Sources And Cycling of Carbonyl Sulfide in the Sargasso Sea

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 L21, showed minor diel variations, and varied little with depth. An OCS precursor, total dissolved organic sulfur (DOS), was lowest at the surface (40 nmol L21) and increased with depth. The photoproduction rate of OCS from in situ incubations averaged 9.6 pmol L21 h21, whereas dark production was 7.0 pmol L21 h21. Apparent quantum yields were 1025–1027 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 L21). Depth profiles in the afternoon showed surface water maxima and decreases with depth, whereas DOS had a surface maximum of 419 nmol L21 and decreased with depth. Dark production was 4.0 pmol L21 h21. Modeling of the diel cycle suggested a photoproduction rate of 16.4 pmol L21 h21. 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. Of the sulfur gases emitted from the ocean (dimethyl sulfide, carbonyl sulfide [OCS], and hydrogen sulfide; Andreae 1990), perhaps the most attention has been paid to dimethyl sulfide because of its potential marine biogeochemistry-climate feedbacks (Charlson et al. 1987). However, the oceanic source of OCS is also quantitatively significant, because this gas has an extremely long tropospheric residence time (1–4 yr; Khalil and Rasmussen 1984) and diffuses into the stratosphere, where it maintains the stratospheric sulfate aerosol layer and thereby affects the planetary radiation budget (Crutzen 1976; Turco et al. 1980; Hofmann 1990). Moreover, the hydrolysis of OCS produces dissolved hydrogen sulfide, which can then affect the cycling of many trace metals (Cutter et al. 1999). Thus, the marine cycling of OCS has both atmospheric and oceanic consequences. Unlike the other marine sulfur gases, OCS has extreme temporal and spatial variations in the ocean. The diel changes in surface OCS first reported by Ferek and Andreae (1984) led those workers to suggest a photochemical production mechanism. Additions of dissolved organic sulfur compounds such as dimethylsulfoniopropionate (DMSP), dimethyl sulfoxide, dimethyl sulfide, methionine, cysteine, and glutathione show that the organic sulfides, particularly glutathione, have the greatest yields of OCS during irradiation (Ferek and Andreae 1984; Zepp and Andreae 1994; Flöck et al. 1997). Studies of the wavelength dependence of OCS photoproduction (apparent quantum yields) in coastal (Zepp 1 Present address: Virginia Institute of Marine Science, P.O. Box 1346, Gloucester Point, Virginia 23062.