Seawater Sulfate Concentrations Research Articles

Methane-derived authigenic carbonate from the lower Doushantuo Formation of South China: Implications for seawater sulfate concentration and global carbon cycle in the early Ediacaran ocean

Authigenic carbonate associated with anaerobic oxidation of methane (AOM), usually via microbial sulfate reduction (MSR) or ferric iron reduction, is generally characterized by extremely low δ13C values (<−30‰, VPDB). This has been used as one of the major diagnostic features for the recognition of hydrocarbon seep carbonate in the geological past. Previous reports on Precambrian authigenic carbonates are rare, limiting our understanding of the effects of their deposition on the Earth's carbon isotopic mass balance. In this study, mainly based on petrographic features and pronounced negative δ13C values as low as −38.1‰, we discovered authigenic calcite cement immediately above the cap dolostone in the basal Ediacaran Doushantuo Formation in the Jiulongwan section, Yangtze Gorges area, South China. Our observations not only provide direct evidence for the involvement of AOM during carbonate precipitation in the early Ediacaran (~635Ma), but also suggest that the seawater sulfate concentrations in the early Ediacaran may have been higher than previously thought.

Sulfate reduction and formation of iron sulfide minerals in nearshore sediments from Qi'ao Island, Pearl River Estuary, Southern China

Sulfate reduction usually leads to the formation of iron sulfide minerals in marine sediments and strongly influences the global coupled carbon-iron-sulfur biogeochemical cycles in marine sediments. Here, we report sulfate reduction and pyrite formation from the coastal sediments near Qi'ao Island, Pearl River Estuary using a combination of sedimentary pore water and solid sediment geochemical analyses. Strong enrichments in reactive iron (276.8–358.2 μmol/g) were observed, especially at the sites located near the river outlets. High contents of reactive iron and low values of the degree of iron pyritization (DOP) indicate that the limiting effect of reactive iron on pyrite formation can be excluded at these sites. In the upper sedimentary layer, conservative mixing of river water and marine water reinforces the fact that sulfate reduction is not appreciable. The limited sulfate supply and low sulfate reduction rates lead to not enough dissolved sulfide available for formation of iron sulfide minerals. However, below this dilution-mixing layer, pore water geochemical profiles indicate that a fraction of sulfate consumption is mediated by upward-diffusing methane. An additional sulfide is produced from anaerobic oxidation of methane (AOM) and later reacts with reactive iron, resulting in enhancement of pyrite formation in the sediments. By contrast, it appears that sulfate concentration is a primary factor controlling for pyrite formation in these estuarine environments. Our study highlights the need in future work for an integrated analysis of the hydrologically driven change in the bottom seawater sulfate concentrations to better understand the regulation factors of global sulfur reservoirs in coastal sediments of estuarine environments, as sulfur burial is an important factor in determining the past atmospheric oxygen level.

Diagenetic overprint to a negative carbon isotope anomaly associated with the Gaskiers glaciation of the Ediacaran Doushantuo Formation in South China

Ediacaran carbon isotope records contain significant and long-lasting negative anomalies for which convincing explanations have not been given. We recognized a diagenetic overprint contributing to the 120-m-thick negative anomaly interval in the upper part of the Ediacaran Doushantuo Formation at Yangjiaping, Hunan province in South China. Coarse-grained carbonate facies deposited in an outer shelf setting of the Yangtze platform provided an opportunity to examine diagenetic components by measuring micro-drilled sub-samples. Scattered δ13C values of bulk samples (+5 to −14‰) in this anomaly interval are here attributed to the secondary precipitation of highly 13C-depleted (up to −33‰) calcite cement on dolomite sedimentary grains of less negative δ13C. This diagenetic overprint was not observed in the intervals above or below the anomaly. In these intervals the δ13C difference between the cements and the grains is insignificant. Strontium isotopic variation in the anomaly interval is identical to that of the interval with negative δ13C anomaly in the middle Doushantuo Formation reported from the Three Gorges area, which is presumably correlated with the Gaskiers glaciation. This correlation scheme indicates a long-term hiatus (∼35myrs) between the anomaly horizon and the overlying Dengying Formation at Yangjiaping. We suggest that the anaerobic oxidization of reduced carbon (organic carbon or methane) by sulfate reduction was the most likely process responsible for 13C-depleted secondary calcite, which progressively filled intergranular porosity. An increase in atmospheric oxygen during Ediacaran and the Gaskiers glaciation enhanced oxidative weathering and elevated seawater sulfate concentrations to a level sufficient for diagenetic alteration. This co-occurred with an increase in 87Sr/86Sr from 0.7080 to 0.7085 in middle Ediacaran. We also show that the δ13C profile of the Doushantuo Formation at Yangjiaping is similar to that at Weng’an in Guizhou province, another section recording an outer shelf depositional environment. Although further study is needed, correlation between the Yangjiaping Unit 3 with the δ13C-scattered interval in Weng’an suggests that the embryo-bearing phosphorites at Weng’an were formed likely before the Gaskiers glaciation.

