Evidence of Atmospheric Sulfur During the MesoArchean

An atmospheric source of S in Mesoarchaean structurally-controlled gold mineralisation of the Barberton Greenstone Belt

Authors:

Agangi et al

Abstract:

The Barberton Greenstone Belt of southern Africa hosts several Mesoarchaean gold deposits. The ores were mostly formed in greenschist facies conditions, and occur as hydrothermal alteration zones around extensional faults that truncate and post-date the main compressional structures of the greenstone belt. Ore deposition was accompanied by the intrusion of porphyries, which has led to the hypothesis that gold may have been sourced from magmas. Because the transport of Au in the hydrothermal fluids is widely believed to have involved S complexes, tracing the origin of S may place strong constraints on the origin of Au. We measured multiple S isotopes in sulfide ore from Sheba and Fairview mines of the Barberton Greenstone Belt to distinguish “deep” S sources (e.g. magmas) from “surface” S sources (i.e. rocks of the volcano-sedimentary succession that contain S processed in the atmosphere preserved as sulfide and sulfate minerals). Ion probe (SIMS) analyses of pyrite from ore zones indicate mass-independent fractionation of S isotopes (Δ33S = −0.6‰ to +1.0‰) and the distribution of the analyses in the Δ33S–δ34S space matches the distribution peak of previously published analyses of pyrite from the entire volcano-sedimentary succession. Notwithstanding that the H2O–CO2 components of the fluids may have been introduced from a deep source external to the greenstone belt rocks, the fact that S bears an atmospheric signature suggests the hypothesis that the source of Au should also be identified in the supracrustal succession of the greenstone belt. Our findings differ from conclusions of previous studies of other Archaean shear-hosted Au deposits based on mineralogical and isotopic evidence, which suggested a magmatic or mantle source for Au, and imply that there is no single model that can be applied to this type of mineralisation in the Archaean.

Anomalies in Archean Sulfate Signatures

Multiple sulfur-isotope signatures in Archean sulfates and their implications for the chemistry and dynamics of the early atmosphere

Authors:

Muller et al

Abstract:

Sulfur isotopic anomalies (∆33S and ∆36S) have been used to trace the redox evolution of the Precambrian atmosphere and to document the photochemistry and transport properties of the modern atmosphere. Recently, it was shown that modern sulfate aerosols formed in an oxidizing atmosphere can display important isotopic anomalies, thus questioning the significance of Archean sulfate deposits. Here, we performed in situ 4S-isotope measurements of 3.2- and 3.5-billion-year (Ga)-old sulfates. This in situ approach allows us to investigate the diversity of Archean sulfate texture and mineralogy with unprecedented resolution and from then on to deconvolute the ocean and atmosphere Archean sulfur cycle. A striking feature of our data is a bimodal distribution of δ34S values at ∼+5‰ and +9‰, which is matched by modern sulfate aerosols. The peak at +5‰ represents barite of different ages and host-rock lithology showing a wide range of ∆33S between −1.77‰ and +0.24‰. These barites are interpreted as primary volcanic emissions formed by SO2 photochemical processes with variable contribution of carbonyl sulfide (OCS) shielding in an evolving volcanic plume. The δ34S peak at +9‰ is associated with non–33S-anomalous barites displaying negative ∆36S values, which are best interpreted as volcanic sulfate aerosols formed from OCS photolysis. Our findings confirm the occurrence of a volcanic photochemical pathway specific to the early reduced atmosphere but identify variability within the Archean sulfate isotope record that suggests persistence throughout Earth history of photochemical reactions characteristic of the present-day stratosphere.

Reworking of atmospheric sulfur in a Paleoarchean hydrothermal system

Reworking of atmospheric sulfur in a Paleoarchean hydrothermal system at Londozi, Barberton Greenstone Belt, Swaziland

Authors:

Roerdink et al

Abstract:

