Sulfur isotopic fractionation during hydrolysis of carbonyl sulfide

Carbonyl Sulfide (OCS) is the most abundant sulfur-containing gas in the atmosphere, and it is used as a proxy for terrestrial gross primary productivity (GPP). Oceans are the major source of OCS to the atmosphere, produced by photochemical and "dark" reactions. Hydrolysis to H2S and CO2 is the major removal process of OCS from the ocean's surface. Measuring the sulfur isotope values (delta S-34) and the isotopic fractionation (epsilon) associated with these major OCS sources and sinks could decrease the uncertainties in its fluxes. In the current study, we aim to determine the epsilon during the hydrolysis process of OCS (epsilon(h)). We used a purge and trap system coupled to a GC/MC-ICPMS to measure delta S-34 values during hydrolysis under different temperatures (4-40 degrees C), salinities (0.2-40 g/L), and pH (4-9), representing various natural environmental conditions. In addition, we use the quantum chemical method to calculate the equilibrium epsilon(h) and compare it to the empirical results. Our results for the low salinity (S =0.2 g/L; pH 8.0) water show a temperature dependency of the epsilon(h) from -3.9 parts per thousand +/- 0.2 parts per thousand (4 degrees C,) to -2.2 +/- 0.6 parts per thousand (40 degrees C). The higher fractionation at low temperatures has implication for ice-core data interpretation. However, in natural seawater at 4 degrees C and 22 degrees C (S = 40 g/L, pH 8.2) there was no such temperature dependency and the epsilon(h) averaged -2.6 +/- 0.3 parts per thousand. Thus, it seems that salinity cancels the temperature effect close to the freezing temperature of water. Varying the pH between 4 and 9 (at 22 degrees C) did not result in any epsilon(h) trend. Ab-initio calculations suggest that OCS hydrolysis is not controlled by equilibrium. The epsilon(h) values we report will aid in quantifying the impact of OCS's hydrolysis on the observable sulfur isotopic signature of OCS in oceanic and in freshwater environments. This in turn will facilitate more accurate mass-balance calculations for the OCS budget from the ocean to the atmosphere.