Ediacaran sulfur cycle: Insights from sulfur isotope measurements (Δ33S and δ34S) on paired sulfate–pyrite in the Huqf Supergroup of Oman

The Ediacaran Period (635–541Ma) was a time of dramatic environmental and ecological change. Sulfur isotopic records preserved in marine strata provide insights into these changes. Previous work using δ34S in pyrite and carbonate-associated sulfate in the Ediacaran-aged Huqf Supergroup (Sultanate of Oman) suggested increases in marine sulfate concentrations, changing microbial pathways and redox conditions, and a long-term increase in δ34S of sulfate delivery to the ocean. However, δ34S-based analyses on their own were insufficient to uniquely reconstruct these environmental and ecological changes. Here we present new paired δ34S and Δ33S data from carbonate-associated sulfate and pyrite in these strata. The results allow us to provide three new constraints on Ediacaran sulfur cycling. First, a notable increase Δ34SCAS-PY, previously attributed to possible disproportionation, in upper Buah strata is associated with Δ33S-evidence for an increase in sulfide reoxidation, indicating a transition to more oxidizing conditions, possibly associated with the onset of bioturbation. Secondly, Δ33S data indicate that a large positive shift in both sulfate and pyrite δ34S in Ara Group strata is mostly likely due to an increase in delivery of 34S-enriched sulfate to the basin. Finally paired δ34S–Δ33S data constrain possible changes in seawater sulfate concentrations, suggesting an increase in sulfate supply related to enhanced evaporite weathering, consistent with geological and geochemical observations that imply high [SO4] (∼17mM) during deposition of the Ara Group.

Reconstruction of secular variation in seawater sulfate concentrations

Abstract. Long-term secular variation in seawater sulfate concentrations ([SO42−]SW) is of interest owing to its relationship to the oxygenation history of Earth's surface environment. In this study, we develop two complementary approaches for quantification of sulfate concentrations in ancient seawater and test their application to late Neoproterozoic (635 Ma) to Recent marine units. The "rate method" is based on two measurable parameters of paleomarine systems: (1) the S-isotope fractionation associated with microbial sulfate reduction (MSR), as proxied by Δ34SCAS-PY, and (2) the maximum rate of change in seawater sulfate, as proxied by &amp;amp;partial; δ 34SCAS/∂ t(max). The "MSR-trend method" is based on the empirical relationship of Δ34SCAS-PY to aqueous sulfate concentrations in 81 modern depositional systems. For a given paleomarine system, the rate method yields an estimate of maximum possible [SO42−]SW (although results are dependent on assumptions regarding the pyrite burial flux, FPY), and the MSR-trend method yields an estimate of mean [SO42−]SW. An analysis of seawater sulfate concentrations since 635 Ma suggests that [SO42−]SW was low during the late Neoproterozoic (&lt;5 mM), rose sharply across the Ediacaran–Cambrian boundary (~5–10 mM), and rose again during the Permian (~10–30 mM) to levels that have varied only slightly since 250 Ma. However, Phanerozoic seawater sulfate concentrations may have been drawn down to much lower levels (~1–4 mM) during short (

Did shifting seawater sulfate concentrations drive the evolution of deep-sea methane-seep ecosystems?

The origin and evolution of the faunas inhabiting deep-sea hydrothermal vents and methane seeps have been debated for decades. These faunas rely on a local source of sulfide and other reduced chemicals for nutrition, which spawned the hypothesis that their evolutionary history is independent from that of photosynthesis-based food chains and instead driven by extinction events caused by deep-sea anoxia. Here I use the fossil record of seep molluscs to show that trends in body size, relative abundance and epifaunal/infaunal ratios track current estimates of seawater sulfate concentrations through the last 150 Myr. Furthermore, the two main faunal turnovers during this time interval coincide with major changes in seawater sulfate concentrations. Because sulfide at seeps originates mostly from seawater sulfate, variations in sulfate concentrations should directly affect the base of the food chain of this ecosystem and are thus the likely driver of the observed macroecologic and evolutionary patterns. The results imply that the methane-seep fauna evolved largely independently from developments and mass extinctions affecting the photosynthesis-based biosphere and add to the growing body of evidence that the chemical evolution of the oceans had a major impact on the evolution of marine life.