Anomalous fractionation of the minor isotopes of sulfur (Δ33S, Δ36S) in Archean pyrite is thought to reflect photochemical reactions in an anoxic atmosphere, with most samples falling along a reference array with Δ36S/Δ33S ≈ −1. Small deviations from this array record microbial sulfate reduction or changes in atmospheric source reactions. Here, we argue that reworking of atmospheric sulfur with distinct minor sulfur isotope ratios (Δ36S/Δ33S ≠ −1) produced additional variability in sulfide Δ33S and Δ36S-values in a 3.52 Ga hydrothermal barite deposit at Londozi, Barberton Greenstone Belt, Swaziland. In situ measurement of the four stable sulfur isotopes in pyrite revealed Δ36S–Δ33S relationships and a Δ36S/Δ33S trend (−3.2 ± 0.4), which is significantly different from the co-variation between Δ36S and Δ33S in the co-existing barite that reflects ambient Paleoarchean seawater sulfate. This argues against biological or thermochemical sulfate reduction at the time of barite deposition, and requires incorporation of sulfide generated in a chemically distinct atmosphere before 3.52 Ga. We propose a model that combines reworking of this sulfur by hydrothermal leaching, deep mixing with juvenile sulfur and surface mixing with biogenic sulfide to explain the observed variation in δ34S, Δ33S and Δ36S. These interactions between abiotic and biological processes in the Londozi hydrothermal system complicate the interpretation of biosignatures based on deviations in Δ33S and Δ36S from the Archean reference array.

Flood Basalts Don't Cause Mass Extinctions Through Sulphuric Acid

Selective environmental stress from sulphur emitted by continental flood basalt eruptions

Authors:

Schmidt et al

Abstract:

Several biotic crises during the past 300 million years have been linked to episodes of continental flood basalt volcanism, and in particular to the release of massive quantities of magmatic sulphur gas species. Flood basalt provinces were typically formed by numerous individual eruptions, each lasting years to decades. However, the environmental impact of these eruptions may have been limited by the occurrence of quiescent periods that lasted hundreds to thousands of years. Here we use a global aerosol model to quantify the sulphur-induced environmental effects of individual, decade-long flood basalt eruptions representative of the Columbia River Basalt Group, 16.5–14.5 million years ago, and the Deccan Traps, 65 million years ago. For a decade-long eruption of Deccan scale, we calculate a decadal-mean reduction in global surface temperature of 4.5 K, which would recover within 50 years after an eruption ceased unless climate feedbacks were very different in deep-time climates. Acid mists and fogs could have caused immediate damage to vegetation in some regions, but acid-sensitive land and marine ecosystems were well-buffered against volcanic sulphur deposition effects even during century-long eruptions. We conclude that magmatic sulphur from flood basalt eruptions would have caused a biotic crisis only if eruption frequencies and lava discharge rates had been high and sustained for several centuries at a time.

Shoaling of Sulfidic Waters During the Guadalupian Extinction (mid permian)

Widespread shoaling of sulfidic waters linked to the end-Guadalupian (Permian) mass extinction

Authors:

Zhang et al

Abstract:

Multiple sulfur isotopes (32S, 33S, 34S, and 36S) measured on pyrites from the Penglaitan section, the Global Stratotype Section and Point (GSSP) for the Guadalupian-Lopingian Series boundary, and from the auxiliary Tieqiao section in South China show a sulfur isotope signal of negative and positive δ34S with negative Δ33S. We suggest that these data indicate mixing of 34S-enriched and 34S-depleted sulfur in the sediments, which may have been driven by shoaling of sulfidic waters. Similar isotopic data of negative δ34S with negative Δ33S were also observed from the EF section in the Delaware Basin of west Texas (USA). The consistency of the minor sulfur isotopic anomalies from both South China and west Texas suggests a causal link between widespread shoaling of sulfidic waters and the end-Guadalupian mass extinction.

Volcanic Sulfur *NOT* Responsible for Early Martian Greenhouse Climate

Sulfur in the early martian atmosphere revisited: Experiments with a 3-D Global Climate Model

Authors:

Kerber et al

Abstract:

Volcanic SO2 in the martian atmosphere has been invoked as a way to create a sustained or transient greenhouse during early martian history. Many modeling studies have been performed to test the feasibility of this hypothesis, resulting in a range of conclusions, from highly feasible to highly improbable. In this study we perform a wide range of simulations using the 3-D Laboratoire de Météorologie Dynamique Generic Global Climate Model (GCM) in order to place earlier results into context and to explore the sensitivity of model outcomes to parameters such as SO2 mixing ratio, atmospheric H2O content, background atmospheric pressure, and aerosol size, abundance, and composition. We conclude that SO2 is incapable of creating a sustained greenhouse on early Mars, and that even in the absence of aerosols, local and daily temperatures rise above 273 K for only for limited periods with favorable background CO2 pressures. In the presence of even small amounts of aerosols, the surface is dramatically cooled for realistic aerosol sizes. Brief, mildly warm conditions require the co-occurrence of many improbable factors, while cooling is achieved for a wide range of model parameters. Instead of causing warming, sulfur in the martian atmosphere may have caused substantial cooling, leading to the end of clement climate conditions on early Mars.

