Geology and stable isotope geochemistry of Paleoarchean sulfur. Formation, preservation and geobiology of ancient pyrite and barite

Abstract

Sulfur isotopes in ancient sulfate and sulfide minerals provide a comprehensive record of microbial processes involved in the early sulfur cycle on Earth. However, the interpretation of these isotopic signatures requires information on the geological context of such samples, because abiotic reactions can produce similar degrees of isotope fractionation to biological pathways. This thesis presents an integrated study of the geology and sulfur isotope geochemistry of Paleoarchean pyrite and barite from the Barberton Greenstone Belt in South Africa and Swaziland, to explore to what extent sulfur isotope ratios in these rocks record ancient microbial activity and how they were affected or overprinted by abiotic processes. Constant mass-independent sulfur isotope signatures in four different barite deposits (3.5-3.2 Ga) support an important role for atmospheric reactions in the production of sulfate, whereas relative 34S-enrichment suggests that the deposits reflect a large-scale residual pool after microbial sulfate reduction. In addition, co-variation between δ34S and Δ33S within individual deposits requires local fractionation by sulfate reducers, instead of abiotic mixing in magmatic-hydrothermal systems. This important role for basin-scale biological processing of sulfate is confirmed by high-resolution quadruple sulfur isotope data from 3.2 Ga sedimentary rocks at Barite Valley, South Africa. Barite-hosted pyrite is strongly depleted in 34S and shows similar negative Δ33S-values as the barite, consistent with microbial reduction at sulfate levels above 200 μM. In contrast, pyrite from barite-free sedimentary rocks defines an array in Δ33S/δ34S that extends from the composition of the barite towards positive Δ33S-values, reflecting suppressed biogenic isotope fractionation at