Mass-independent Fractionation Of Sulfur Isotopes Research Articles

A new Re-Os age constraint informs the dynamics of the Great Oxidation Event

Abstract The early Paleoproterozoic (ca. 2.5–2.2 Ga) represents a critical juncture in Earth history, marking the inception of an oxygenated atmosphere while bearing witness to potentially multiple widespread and severe glaciations. Deciphering the nature of this glacial epoch and its connection with atmospheric oxygenation has, however, proven difficult, hindered by a reliance on disputed stratigraphic correlations given the paucity of direct radiometric age constraints. Nowhere is this more acute than within the South African Transvaal Supergroup: Here, while the loss of oxygen-sensitive mass-independent sulfur isotope fractionation (S-MIF) has been reported from both the Duitschland and Rooihoogte formations, divided opinion surrounding the time-equivalence of these units has prompted authors to argue for vastly different oxygenation trajectories. Addressing this debate, we present a depositional Re-Os age (2443 ± 33 Ma) from diamictite samples preserved in drillcore of the upper Duitschland Formation. The 100-million-year separation between the Duitschland Formation and its previously presumed equivalent reveals at least two isolated disappearances of S-MIF, requiring that the Great Oxidation Event was dynamic and proceeded via discrete oxygenation episodes whose structure remains incompletely understood. Importantly, our revised framework aligns the lower Duitschland diamictite with the low-latitude glacigenic Makganyene Formation, supporting hypotheses of widespread regional, and potentially global, early Paleoproterozoic glaciation.

Mass-independent fractionation of oxygen and sulfur isotopes

Mass-independent fractionation of oxygen and sulfur isotopes

Sedimentary and stratigraphic architecture of the Duitschland and Rooihoogte formations (Palaeoproterozoic, South Africa): implications for tempo of the Great Oxidation Event

Abstract Determining the tempo and causality of key palaeoclimate events recorded in sedimentary strata depends on high-resolution numerical ages and well-constrained stratigraphic correlations at regional and global scale. This requirement is not necessarily met in Precambrian strata due to poorer age resolution, limited preservation, and secondary overprints. A good example includes the Palaeoproterozoic Rooihoogte and Duitschland formations in South Africa, which document the disappearance of mass-independent fractionation of sulphur isotopes (MIF-S) and contain glacial diamictites at their base. They are thus key records of Earth’s surface oxygenation during the Great Oxidation Event (GOE). However, previous studies have either correlated these units, resulting in a unidirectional oxidation trend; or have regarded them as successive strata, causing an interpretation of oscillating oxidation. This study uses extensive outcrop and new core material to investigate correlation between these two units, and to establish depositional models. Results show that key stratigraphic markers can be traced around the entire Kaapvaal craton both in outcrop and the subsurface. In particular, the basal Bevets breccia and the top Duitschland breccia are here re-interpreted as two separate units that are present at the base and top of both formations, supporting correlation of the formations. Consequently, the base of both formations records a major craton-wide event of uplift and karstification, leading to carbonate dissolution and chert brecciation. Erosion of older rocks from across the craton also delivered material for the basal glacial diamictite. The majority of mixed siliciclastic-carbonate sediments were deposited on a storm- and/or delta-influenced shelf. Depositional packages in both formations reflect post-glacial relative sea level rise, followed by progradation of a deltaic, storm or shoreline depositional system. There is a relatively short-lived depositional hiatus to overlying shales of the Timeball Hill Formation. Both Rooihoogte and Duitschland formations thus record only a single glacial event at their base, and a unidirectional trend of oxidation.

Atmospheric ozone destruction and the end-Permian crisis: Evidence from multiple sulfur isotopes

