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Abstract: Detailed iron, sulfur and carbon chemistry through the >742million year old Chuar Group reveals a marine basin dominated by anoxic and ferrous iron-rich (ferruginous) bottom waters punctuated, late in the basin''s development, by an intrusion of sulfide-rich (euxinic) conditions. The observation that anoxia occurred frequently in even the shallowest of Chuar environments (10s of meters or less) suggests that global atmospheric oxygen levels were significantly lower than today. In contrast, the transition from ferruginous to euxinic subsurface water is interpreted to reflect basinal control—specifically, increased export of organic carbon from surface waters. Low fluxes of organic carbon into subsurface water masses should have been insufficient to deplete oxygen via aerobic respiration, resulting in an oxic oxygen minimum zone (OMZ). Where iron was available, larger organic carbon fluxes should have depleted oxygen and facilitated anaerobic respiration using ferric iron as the oxidant, with iron carbonate as the expected mineralogical signature in basinal shale. Even higher organic fluxes would, in turn, have depleted ferric iron and up-regulated anaerobic respiration by sulfate reduction, reflected in high pyrite abundances. Observations from the Chuar Group are consistent with these hypotheses, and gain further support from pyrite and sulfate sulfur isotope abundances. In general, Chuar data support the hypothesis that ferruginous subsurface waters returned to the oceans, replacing euxinia, well before the Ediacaran emergence of persistently oxygenated conditions, and even predating the Sturtian glaciation. Moreover, our data suggest that the reprise of ferruginous water masses may relate to widespread rifting during the break-up of Rodinia. This environmental transition, in turn, correlates with both microfossil and biomarker evidence for an expanding eukaryotic presence in the oceans, suggesting a physiologically mediated link among tectonics, environmental chemistry and life in the dynamic Neoproterozoic Earth system. [Copyright &y& Elsevier]

Triple oxygen isotopes within post-Marinoan barites have played an integral role in our understanding of Cryogenian glaciations. Reports of anomalous Δ O 17 values within cap carbonate hosted barites however have remained restricted to South China and Mauritania. Here we extend the Δ O 17 anomaly to northwest Canada with our new measurements of barites from the Ravensthroat cap dolostone with a minimum Δ O 17 value of − 0.75 ‰ . For the first time we pair triple oxygen with multiple sulfur isotopic data as a tool to identify the key processes that controlled the post-Marinoan sulfur cycle. We argue using a dynamic 1-box model that the observed isotopic trends both in northwest Canada and South China can be explained through the interplay between sulfide weathering, microbial sulfur cycling and pyrite burial. An important outcome of this study is a new constraint placed on the size of the post-Marinoan sulfate reservoir (≈0.1% modern), with a maximum concentration of less than 10% modern. Through conservative estimates of sulfate fluxes from sulfide weathering and under a small initial sulfate reservoir, we suggest that observed isotopic trends are the product of a dynamic sulfur cycle that saw both the addition and removal of the Δ O 17 anomaly over four to five turnovers of the post-Marinoan marine sulfate reservoir. [ABSTRACT FROM AUTHOR]

Accurately reconstructing the temperature of the ocean through time carries implications for Earth's climate and early habitability. Attempts to build these reconstructions using oxygen isotope records have led to three end-member interpretations. Namely, that the observed enrichment over time in 18O relative to 16O in ancient chemical sediments reflects a change in sea-surface temperatures (SST), a change in the 18O composition of the contemporaneous water, or that the primary signal has been subsequently overprinted. These questions become most salient in the Archean, where estimates of the isotopic composition of the ocean span ∼20‰, with a correspondingly wide range in estimated SSTs. Here, we introduce barite (BaSO 4) as a robust new proxy for the oxygen isotope composition of the Archean ocean. We compile new and existing triple oxygen isotope and sulfur isotope data from the Fig Tree Group barite deposits in the Barberton Greenstone Belt, South Africa, with the goal of identifying the primary sources of sulfate to the Archean ocean. Using a simple Monte Carlo approach, we then constrain the possible isotopic composition of contemporaneous seawater. Our results suggest that microbial sulfur cycling played a limited role in setting the isotopic composition of Archean seawater sulfate. Additionally, it is likely that a significant flux of sulfate to the marine reservoir was atmospherically derived and carried a significant positive triple oxygen isotope signal. Importantly, our results support an Archean ocean with a somewhat enriched oxygen isotope composition (∼0-5‰), with the exact composition dependent on the relative contribution from each sulfate production pathway. This result points either to the decreased significance of low-temperature weathering and/or to elevated SSTs in the early Archean. • Oxygen isotope composition of Archean seawater. • Constraints on Archean atmospheric sulfate. • Minimal Archean microbial sulfur cycling. [ABSTRACT FROM AUTHOR]

