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

Abstract

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.