Diagenetic barite deposits in the Yurtus Formation in Tarim Basin, NW China: Implications for barium and sulfur cycling in the earliest Cambrian

Barite concretions and bands are widely distributed in black shale–chert horizons in the Yurtus Formation of Lower Cambrian in Aksu area, northwestern Tarim Basin, NW China. They mainly consist of coarse-grained anhedral to euhedral barite crystals with minor dolomites and pyrites. Petrological features indicate these concretions grew from the porewater in unconsolidated sediments at shallow burial below sediment-water interface. The slight deviation of 87Sr/86Sr ratios (0.7083 to 0.7090) and significant elevated δ34S values (56.8–76.4‰ CDT) of barite samples with respect to those of the Early Cambrian seawater further support that barite deposits precipitated from the enclosed porewater in sediment column, which evolved from the penecontemporaneous seawater with weak interaction with the host fine-grained siliciclastic sediments and highly-depleted sulfate in response to prolonged strong bacterial sulfate reduction without necessary renewal. The abundant organic matters in the basal Yurtus Formation should have facilitated developing sulfate-depleted methanogenesis zone and sulfate–methane transition zone (SMTZ) slightly after deposition. Therefore, barite deposits in the Yurtus Formation most likely resulted from diagenetic barium cycling and persistently grew from the porewater in the static SMTZ with a low sedimentation rate in the Early Cambrian. In comparison with the distribution of sedimentary barites in geological records, we tentatively proposed that a transition in diagenetic barium cycling and associated mineralization may have occurred from the Precambrian to Cambrian periods; this scenario may be causally linked to the changes in marine ecology (the advent of mesozooplankton and associated faecal pellet) and geochemistry (the increase of seawater sulfate concentration). Thus, the occurrence of diagenetic barite deposits in the Yurtus Formation implies that diagenetic barium cycling and more effective scavenging of barium from CH4- and Ba-rich porewaters within sediments might have become an nonnegligible process in continental margin areas, at least, since the earliest Cambrian, which could have significantly impacted the marine barium cycling.

Decline in oceanic sulfate levels during the early Mesoproterozoic

Multiple-sulfur isotope compositions (32S, 33S, 34S and 36S) were analyzed for paired carbonate-associated sulfate (CAS) and disseminated pyrite (PY) from the ∼1.6-Ga Gaoyuzhuang Formation of the North China Craton to reconstruct the history of sulfate levels in Proterozoic oceans. The 200-m-thick study interval yielded relatively constant values for δ34SCAS (13.0±1.8‰), δ34SPY (8.0±2.3‰), and Δ34SCAS-PY (∼5‰), as well as relatively constant Δ33S (0±0.05‰) and Δ36S (0.35±0.15‰) for both CAS and pyrite. Limited variation in δ34SPY and slightly lower Δ33S of pyrite relative to CAS suggest water-column precipitation of pyrite. Limited fractionation of sulfur during microbial sulfate reduction (as documented by Δ34SCAS-PY) implies low seawater sulfate concentrations in the early Mesoproterozoic ocean. We quantitatively constrained paleo-seawater [SO42−] using a novel modeling approach based on measured values of Δ34SCAS-PY and ∂δ34SCAS/∂t(max). For the study unit, Δ34SCAS-PY is 5.4±1.4‰ (n=17), and ∂δ34SCAS/∂t(max) is 6.8–34‰Myr−1 based on sedimentation rates of 30–150mMyr−1. These data indicate early Mesoproterozoic seawater [SO42−] of ∼<0.1 to 0.35mM (with a maximum possible concentration of 1.8mM), a range that is lower and more tightly constrained than earlier estimates for the Mesoproterozoic. Compilation of published data suggests that low seawater sulfate concentrations began about ∼1.7Ga and persisted until at least the mid-Mesoproterozoic (∼1.4Ga), documenting a distinct early Mesoproterozoic perturbation in ocean chemistry that may have been related to a decline in atmospheric pO2 after Great Oxidation Event I.