Abstract: We use sulfur (S) isotope signatures within ancient sediments and a photochemical model of sulfur dioxide (SO2) photolysis to interpret the evolution of the atmosphere over the first half of Earth''s history. A decrease in mass-independent sulfur isotope fractionation has been reported in Archean rocks deposited between ~ 2.7 Ga and ~ 3.2 Ga, and is reinforced by new S isotope data that we report here. This pattern has been interpreted by some as evidence that atmospheric oxygen (O2) was elevated during this time. In this paper, we argue against that conclusion, and show that it is inconsistent with other geochemical data. In its place, we propose a new model that can explain the sulfur isotope record that can also avoid conflicts with independent constraints on O2 and account for concurrent glacial deposits. Specifically, we suggest that prior to the rise of O2 excursions in the sulfur isotope record were modulated by the thickness of an organic haze. This haze would have blocked the lower atmosphere from the UV fluxes responsible for the anomalous sulfur photochemistry and would have caused an anti-greenhouse effect that triggered the glaciations. We used a photochemical model to verify that a haze could have affected the isotopic signal in this manner, and to examine how changes in atmospheric methane (CH4) and carbon dioxide (CO2) concentrations could have controlled haze thickness. Finally, we combined the resulting relationships with climate models and sulfur isotope and glacial records to deduce a new evolutionary sequence for Archean climate, surface chemistry, and biology. [Copyright &y& Elsevier]

• Extensive new Ba isotopic survey of modern barites. • First Proterozoic Ba isotopic results. • Provides genetic model for enigmatic post-Marinoan barite formation. • Provides evidence for stratified post-Marinoan ocean. • Provides arguments for why barite horizons are confined to Marinoan and not Sturtian or Paleoproterozoic strata. Measurements of triple oxygen isotope ratios in barite horizons within post-Marinoan cap carbonates have provided some of the most compelling evidence that the Marinoan glaciation was a Snowball Earth event. However, the origin of these barite horizons remains unresolved. To constrain the Ba sources, and thus formation mechanisms of these horizons, we analyzed the Ba isotope composition of post-Marinoan barite deposits from Northwest Canada, Northern Norway, Brazil and South China. We augment these analyses with a Ba isotope survey of almost 100 modern and ancient additional barite measurements, including samples from pelagic (or 'marine'), hydrothermal, terrestrial, Proterozoic stratiform and cold seep environments. Unlike modern cold seep or terrestrial barites, we find that globally-distributed post-Marinoan barites exhibit a relatively narrow isotopic range, suggesting a well-mixed, effectively limitless Ba source. Moreover, post-Marinoan deposits exhibit a similar mean Ba isotope composition to modern marine barites, which we interpret as evidence of a marine Ba source. Considered alongside existing geochemical, geological, and new Ba isotope survey data, we conclude that Ba in barite horizons was sourced from a well-mixed, Ba-replete but SO 4 -poor reservoir that accumulated during the Marinoan Snowball Earth interval. This deep Ba reservoir was then transported upward—either by ocean circulation or dolomitization of underlying cap carbonates—and was brought into contact with continental weathering-derived sulfate in a post-glacial meltwater surface layer. Thus, in addition to providing a plausible mechanism for generating globally-synchronous deposition of post-Marinoan barite horizons that reconciles all existing geochemical and geological data, our results demonstrate the utility of Ba isotopes to interrogate the origin of enigmatic barite deposits throughout the sedimentary record. [ABSTRACT FROM AUTHOR]

