THE AECHEAN SULFUR CYCLE

The presence of mass-independent fractionation of sulfur isotopes (MIF-S) in some sedimentary rocks older than ~2.45 Ga, and absence of MIF-S in sediments younger than ~2.32 Ga, have been used to suggest a dramatic change from an anoxic to oxic atmosphere around 2.35 Ga. This is because the only mechanism to create MIF-S has been believed to be photochemical reactions of volcanic SO2 by UV light in the absence of an ozone shield. Current popular models for the Archean sulfur cycle also postulate that: (i) the sulfate contents of oceans were less than 1/100 of today, because sulfide minerals were not weathered to produce sulfate under an anoxic atmosphere; and (ii) most of sulfides in sedimentary rocks were not produced by bacterial sulfate reduction, but they were detrial grains of igneous/hydrothermal sulfides and/or products of atmospheric photochemical reactions of volcanic SO2 by UV.

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.