Radiocarbon in the Arabian Sea water column: Temporal variations in bomb 14C inventory since the GEOSECS and CO2 air-sea exchange rates

Results of measurements of 14C in the upper 1500 m water column in 10 stations of the Arabian Sea are presented. The study carried out during 1994–1995, which includes reoccupation of three Geochemical Ocean Sections Study (GEOSECS) stations (1977–1978), provides an opportunity to assess the temporal variations in the distribution and inventory of bomb radiocarbon over a period of ∼2 decades. The Δ14C values of surface waters (5 m) range between 31 and 58‰, ∼20‰ less than those measured during 1978 at the three GEOSECS stations. The higher Δ14C values measured during the present (as well as GEOSECS) study occur in the southern Arabian Sea, which can be attributed to the influx of low‐salinity and high‐Δ14C waters from the Bay of Bengal. The Δ14C in the Persian Gulf Water (PGW) and the Red Sea Water (RSW) are in the range of −34 to −59‰ and −89 to −113‰, respectively, and do not show any significant differences from those measured during the GEOSECS. The bomb 14C inventories at the three GEOSECS stations during 1977 and 1995 show that at station 417 in the central Arabian Sea it has increased by ∼30%, whereas at the other two stations, 416 and 418 in the north and south central Arabian Sea, it is nearly the same. In general, the bomb 14C inventory in the region ranges between 3.5 and 7.7×109 atoms cm−2. The CO2 air‐sea exchange rates, derived from the bomb 14C inventory, yield values of 7–16 mol m−2 yr−1. These exchange rates coupled with reported pCO2 measurements in surface Arabian Sea waters yield CO2 evasion rates in the range of 50–180 Tg‐C yr−1 for the Arabian Sea.