Radiocarbon evidence for reduced deep water ventilation of the northern Indian Ocean during the last glacial maxima and early deglaciation

Abstract

Enhanced outgassing of sequestered carbon to the atmosphere by upwelling of deep water in the Southern Ocean during the last deglaciation is assumed to be the main factor that modulated changes in atmospheric CO2 and atmospheric radiocarbon activity; however, the exact mechanism behind this deglacial CO2 release still remains unclear. In this study, we have investigated ventilation changes in the deep waters of the northern Indian Ocean over the past 25 kyr BP using paired benthic and planktic foraminiferal radiocarbon, along with stable carbon and oxygen isotopes of benthic foraminifera, sedimentary CaCO3 content, and coarse fraction from the gravity core SSD-044/GC-01 (3160 m water depth) collected from Central Equatorial Indian Ocean. The results indicate a greater proportion of southern sourced CO2-rich, poorly-ventilated Antarctic Bottom Water (AABW) during the Last Glacial Maximum (LGM) and Heinrich Stadial (HS1), in the northern Indian Ocean. The observed poor ventilation during the last glaciation shows that the northern Indian Ocean was a part of the glacial ocean aged carbon pool. Improved ventilation during the B�lling-Aller�d (B-A) is consistent with the rapid resumption of Atlantic overturning circulation and inflow of North Atlantic Deep Water (NADW) into the northern Indian Ocean at the onset of the B-A, which caused flushing of the deep-carbon pool with evidence of degassing from the northern Indian Ocean. This study thus supports the role of Southern Ocean in modulating atmospheric CO2 variations during the last deglaciation.