Sulfate was a trace constituent of Archean seawater.

In the low-oxygen Archean world (>2400 million years ago), seawater sulfate concentrations were much lower than today, yet open questions frustrate the translation of modern measurements of sulfur isotope fractionations into estimates of Archean seawater sulfate concentrations. In the water column of Lake Matano, Indonesia, a low-sulfate analog for the Archean ocean, we find large (>20 per mil) sulfur isotope fractionations between sulfate and sulfide, but the underlying sediment sulfides preserve a muted range of δ(34)S values. Using models informed by sulfur cycling in Lake Matano, we infer Archean seawater sulfate concentrations of less than 2.5 micromolar. At these low concentrations, marine sulfate residence times were likely 10(3) to 10(4) years, and sulfate scarcity would have shaped early global biogeochemical cycles, possibly restricting biological productivity in Archean oceans.

Dissecting ancient microbial sulfur cycling

Early Earth Before the rise of oxygen, life on Earth depended on the marine sulfur cycle. The fractionation of different sulfur isotopes provides clues to which biogeochemical cycles were active long ago (see the Perspective by Ueno). Zhelezinskaia et al. found negative isotope anomalies in Archean rocks from Brazil and posit that metabolic fluxes from sulfate-reducing microorganisms influenced the global sulfur cycle, including sulfur in the atmosphere. In contrast, Paris et al. found positive isotope anomalies in Archean sediments from South Africa, implying that the marine sulfate pool was more disconnected from atmospheric sulfur. As an analog for the Archean ocean, Crowe et al. measured sulfur isotope signatures in modern Lake Matano, Indonesia, and suggest that low seawater sulfate concentrations restricted early microbial activity. Science , this issue p. [703][1], p. [742][2], p. [739][3]; see also p. [735][4] [1]: /lookup/doi/10.1126/science.1261676 [2]: /lookup/doi/10.1126/science.1256211 [3]: /lookup/doi/10.1126/science.1258211 [4]: /lookup/doi/10.1126/science.1258966

Large sulfur isotope fractionations associated with Neoarchean microbial sulfate reduction.

The minor extent of sulfur isotope fractionation preserved in many Neoarchean sedimentary successions suggests that sulfate-reducing microorganisms played an insignificant role in ancient marine environments, despite evidence that these organisms evolved much earlier. We present bulk, microdrilled, and ion probe sulfur isotope data from carbonate-associated pyrite in the ~2.5-billion-year-old Batatal Formation of Brazil, revealing large mass-dependent fractionations (approaching 50 per mil) associated with microbial sulfate reduction, as well as consistently negative Δ(33)S values (~ -2 per mil) indicative of atmospheric photochemical reactions. Persistent (33)S depletion through ~60 meters of shallow marine carbonate implies long-term stability of seawater sulfate abundance and isotope composition. In contrast, a negative Δ(33)S excursion in lower Batatal strata indicates a response time of ~40,000 to 150,000 years, suggesting Neoarchean sulfate concentrations between ~1 and 10 μM.

Neoarchean carbonate–associated sulfate records positive Δ 33 S anomalies

Mass-independent fractionation of sulfur isotopes (reported as Δ(33)S) recorded in Archean sedimentary rocks helps to constrain the composition of Earth's early atmosphere and the timing of the rise of oxygen ~2.4 billion years ago. Although current hypotheses predict uniformly negative Δ(33)S for Archean seawater sulfate, this remains untested through the vast majority of Archean time. We applied x-ray absorption spectroscopy to investigate the low sulfate content of particularly well-preserved Neoarchean carbonates and mass spectrometry to measure their Δ(33)S signatures. We report unexpected, large, widespread positive Δ(33)S values from stratigraphic sections capturing over 70 million years and diverse depositional environments. Combined with the pyrite record, these results show that sulfate does not carry the expected negative Δ(33)S from sulfur mass-independent fractionation in the Neoarchean atmosphere.

Diversity and regulation of ATP sulfurylase in photosynthetic organisms.

