Radiocarbon in the Northern Indian Ocean two decades after GEOSECS

Abstract

The 14C measurements in the Arabian Sea and the Bay of Bengal during the late 1990s offer a way to assess the temporal changes in the inventories of bomb-14C and its penetration into the ocean, in two decades since GEOSECS expeditions (1977-1978). The mean penetration depth of bomb radiocarbon during GEOSECS (1977-1978) was 270 m, which increased by ∼40% to 381 m in 1994-1998. The small changes in bomb-14C inventories, significant increase in the mean penetration depths and lowering of the surface 14C values in the northern Indian Ocean indicate the temporal variation of bomb-14C in two decades is mainly through downward transfer through mixing with deeper waters. The observed bomb- 14C inventory in the northern Indian Ocean agrees with numerical model simulated values, except at the equatorial Indian Ocean. The high bomb-14C inventory at the equator can be attributed to lateral advection of 14C-enriched waters from the Pacific Ocean through the Indonesian archipelago. The air-sea CO2 exchange rates in the northern Indian Ocean calculated from the bomb-14C inventories range from ∼7 mol m-2 yr-1 (in the northern Bay of Bengal) to 20 mol m-2 yr-1 (in the equatorial Indian Ocean). Net sea-air flux of CO2 estimated for the northern Indian Ocean between 0 and 25N is ∼104 30 TgC yr-1. The Bay of Bengal is a net sink of atmospheric CO2 (∼-1 0.4 TgC yr-1), while the Arabian Sea is a source of CO2 (∼69 21 TgC yr-1). © Copyright 2012 by the American Geophysical Union.