Sulfur isotopes reservoirs

Another factor that is of great importance for the observed sulfur isotope variations of natural sulfides is whether sulfate reduction takes place in an open or closed system. An open system has an infinite reservoir of sulfate in which continuous removal from the source produces no detectable loss of material. Typical examples are the Black Sea and local oceanic deeps. In such cases, H2S is extremely depleted in " S while consumption and change in " S remain negligible for the sulfate. In a closed system, the preferential loss of the lighter isotope from the reservoir has a feedback on the isotopic composition of the unreacted source material. The changes in the " S-content of residual sulfate and of the H2S are modeled in Fig. 2.21, which shows that 5 S-values of the residual sulfate steadily increase with sulfate consumption (a linear relationship on the log-normal plot). The curve for the derivative H2S is parallel to the sulfate curve at a distance which depends on the magnitude of... 

Characteristics often ascribed to MVT deposits include temperatures generally <200°C and deposition from externally derived fluids, possibly basinal brines. Sulfur isotope valnes from MVT deposits suggest two major sulfide reservoirs, one between -5 and +15%c and one greater than +20%c (Seal 2006). Both sulfide reservoirs can be related, however, to a common sea water sulfate source that has undergone different sulfur fractionation processes. Reduction of sulfate occurs either bacterially or by abiotic thermochemical reduction. High 5 S-values should reflect minimal fractionations associated with thermochemical reduction of sea water sulfate (Jones et al. 1996). 

Figure 10.12 illustrates the carbon and sulfur isotopic compositions of a variety of materials. For both carbon and sulfur, there is an important fractionation that obtains when organic processes are involved. Organic material is depleted in 3c, and sulfide produced from bacterial reduction of sulfate is depleted in 34s. In the exogenic carbon cycle, there are two principal reservoirs of carbon the oxidized inorganic carbon reservoir, which is mostly carbonate... 

There are several review papers on mass-independent chemical processes and their applications. Thiemens and Weston reviewed the progress in understanding the physical chemistry of gas-phase mass-independent processes and their observation on Earth and meteorites. Thiemens et al. (2001) reviewed the observations of mass-independent isotopic composition in various solid reservoirs of Earth and Mars, including both oxygen and sulfur isotopes. A more recent review (Thiemens, 2002) has summarized the theoretical and laboratory studies of the physical chemistry of mass-independent isotope effects and their observation on Earth and Mars, subsequent to the review of Thiemens et al. (2001). 

The reduction is typically limited by the availability of organic carbon and often occurs in shallow waters at continental margins. Thus, global sulfide production would be dependent on the availability of biological productive areas over geological time. Sulfur-isotope data can be used to constrain simple models of the sulfur cycle over geological time and establish the size of the reservoirs as shown in Figure 5(b). 

Table 4. Sulfur isotopic composition in various reservoirs.

Sulfur has four stable isotopes , gS, j S, which are -95.02%, -0.75%, -4.21% and 0.02% abundant, respectively. Sulfur is ubiquitous in the global environment and major sulfur reservoirs include sulfate in the oceans, evaporites, sulfide ore deposits, and organic sulfur compounds. Sulfur isotope abundance variations are shown in Figure 11. 

The prevalence of sulfur s second most abundant isotope, S, along with the fractionation known to occur in many biogeochemical processes, make isotopic studies of sulfur a potentially fruitful method of unraveling its sources and sinks within a given reservoir. 

The biochemical reduction of sulfate to sulfide by bacteria of the genus Desulfovibrio in anoxic waters is a significant process in terms of the chemistry of natural waters since sulfide participates in precipitation and redox reactions with other elements. Examples of these reactions are discussed later in this paper. It is appropriate now, however, to mention the enrichment of heavy isotopes of sulfur in lakes. Deevey and Nakai (13) observed a dramatic demonstration of the isotope effect in Green Lake, a meromictic lake near Syracuse, N. Y. Because the sulfur cycle in such a lake cannot be completed, depletion of 32S04, with respect to 34S04, continues without interruption, and 32S sulfide is never returned to the sulfate reservoir in the monimolimnion. Deevey and Nakai compared the lake to a reflux system. H2S-enriched 32S diffuses to the surface waters and is washed out of the lake, leaving a sulfur reservoir depleted in 32S. The result is an 34S value of +57.5% in the monimolimnion. 

Bioturbation and other physical processes associated with the upper portions of marine sediments may lead to rapid exchange between pore-water and overlying depositional water. Depending on the intensity of bioturbation, sulfate in depth zones 1 and 11 and the uppermost part of zone 111 (Figure 4) may be effectively in contact with an infinite reservoir of seawater sulfate. When this is the case, pore-water SO will have a nearly constant 8 value with depth regardless of the withdrawal of isotopically light sulfur to form H2S. The initial isotopic composition of H2S produced by SRB in zones 1 and 11 will be equal to the instantaneous isotopic separation between seawater sulfate and bacterial sulfide (i.e., up to about Aso -HjS = 45%o). Metastable iron sulfides and pyrite formed from this H2S will have an isotopic composition very close to this initial H2S because of the small fractionation observed during sulfidization of iron minerals. 

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