Sulfate Rayleigh fractionation

Figure 2. Rayleigh fractionation curves of 834S as a function of the percent of the initial sulfate reduced.

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. 

The isotopic composition of chemical species in such a system evolves quite differently than in a closed system experiencing Rayleigh fractionation. For example, the progressive reduction of sulfate, with an associated fractionation of 40 %o, produces a linear relationship between the isotopic composition of the sulfate and the extent of the original sulfate remaining (Fig. 12). The equation expressing relationship in Figure 12 (Eqn. 19) is derived from Equation (16) with the additional mass balance constraints (Eqns. 17 and 18) ... 

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.

---