Evidence of poorly ventilated deep central Indian ocean during the last glaciation

Abstract

The Indian Ocean accounts for over 20% of the global ocean volume, nearly at par with the Atlantic, and possesses a unique hydrography characterized by turn-over entirely through exchange with the Southern Ocean, Pacific and Atlantic. Despite its volumetric and hydrographic 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 have been used to decipher past changes in the 'radiocarbon ventilation age' of deep waters from the Central Indian Ocean (CIO) basin. Time-series spanning the last 37 ka show coherent variations in both sediment cores, and indicate greatly enhanced ocean-atmosphere radiocarbon disequilibrium in the region during the last glaciation, with peak ocean-atmosphere radiocarbon age offsets occurring during Heinrich Stadial-1 (HS-1) and Heinrich Stadial-2 (HS-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 Benthic-Atmosphere (B-Atm) offsets remaining >3000 14C yrs during the Bølling-Allerød ~15 period BP. The more gradual rejuvenation of the CIO is supported by parallel oxygenation indicators, as well as existing stable isotope data and Nd isotope trends. Together, the data suggest that the CIO was isolated from well-ventilated North Atlantic sourced deep waters during the last glacial, and particularly during Heinrich stadials 2 and 1. These findings underline the important role played by the Indian Ocean in deglacial carbon cycle change, particularly in the latter half of the last deglaciation.