Antiquity and evolutionary status of bacterial sulfate reduction: Sulfur isotope evidence

Abstract

The presently available sedimentary sulfur isotope record for the Precambrian seems to allow the following conclusions: (1) In the Early Archaean, sedimentary δ^{3 4} patterns attributable to bacteriogenic sulfate reduction are generally absent. In particular, the δ^{3 4} spread observed in the Isua banded iron formation (3.7×10⁹ yr) is extremely narrow and coincides completely with the respective spreads yielded by contemporaneous rocks of assumed mantle derivation. Incipient minor differentiation of the isotope patterns notably of Archaean sulfates may be accounted for by photosynthetic sulfur bacteria rather than by sulfate reducers. (2) Isotopic evidence of dissimilatory sulfate reduction is first observed in the upper Archaean of the Aldan Shield, Siberia (∼3.0×10⁹ yr) and in the Michipicoten and Woman River banded iron formations of Canada (2.75×10⁹ yr). This narrows down the possible time of appearance of sulfate respirers to the interval 2.8–3.1×10⁹ yr. (3) Various lines of evidence indicate that photosynthesis is older than sulfate respiration, the SO ₄ ²⁻ utilized by the first sulfate reducers deriving most probably from oxidation of reduced sulfur compounds by photosynthetic sulfur bacteria. Sulfate respiration must, in turn, have antedated oxygen respiration as O₂-respiring multicellular eucaryotes appear late in the Precambrian. (4) With the bulk of sulfate in the Archaean oceans probably produced by photosynthetic sulfur bacteria, the accumulation of SO ₄ ²⁻ in the ancient seas must have preceded the buildup of appreciable steady state levels of free oxygen. Hence, the occurrence of sulfate evaporites in Archaean sediments does not necessarily provide testimony of oxidation weathering on the ancient continents and, consequently, of the existence of an atmospheric oxygen reservoir.