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Pathways for Neoarchean Pyrite Formation Constrained by Mass

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Pathways for Neoarchean Pyrite Formation Constrained by Mass Pathways for Neoarchean pyrite formation constrained SPECIAL FEATURE by mass-independent sulfur isotopes James Farquhara,b,1, John Cliffb, Aubrey L. Zerklec, Alexey Kamyshnyd, Simon W. Poultone, Mark Clairef, David Adamsb, and Brian Harmsa aDepartment of Geology and Earth System Science Interdisciplinary Center, University of Maryland, College Park, MD 20742; bCentre for Microscopy and Microanalysis, University of Western Australia, Perth, WA 6009, Australia; cSchool of Civil Engineering and Geosciences, Newcastle University, Newcastle upon Tyne NE1 7RU, United Kingdom; dDepartment of Geological and Environmental Sciences, Faculty of Natural Sciences, Ben-Gurion University of the Negev, Beer Sheva 84105, Israel; eSchool of Earth and Environment, University of Leeds, Leeds LS2 9JT, United Kingdom; and fSchool of Environmental Sciences, University of East Anglia, Norwich NR4 7TJ, United Kingdom Edited by Mark H. Thiemens, University of California at San Diego, La Jolla, CA, and approved December 28, 2012 (received for review November 1, 2012) It is generally thought that the sulfate reduction metabolism is range of variability for Δ33S is significantly greater in samples older ancient and would have been established well before the Neo- than ∼2.4 Ga than in younger samples (e.g., compilation in refs. archean. It is puzzling, therefore, that the sulfur isotope record of 4 and 5). This observation has been linked to the production, the Neoarchean is characterized by a signal of atmospheric mass- transfer, and preservation of mass-independent sulfur isotope independent chemistry rather than a strong overprint by sulfate signals (presumably of atmospheric origin) early in Earth history. reducers. Here, we present a study of the four sulfur isotopes The production of this signal in the atmosphere and its subsequent obtained using secondary ion MS that seeks to reconcile a number transfer to the Earth surface is sensitive to atmospheric O2 levels of features seen in the Neoarchean sulfur isotope record. We and the redox state of sulfur in the atmosphere (6). Furthermore, suggest that Neoarchean ocean basins had two coexisting, signif- the preservation of a mass-independent signal in the sedimentary icantly sized sulfur pools and that the pathways forming pyrite record is sensitive to the intensity of redox cycling of sulfur in precursors played an important role in establishing how the isotopic surface environments, which is greater in an oxidized world (4, 7, 8). The disappearance of the large range in Δ33S variability from characteristics of each of these pools was transferred to the the geologic record at ∼2.4 Ga has, therefore, been attributed to sedimentary rock record. One of these pools is suggested to be the rise of atmospheric oxygen. Studies that have looked at the a soluble (sulfate) pool, and the other pool (atmospherically derived EARTH, ATMOSPHERIC, late Archean sulfur cycle in detail have also argued that the mean AND PLANETARY SCIENCES elemental sulfur) is suggested to be largely insoluble and unreactive Δ33 fi value for the samples analyzed thus far shows a positive S bias until it reacts with hydrogen sul de. We suggest that the relative (9). The origin of this bias is not understood but may be linked contributions of these pools to the formation of pyrite depend on to the identity of the sulfur pools in the early oceans and the fi both the accumulation of the insoluble pool and the rate of sul de transfer of sulfur from these pools to sedimentary pyrite in the production in the pyrite-forming environments. We also suggest late Archean. that the existence of a significant nonsulfate pool of reactive sulfur In addition to δ34S and Δ33S records, Δ36S records for the late has masked isotopic evidence for the widespread activity of sulfate Archean have also received a great deal of attention. The ma- reducers in the rock record. jority of Δ33S vs. Δ36S data presented for the late Archean converges on the origin (Δ33S ∼ Δ36S ∼ 0) and displays greater Neoarchean polysulfide production pathways | δ34S | Δ33S | Δ36S variability when Δ33S and Δ36S values are large. Some studies have noted (10, 11) that data from discrete stratigraphic intervals 36 33 ver the past 30 y, a significant amount of sulfur isotope seem to define linear arrays with a constant slope (Δ S/Δ S). These arrays do not all intersect the origin but instead, often Odata has been collected for sedimentary rocks and used as 33 36 cross the axes at Δ S = 0 and Δ S ≠ 0. Although the different a way to study past atmospheric chemistry and biological ac- 36 33 tivity. One feature of the sulfur isotope record that has been relationships between Δ S and Δ S have been attributed to an well-documented in the literature