Lack of Fractionation of Sulfur Isotopes

There is a comparative lack of fractionation of sulphur isotopes between coexisting sulphide and sulphates in Archean and early Proterozoic sediments relative to Phanerozoic sediments (figure 7-7) (Ohmoto and Felder 1987; Knoll 1990; Bottomley et al. 1992). Knoll (1990) reviews possible explanations for this lack of fractionation in sediments dated at 3.5 Ga, concluding:

. . . perhaps almost all sulphate in pore fluids was reduced biologically to sulphide in an essentially closed system with little fractionation because of high ambient temperatures (70°C or more)—a theory for which the geological record provides little supporting evidence. A generally acceptable solution to this problem has not yet been proposed (p. 12).

We cite here and in other published work just such supporting evidence.

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