian Ocean during the LGM, suggesting that the northern Indian Ocean basin was a part of the glacial ocean aged carbon pool. The condition changed to better-ventilated water during the deglaciation, probably due to increased contribution of the northern sourced deep water to the northern Indian Ocean and outgassing the glacially stored CO₂.