Sources of marine carbonyl sulfide and its precursors traced by sulfur isotopes

Carbonyl sulfide (OCS) is a major precursor of stratospheric sulfate aerosols and a proxy for terrestrial photosynthesis. In recent years, sulfur-isotope measurements (δ³⁴S) of OCS emerged as an approach to constrain the OCS budget. Yet, such measurements are still scarce for aquatic OCS. Here we present a large dataset of δ³⁴S values of marine OCS. In addition, we present δ³⁴S values of marine carbon disulfide (CS₂) and dimethyl sulfide (DMS), which in the air, act as important precursors of tropospheric OCS. Samples were collected at the Atlantic Ocean, the Red Sea, the Mediterranean Sea, the Wadden Sea, and the North Sea. The gases were sampled by a water–air equilibrator, preserved in canisters, and analyzed via a preconcentration system coupled to a gas chromatograph connected to a multi-collector inductively coupled plasma mass spectrometer. We found δ³⁴S values of −3.8‰ to 19.4‰ for OCS, −10.5‰ to 20‰ for CS₂, and 14–23‰ for DMS. These δ³⁴S values are controlled mainly by two endmembers: production in the water column and production in sediments. Lab experiments suggest that the ³⁴S-fractionation of OCS photo-production is 0.8‰ ± 0.5‰. In addition, based on measurements from the Atlantic Ocean, we calculated the ³⁴S-fractionation of OCS dark-production as −6‰ ± 2‰. This new data significantly improves our knowledge of the sulfur isotope distribution of marine OCS and helps identify its different sources, sinks, and production pathways.