Radiocarbon depleted intermediate water masses during the LGM in the equatorial Indian Ocean

Ocean circulation and changes in ventilation represent one of the crucial regulation screws in the Earth´s climate system. For the Last Glacial Maximum (LGM, 21 kyr BP), characterized by a 100 ppm lower atmospheric CO2 concentration, growing evidence exists for enhanced storage of CO2 in the deep-sea. However, inasmuch the intermediate depth of the oceans opposed or amplified this sequestration is poorly constrained.

Here we present coupled 230Th/U and 14C measurements on scleractinian cold-water corals retrieved from ~450m water depth off the Maldives in the Indian Ocean. Based on these measurements we calculate ∆14C, ∆∆14C and Benthic-Atmosphere (Batm) ages in order to understand ventilation dynamics of Indian Ocean intermediate water masses. Our results exhibit radiocarbon depleted intermediate water masses as low as -340 ‰ (∆∆14C), corresponding to ~2100 years (Batm) at the LGM. Such extremely radiocarbon depleted intermediate water masses suggest abyssal upwelling of southern-sourced deep-water masses, being strongly enriched in respired carbon and thus highlighting the oceanic carbon storage capacity even at upper thermocline depth. Nevertheless, Batm ages reveal a pronounced short-term centennial variability, that demonstrates the dynamic nature of thisoceanographic phenomena.

Thus, reduced intermediate ocean ventilation off the Maldives and resulting carbon storage may help to improve our understanding of the role of the thermocline Ocean with respect to the Earth´s carbon cycle.