Sulfur isotopic fractionation during hydrolysis of carbonyl sulfide

Abstract

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 (δ³⁴S) and the isotopic fractionation (ε) associated with these major OCS sources and sinks could decrease the uncertainties in its fluxes. In the current study, we aim to determine the ε during the hydrolysis process of OCS (ε_h). We used a purge and trap system coupled to a GC/MC-ICPMS to measure δ³⁴S values during hydrolysis under different temperatures (4–40 °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 ε_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 ε_h from −3.9 ‰ ± 0.2 ‰ (4 °C,) to −2.2 ± 0.6 ‰ (40 °C). The higher fractionation at low temperatures has implication for ice-core data interpretation. However, in natural seawater at 4${}^{°}C$ and 22 °C (S = 40 g/L, pH 8.2) there was no such temperature dependency and the ε_h averaged −2.6 ± 0.3 ‰. 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 °C) did not result in any ε_h trend. Ab-initio calculations suggest that OCS hydrolysis is not controlled by equilibrium. The ε_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.