Sedimentary and dark production sources of COS and CS2 identified by their sulfur isotopic values

Carbonyl sulfide (COS) is a long-lived trace gas, and an important precursor for stratospheric sulfate aerosols, which reduce solar radiation reaching earth surface and may regulates earth's climate. The main sink of COS is the uptake by terrestrial plants similar to CO₂. Thus, COS is used as a proxy for CO₂ removal by terrestrial plants (gross primary production, GPP). Oceans are the major source of COS to the atmosphere , either directly or indirectly by emitting other volatile sulfur compounds such as CS₂ and DMS that partially oxidize to COS in the atmosphere.  In the surface ocean, COS is produced by photochemical reactions and by "dark production" deeper in the water column and from sediments. In the present study we aim to determine and quantifying the COS and CS₂ “dark production” by using sulfur isotopes measurements (³⁴S/³²S; δ³⁴S) of surface, deep water and sediment samples. In addition, laboratory experiments were conducted to follow the productions of COS and CS₂ by direct reactions with CO and HS^-/S_X^2- and by incubations experiments of seawater. Our preliminary results from the Atlantic Ocean, the Mediterranean, North, Wadden, and Red Seas show surface δ³⁴S values in the range of -4 to 20‰ for COS, and -10 to 22‰ for CS₂ while DMS was 18-21‰. The δ³⁴S values of DMS are in line with previous measurements of the surface ocean and reflect its biological source with small isotopic fractionation relative to marine sulfate (21‰). This was also expected for COS and CS₂ that also produced from biological sources. However, their δ³⁴S values extended over large ranges up to 30‰, while their heaviest δ³⁴S value are closed to DMS. There are clear mixing lines for COS and CS₂ between the surface ocean sources (heavy) and the sedimentary sources (light) in shallow water. The isotopic values of sedimentary-production are calculated as -4‰ for COS and -10‰ for CS₂, based on the samples from the sediment rich waters of the Wadden Sea. These values suggest abiotic sulfurization of light organic compounds by ³⁴S depleted HS^-/S_X^2- from the microbial sulfate reduction (MSR) in the sediment. Indeed, the intertidal sands of the Wadden Sea are known to host intense MSR activity and produce large amounts of H₂S and polysulfides. The specific organic precursors are still unknown and will be the subject of our upcoming experiments. Also, the “dark production” isotopic signals of the surface water is not yet well resolved, but seems also to be isotopically lighter then DMS and marine sulfate. These new findings show that the COS/CS₂ sources in the ocean are complex combining contributions from several biotic and abiotic processes which seem to have unique isotopic signatures.