ATP sulfurylase (ATPS) catalyzes the first committed step in the sulfate assimilation pathway, the activation of sulfate prior to its reduction. ATPS has been studied in only a few model organisms and even in these cases to a much smaller extent than the sulfate reduction and cysteine synthesis enzymes. This is possibly because the latter were considered of greater regulatory importance for sulfate assimilation. Recent evidences (reported in this paper) challenge this view and suggest that ATPS may have a crucial regulatory role in sulfate assimilation, at least in algae. In the ensuing text, we summarize the current knowledge on ATPS, with special attention to the processes that control its activity and gene(s) expression in algae. Special attention is given to algae ATPS proteins. The focus on algae is the consequence of the fact that a comprehensive investigation of ATPS revealed that the algal enzymes, especially those that are most likely involved in the pathway of sulfate reduction to cysteine, possess features that are not present in other organisms. Remarkably, algal ATPS proteins show a great diversity of isoforms and a high content of cysteine residues, whose positions are often conserved. According to the occurrence of cysteine residues, the ATPS of eukaryotic algae is closer to that of marine cyanobacteria of the genera Synechococcus and Prochlorococcus and is more distant from that of freshwater cyanobacteria. These characteristics might have evolved in parallel with the radiation of algae in the oceans and the increase of sulfate concentration in seawater.

Fluctuation of shelf basin redox conditions in the early Ediacaran: Evidence from Lantian Formation black shales in South China

Finely laminated, pyrite- and organic-rich black shales that bear a euxinic interpretation dominate the Ediacaran Lantian Formation in southern Anhui, South China. However, these black shales preserve benthic sessile algae and possible metazoans in situ, suggesting that the Lantian shelf basin may have had at least episodically oxidized bottom water during the early Ediacaran Period. Redox conditions of the fossiliferous Lantian black shales were examined by petrological analysis, pyrite framboid and δ34S measurements, and redox-sensitive trace element (RSTE) analysis. Lamina-by-lamina pyrite framboid measurements show that Lantian black shales were deposited under fluctuating redox conditions. The δ34S values of pyrites range from −20.5‰ to 2.3‰ for different laminae and display less negative values than the earliest Ediacaran black shales, supporting a return to a relatively low seawater sulfate concentration after the early Ediacaran oxidation event. Similarly, the Lantian black shale RSTE concentrations (e.g., Mo, U, and V) are low compared to previously published results from the early Ediacaran and constant from lamina to lamina, suggesting a drop in ocean wide RSTE concentrations occurring independently of frequent local redox condition changes. These frequent redox condition changes in the Lantian shelf basin may have facilitated the productivity and preservation of macroscopic Lantian biota.

Early Triassic seawater sulfate drawdown

The marine sulfur cycle is intimately linked to global carbon fluxes, atmospheric composition, and climate, yet relatively little is known about how it responded to the end-Permian biocrisis, the largest mass extinction of the Phanerozoic. Here, we analyze carbonate-associated-sulfate (CAS) from three Permo–Triassic sections in South China in order to document the behavior of the C–S cycle and its relationship to marine environmental changes during the mass extinction and its aftermath. We find that δ34SCAS varied from +9‰ to +44‰ at rates up to 100‰Myr−1 during the Griesbachian–Smithian substages of the Early Triassic. We model the marine sulfur cycle to demonstrate that such rapid variation required drawdown of seawater sulfate concentrations to ⩽4mM and a reduction in its residence time to ⩽200kyr. This shorter residence time resulted in positive covariation with δ13Ccarb due to strong coupling of the organic carbon and pyrite burial fluxes. Carbon and sulfur isotopic shifts were associated with contemporaneous changes in climate, marine productivity, and microbial sulfate reduction rates, with negative shifts in δ13Ccarb and δ34SCAS linked to warming, decreased productivity, and reduced sulfate reduction. Sustained cooling during the Spathian re-invigorated oceanic overturning circulation, reduced marine anoxia, and limited pyrite burial. As seawater sulfate built to higher concentrations during the Spathian, the coupling of the marine C and S cycles came to an end and a general amelioration of marine environmental conditions set the stage for a recovery of invertebrate faunas. Variation in seawater sulfate during the Early Triassic was probably controlled by climate change, possibly linked to major eruptive phases of the Siberian Traps.

