Sulfate isotope ratios

Isotope effects also play an important role in the distribution of sulfur isotopes. The common state of sulfur in the oceans is sulfate and the most prevalent sulfur isotopes are (95.0%) and (4.2%). Sulfur is involved in a wide range of biologically driven and abiotic processes that include at least three oxidation states, S(VI), S(0), and S(—II). Although sulfur isotope distributions are complex, it is possible to learn something of the processes that form sulfur compounds and the environment in which the compounds are formed by examining the isotopic ratios in sulfur compounds. 

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... 

Johnson TM, Bullen TD (2003) Selenium isotope fractionation during reduction by Fe(ll)-Fe(lll) hydroxide-sulfate (green rust). Geochim Cosmochim Acta 67 413-419 Johnson TM, Bullen TD, Zawislanski PT (2000) Selenium stable isotope ratios as indicators of sources and cycling of selenium Results from the northern reach of San Francisco Bay. Environ Sci Tech 34 ... 

Rabinovich AL, Grinenko VA (1979) Sulfate sulfur isotope ratios for USSR river water. Geochemistry 16 No 2 68-79... 

Profiles of rates and forms of S accumulation in sediments may preserve records of past conditions of climate, lake chemistry, or atmospheric deposition. Specifically, it has been suggested that S content, speciation, and isotopic ratio can serve as paleosalinity indicators (18, 20, 21, 231, 232) that S forms can indicate oxygenation of bottom waters (18, 226) that S content and isotopic signature can indicate historical rates of atmospheric deposition of sulfate (26, 29, 30, 49-51, 61, 199) and that S content records the eutrophication history of lakes (24, 25). These hypotheses are examined briefly in the following sections. 

Figure 7.4 Generalized profiles of concentration and isotope ratio changes for dissolved sulfate and carbon species in anoxic marine sediments. Depth scale is arbitrary with depth units ranging from 10 1 to 102 m. (Reproduced from Claypool, G.E., Kvenvolden, K.A., Ann. Rev. Earth Planet Sci., 11, 299 (1983). With permission from Annual Reviews, Inc.)...

Mass Spectral Techniques. Samples for isotope ratio analysis are typically converted to sulfates or sulfides, then to S02(g) for analysis on a mass spectrometer (MS). The precision of the S02 measurement is commonly reported as 0.1 to 0.2 0/00 (16.241. yet systematic errors of 1 0/00 or larger may result from 1) memory effects due to adsorption of S02 on the walls of the MS, and 2) secondary isotope effects due to the existence of two stable isotopes of oxygen, 160 and lsO (251. Both of these errors can be eliminated by using SF6 rather than S02 as the analyte in the MS (25.261. However, existing sulfur fluorination procedures are relatively dangerous and tedious, making the SF6 method less desirable as a routine environmental technique (261. 

Isotope ratios have been used with some success in the past to determine the importance of gas phase (Equation 4) verses aqueous phase (Equations 2A,2B,2C) oxidation of SO2. Saltzman et al. (24) compared the S34S values for SO2 and sulfate from samples collected from Hubbard Brook Experimental Forest (HBEF) in the non-urban northeastern US. They found discriminations which were intermediate to those expected for the individual oxidation mechanisms and suggested that both gas and aqueous phase oxidation were important. Newman (50) found that 6 S values for SO2 in the plume of an oil fired power plant decreased with distance (and time) from the stack which they attributed to equilibrium isotope effects. 

Summary of Non-Marine Sulfur Emissions. S S values for continental sources of atmospheric sulfur dioxide vary, ranging between -32 and +10 0/00. This makes it difficult to use sulfur isotope ratios to distinguish sulfate from these individual sources. It appears that the f S value for marine biogenic sulfur is much more enriched in the heavier isotope than sulfur from continental origins. Therfore, it should be possible to isotopically distinguish between marine biogenic and continentally-derived sulfur. 

In the present work, twenty-seven of these oils were separately analyzed for sulfur content and sulfur isotope ratio (Parr Instrument Company bomb. Sulfate in washings from the bomb were precipitated with Ba2+. The BaSOA precipitate served for gravimetric determination of the S-content conversion to S02 for mass spectrometry (4). The 3AS/32S abundance ratios are presented in the usual 63AS notation. 

