Multiple sulfur isotopes in Paleoarchean barites identify an important role for microbial sulfate reduction in the early marine environment
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It is generally thought that the sulfate reduction metabolism is ancient and would have been established well before the Neoarchean. It is puzzling, therefore, that the sulfur isotope record of the Neoarchean is characterized by a signal of atmospheric mass-independent chemistry rather than a strong overprint by sulfate reducers. Here, we present a study of the four sulfur isotopes obtained using secondary ion MS that seeks to reconcile a number of features seen in the Neoarchean sulfur isotope record. We suggest that Neoarchean ocean basins had two coexisting, significantly sized sulfur pools and that the pathways forming pyrite precursors played an important role in establishing how the isotopic characteristics of each of these pools was transferred to the sedimentary rock record. One of these pools is suggested to be a soluble (sulfate) pool, and the other pool (atmospherically derived elemental sulfur) is suggested to be largely insoluble and unreactive until it reacts with hydrogen sulfide. We suggest that the relative contributions of these pools to the formation of pyrite depend on both the accumulation of the insoluble pool and the rate of sulfide production in the pyrite-forming environments. We also suggest that the existence of a significant nonsulfate pool of reactive sulfur has masked isotopic evidence for the widespread activity of sulfate reducers in the rock record.
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Abstract —In the present paper we demonstrate that most sulfides of the studied deposits of the Archean Sumozero–Kenozero greenstone belt within the Karelian Craton on the Fennoscandian Shield have nonzero Δ33S values. This indicates that proportions of seawater sulfate and elemental sulfur in Mesoarchean, included into the ores and resulting from UV photolysis, are different. Our results show that systematics of sulfur isotopes of sulfides generally reflects the mixing of mass-independently fractionated sulfur reservoirs with positive and negative Δ33S values. Pyrite is depleted in 34S isotope, which was interpreted as evidence for microbial sulfate reduction. Variations in the positive Δ33S anomalies of the Leksa deposit and the general tendency for Δ33S sulfide content to increase with stratigraphic levels in certain boreholes most likely reflect the change in temperature and the fluid mixing throughout the life of the hydrothermal system. The presence of sulfides with strongly negative Δ33S anomalies suggests that atmospheric sulfur and seawater sulfate, rather than volcanic sulfur, were the prevailing source for mineral systems of the studied deposits. The presented data require the Mesoarchean seawater to contain sulfates at least locally.
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The environmental expression of sulfur compound disproportionation has been placed between 640 and 1050 million years ago (Ma) and linked to increases in atmospheric oxygen. These arguments have their basis in temporal changes in the magnitude of 34S/32S fractionations between sulfate and sulfide. Here, we present a Proterozoic seawater sulfate isotope record that includes the less abundant sulfur isotope 33S. These measurements imply that sulfur compound disproportionation was an active part of the sulfur cycle by 1300 Ma and that progressive Earth surface oxygenation may have characterized the Mesoproterozoic.
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