THE AECHEAN SULFUR CYCLE
We have recently discovered that MIF-S is absent in two major shale formations in the Pilbara Craton, Australia: the 2.76 Ga Hardey lacustrine shales and the 2.92 Ga Mosquito Creek marine shales. The S/C relationships of these shales and morphologies of the pyrite crystals suggest that the pyrite crystals were products of bacterial sulfate reduction in the overlying water bodies or in pore water of sediments. We have also recognized that the mineralogical and geochemical characteristics (e.g., concentration ratios among S, organic C, N, Fe, P, V, U, Mo; isotope ratios of C, N, and S) of many major Archean and Paleoproterozoic black shale formations are essentially the same as those of Phanerozoic black shales.
Based on these data, we conclude that the Archean sulfur cycle was essentially the same as today. For the connection between the atmospheric pO2 level and MIF-S, we suggest the following three possibilities: (1) The atmospheric pO2 level frequently fluctuated from anoxic to oxic during the Archean; (2) The atmosphere remained oxic since ~3.8 Ga, and the MIF-S signatures represent periods when volcanic eruptions ejected large volumes of SO2 to the stratosphere; and (3) The atmospheric pO2 history was not linked to the geologic record of MIF-S.