IMPACT OF DIMETHYLSULFIDE PHOTOCHEMISTRY ON METHYL SULFUR CYCLING IN THE EQUATORIAL PACIFIC-OCEAN

Shipboard experiments were conducted in the equatorial Pacific Ocean t o ascertain the relative importance of atmospheric ventilation, biolog ical consumption, and photolysis in the removal of dimethylsulfide (DM S) from seawater. Comparisons were made at a series of sampling locati ons in a transect from 12 degrees N 140 degrees W to 12 degrees S 135 degrees W, as part of the International Global Atmospheric Chemistry p roject's Marine Aerosol and Gas Exchange cruise in February-March 1992 . Turnover rate constants for DMS were used to compare the different r emoval pathways over three depth intervals (0-1 m, 0-20 m, and 0-60 m) . In the surface mixed layer (0-60 m) the DMS turnover rate constants ranged from 0.02 to 0.19 day(-1) for atmospheric ventilation, 0.04 to 0.66 day(-1) for biological consumption, and 0.05 to 0.15 day(-1) for photolysis. When all three processes are considered, the corresponding turnover time for DMS ranges from 1 to 4 days, with photolysis accoun ting for 7%-40% of the total turnover of DMS. Laboratory irradiations were conducted with stored seawater samples to study the kinetics and wavelength dependence of DMS photolysis. Salient results were (1) the photolysis of DMS followed pseudo first-order kinetics, (2) dimethylsu lfoxide was a minor (14%) product of DMS photolysis, and (3) the photo lysis of DMS in seawater under natural light conditions occurred prima rily at wavelengths between 380 and 460 nm. On the basis of these resu lts, we predict that the photolysis of DMS will occur at appreciable d epths in the photic zone in oligotrophic marine environments (similar to 60 m). An important finding of this study is that atmospheric loss, biological consumption, and photolysis are all important removal path ways for DMS in the photic zone of the equatorial Pacific Ocean. The r elative importance of each pathway is a function of the depth interval considered, sampling location, and meteorological conditions.