Sources of oceanic carbonyl sulfide revealed by sulfur isotopes measurements

Carbonyl sulfide (COS) is the major long-lived sulfur gas in the troposphere, and an important precursor for stratospheric sulfate aerosols, which increases earth’s albedo. The main sink of COS is the uptake by terrestrial plants, in a similar pathway to CO₂. Therefore, COS is used as a promising proxy for CO₂ removal by terrestrial plants (gross primary production, GPP), which regulates the earth’s climate. Currently, COS budget estimates have large uncertainties associated with the magnitude of COS sources and sinks. The COS ocean-atmosphere flux is the largest natural source of tropospheric COS, however, its magnitude is at the heart of a scientific debate with estimates ranging between  200 to 800 Ggr S Yr^-1 [1-2].

Sulfur isotopes measurements (³⁴S/³²S; δ³⁴S) are recently used in an isotopic mass-balance to constrain the COS budget, assuming each end-member has a unique isotopic signature [3]. However, in our previous work [3], we estimated the isotopic signature of the ocean-atmosphere COS flux, based on limited samples from the Mediterranean and Red Seas, which may not be representative of the oceans. In the current work, we present measurements of photochemistry experiments and natural samples from the Atlantic Ocean, sampled during dawn, afternoon, and sunset. Atlantic Ocean samples that were taken during dawn (min COS concentrations) show δ³⁴S value of 14±2‰ (n=8, one outlier with δ³⁴S value of 19.2‰ was excluded). However, samples taken during the afternoon (max concentration) show heavier δ³⁴S values of 18±1‰ (n=6).  This significant difference in δ³⁴S values between dawn and afternoon (P-value 0.0003) indicates that COS “dark production” is associated with an isotopic fractionation that produces isotopically lighter COS, supporting the hypothesis that COS “dark production” is related to biotic processes. While COS photoproduction is associated with heavier isotopic values, which we assume are closer to the δ³⁴S value of its biogenic source. This assumption is also supported by our photochemistry experiments, which indicate a small isotopic fractionation of COS photoproduction from cysteine (≤1‰). The isotopic signatures we present here will be used to better understand the main processes controlling oceanic COS production, and better constrain the ocean-atmosphere COS flux.   

 

[1] Lennartz, Sinikka T., et al. "Marine carbonyl sulfide (OCS) and carbon disulfide (CS2): a compilation of measurements in seawater and the marine boundary layer." Earth system science data 12.1 (2020): 591-609.

[2] Berry, Joe, et al. "A coupled model of the global cycles of carbonyl sulfide and CO2: A possible new window on the carbon cycle." Journal of Geophysical Research: Biogeosciences 118.2 (2013): 842-852.

[3] Davidson, Chen, Alon Amrani, and Alon Angert. "Tropospheric carbonyl sulfide mass balance based on direct measurements of sulfur isotopes." Proceedings of the National Academy of Sciences 118.6 (2021): e2020060118.