The end-Permian mass extinction (EPME) is considered the largest biotic crisis of the Phanerozoic. To explain the worldwide destruction and mutation of land plants, previous work has emphasized the role of enhanced UV irradiation linked to volcanism-induced disruption of the ozone shield. However, direct evidence for a link between volcanic SO2 emission and stratospheric ozone deterioration is missing. Previous bulk analysis (i.e., SF6 method) found no Mass-Independent Fractionation of sulfur isotopes (MIF-S, noted Δ33S). However, possible grain scale sulfur isotope anomaly can be easily overlooked during bulk analysis. To detect possible sulfur isotope anomaly in grain scale, we applied in-situ multiple sulfur isotope analysis of the sulfide using secondary ion mass spectrometry (SIMS). We found a marked positive shift (higher than +0.30‰ and up to +0.94‰) just before the end-Permian extinction in the Meishan section. Modelling shows that these positive Δ33S anomalies cannot be explained by Mass Dependent Fractionation (MDF) processes alone, rather by UV-induced photolysis of volcanic SO2. The formation and preservation of MIF-S anomalies in the atmosphere and subsequent transport to the oceans require intense and prolonged irradiation of the Earth's surface with solar UV, thus fingerprinting severe disruption of the atmospheric ozone balance at the onset of Earth's largest mass extinction.

Toward the Origins of Quadruple Sulfur Isotope Anomalies in Modern Sulfate: A Multitracer Approach and Implications for Paleo- and Planetary Atmospheres

Sulfur isotope mass-independent fractionation signatures (S-MIF) have been widely found in modern sulfate aerosols. Their mechanistic origins and potential implications for atmospheric sulfate formation chemistry are however poorly understood. Of particular importance is that the mystery of modern sulfate S-MIF hinders precise interpretations of S-MIF in geological sediment and meteorite samples. Here, we examine several proposed origins of modern sulfate S-MIF using a multiple-tracer approach. Quadruple sulfur isotopes of sulfate aerosols collected from Guangzhou, a megacity in South China, were measured along with various chemical and isotopic tracers for high-altitude air masses, biomass burning, and mineral dusts in the same samples. Similar to previous studies, we found that negative Δ36S values in modern sulfate aerosols are linked to combustions, and to a larger extent, to high-temperature processes that may involve symmetry-dependent recombination reactions of sulfur. The origins of Δ33S anomalies widely observed in modern sulfate aerosols remain elusive and require further investigation before we can unambiguously use this proxy to quantify sulfur emission and transformation pathways in the modern atmosphere. From a comparative planetology perspective, we highlight that a complete understanding of S-MIF in modern aerosols is crucial for tracking paleo- and planetary atmospheres by multiple sulfur isotope measurements in Phanerozoic sediments and Martian meteorites because their S-MIF magnitudes are similar.

Reconciling discrepant minor sulfur isotope records of the Great Oxidation Event

Understanding the timing and trajectory of atmospheric oxygenation remains fundamental to deciphering its causes and consequences. Given its origin in oxygen-free photochemistry, mass-independent sulfur isotope fractionation (S-MIF) is widely accepted as a geochemical fingerprint of an anoxic atmosphere. Nevertheless, S-MIF recycling through oxidative sulfide weathering—commonly termed the crustal memory effect (CME)—potentially decouples the multiple sulfur isotope (MSI) record from coeval atmospheric chemistry. Herein, however, after accounting for unrecognised temporal and spatial biases within the Archaean–early-Palaeoproterozoic MSI record, we demonstrate that the global expression of the CME is barely resolvable; thereby validating S-MIF as a tracer of contemporaneous atmospheric chemistry during Earth’s incipient oxygenation. Next, utilising statistical approaches, supported by new MSI data, we show that the reconciliation of adjacent, yet seemingly discrepant, South African MSI records requires that the rare instances of post-2.3-billion-year-old S-MIF are stratigraphically restricted. Accepting others’ primary photochemical interpretation, our approach demands that these implied atmospheric dynamics were ephemeral, operating on sub-hundred-thousand-year timescales. Importantly, these apparent atmospheric relapses were fundamentally different from older putative oxygenation episodes, implicating an intermediate, and potentially uniquely feedback-sensitive, Earth system state in the wake of the Great Oxidation Event.

