Deep-Sea Research, Vol. 36, No. 4, pp. 577-585, 1989. 0198-0149/89 $3.00 + 0.00 Printedin Great Britain. © 1989 Pergamon Pressplc. Dimethylsulphide oxidation in the ocean DAVID SHOOTER*t and PETER BRIMBLECOMBE* (Received 19 April 1988; in revised form 4 October 1988; accepted 26 October 1988) Abstract--Dimethylsulphide (DMS) is generally thought to be lost from the ocean by transfer to the atmosphere. However DMS is slowly autoxidized in saline solution (k' = 2.2 x 10 ~ s-1 at pH 8 and 20"C). The rate of autoxidation decreases with increasing pH and has an activation energy of 78 kJ mo1-1 at pH 8. Although slow, this autoxidation is fast enough to require a deep- sea source (precipitating detrital material?) for the low concentrations of DMS found at great depths. DMS does not bind significantly to Cu, Cd, Zn and Pb in seawater, although Cu was observed to increase the rate of autoxidation. INTRODUCTION THE compound dimethylsulphide (DMS) is produced by marine phytoplankton in the upper layers of the ocean. Concentrations of DMS in surface waters are highly variable, ranging from a few nanograms (S) per litre to about a microgram (S) per litre during blooms of marine organisms such as Phaeocystis poucheti (LOVELOCK et al., 1972; NGUVEN et al., 1978; BARNARD et al., 1982; BATES et al., 1987). Concentrations in the deep ocean are very much lower, probably no more than a few nanograms (S) per litre (NGUYEN et aL, 1983; ANDREAE et al., 1983; CLINE and BATES, 1983; ANDREAE and BARNARD, 1984; ANDREAE, 1985; COOPER et al., 1987). It appears likely that DMS is produced by the hydrolysis of beta-dimethylsulphonio- propionate (DMSP) to DMS and acrylic acid (ANDREAE, 1980; CLINE and BATES, 1983). The DMS is rather stable towards autoxidation in seawater, so there have been rather fewer studies of its reactions in solution compared with observations of processes such as its loss from surface waters by sea-air transfer. The rapid decrease in DMS concentration with depth implies downward transport by eddy diffusion. Low, but finite DMS concentrations in deep water require either downward transport or a deep ocean source. Gaseous DMS has been shown to adsorb onto a variety of adsorbers, including activated carbon and clays (TANADA et al., 1978). Therefore vertical transport of DMS might also take place through adsorption of DMS onto the surface of descending particles. In addition to this, dialkylsulphides are known to form transition metal complexes (WORTH and HAENDLER, 1942; TROCHA-GRIMSHAWand HENBEST, 1968). Despite the apparent stability of DMS, it is likely to be oxidized by bacterial processes in seawater (ANDREAE, 1980), and BRIMBLECOMBE and SHOOTER(1986) have more recently shown that aqueous solutions of DMS are susceptible to photo-oxidation by visible light * School of Environmental Sciences, University of East Anglia, Norwich, NR4 7TJ, U.K. l Permanent address: Chemistry Department, University of Auckland, Private Bag, Auckland, New Zealand. 577