Abstract
The cycling of the radiatively important gas carbonyl sulfide (OCS) was studied in surface waters of the Sargasso Sea. In August 1999, surface OCS concentrations averaged 8.6 pmol L −1 , showed minor diel variations, and varied little with depth. An OCS precursor, total dissolved organic sulfur (DOS), was lowest at the surface (40 nmol L −1 ) and increased with depth. The photoproduction rate of OCS from in situ incubations averaged 9.6 pmol L −1 h −1 , whereas dark production was 7.0 pmol L −1 h −1 . Apparent quantum yields were 10 −5 −10 −7 from 313–436 nm and varied with the water depth irradiated. In March 2000, there were strong diel variations in surface OCS (highest in late afternoon; overall average, 16.9 pmol L −1 ). Depth profiles in the afternoon showed surface water maxima and decreases with depth, whereas DOS had a surface maximum of 419 nmol L −1 and decreased with depth. Dark production was 4.0 pmol L −1 h −1 . Modeling of the diel cycle suggested a photoproduction rate of 16.4 pmol L −1 h −1 . Overall, the photochemical production of OCS strongly depended on DOS and chromophoric dissolved organic matter, whereas dark production was influenced by the presence of particles and perhaps microbial respiration, showing a direct biotic influence on OCS cycling.