Rapid timescale for an oxic transition during the Great Oxidation Event and the instability of low atmospheric O2

The Great Oxidation Event (GOE), arguably the most important event to occur on Earth since the origin of life, marks the time when an oxygen-rich atmosphere first appeared. However, it is not known whether the change was abrupt and permanent or fitful and drawn out over tens or hundreds of millions of years. Here, we developed a one-dimensional time-dependent photochemical model to resolve time-dependent behavior of the chemically unstable transitional atmosphere as it responded to changes in biogenic forcing. When forced with step-wise changes in biogenic fluxes, transitions between anoxic and oxic atmospheres take between only 102 and 105 y. Results also suggest that O2 between [Formula: see text] and [Formula: see text] mixing ratio is unstable to plausible atmospheric perturbations. For example, when atmospheres with these O2 concentrations experience fractional variations in the surface CH4 flux comparable to those caused by modern Milankovich cycling, oxygen fluctuates between anoxic ([Formula: see text]) and oxic ([Formula: see text]) mixing ratios. Overall, our simulations are consistent with possible geologic evidence of unstable atmospheric O2, after initial oxygenation, which could occasionally collapse from changes in biospheric or volcanic fluxes. Additionally, modeling favors mid-Proterozoic O2 exceeding [Formula: see text] to [Formula: see text] mixing ratio; otherwise, O2 would periodically fall below [Formula: see text] mixing ratio, which would be inconsistent with post-GOE absence of sulfur isotope mass-independent fractionation.

Sulfur isotopes as biosignatures for Mars and Europa exploration

Sulfur (S) isotopes are used to trace metabolic pathways associated with biological S-cycling in past and present environments on Earth. These pathways (sulfate reduction, sulfur disproportionation and sulfide oxidation) can produce unique S isotope signals that provide insight into biogeochemical S-cycling. The S cycle is also relevant for extraterrestrial environments and processes. On early Mars, sulfur existed in different redox states and was involved in a large range of surface processes (e.g. volcanic, atmospheric, hydrothermal and aqueous brines). Sulfur compounds have also been detected on Europa's icy moon surface, with the S cycle implicated in Europa's surface and ocean geochemistry. Given the well-established utility of S isotopes in providing a record for past life on Earth, S isotopes are a valuable tool for identifying biosignatures on Mars and Europa. Here, we review S isotopes as a biosignature, in light of two recent advances in understanding the S cycle in both Mars and Europa: (1) the measurements of δ 34 S in situ at Gale Crater and quadruple S isotopes (QSI) in Martian meteorites; (2) the identification of a likely exogenous origin of sulfur on Europa's surface. We discuss important considerations for unravelling QSI biosignatures in Martian environments, considering high- and low-sulfur environments, atmospheric mass-independent fractionation of sulfur isotopes (S-MIF) signals and metabolic energy-limited niches. For Europa, we describe the potential for S isotopes to probe biogeochemistry, and identify key knowledge gaps to be addressed to unlock S isotopic tools for future life detection efforts. The resulting picture demonstrates how S isotopes will be a valuable tool for Mars sample return, and how future missions can focus on the search for environments where QSI signatures of microbial S-cycling processes have a greater chance of being preserved. For Europa, the first step will be to account for the S isotope composition of the various S pools, to recognize or rule out non-biologically mediated S isotope values, with a focus on experimental examination of potential S isotope signatures from exogenous sulfur sources. Thematic collection: This article is part of the Sulfur in the Earth system collection available at: https://www.lyellcollection.org/cc/sulfur-in-the-earth-system

Sulfur mass-independent fractionation during SO2 photolysis in low-temperature/pressure atmospheres

Mass-independent fractionation of sulfur isotopes (MIF-S) has been observed in Archean sedimentary rocks and modern stratospheric sulfate aerosol (SSA). Photolysis of SO2 is known to cause significant MIF-S and could be related to the observed MIF-S. However, previous experiments with SO2 photolysis at room temperature, or at low temperatures and atmospheric pressure, did not quantitatively explain quadruple sulfur isotopic compositions (δ34S, Δ33S, and Δ36S values) of the Archean sedimentary rocks and the modern SSA. Here we describe sulfur isotopic fractionation during SO2 photolysis including isotopic self-shielding at low temperatures (down to 228 K) and low pressures (from 5.9 to 9.8 kPa) where pressure broadening of SO2 becomes negligible. Results indicate that magnitudes of sulfur isotopic fractionation factors (34ε, 33E, and 36E values, where 33E = 33ε – 1000 × [(1 + 34ε/1000)0.515 − 1] and 36E = 36ε − 1000 × [(1 + 36ε/1000)1.90 − 1]) increase with decreasing temperature, with values at 228 K being about four times those at 296 K (34ε of up to +344‰). Meanwhile, 33E/34ε and 36E/33E ratios are roughly independent of temperature over the temperature range (by approximately +0.1 and − 3.1, respectively), although the 33E/34ε ratios slightly increase with decreasing temperature, ranging from ~ + 0.08 to ~ + 0.13. A two-component mixing model involving SO2 oxidation by OH and SO2 photolysis in the stratosphere reproduces δ34S and Δ33S values of modern SSA when the contribution ratio of SO2 photolysis to SO3 production is ~20%. The contribution ratio of ~20% is consistent with a previous estimation by Whitehill et al. (2015). However, Δ33S/δ34S ratios at low temperatures are far from those of Archean sedimentary rocks, calling for an additional mechanism (or mechanisms) to explain Archean MIF-S.