Sulfate availability and the geological record of cold-seep deposits

Cold-seep deposits are the remnants of ancient chemosynthetic ecosystems that derive energy from microbial anaerobic oxidation of methane (AOM) using seawater sulfate. They provide a physical record of a microbial process that plays a critical role in regulating biospheric methane. Although highly ^(13)C-depleted kerogen suggests that AOM dates back 2.7 b.y., puzzlingly, the oldest reported cold seeps only appear at 635 Ma and lack carbon isotopic signals (<–30‰ Peedee belemnite) that are diagnostic of AOM in examples younger than 350 Ma. Using a one-dimensional biogeochemical reaction-transport model, we confirm that these discrepancies are an expected consequence of changes in seawater chemistry. More specifically, sub-millimolar (mM) to millimolar seawater sulfate concentrations ([SO_4^(2–)]SW) and elevated concentrations of dissolved inorganic carbon that characterized seawater through much of the Precambrian limited AOM-driven carbonate supersaturation and 13C depletion, making seep carbonates less likely to form and more challenging to identify. Moderate ^(13)C depletions observed in 420–370-m.y.-old cold-seep carbonates (independently identified by fossil assemblages and contextual and textural observations) indicate [SO_4^(2–)]SW < 5 mM in this interval. This is significant because low [SO_4^(2–)]SW has been linked to widespread ocean anoxia in the early Paleozoic, an environmental condition thought to have influenced the evolution, extinction, and recovery of early animals.

Transcriptome analysis of the sulfate deficiency response in the marine microalga Emiliania huxleyi

The response to sulfate deficiency of plants and freshwater green algae has been extensively analysed by system biology approaches. By contrast, seawater sulfate concentration is high and very little is known about the sulfur metabolism of marine organisms. Here, we used a combination of metabolite analysis and transcriptomics to analyse the response of the marine microalga Emiliania huxleyi as it acclimated to sulfate limitation. Lowering sulfate availability in artificial seawater from 25 to 5 mM resulted in significant reduction in growth and intracellular concentrations of dimethylsulfoniopropionate and glutathione. Sulfate-limited E. huxleyi cells showed increased sulfate uptake but sulfate reduction to sulfite did not seem to be regulated. Sulfate limitation in E. huxleyi affected expression of 1718 genes. The vast majority of these genes were upregulated, including genes involved in carbohydrate and lipid metabolism, and genes involved in the general stress response. The acclimation response of E. huxleyi to sulfate deficiency shows several similarities to the well-described responses of Arabidopsis and Chlamydomonas, but also has many unique features. This dataset shows that even though E. huxleyi is adapted to constitutively high sulfate concentration, it retains the ability to re-program its gene expression in response to reduced sulfate availability.

Stable isotope (S, C) chemostratigraphy and hydrocarbon biomarkers in the Ediacaran upper section of Sierras Bayas Group, Argentina

The Neoproterozoic sedimentary successions of the Sierras Bayas Group (SBG) are situated within the Tandilia Belt, Buenos Aires Province, Argentina. The Loma Negra Formation (LNF) from the upper part of the SBG has been investigated using a combination of inorganic, organic and isotopic geochemical methods in order to determine the stable isotope composition of carbonates (δ13Ccar, δ18Ocar), kerogen (δ13Cker), pyrite and carbonate associated sulfur (δ34Spy, δ34SCAS). Concentrations of major, trace and rare earth elements and hydrocarbon distributions were also investigated. This data set was used to further the understanding of the interactions between paleo-biodiversity and paleoenvironmental conditions for terminal Ediacaran (post-Gaskiers) shelf deposits. The high δ34Spy values exceeding the coeval δ34SCAS values in green micritic limestones of the lower LNF may be explained by a combination of different events such as globally low seawater sulfate concentrations, an increased rate of bacterial sulfate reduction, and a decrease in sulfate levels in the near-bottom deepest water layer inherited from glaciations. Negative Ce anomalies in the dark grey micritic limestones of the upper LNF suggest they were deposited under oxidizing conditions. However, higher concentrations of Fe, Mo, Zn and REE, and the occurrence of authigenic 34S-enriched pyrite suggest that the sediments were maintained under reducing conditions. The distributions of hydrocarbon biomarkers (e.g. hopanes maximizing at C29) are in line with this hypothesis and indicate a diverse microbial community including primary producers such as cyanobacteria (e.g. terminally-branched monomethyl alkanes, hopanoid distribution), phototrophic bacteria (e.g. acyclic isoprenoids C<21) and green bacteria (e.g. n-C18≫Phytane). The unsteady Δ13Ccar-ker values reflect changes in primary biomass due to relative contributions of bacterial microorganisms using different photosynthetic carbon-fixation pathways. The lowest Δ13Ccar-ker values coincided with the latest biomarker signal interpreted as the signature of green non-sulfur bacteria using a less 13C fractionation pathway. The combined biogeochemical features of the Loma Negra Formation, indicated a well-stratified water column with oxygenated surface waters, oxygen-poor bottom waters and anoxic sediments which helped to refine a correlation with the Polanco Formation from the Arroyo del Soldado Group in Uruguay.