is a change from a relatively atmospheric origin, it is unclear how important additional pro- 36 small range of variability for δ34S in Archean sediments to a cesses that are known to produce variations in Δ S (like sulfate larger range in younger sediments. Interpretations of the δ34S reduction and pyrite formation) are for producing the observed ½ð34S=32SÞ =ð34S=32SÞ − 1 record have linked this pat- compositional variability. sample V​-​CDT Here, we present results of secondary ion MS (SIMS) analyses tern primarily to changes in the way that organisms metabolize (spot analyses) of sulfur isotope values in Neoarchean pyrites sulfur. Experiments have shown that the magnitude of sulfur iso- and explore these data with the specific goal of addressing some tope fractionation diminishes as sulfate concentrations decrease of the outstanding issues outlined above. This study has three (1). At low sulfate concentrations, sulfate reducers express only aims. The first aim is to examine the origin and significance of small fractionations, because the sulfate reduction metabolism Δ36 ½ð36 =32 Þ =ð36 =32 Þ − operates as a nearly unidirectional chain of metabolic steps, and the relationship between S S S sample S S V-CDT ½ð34 =32 Þ =ð34 =32 Þ 1:9 Δ33 ½ð33 =32 Þ = the isotope effects associated with each of the individual metabolic S S sample S S V-CDT and S S S sample steps are not expressed in the final metabolic product (cf. 2). This 33 32 34 32 34 32 0:515 ð S= SÞ − ½ð S= SÞ =ð S= SÞ in Neo- microbial response to sulfate concentration has been invoked as V-CDT sample V-CDT the principle reason for the small range of δ34S observed early in archean rocks. The second aim is to explore the nature of the geologic record, suggesting that sulfate concentrations were sulfur isotope evidence for sulfate reduction in Archean lower in Earth’s early oceans. It is not known if other factors also played a part in generating the small range of δ34S fractionations observed in the Neoarchean. We note that very large fractiona- Author contributions: J.F. and J.C. designed research; J.F., J.C., A.L.Z., A.K., S.W.P., M.C., tions can occur at only slightly higher sulfate concentrations (1.1–2 D.A., and B.H. performed research; J.C. and D.A. contributed new reagents/analytic tools; mM) (3) and that the low concentration limit for generation of J.F. and J.C. analyzed data; and J.F., J.C., A.L.Z., A.K., S.W.P., M.C., and B.H. wrote the paper. large fractionations associated with sulfate reduction may even- The authors declare no conflict of interest. tually be revised. This article is a PNAS Direct Submission. Another feature of the sulfur isotope record is revealed by the 1To whom correspondence should be addressed. E-mail: [email protected]. 33 Δ S values of pyrite and sedimentary sulfate, which provide ad- This article contains supporting information online at www.pnas.org/lookup/suppl/doi:10. ditional information about the evolution of the sulfur cycle. The 1073/pnas.1218851110/-/DCSupplemental. www.pnas.org/cgi/doi/10.1073/pnas.1218851110 PNAS Early Edition | 1of6 Downloaded by guest on September 28, 2021 environments, particularly whether the four isotopes of sulfur provide widespread evidence for sulfate reduction, even when the evidence provided by δ34S alone is minor. The third aim is to explore the nature of the connections between different sulfur pools in the Archean sulfur cycle and to determine if there is evidence for an additional large standing sulfur pool in early oceanic and sedimentary environments. We examine whether these data are coherent at the grain scale, and what these data tell us about pathways for pyrite formation. We suggest that the generation of polysulfide in the Neoarchean may explain many poorly understood features of the sedimentary record of mass independent sulfur. Samples The samples investigated here are from the GKF01 drill core, which intersects well-preserved sediments of the ∼2.65- to 2.5-Ga Camp- bellrand–Malmani carbonate platform (Ghaap Group, Transvaal Supergroup, South Africa) (12). Sample selection was guided by a previous study undertaken by Zerkle et al. (11). A subset of the strata studied by Zerkle et al. (11) was selected for more detailed analysis, with the aim of evaluating variations in the four sulfur iso- topes at the grain and subgrain scale. Samples were also selected to test whether the nature of grain-scale isotopic heterogeneity mea- sured by SIMS supports or contradicts hypotheses developed in the study by Zerkle et al. (11) on the basis of whole-rock SF6 analyses. Two types of samples were chosen for this study. One subset of samples consists of fine-grained laminated clastic lithologies (shales and siltstones) with small disseminated pyrites distributed Fig. 1. Electron probe maps of ovoid (type 1) pyrite grains and euhedral along bedding planes. The pyrite grains range in size from a few (type 2) grains.
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