Abstract: We present 28 multiple sulfur isotope measurements of seawater sulfate ( and ) from the modern ocean over a range of water depths and sites along the eastern margin of the Pacific Ocean. The average measured is 21.24‰ ( ) with a calculated of ( ). With these values, we use a box-model to place constraints on the gross fraction of pyrite burial in modern sediments. This model presents an improvement on previous estimates of the global pyrite burial flux because it does not rely on the assumed value of , which is poorly constrained, but instead uses new information about the relationship between and in global marine sulfate. Our calculations indicate that the pyrite burial flux from the modern ocean is between 10% and 45% of the total sulfur lost from the oceans, with a more probable range between 20% and 35%. [Copyright &y& Elsevier]

Abstract: Multiple eukaryotic clades make their first appearance in the fossil record between ~810 and 715Ma. Molecular clock studies suggest that the origin of animal multicellularity may have been part of this broader eukaryotic radiation. Animals require oxygen to fuel their metabolism, and low oxygen levels have been hypothesized to account for the temporal lag between metazoan origins and the Cambrian radiation of large, ecologically diverse animals. Here, paleoredox conditions were investigated in the Fifteenmile Group, Ogilvie Mountains, Yukon, Canada, which hosts an 811Ma ash horizon and spans the temporal window that captures the inferred origin and early evolution of animals. Iron-based redox proxies, redox-sensitive trace elements, organic carbon percentages and pyrite sulfur isotopes were analyzed in seven stratigraphic sections along two parallel basin transects. These data suggest that for this basin, oxygenated shelf waters overlay generally anoxic deeper waters. The anoxic water column was dominantly ferruginous, but brief periods of euxinia likely occurred. These oscillations coincide with changes in total organic carbon, suggesting euxinia was primarily driven by increased organic carbon loading. Overall, these data are consistent with proposed quantitative constraints on Proterozoic atmospheric oxygen being greater than 1% of modern levels, but less than present levels. Comparing these oxygen levels against the likely oxygen requirements of the earliest animals, both theoretical considerations and the ecology of modern oxygen-deficient settings suggest that the inferred oxygen levels in the mixed layer would not have been prohibitive to the presence of sponges, eumetazoans or bilaterians. Thus the evolution of the earliest animals was probably not limited by the low absolute oxygen levels that may have characterized Neoproterozoic oceans, although these inferred levels would constrain animals to very small sizes and low metabolic rates. [Copyright &y& Elsevier]

Abstract: Mineralogical, petrographic and sedimentological observations document early diagenetic talc in carbonate-dominated successions deposited on two early Neoproterozoic (~800–700million years old) platform margins. In the Akademikerbreen Group, Svalbard, talc occurs as nodules that pre-date microspar cements that fill molar tooth structures and primary porosity in stromatolitic carbonates. In the upper Fifteenmile Group of the Ogilvie Mountains, NW Canada, the talc is present as nodules, coated grains, rip-up clasts and massive beds that are several meters thick. To gain insight into the chemistry required to form early diagenetic talc, we conducted precipitation experiments at 25°C with low-SO4 synthetic seawater solutions at varying pH, Mg2+ and SiO2(aq). Our experiments reveal a sharp and reproducible pH boundary (at ~8.7) only above which does poorly crystalline Mg-silicate precipitate; increasing Mg2+ and/or SiO2(aq) alone is insufficient to produce the material. The strong pH control can be explained by Mg-silica complexing activated by the deprotonation of silicic acid above ~8.6–8.7. FT-IR, TEM and XRD of the synthetic precipitates reveal a talc-like 2:1 trioctahedral structure with short-range stacking order. Hydrothermal experiments simulating burial diagenesis show that dehydration of the precipitate drives a transition to kerolite (hydrated talc) and eventually to talc. This formation pathway imparts extensive layer stacking disorder to the synthetic talc end-product that is identical to Neoproterozoic occurrences. Early diagenetic talc in Neoproterozoic carbonate platform successions appears to reflect a unique combination of low Al concentrations (and, by inference, low siliciclastic input), near modern marine salinity and Mg2+, elevated SiO2(aq), and pH>~8.7. Because the talc occurs in close association with microbially influenced sediments, we suggest that soluble species requirements were most easily met through microbial influences on pore water chemistry, specifically pH and alkalinity increases driven by anaerobic Fe respiration. [Copyright &y& Elsevier]