Evidence of the PaleoArchean Sulfur Cycle

Paleoarchean sulfur cycling: Multiple sulfur isotope constraints from the Barberton Greenstone Belt, South Africa

Authors:

Montinaro et al

Abstract:

Mass-dependent and mass-independent sulfur isotope fractionation archived in volcanic and sedimentary rocks from the Barberton Greenstone Belt (3550–3215 Ma), South Africa, provide constraints for sulfur cycling on the early Earth. Four different sample suites were studied: komatiites and tholeiites, barite, massive and disseminated sulfide ores, and non-mineralized black shales.

Variable but generally slightly positive δ34S values between −0.7 and +5.2‰, negative Δ33S values between −0.50 and −0.09‰, and a negative correlation between δ34S and Δ33S as well as between Δ33S and Δ36S for komatiites and tholeiites from the Komati Formation and from the Weltevreden Formation are outside the expected range of unfractionated juvenile sulfur. Instead, results suggest alteration of oceanic crustal rock sulfur through interactions with fluids that most likely derived their sulfur from seawater.

Barite from the Mapepe Formation displays positive δ34S values between +3.1 and +8.1‰ and negative Δ33S values between −0.77 and −0.34‰. The mass-independent sulfur isotope fractionation indicates an atmospheric sulfur source, notably photolytic sulfate, whereas the positive δ34S values suggest bacterial sulfate reduction of the marine sulfate reservoir.

Non-mineralized black shale samples from the presumed stratigraphic equivalent of the Mapepe Formation show positive δ34S values between 0.0 and +1.3‰ and positive Δ33S values between +0.59 and +2.45‰. These results are interpreted to result from the reduction of photolytic elemental sulfur, carrying a positive Δ33S signature.

Positive δ34S values ranging from +0.7 to +3.5‰ and slightly negative Δ33S values between −0.17 and −0.12‰ characterize massive and disseminated sulfides from the Bien Venue Prospect. Results suggest unfractionated juvenile magmatic sulfur source as the primary sulfur source, but a contribution from recycled seawater sulfate, which would be indicative of submarine hydrothermal activity, cannot be ruled out.

Massive and disseminated sulfides from the M’hlati prospect are distinctly different from massive and disseminated sulfide from the Bien Venue Prospect. They show negative δ34S values between −1.2 and −0.1‰ and positive Δ33S values between +2.66 and +3.17‰, thus, displaying a sizeable mass-independent sulfur isotopic fractionation. Again, these samples clearly exhibit the incorporation of an atmospheric MIF-S signal. The source of sulfur for these samples has positive Δ33S values, suggesting a connection with photolytic elemental sulfur.

In conclusion, the sulfur isotope signatures in Paleoarchean rocks from the Barberton Greenstone Belt are diverse and indicate the incorporation of different sources of sulfur. For komatiites and tholeiites, barite and massive and possibly also disseminated sulfides from Bien Venue, multiple sulfur isotopes are related to ambient seawater sulfate and its photolytic origin, while massive and disseminated sulfides from M’hlati and non-mineralized black shales are related to a second (photolytic elemental sulfur) end member.

Evidence of a Gradual Oxygenation of the Calymmian MesoProterozoic PaleoAtmosphere From Sulfur Isotopes

Sulfur isotope composition of carbonate-associated sulfate from the Mesoproterozoic Jixian Group, North China: Implications for the marine sulfur cycle

Authors:

Guo et al

Abstract:

The Mesoproterozoic has been traditionally viewed as a period of prolonged stability in terms of environmental and biological evolution, yet growing body of evidence suggests subtle, yet dynamic changes in marine biogeochemical cycles. In this study, we present a high-resolution analysis of carbonate-associated sulfate (CAS) from the Mesoproterozoic Jixian Group (1.6–1.4 Ga), North China. Combined with previously reported carbon isotopes from the same sections, sulfur isotope data provide insight into both oceanic chemistry and biospheric evolution during this period.

