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 Δ14C 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 CO2exchange 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 25°N 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).