Impact of dimethylsulfide photochemistry on methyl sulfur cycling in the equatorial Pacific Ocean

Abstract

Shipboard experiments were conducted in the equatorial Pacific Ocean to ascertain the relative importance of atmospheric ventilation, biological consumption, and photolysis in the removal of dimethylsulfide (DMS) from seawater. Comparisons were made at a series of sampling locations in a transect from 12øN 140øW to 12øS 135øW, as part of the International Global Atmospheric Chemistry project's Marine Aerosol and Gas Exchange cruise in February-March 1992. Turnover rate constants for DMS were used to compare the different removal 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 -• for atmospheric ventilation, 0.04 to 0.66 day -• for biological consumption, and 0.05 to 0.15 day -• for photolysis. When all three processes are considered, the corresponding turnover time for DMS ranges from 1 to 4 days, with photolysis accounting 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) dimethylsulfoxide was a minor (14%) product of DMS photolysis, and (3) the photolysis of DMS in seawater under natural light conditions occurred primarily at wavelengths between 380 and 460 nm. On the basis of these results, we predict that the photolysis of DMS will occur at appreciable depths in the photic zone in oligotrophic marine environments (-60 m). An important finding of this study is that atmospheric loss, biological consumption, and photolysis are all important removal pathways for DMS in the photic zone of the equatorial Pacific Ocean. The relative importance of each pathway is a function of the depth interval considered, sampling location, and meteorological conditions. precursor of aerosols and cloud condensation nuclei and thus affects the Earth's radiation balance and climate [Bonsang et al., 1980; Bigg et al., 1984; Chadson et al., 1987; Prospero et al., 1991; Ayers and Gras, 1991; Berresheim et al., 1993]. Oceanic DMS emissions are partly a function of surface concentrations of DMS in seawater, which, in turn, are regulated by imbalances in the rates of biological, physical, and chemical sources and removal processes for DMS in the water column. Currently, we have a poor understanding of the dynamics of these processes in seawater. The primary source of DMS in seawater is through the enzy-Paper number 95JC03624. 0148-0227/96/95 JC-03624505.00 matic decomposition of dimethylsulfoniopropionate (DMSP), which is derived from algae in the photic zone [Burgermeister et al., 1990, and references therein]; abiotic decomposition of DMSP is negligible [Dacey and Blough, 1987]. Culture studies support the supposition of a biological source of DMS in seawater [Vairavamurthy et al., 1985; Dickson and Kirst, 1986, 1987a, b; Keller et al., 1989], but often DMS concentrations are poorly correlated with algal parameters...

Open ocean DMS air/sea fluxes over the eastern South Pacific Ocean

“…However, very few studies have examined the validity of this assumption. Near surface gradients can arise from photochemical loss and biological production/loss (Kieber and Jiao, 1996). Stratification caused by strong salinity gradients or solar heating can isolate the surface from waters below, leading to depletion of DMS.…”

Assessing the flux of different volatile sulfur gases from the ocean to the atmosphere

“…The 6-day back trajectory analysis (Figure lb) The formation of CH3SH is the favored pathway because it can be more easily incorporated into bacterial sulfur amino acids (methionine, cysteine, and homocysteine) than is possible with DMS. In spite of being the favored DMSP degradation product, CH3SH usually has a lower concentration in the upper ocean because of its high biological and chemical reactivity [Kiene, 1996 graded mainly to CH3SH. However, the disproportionately high CH3SH concentrations may also be due to spatial differences in the relative rates of disappearance of CH3SH and DMS [Kiene, 1996].…”

“…In spite of being the favored DMSP degradation product, CH3SH usually has a lower concentration in the upper ocean because of its high biological and chemical reactivity [Kiene, 1996 graded mainly to CH3SH. However, the disproportionately high CH3SH concentrations may also be due to spatial differences in the relative rates of disappearance of CH3SH and DMS [Kiene, 1996]. At this time, it is not possible to determine whether the variation in the ratio of the concentrations of CH3SH and DMS is due to differences in the production or the destruction terms.…”

“…No investigations have been reported on the growth rate of algae on the inside of Weiss equilibrators made from Teflon, or its impact on the measurement of reduced sulfur gases. However, the level 1 model of Johnson [ 1999] was reworked to include a production term inside the equilibrator and, even using the highest aqueous DMSP turnover rate measured by Kiene [ 1996] and 100% conversion into DMS, the possible contribution due to fouling is calculated to be negligible.…”

Photochemical oxidation of dimethylsulphide to dimethylsulphoxide in estuarine and coastal waters

“…Results from 35 S-DMS tracer experiments indicate that microbial DMS consumption in surface waters primarily yields dimethylsulphoxide (DMSO) (Del Valle et al, 2007b). By contrast, the DMSO yield from DMS photo-oxidation apparently varies between shelf seas (25%), polar waters (39%) and the open ocean (14%) (Kieber et al, 1996;Hatton, 2002;Toole et al, 2004). Even so, DMSO concentrations frequently exceed those of DMS (Lee et al, 1999), possibly due to a lack of photochemical removal (Toole et al, 2004), slow microbial consumption (Tyssebotn et al, 2017), significant biological production (Del Valle et al, 2007a) or an aggregate of all three.…”