Precambrian sulfur isotopes and a possible role for sulfite in the evolution of biological sulfate reduction

Abstract

Abstract This paper examines how and when the biochemical process of dissimilatory sulfate reduction evolved. During sulfate reduction by modern sulfate-reducing bacteria the sulfide produced is enriched in 32S compared to the starting sulfate. This phenomenon is used to interpret the origin of sulfide in both modern and ancient environments. The isotopic compositions of Precambrian sulfide and sulfate minerals suggests that until ca. 2.0 Ga before present (B.P.) sulfide was the major S-component in sediments and the hydrosphere and was formed mainly by volcanogenic processes. After ca. 2.0 Ga B.P., when free oxygen was maintained in the atmosphere by the biological photolysis of water, sulfate became the major sulfur component of the seas and was the main sulfur source for both biological and abiological sulfide formation. Dissimilatory sulfate reduction then became widespread. The extent of 34S-enrichment of a minor proportion of Archaean and Lower Proterozoic volcanogenic sulfides indicates their formation by reduction of an oxidised sulfur source. This source was not sulfate, which formed the Archaean deposits ( δ 34 S(SO 4 2− ) ⋍ 2 ± 2‰ ), but most probably SO2(SO32− or HSO3−) introduced into the hydrosphere in exhalations in which H2S ⪢ SO2 and where isotopic equilibria favour 34S-enrichment in the more oxidised sulfur compound. It is suggested that: 1. (1) In the anoxic environments of the primitive Earth, sulfite was the most readily available and persistent S-source to evolving procaryotes. 2. (2) The biochemical evolution of sulfite reduction occurred early among procaryotes and was the initial event leading to the evolution of dissimilatory sulfate reduction. 3. (3) Dissimilatory (and assimilatory) sulfate reduction evolved when sulfate became the predominant sulfur compound available to the evolving procaryotes around 2.0 Ga B.P.