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 massindependent 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.Neoarchean polysulfide production pathways | δ 34 S | Δ 33 S | Δ 36 S O ver the past 30 y, a significant amount of sulfur isotope data has been collected for sedimentary rocks and used as a way to study past atmospheric chemistry and biological activity. One feature of the sulfur isotope record that has been well-documented in the literature is a change from a relatively small range of variability for δ 34 S in Archean sediments to a larger range in younger sediments. Interpretations of the δ 34 S ½ð 34 S= 32 SÞ sample =ð 34 S= 32 SÞ V -CDT − 1 record have linked this pattern primarily to changes in the way that organisms metabolize sulfur. Experiments have shown that the magnitude of sulfur isotope fractionation diminishes as sulfate concentrations decrease (1). At low sulfate concentrations, sulfate reducers express only small fractionations, because the sulfate reduction metabolism operates as a nearly unidirectional chain of metabolic steps, and the isotope effects associated with each of the individual metabolic steps are not expressed in the final metabolic product (cf. 2). This microbial response to sulfate concentration has been invoked as the principle reason for the small range of δ 34 S observed early in the geologic record, suggesting that sulfate concentrations were lower in Earth's early oceans. It is not known if other factors also played a part in generating the small range of δ 34 S fractionations observed in the Neoarchean. We note that very large fractionations can occur at only slightly higher sulfate concentrations (1.1-2 mM) (3) and that the low concentration limit for generation of large fractionations associated ...

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The minimum oxygen fugacity (fO2) of Earth’s upper mantle probably is controlled by metal saturation, as defined by the iron-wüstite (IW) buffer reaction (FeO → Fe + O). However, the widespread occurrence of moissanite (SiC) in kimberlites, and a suite of super-reduced minerals (SiC, alloys, native elements) in peridotites in Tibet and the Polar Urals (Russia), suggest that more reducing conditions (fO2 = 6–8 log units below IW) must occur locally in the mantle. We describe pockets of melt trapped in aggregates of corundum crystals ejected from Cretaceous volcanoes in northern Israel which contain high-temperature mineral assemblages requiring extremely low fO2 (IW < −10). One abundant phase is tistarite (Ti2O3), previously known as a single grain in the Allende carbonaceous chondrite (Mexico) and believed to have formed during the early evolution of the solar nebula. It is associated with other reduced phases usually found in meteorites. The development of super-reducing conditions in Earth’s upper mantle may reflect the introduction of CH4 + H2 fluids from the deep mantle, specifically related to deep-seated volcanic plumbing systems at plate boundaries.

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