Air-sea exchange of CO₂ in the Gulf of Kutch, northern Arabian Sea based on bomb-carbon in corals and tree rings

Abstract

Radiocarbon analyses were carried out in the annual bands of a 40 year old coral collected from the Gulf of Kutch (22.6°N, 70°E) in the northern Arabian Sea and in the annual rings of a teak tree from Thane (19°14'N, 73°24'E) near Bombay. These measurements were made in order to obtain the rates of air-sea exchange of CO₂ and the advective mixing of water in the Gulf of Kutch. The Δ¹⁴C peak in the Thane tree occurs in the year 1964, with a value of ~630%, significantly lower than that of the mean atmospheric Δ¹⁴C of the northern hemisphere (~ 1000%). The radiocarbon time series of the coral was modelled considering the supply of carbon and radiocarbon to the gulf through air-sea exchange and advective water transport from the open Arabian Sea. A reasonable fit for the coral data was obtained with an air-sea CO₂ exchange rate of 11-12 mol m^-2 yr^-1, and an advective velocity of 28 m yr^-1 between the Arabian Sea and the Gulf of Kutch; this was based on a model generated time series for radiocarbon in the Arabian Sea. The deduced velocity (~ 28 m yr^-1) of the advective transport of water between the gulf and the Arabian Sea is much lower than the surface tidal current velocity in this region, but can be understood in terms of net fluxes of carbon and radiocarbon to the gulf to match the observed coral Δ¹⁴C time series.