Bulk and grain-scale minor sulfur isotope data reveal complexities in the dynamics of Earth’s oxygenation

SignificanceThe permanent disappearance of mass-independent sulfur isotope fractionation (S-MIF) from the sedimentary record has become a widely accepted proxy for atmospheric oxygenation. This framework, however, neglects inheritance from oxidative weathering of pre-existing S-MIF-bearing sedimentary sulfide minerals (i.e., crustal memory), which has recently been invoked to explain apparent discrepancies within the sulfur isotope record. Herein, we demonstrate that such a crustal memory effect does not confound the Carletonville S-isotope record; rather, the pronounced Δ33S values identified within the Rooihoogte Formation represent the youngest known unequivocal oxygen-free photochemical products. Previously observed 33S-enrichments within the succeeding Timeball Hill Formation, however, contrasts with our record, revealing kilometer-scale heterogeneities that highlight significant uncertainties in our understanding of the dynamics of Earth's oxygenation.

A prolonged, two-step oxygenation of Earth's early atmosphere: Support from confidence intervals

Abstract The Great Oxidation Event (GOE), among Earth's most transformative events, marked the sustained presence of oxygen above 10−5 times the present atmospheric level. Estimates of the onset of the GOE span 2501–2225 Ma and are based primarily on the loss of mass-independent fractionation of sulfur isotopes (MIF-S) in pyrite. To better constrain the timing of the GOE, we apply probabilistic techniques to estimate the confidence intervals of four proxies: MIF-S, redox-sensitive detrital minerals, “red beds,” and I/(Ca + Mg). These GOE proxies are drawn from a highly fragmentary geologic record, and consequently, estimates of the 95% confidence intervals span tens to hundreds of millions of years—orders of magnitude larger than suggested by radiometric constraints on individual successions. Confidence interval results suggest that red beds and nonzero I/(Ca + Mg) values may have appeared earlier than 2480 Ma and 2460 Ma, respectively, whereas redox-sensitive detrital minerals and MIF-S may have disappeared after 2210 Ma and 2190 Ma, respectively. These data suggest a delay of potentially >300 m.y. between initial and permanent oxygenation of the atmosphere and a delay of tens of millions of years between onset of the Lomagundi-Jatuli carbon isotope excursion and permanent oxygenation of the atmosphere.

Mass-Independent Sulfur Isotope Fractionation in the Photochemical SO2 Processes under the UV Radiation of Different Wave Length

A series of photochemical experiments with sulfur dioxide were carried out using a xenon lamp (λ > 200 nm), a low-pressure mercury lamp (λ = 184.9 and 253.7 nm), and an UP-213 laser system (λ = 213 nm) as light sources. Based on the isotope data obtained for elemental sulfur (the end product of SO2 photolysis), we revealed specifics in correlations between δ34S, ∆33S, and ∆36S values depending on the spectral characteristics of the incident radiation. Our studies showed that the Archean Δ33S/δ34S and Δ36S/Δ33S ratios could be reproduced in the SO2 photolysis experiments by combining spectral composition of the radiation, the intensity of the spectral components, and the partial pressure of SO2. The results of the experiments suggest that the UV radiation with wavelengths less than 200 nm was a determining factor in the production of sulfur isotopic anomalies in the Archean atmosphere.

