Methane dynamics in the Weddell Sea determined via stable isotope ratios and CFC‐11

Abstract

The physical, chemical/biological processes that control the methane dynamics in the Weddell Sea are revealed by the distributions of methane (CH4), its stable carbon isotope ratio, δ13C‐CH4, and the conservative transient tracer, chlorofluorocarbon‐11 (CFC‐11, CCl3F). In general, a nearly linear correlation between CH4 and CFC‐11 concentrations was observed. Air‐sea exchange is the major source of methane to this region, and the distribution of methane is controlled mainly by mixing between surface water and methane‐poor Warm Deep Water. A significant influence of methane oxidation over the predominant two end‐member mixing was only found in the Weddell Sea Bottom Water (WSBW) of the deep central Weddell Basin, where the turnover time of methane appears to be about 20 years. Mixing also controls most of the δ13C‐CH4 distribution, but lighter than expected carbon isotopic ratios occur in the deep WSBW of the basin. From box model simulations, it appears that this “anomaly” is due to methane oxidation with a low kinetic isotope fractionation of about 1.004. The surface waters in the Weddell Sea and the Antarctic Circumpolar Current showed a general methane undersaturation of 6 to 25% with respect to the atmospheric mixing ratio. From this undersaturation and model‐derived air‐sea exchange rates, we estimate a net uptake of CH4 of roughly −0.5 μmol m−2 d−1 during austral autumn.