Abstract
Sulfate-reducing bacteria (SRB) are considered to be one of the oldest micro-organisms in the Earth history. Sulfur isotope data of sedimentary pyrite indicate that SRB have been active at least since 3.5 Ga. However, living environments of SRB, such as concentration level of sulfate in the Archean oceans and sulfur flux into Archean oceans, have been debated vigorously.
Detailed sulfur isotope studies were performed on individual pyrite crystals in black shale and chert samples collected from several Archean greenstone belts. Sulfur isotope compositions of pyrite in all samples are fractionated, suggesting their biogenic origin. It is found that some representative sample sets from 2.7 Ga greenstone belts show a good correlation between organic carbon and pyrite sulfur concentrations. These results suggest that Archean oceans have been sulfate-rich at least since 2.7 Ga. This sulfate-rich ocean model conflicts with the sulfate-poor ocean model based on the mass independent fractionation of sulfur isotopes.