Chemical evolution of seawater in the Transvaal Ocean between 2426 Ma (Ongeluk Large Igneous Province) and 2413 Ma ago (Kalahari Manganese Field)

Drill core and outcrop samples of pure marine chemical sediments (banded iron formation (BIF), manganese formation (MnF), jaspilites, lutites, and cherts) from the transition of the ~2426 Ma old Ongeluk Formation into the 2413 Ma old Hotazel Formation, Transvaal Supergroup, South Africa, reveal remarkable changes of seawater chemistry in the Transvaal Ocean. Similar to pre-Ongeluk chemical sediments, the shale-normalized rare earths and yttrium (REYSN) patterns of jaspilites intercalated with the volcanic rocks of the Ongeluk large igneous province and directly overlying cherts do not show positive EuSN anomalies, indicating that high-temperature (>250 °C) hydrothermal fluids did not contribute significantly to the REY budget of ambient waters. However, a 10 cm drill core section in the lower Hotazel Formation is characterized by conspicuous positive EuSN anomalies, revealing temporary inflow of water masses strongly affected by high-temperature hydrothermal fluids. After this short episode, the REYSN pattern of Transvaal seawater returned to that of pre-Ongeluk times, showing heavy REYSN enrichment, positive LaSN, GdSN and YSN anomalies, but no CeSN or EuSN anomalies. Higher up in the stratigraphy, the Hotazel Formation shows negative CeSN anomalies in some of the lutites, BIFs and MnFs, reflecting Ce depletion in ambient seawater. All Hotazel lutite, BIF, and MnF samples studied show unradiogenic εNd(t) values (−0.5 ± 0.2 to −2.4 ± 0.2), indicating a mostly continental REY source. The REY distribution and Nd isotope data combined suggest that oxidative terrestrial weathering of this continental crustal source supplied most of the dissolved REY to local “Transvaal seawater”. Precipitation of the Hotazel lutites, BIFs and MnFs with negative CeSN anomalies, therefore, suggests that oxic conditions prevailed on the Kaapvaal Craton and in Hotazel seawater already at ~2.413 Ga, i.e. 80 m.y. before the disappearance of mass-independent sulfur isotope fractionation (MIF-S) that defines the Great Oxidation Event at ~2.33 Ga.

Tracing the Early Emergence of Microbial Sulfur Metabolisms

Hydrogen sulfide (nH2S) and sulfur oxide (SO n ; n = 1, 2, 3) gases in early Earth’s globally anoxic atmosphere were subjected to gas-phase chemical transformations by UV light. A principal photolysis pathway at that time produced elemental sulfur aerosols with mass-independently fractionated (MIF) isotopic values carrying variable minor isotope (33S, 36S) compositions. These rained into the sulfate-deficient Archean (ca. 3.85–2.5 Ga) oceans to react with [Fe2+]aq and form sedimentary sulfides. The MIF-bearing sulfides were incorporated into Archean sediments, including banded iron formations (BIF). Such aerosols may also have fueled microbial sulfur metabolisms, and thus are traceable by the MIF sulfur isotopes. Yet, data show that before ∼3.5 Ga mass-dependent34S/32S values in Early Archean sediments tend to fall within a narrow (±0.1%) range even as they carry mass-independent values. By about 3.5 Ga, 34S/32S values show much greater changes (>1%) in range congruent with microbial metabolic processing. Here, we trace probable pathways of elemental sulfur aerosols into Archean sediments, and couple our study with analysis of the evolutionary relationships of enzymes involved in sulfur metabolism to explain the observed trends. Our model explains why elemental sulfur aerosols were apparently not utilized by the Eoarchean (pre-3.65 Ga) biosphere even though an immediate precursor to the required enzyme may have already been present. Highlights Evolution of microbial sulfur metabolisms is tracked by multiple sulfur isotopes Alkaline hydrothermal vents were an abode for early microbial life Sulfite detoxification prompted anaerobic respiration Reversal of respiratory electron transport chain (ETC) stimulated photothiotrophy Surplus e- acceptors permitted the emergence of elemental sulfur reduction