• Messinian sulfate evaporites are offset from contemporaneous seawater sulfate composition toward higher δ 18 O and lower Δ'17O values. • Normal distributions of sulfate δ 18 O and Δ'17O reflect a well-mixed sulfate reservoir. • Evaporite sulfate reflects both original seawater and post-restriction alteration. The triple oxygen isotope composition of seawater sulfate, as recorded in marine sulfate evaporites and barites, is commonly used to interpret past changes in atmospheric p O 2 / p CO 2 and gross primary production (GPP). In practice, the most-negative measured triple oxygen isotope value (Δ'17O) of sulfate from a marine evaporite deposit is thought to most closely represent contemporaneous seawater sulfate and is used to calculate atmospheric composition. However, a range of triple oxygen isotope compositions are typically measured within a single marine evaporite basin. Here, we characterize in detail the variability in the triple oxygen isotope composition of the sulfate in gypsum sampled from three Messinian (5-6 Ma) marine evaporite sub-basins from the Western Mediterranean Basin. Evaporite sulfate is offset from contemporaneous seawater sulfate and reflects mixing between two end-member sulfate populations: the original seawater sulfate and sulfate that has been isotopically reset after basin restriction. The combined Δ'17O and δ 18 O compositions of sulfate within a stratigraphic context offer the opportunity to better constrain the degree to which marine sulfate evaporites preserve the original isotopic composition of open ocean seawater sulfate. This study encompasses an exploration of mass-dependent fractionation, isotope equilibrium with water, and various scenarios of mixing. Our results calibrate the utility of marine sulfate evaporites in constraining the contemporaneous, open ocean triple oxygen isotope composition of seawater sulfate. [ABSTRACT FROM AUTHOR]

• The O-isotope composition of minerals provides information about the minerals life-history. • For chert, there is a significant contribution from alteration by meteoric fluids. • The data are not in keeping with significant temperature changes at Earth's surface. The temperature and chemistry of early seawater have both been inferred from the isotopic composition of Precambrian chert (SiO 2), a precipitated mineral formed on or within marine sediments. The δ 18 O of chert shows a robust quasi-linear increase through time - a signal that has been interpreted in a number of conflicting ways. For example, changing δ 18 O has been hypothesized to reflect the product of cooling surface ocean temperatures, a signature of evolving seawater δ 18 O composition, or the product of later stage diagenesis (where measured δ 18 O reflects the composition of diagenetic fluids). We suggest this uncertainty can be resolved through the additional measurement and interpretation of the minor oxygen isotope 17O (noted as Δ'17O) in conjunction with δ 18 O. In this study, we present a suite of triple oxygen isotope data on stratigraphically constrained Precambrian chert (both peritidal chert nodules in carbonates and iron formation silica). These mineralogically well-defined data allow for the first stratigraphic tests of the fidelity of 17O in SiO 2. We then apply a Monte Carlo resampling technique to test the features of the competing hypotheses noted above, here now including critical constraints from 17O. The most parsimonious interpretation of these data suggests that secondary alteration with higher-temperature, meteoric-derived groundwater has skewed an original geochemical signature. This treatment can allow for some change in the oxygen isotope composition of seawater itself, however this does not appear to be the most statistically defensible single solution - clearly, some combination of multiple mechanisms is always possible and even likely. What is definitively the case is an equitable, modern-like Archean surface ocean temperature. [ABSTRACT FROM AUTHOR]