Sulfide Rayleigh fractionation

In a low-sulfate lake (lower Rayleigh fractionation curve in Figure 10), the isotopic composition of sulfur in sulfide minerals is fixed at a 834S value near the composition of the sulfate in the lake by essentially complete reduction within the sediment column. The Smin concentrations reflect the lake s low sulfate concentration. Preferential loss of 34S-depleted H2S from the sediment moves the 834Smin values to the right. 

Figure 10. This graph shows how the isotopic composition of sulfate and sulfide evolve in a closed system where sulfide is produced by sulfate reduction with an a of 1.040 and an initial isotopic composition of sulfate of 20 %o. Sulfide is assumed to accumulate quantitatively as sulfate becomes depleted. The parameter /so4 expresses the remaining fraction of the original sulfate in the system. A Rayleigh distillation model was used to calculate these results. See text for details.

Figure 11. This graph shows how both the concentrations and isotopic compositions of snlfate and snffide evolve with sediment depth in the sapropelic sediments of Mangrove Lake, Bermnda. Whereas the isotopic compositions of snlfate and sulfide appeared to evolve as in a Rayleigh distillation model (see Fig. 10), snch a model is inappropriate for determining fractionations. This is because marine sediments are open with respect to the exchange of chemical species. See text for details. Data are replotted from Canfield et al. (1998b).

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