High Activity of Sulphate-Reducing Bacteria in Archaean and Proterozoic Oceans and an Early Development of the Phanerozoic-Style Sulphur Cycles

Abstract

Pyrite crystals in Precambrian sedimentary rocks are characterized by 834S values around 0%0. Two different models have been suggested to explain these sulphur isotope data: sulphide formation in sulphate-poor ocean (Cameron, 1982; Canfield and Teske, 1996); sulphide formation by extensive microbial sulphate reduction in sulphate-rich ocean (Ohmoto et al., 1993). Changes in the sulphate content of the past oceans were likely to have been directly related to the oxygen level in the atmosphere, because sulphate is a product of oxic weathering of sulphide minerals on continents, as well as of weathering of anhydride. The surface environments (temperature, redox state) of the Precambrian Earth can be constrained by examining whether pyrites in Precambrian sedimentary rocks were formed in sulphate-rich or sulphate-poor oceans. For that purpose, micro-analyses of sulphur isotopes were performed on large numbers of single grains of pyrite crystals in the Archaean and Proterozoic sedimentary rocks.