Dependence of the 180/160 ratio in sulfate on the 180/160 ratio in the H2O2 by which the sulfate was formed in aqueous oxidation of SO2 was determined. From this and a previously determined dependence of the oxygen isotope ratio in sulfate on that of the solvent water, a comprehensive relation-ship 18Osoa = 0.57 18Oh3o + 0.43 S180Ha0a + 8.4%,... 

The most recent studies of the isotopic composition of the sulfate and dissolved sulfide in the BCL conducted in 2004, however, did not confirm the existence of the lower isotopic trend in the isotopic composition of sulfide. The average sulfur isotopic composition of sulfide below 1750 m was - 40.0%o and varied between - 39.1 and - 41.5% [52], which is very similar to the isotopic composition of most of the anoxic zone (Fig. 6). In contrast, the isotope composition of sulfate in the BCL varied between + 19.9 and + 21.6% (average + 20.8%o) suggesting a slight 34S enrichment of about 1.3%o compared to the entire anoxic zone. Volkov and Rimskaya-Korsakova [52] hypothesized that the observed enrichment in sulfate sulfur was a result of the relative depletion of sulfate during bacterial sulfate reduction in this zone. These data are supported by the 2% decrease in the sulfate/chlorinity ratio observed in the BCL. 

The connection between weathering, evaporation, bacterial activity in marine sediments in shallow water areas and stable sulfur isotope distribution was studied by Holser and Kaplan (1966), Nielsen and Ricke (1969), Nielsen (1968), and Eremenko and Pankina (1972). The sulfur isotope ratios of evaporite sulfates show enrichment of 34S in the Devonian deposits (5 +23 °/oo), a pronounced dip in the Permian ( 5 +12 °/oo), values of +20 °/oo in the Triassic, a fall (+17 to +15 °/oo) in the Jurassic and Cretaceous, stabilizing in the Tertiary to the Recent value of +20 °/oo. 

The isotope ratios of the sulfur isotopes are also affected by kinetic and equilibrium isotope effects. Kinetic isotope effects are marked in the reduction of sulfates to hydrogen sulfide by bacteria (enrichment of the lighter isotopes in H2S). The equilibrium isotope effect in the reaction... 

This phenomenon forms the basis for the formulations of Urey (1947) and Bigeleisen and Mayer (1947) for the temperature dependence of isotopic exchange between two molecules. With the nearly simultaneous development of the isotope-ratio mass spectrometer by Nier et al. (1947), the potential for application of stable isotopes was created. Other isotopic fractionation processes are observed in kinetics, diffusion, evaporation-condensation, crystallization, and biology (e.g., photosynthesis, respiration, nitrogen fixation, sulfate reduction, and transpiration). The concomitant isotopic fractionations can also be used to provide details of the relevant process. 

Sulfur isotope studies have also provided insights into the transition from Archean low Po to higher values in the Proterozoic. In the same studies that revealed extremely low Archean ocean sulfate concentrations, it was found that by —2.2 Ga, isotopic compositions of sedimentary sulfates and sulfides indicate bacterial sulfate reduction under more elevated seawater sulfate concentrations compared with the sulfate-poor Archean (Habicht et al., 2002 Canfield et al., 2000). As described above, nitrogen isotope ratios in sedimentary kerogens show a large and permanent shift at —2.0 Ga, consistent with denitrification, significant seawater nitrate concentrations, and thus available atmospheric O2. 

Vengosh et ai, 1994). The anthropogenic sulfate is also isotopically distinguished, as indicated by the pollution of the Arno River in northern Tuscany with a distinctive sulfur isotopic ratio = 6-8%c Cortecci et al, 2002). 

N.V. Grassineau, D.P. Mattey, D. Lowry (2001) Sulfur isotope analysis of sulfide and sulfate minerals by continuous flow-isotope ratio mass spectrometry. Anal. Chem. 72, 220-225... 

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