Sustained low marine sulfate concentrations from the Neoproterozoic to the Cambrian: Insights from carbonates of northwestern Mexico and eastern California

Stratigraphic carbonate-carbon isotope trends are similar for correlative Ediacaran and Cambrian carbonates of Sonora, Mexico, and Death Valley, California. In contrast, the sulfur isotope compositions of both carbonate-associated sulfate (CAS) and pyrite in the two regions exhibit unique trends with high degrees of stratigraphic variability. We have established that the sulfur records are coeval using δ13C chemostratigraphy and biostratigraphic markers, where present. Over short stratigraphic intervals, δ34SCAS variability is consistent with regionally low marine sulfate concentrations during this period. Values of Δ34S (δ34SCAS–δ34Spyr) range from −5.8‰ to +27.1‰ and average ∼+11‰, consistent with limited net fractionation during bacterial sulfate reduction, which is additional evidence for low sulfate concentrations. Modeling based on these regional sulfur isotope trends suggests sustained low sulfate conditions throughout the Neoproterozoic and well into the Cambrian, with concentrations of ∼2mM or lower.When all of the available sulfate proxy data from our work and previously published studies are considered, most Neoproterozoic and Cambrian successions exhibit trends consistent with low seawater sulfate. The persistent and complete disagreement in δ34Ssulfate among multiple basins was briefly interrupted ∼580 million years ago, coincident with the onset of the Wonoka-Shuram carbon isotope anomaly and again near the termination of Series 3 of the Cambrian—characteristics generally unrecognized in older rock units. During these two intervals, similar stratigraphic trends in δ34SCAS are recorded globally, whereas absolute values remain distinct among individual basins. However, these periods of broad isotopic trend agreement coincide with large-magnitude sulfur isotope excursions, which also point to low seawater sulfate concentrations. Therefore, although brief intervals of isotopic homogeneity exist, the Neoproterozoic and Cambrian ocean must have been dominated by low sulfate throughout. Ultimately, the recognition of persistently low sulfate well into the Paleozoic raises questions about the relationships between sulfate concentration in seawater and its primary controls, including ocean oxygenation and its influence on metazoan evolution.

Constraining atmospheric oxygen and seawater sulfate concentrations during Paleoproterozoic glaciation: In situ sulfur three-isotope microanalysis of pyrite from the Turee Creek Group, Western Australia

Previous efforts to constrain the timing of Paleoproterozoic atmospheric oxygenation have documented the disappearance of large, mass-independent sulfur isotope fractionation and an increase in mass-dependent sulfur isotope fractionation associated with multiple glaciations. At least one of these glacial events is preserved in diamictites of the ∼2.4 Ga Meteorite Bore Member of the Kungarra Formation, Turee Creek Group, Western Australia. Outcrop exposures of this unit show the transition from the Boolgeeda Iron Formation of the upper Hamersley Group into clastic, glaciomarine sedimentary rocks of the Turee Creek Group. Here we report in situ multiple sulfur isotope and elemental abundance measurements of sedimentary pyrite at high spatial resolution, as well as the occurrence of detrital pyrite in the Meteorite Bore Member. The 15.3‰ range of Δ 33S in one sample containing detrital pyrite (−3.6‰ to 11.7‰) is larger than previously reported worldwide, and there is evidence for mass-independent sulfur isotope fractionation in authigenic pyrite throughout the section (Δ 33S from −0.8‰ to 1.0‰). The 90‰ range in δ 34S observed (−45.5‰ to 46.4‰) strongly suggests microbial sulfate reduction under non-sulfate limiting conditions, indicating significant oxidative weathering of sulfides on the continents. Multiple generations of pyrite are preserved, typically represented by primary cores with low δ 34S (<−20‰) overgrown by euhedral rims with higher δ 34S (4–7‰) and enrichments in As, Ni, and Co. The preservation of extremely sharp sulfur isotope gradients (30‰/<4 μm) implies limited sulfur diffusion and provides time and temperature constraints on the metamorphic history of the Meteorite Bore Member. Together, these results suggest that the Meteorite Bore Member was deposited during the final stages of the “Great Oxidation Event,” when pO 2 first became sufficiently high to permit pervasive oxidative weathering of continental sulfides, yet remained low enough to permit the production and preservation of mass-independent sulfur isotope fractionation.