Carbonyl sulfide as a tracer for terrestrial net primary production: The oceanic perspective

Carbonyl sulfide (OCS) is the most abundant sulfur gas in the atmosphere, and the ocean is thought to contribute the major part to its atmospheric budget. As OCS is a CO₂ analog and is taken up by plants during photosynthesis, it is currently discussed as a tool to constrain terrestrial gross primary production. An important prerequisite for that is a balanced atmospheric budget, but sources and sinks are currently not well explained. One major uncertainty in the atmospheric OCS budget is the role of the ocean. To quantify terrestrial net primary production using carbonyl sulfide as a tracer, the quantification of the ocean’s direct and indirect OCS emissions is crucial. A large missing source indicated by atmospheric modelling studies and satellite measurements has been attributed to the equatorial ocean that requires more than twice the amount of today’s known oceanic OCS emissions. We investigate the role of the equatorial ocean in the global OCS budget using oceanic measurements of OCS and related trace gases (CS₂, DMS) together with box modelling. Results from our cruise to the tropical Indian Ocean in summer 2014 show that contrasting to results from previous inverse modelling studies, the tropical open ocean is rather a net sink than a source for atmospheric OCS. This contribution discusses possibilities to close the atmospheric budget with a focus on oceanic emissions and identifies key questions for a better understanding of the global oceanic source strength of OCS.