Modeling the sulfur isotopic signature of marine carbonyl sulfide emissions

Carbonyl sulfide (OCS), the most abundant sulfur-containing trace gas in Earth's atmosphere, plays a central role in stratospheric aerosol formation and can serve as a proxy for terrestrial carbon dioxide uptake. In this context, quantifying its atmospheric sources and sinks is of great interest, but especially the role of marine emissions is poorly constrained. Analysis of sulfur isotopic ratios (³⁴S/³²S; d³⁴S) is a valuable tool to quantify the relative contributions of different sources to the atmospheric budget of OCS. However, the d³⁴S values for marine OCS emissions are based on a data set that has so far been limited to a few measurements in coastal and shelf areas. Here, we present a first global ocean mixed-layer model of OCS sulfur isotopes, building on experimentally derived fractionation factors for the most important biogeochemical processes of marine OCS cycling, i.e. photochemical production, dark production and degradation by hydrolysis. The model is tested against incubation experiments and novel measurements along an Atlantic transect. We calculate the d³⁴S values of marine OCS emissions, with the ultimate aim to decipher their relative contributions to the atmospheric budget. Our simulations show regional and temporal variations in the d³⁴S values of OCS, suggesting a distinct latitudinal gradient with lower d³⁴S in the tropics and higher d³⁴S in high latitudes. The spatially weighted average of d³⁴S values of OCS is used to update a global mass balance approach to infer the role of direct marine emissions of OCS in the atmospheric budget.