Sulfur elemental isotope fractionation

R. W. FairbriiXjE, Encyclopedia of Geochemistry and Environmental Sciences, Van Nostrand, New York, 1972.. See sections on Geochemical Classification of the Elements Sulfates Sulfate Reduction-Microbial Sulfides Sulfosalts Sulfur Sulfur Cycle Sulfur Isotope Fractionation in Biological Processes, etc., pp. 1123 - 58. 

It is possible that a branching reaction could cause isotopic fractionation of Se removed from a Se(0) precipitate, but no evidence exists for this at present. Elemental sulfur can be converted to sulfate and sulfide through a branching reaction mediated by disproportionating bacteria, and the produced sulfate s ratio is shifted +17%o to +31%o relative to the... 

Bottcher ME, Thamdrup B, Vennemann TW (2001) Oxygen and sulfur isotope fractionation during anaerobic bacterial disproportionation of elemental sulfur. Geochim Cosmochim Acta 65 ... 

Naturally occurring stable isotopes of C, N, and S have been used extensively for over a decade as direct tracers of element cycling in marine and terrestrial food webs (34-39). Carbon and sulfur isotopes fractionate very little between food and consumer thus their measurement indicates which primary producers or detrital pools are sources of C and S for consumers. For example, a study of plants and animals in Texas sand dunes showed that insect species had 813C values either like those of C3 plants or like those of C4 plants (-27 and -13%o, respectively). Rodent species had intermediate values near -20%o that indicated mixed diets of both C3 and C4 plants (40). The 13C measurements, used in simple linear mixing models, proved to be quick and reliable indicators of which plant sources provided the carbon assimilated by higher trophic levels. 

A summary of the available results on the extent of isotope fractionation during sulfide oxidation is summarized in Table 3. The phototrophic oxidations of sulfide to elemental sulfur and of elemental sulfur to sulfate yield only small or negligible fractionations. Small fractionations also accompany the non-phototrophic, biologically-mediated, oxidation of sulfide to elemental sulfur, as well as the oxidation of sulfur intermediate compounds to sulfate (Table 3). However, significant depletion of sulfate in... 

Studies of isotope fractionation during sulfide oxidation are rather scant and important sulfide-oxidizing organisms like, for example, Beggiatoa sp. and Thioploca sp., have yet to be studied. Nevertheless, the available results suggest that biologically mediated oxidation of sulfate to elemental sulfur and sulfate lead to only minimal isotope fractionation. 

Table 4. Isotope fractionation (%o) during elemental sulfur (S ) disproportionation by pure and enrichment cultures.

Canfield DE, Thamdrup B, Fleisher S (1998a) Isotope fractionation and sulfur metabolism by pure and enrichment cultures of elemental sulfur-disproportionating bacteria. Limnol Oceanogr 43 253-264... 

Stable isotopes serve as naturally occurring tracers that can provide much information about how chemical reactions proceed in nature, such as which reactants are consumed and at what temperature reactions occur. The stable isotopes of several of the lighter elements are sufficiently abundant and fractionate strongly enough to be of special usefulness. Foremost in importance are hydrogen, carbon, oxygen, and sulfur. 

In react, dataset isotope.dat contains polynomial coefficients that define temperature functions for the fractionation factors of species, minerals, and gases. The factors describe fractionation relative to a reference species chosen for each element. The reference species for oxygen and hydrogen is solvent water, H2O. CO2 and H2S, in either aqueous or gaseous form, serve as reference species for carbon and sulfur. 

Over the last 30 years the study of the stable isotope composition of carbonates has been one of the more active areas of research in carbonate geochemistry. These studies have particular application to later discussion of carbonate diagenesis and historical geochemistry of carbonate rocks. Many of the same considerations involved in understanding elemental distribution coefficients apply to the fractionation of stable isotopes. Consequently, we have included a discussion of the chemical principals associated with isotope behavior in this chapter. Only a relatively brief summary of these basic chemical considerations will be presented here, because recent books and extensive reviews are available on this topic (e.g., Arthur et al., 1983 Hoefs, 1987). Also, our discussion will be restricted to carbon and oxygen isotopes, because these isotopes are by far the most important for the study of carbonate geochemistry. The principles, however, apply to other stable isotopes (e.g., sulfur). 

A significant fraction of H2S produced by dissimilatory processes will also be oxidized by other reactions (Jprgensen, 1982). These intermediate sulfur species (e.g., elemental sulfur) from H2S oxidation may be enriched in 34S, also contributing to the overall 34S signal of H2S (Fry et al., 1988 Canfield and Thamdrup, 1994). Stable sulfur isotopes in... 

Kaplan, I.R. and Rittenberg, S.C., 1962. Fractionation of isotopes in relation to the problem of elemental sulfur transport by microorganisms. Nature, 194 1098—1099. 

Due to the presence of iron oxides in the experiments, the observed fractionations do not necessarily represent the true cellular level fractionations characteristic of these organisms. This is because additional elemental sulfur may form as a result of the reaction of iron oxides with the sulfide formed during disproportionation. The observed isotopic compositions of the sulfide and sulfate depend on the extent to which this newly formed elemental sulfur either accumulates during the experiment or is re-utilized by the organism. Therefore, the experimental results have been corrected to reconstruct the cellular level fractionations during the disproportionation process (for details see Canfield et al. 1998a). In fact, in most cases the correction is quite small (Table 4). 

The basic difference between the formation of sulfur and the carbon gases and its influence on the isotopic ratios of kerogen, bitumen, oil and gas lies in the chemical activity of the thermally derived fragments. Both elemental sulfur and hydrogen sulfide formed at elevated temperatures could re-react with all fractions (Stoler et al., 2003). Research on changes in both carbon and... 

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