Sulfate isotopic composition

Other Mass-independent Sulfate Isotopic Compositions 166... 

After cooling, the reaction mixture is diluted with ether, the organic phase washed with ice-cold water and dried over anhydrous sodium sulfate. Evaporation of the solvent and crystallization of the residue from ether-hexane gives 6,6,8j5-d3-5a-androstan-3j5-ol-7-one (11) in 84% yield mp 141-142,5° isotopic composition 6% d2,93% da and 1 %... 

Sulfur isotopic compositions (S S) of sulfides and sulfate (barite) from the Se-type and Te-type are summarized in Fig. 1.122. Almost all S S values from the Se-type and Te-type fall in a range from —3%o to - -6%o (Fig. 1.122). In general, the 5 S values from the Se-type are similar to those of the Te-type. However, some S S values from the Se-type are lower than those from the Te-type. 

The ground water from well 6 (site 1) yielded both the most depleted S13C and most enriched 834S. Isotopic compositions of this dual nature are consistent with methanogenesis and bacterial sulfate reduction, arising form interaction with organic-rich (bog) soils from below. 

This may seem paradoxical, as the kinetic isotope effect induced by S-O bond breakage still exists. How can the overall reaction have little isotopic fractionation when one step within it has a large kinetic isotope effect The key to understanding this is in the isotopic composition of the intermediate species in the reaction chain. An intermediate may become enriched in heavier isotopes if the next step in the reaction chain preferentially consumes lighter isotopes. In the hypothetical case described above, at steady state the sulfate within the cell is enriched in the heavy isotope by an amount equal to the kinetic isotope effect occurring at step 2. Thus, the isotopic composition of the flux of S through step 2 is the same as that of the flux of S into the cell and the kinetic isotope effect occurring at step 2 has no effect on the overall isotopic fractionation. 

Startup effects. Startup effects must also be considered in the interpretation of laboratory experiments. For example, during sulfate reduction, the first small amormt of sulfur to pass through the chain of reaction steps would be subject to the kinetic isotope effects of all of the reaction steps. This is because it takes some time for the isotopic compositions of the pools of intermediates to become enriched in heavier isotopes as described above for the steady-state case. Accordingly, the first HjS produced would be more strongly enriched in the lighter isotopes than that produced after a steady state has been approached. This principle was modeled by Rashid and Krouse (1985) to interpret kinetic isotope effects occurring during abiotic reduction of Se(IV) to Se(0) (see below). Startup effects may be particularly relevant in laboratory experiments where Se or Cr concentrations are very small, as is the case in some of the studies reviewed below. 

Young et al. (in press) conducted related experiments. They examined the Cu isotope compositions of the solutions produced during acid sulfate leaching experiments aimed at extracting Cu from ore minerals (chalcopyrite, chalcocite, djurleite, bomite). The leaching experiment for chalcopyrite reads ... 

Sulfur isotope relative fractionation factors between various minerals and py-rite are shown in figure 11.40. As we can see, the isotopic compositions of sulfates reflect the relative fractionation effects induced by the 804 groups (compare figures 11.40 and 11.39). 

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... 

The most notable feature of the sulfur isotope geochemistry of lunar rocks is the uniformity of 8 " S-values and their proximity to the Canyon Diablo standard. The range of published 8 " S-values is between -2 to +2.5%o. However, as noted by Des Marais (1983), the actual range is likely to be considerably narrower than 4.5%o due to systematic discrepancies either between laboratories or between analytical procedures. The very small variation in sulfur isotope composition supports the idea that the very low oxygen fugacities on the Moon prevent the formation of SO2 or sulfates, thus eliminate exchange reactions between oxidized and reduced sulfur species. 

Elucidation of the origin of sulfur in volcanic systems is complicated by the fact that next to SO2, significant amounts of H2S, sulfate and elemental sulfur can also be present. The bulk sulfur isotope composition must be calculated using mass balance constraints. The principal sulfur gas in equilibrium with basaltic melts at low pressure and high temperature is SO2. With decreasing temperature and/or increasing... 

Coplen TB, Brand WA, Gehre M, Groning M, Meijer HA, Toman B, Verkouteren RM (2006) New guidelines for S C measurements. Anal Chem 78 2439-2441 Cortecci G, Longinelli A (1970) Isotopic composition of sulfate in rain water, Pisa, Italy. Earth Planet Sci Lett 8 36 0... 

Ku TCW, Walter LM, Coleman ML, Blake RE, Martini AM (1999) Coupling between sulfur recycling and syndepositional carbonate dissolution evidence from oxygen and sulfur isotope composition of pore water sulfate, South Florida Platform, USA, Geochim Cosmochim Acta 63 2529-2546... 

Mekhtiyeva VL, Pankina GR (1968) Isotopic composition of sulfur in aquatic plants and dissolved sulfates. Geochemistry 5 624... 

Paytan A, Kastner M, Campbell D, Thiemens MH (1998) Sulfur isotope composition of Cenozoic seawater sulfate. Science 282 1459-1462... 

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