Sulfur compounds concentration ocean

Current research on the atmospheric cycling of sulfur compounds involves the experimental determination of reaction rates and pathways (see Plane review, this volume) and the field measurement of ambient concentrations of oceanic emissions and their oxidation products. Photochemical models of tropospheric chemistry can predict the lifetime of DMS and H2S in marine air however there is considerable uncertainty in both the concentrations and perhaps in the identity of the oxidants involved. The ability of such models to simulate observed variations in ambient concentrations of sulfur gases is thus a valuable test of our assumptions regarding the rates and mechanisms of sulfur cycling through the marine atmosphere. 

The third, fourth, and fifth sections investigate the distribution and biological and chemical transformations of reduced sulfur compounds in the oceans. The third section focuses primarily on dimethyl sulfide, which is the predominant form of volatile sulfur in the ocean. Research in the past has concentrated on documenting the distribution of DMS in various oceanic environments. The factors controlling this distribution are not well understood. These chapters examine laboratory and field investigations relating DMS production to productivity and spedation. 

Although there have only been a few studies to date, it has been suggested that coastal plumes (Turner et al., 1996 Simo et al., 1997) and estuaries (Iverson et al., 1989 Cerqueira and Pio, 1999) may be important atmospheric sources of DMS. DMS, a compound produced by certain phytoplankton, has been shown to have possible implications for climate control once released into the atmosphere (Charlson et al., 1987). DMS is formed by cleavage of dimethylsulfoniopropionate (DMSP) (Kiene, 1990). In fact, DMSP, shown to be correlated with bacterial activity, may provide as much as 100% of the sulfur and 3.4% of the carbon required for bacterial growth in oceanic waters (Kiene and Linn, 2000). Other sulfur compounds such as COS and carbon disulfide (CS2) have also been shown to be possible sources of S in estuaries. For example, significant concentrations of COS and CS2 were found in four European estuaries, 220 150 and 25 6 pM (Sciare et al., 2002). COS is the most abundant sulfur compound in the... 

Inorganic sulfur compounds are at a much lower concentration in surface waters than in the ocean. Nevertheless, rivers move a large amount of dissolved sulfate to the sea each year (Meybeck, 1987) and industrial activities and agriculture have added much to this flux. Typically estimates are perhaps a little less than a lOOTg(S) yr with additional loads of the same magnitude as those from industrial and agricultural sources. There is also some... 

In aquatic settings, sulfate reducers are intimately associated with various heterotrophs and autotrophs. These include purple and green sulfur bacteria and thiobacUli which affect the availability of organic matter and alter the distribution of sulfur compounds in their various valence states. In the majority of the sites where elemental sulfur is formed (see Chapter 6.2), oxidation to sulfate also occurs and other intermediate oxidation states may also be found. For example, Volkov et al. (1972) reported the presence of thiosulfate in waters of some sediments from the Pacific Ocean east of Japan. These are interpreted as oxidation products of sulfide rather than intermediates of sulfate reduction since their concentration increased with increasing free sulfide ion content. 

We know rather well the concentration of oxidized sulfur compounds (S02 and SOJ") over industralized North America and Europe as well as over the Atlantic Ocean. However, our knowledge of the concentration of reduced sulfur is much poorer because of analytical problems. 

Meszaros, E., 1978 Concentration of sulfur compounds in remote continental and oceanic areas. Atmospheric Environment 12, 699-705. 

Dimethylsulfide (DMS) is the most abundant volatile sulfur compound in sea water and constitutes about half of the global biogenic sulfur flux to the atmosphere. Studies of the concentration of DMS in the ocean have shown that average surface water concentrations may vary by up to a factor of 50 between summer and winter in mid and high latitudes. Furthermore, there are large-scale variations in DMS concentration associated with phytoplankton biomass, although there are generally poor correlations between local oceanic DMS concentrations and the biomass and productivity of phytoplankton (due to differences between plankton species in ability to produce DMS). 

COS is produced in the ocean by photochemical oxidation of organic sulfur compounds whereby dissolved organic matter acts as a photosensitizer. The aqueous concentration of COS manifests a strong diel cycle, with the highest concentrations in daytime (concentration range on the order of 0.03-0.1 nmol U ). COS hydrolyzes in water to H2S at rates dependent on water temperature and pH. The flux of oceanic COS to the atmosphere may represent about one-third of the global COS flux. 

Under low-dose conditions, forest ecosystems act as sinks for atmospheric pollutants and in some instances as sources. As indicated in Chapter 7, the atmosphere, lithosphere, and oceans are involved in cycling carbon, nitrogen, oxygen, sulfur, and other elements through each subsystem with different time scales. Under low-dose conditions, forest and other biomass systems have been utilizing chemical compounds present in the atmosphere and releasing others to the atmosphere for thousands of years. Industrialization has increased the concentrations of NO2, SO2, and CO2 in the "clean background" atmosphere, and certain types of interactions with forest systems can be defined. 

Other processes which may explain changes in DMS concentration with depth, on a short time scale, are those related to bacteria. Studies conducted in sediments (31-39.62) have shown that DMS can be consumed by microorganisms. Based on sulfur requirements of microorganisms, the availability of sulfur from sulfate in these environments far exceeds that of DMS. However, as a carbon source, the concentration of DMS is similar to other compounds of low molecular weight and may be cycled by microorganisms to serve as both a carbon ana sulfur source. Although the utilization of DMS aerobically has been reported (63.64). the extent to which similar processes exist in oceanic environments is not known. For futher discussions of the microbial processes related to DMS the reader is referred to another Chapter of this book (65). 

Hydrogen sulfide formation through dissimilatoiy sulfur reduction has for years been known as a source of environmental sulfur. This compound has invited recent study because of its possible effect on the redox chemistry of sea water. Both the lifetime and the oceanic concentrations of this reactive and highly toxic compound are the focus of the fifth section. 

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