The sulfur isotopic composition of CAS in the Jixian Group displays high-amplitude variations with a total range from +3.7‰ to +51.9‰. Stratigraphic variation in δ34SCAS is most noted in the Wumishan Formation and records shifts of greater than 10‰ over stratigraphic thickness of 100–200 m, indicating a short residence time resulting from a limited oceanic sulfate reservoir. Differences in the pattern of isotopic variability between the Gaoyuzhuang and Wumishan formations suggest complex behavior of the marine sulfur cycle based on proximity of depositional environments to a local or regional chemocline, and the degree of mixing between adjacent environments. Deposition under predominantly oxic conditions favored a stable sulfate reservoir whose isotopic composition was controlled primarily by pyrite burial. By contrast, depositional environments proximal to euxinic waters were subject to rapid changes in both sulfate reservoir size and isotopic composition, resulting from growth and subsequent oxidation of a reactive hydrogen sulfide reservoir. Together, these units demonstrate the difficulty of interpreting marine sulfur cycling in a low oxygen, low sulfate world subject to a range of environmental controls.

Data also reveal a long-term positive trend in average δ34SCAS and δ13Ccarb from approximately +10‰ to ∼+20‰, and ∼−0.5‰ to +0.5‰, respectively, which is interpreted as reflecting prolonged organic carbon and pyrite burial. Burial of reduced carbon and sulfur may have resulted in a slow, but steady release of oxygen to the biosphere, ultimately driving a gradual increase in oceanic sulfate levels. Our study, combined with previous contributions from the late Mesoproterozoic, supports hypotheses of protracted biospheric oxygenation marked by a modest increase in oceanic sulfate concentrations through the Mesoproterozoic.

Declinate in Oceanic Sulfate During Calymmian MesoProterozoic

Decline in oceanic sulfate levels during the early Mesoproterozoic

Authors:

Luo et al

Abstract:

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 30-150 m Myr−1. These data indicate early Mesoproterozoic seawater [SO42−] of ∼ < 0.1 to 0.35 mM (with a maximum possible concentration of 1.8 mM), 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 ∼1.7 Ga and persisted until at least the mid-Mesoproterozoic (∼1.4 Ga), 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.

Evidence of Sulfur Eating Bacteria From NeoArchean Brazil

Large sulfur isotope fractionations associated with Neoarchean microbial sulfate reduction

Authors:

Zhelezinskaia et al

Abstract:

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 Δ33S values (~ –2 per mil) indicative of atmospheric photochemical reactions. Persistent 33S depletion through ~60 meters of shallow marine carbonate implies long-term stability of seawater sulfate abundance and isotope composition. In contrast, a negative Δ33S 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.

Evidence of a Sulfur Anomalies From NeoArchean

Neoarchean carbonate–associated sulfate records positive Δ33S anomalies

Authors:

Paris et al

Abstract:

Mass-independent fractionation of sulfur isotopes (reported as Δ33S) 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 Δ33S 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 Δ33S signatures. We report unexpected, large, widespread positive Δ33S 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 Δ33S from sulfur mass-independent fractionation in the Neoarchean atmosphere.

Sulfate Data From Geological Record may be Widely & Thoroughly Compromised

Widespread contamination of carbonate-associated sulfate by present-day secondary atmospheric sulfate: Evidence from triple oxygen isotopes

Authors:

Peng et al

Abstract:

The isotope composition of seawater sulfate is an important tracer of sulfur, carbon, and oxygen cycles in Earth's deep past. Carbonate-associated sulfate (CAS) extracted by acid digestion is widely used as a proxy for sulfate in paleo-seawater from which the carbonate minerals precipitated. Early and late diagenesis, weathering, and laboratory processing can in some cases compromise original seawater sulfate signals. Here, we report that extracted CAS can also be severely contaminated by recent atmospheric sulfate, especially when the sampled carbonates are from outcrops in arid to semi-arid climates or in heavily polluted regions. Our evidence comes from triple oxygen isotope compositions of sequentially extracted water-leachable sulfate and acid-leachable sulfate from carbonates of diverse ages from northwestern and north-central China and southwestern North America. Independent of the age of the rocks, almost all the water-leachable sulfates and half of the acid-leachable sulfates bear positive 17O anomalies, clearly distinguishable from those of typical seawater sulfate. Because secondary atmospheric sulfate (SAS) is the only source of sulfate known to bear positive 17O anomalies, we conclude that sulfate extracted from carbonate outcrops in these regions has a significant component of SAS. Because SAS generally has a much lower δ34S value than paleo-seawater sulfate, it could shift the δ34S of the extracted CAS to lower values and in some cases even lower than that of the co-occurring pyrite, i.e., the "super-heavy pyrite" enigma reported in geological records. Our findings call for a re-evaluation of many published, outcrop-based CAS data and conclusions.