A seawater-sulfate origin for early Earth’s volcanic sulfur

Mass-independent fractionation of sulfur isotopes (MIF-S)—as recorded primarily in pre-2.5 billion years ago (Ga) sedimentary rocks—has been interpreted as evidence of photolysis of volcanic SO2 in an anoxic troposphere. Here, I present thermodynamic and kinetic calculations, combined with data on the geology, mineralogy and chemical and isotopic compositions of modern and Archaean (3.8–2.5 Ga) aged volcanic samples from different tectonic settings, to examine early Earth’s sulfur cycle. Based partly on the similarities between submarine hydrothermal deposits and arc volcanic rocks in pyrite (FeS2) abundances and sulfur isotopic compositions (for example, the presence of both positive and negative δ34S values), I conclude that degassing of sulfur (mostly as SO2) into the atmosphere has been carried out primarily by subaerial eruptions of oxidized, arc-like magmas since at least 3.5 Ga. The generation of volcanic SO2 requires plate tectonics and the involvement of sulfate-rich seawater, which requires large exposed lands and an oxygenated atmosphere. I propose that the MIF-S signatures in sedimentary rocks were created by ultraviolet photochemical reactions between SO2 from explosive volcanic eruptions and O2 in the stratosphere, above an oxygen-rich troposphere, or by high-temperature reactions between organic compounds and sulfate in the oceans. Formation of mass-independent isotope fractionation of sulfur signatures recorded in Archaean sedimentary rocks could have occurred in an oxygen-rich atmosphere, according to thermodynamic and kinetic calculations and analysis of Earth’s early sulfur cycle.

The Great Oxidation Event preceded a Paleoproterozoic “snowball Earth”

The inability to resolve the exact temporal relationship between two pivotal events in Earth history, the Paleoproterozoic Great Oxidation Event (GOE) and the first "snowball Earth" global glaciation, has precluded assessing causality between changing atmospheric composition and ancient climate change. Here we present temporally resolved quadruple sulfur isotope measurements (δ34S, ∆33S, and ∆36S) from the Paleoproterozoic Seidorechka and Polisarka Sedimentary Formations on the Fennoscandian Shield, northwest Russia, that address this issue. Sulfides in the former preserve evidence of mass-independent fractionation of sulfur isotopes (S-MIF) falling within uncertainty of the Archean reference array with a ∆36S/∆33S slope of -1.8 and have small negative ∆33S values, whereas in the latter mass-dependent fractionation of sulfur isotopes (S-MDF) is evident, with a ∆36S/∆33S slope of -8.8. These trends, combined with geochronological constraints, place the S-MIF/S-MDF transition, the key indicator of the GOE, between 2,501.5 ± 1.7 Ma and 2,434 ± 6.6 Ma. These are the tightest temporal and stratigraphic constraints yet for the S-MIF/S-MDF transition and show that its timing in Fennoscandia is consistent with the S-MIF/S-MDF transition in North America and South Africa. Further, the glacigenic part of the Polisarka Formation occurs 60 m above the sedimentary succession containing S-MDF signals. Hence, our findings confirm unambiguously that the S-MIF/S-MDF transition preceded the Paleoproterozoic snowball Earth. Resolution of this temporal relationship constrains cause-and-effect drivers of Earth's oxygenation, specifically ruling out conceptual models in which global glaciation precedes or causes the evolution of oxygenic photosynthesis.

An analytical formulation of isotope fractionation due to self-shielding

Isotope fractionation due to photochemical self-shielding is believed to be responsible for the enrichment of inner solar system planetary materials in the rare isotopes of carbon, nitrogen, and oxygen relative to the Sun. Self-shielding may also contribute to sulfur isotope mass-independent fractionation in modern atmospheric sulfates, although its role in the early Earth atmosphere has not yet been convincingly established. Here, I present an analytical formulation of isotopic photodissociation rate coefficients that describe self-shielding isotope signatures for 3 and 4-isotope systems broadly representative of O and S isotopes. The analytic equations are derived for idealized molecular spectra, making an analytic formulation tractable. The idealized spectra characterize key features of actual isotopologue spectra, particularly for CO and SO2, but are applicable to many small molecules and their isotopologues. The analytic expressions are convenient for evaluating the magnitude of isotope effects without having to pursue involved numerical solutions. More importantly, the analytic expressions illustrate the origin of particular isotope signatures, such as the previously unexplained large mass-dependent fractionation associated with photodissociation of optically-thick SO2. The formulation presented here elucidates the origin of some of these important isotopic fractionation processes.

Positive cerium anomalies imply pre-GOE redox stratification and manganese oxidation in Paleoproterozoic shallow marine environments

The Paleoproterozoic Koegas Subgroup (Transvaal Supergroup, South Africa) was deposited in the immediate prelude to the Great Oxidation Event (GOE), and can therefore shed light on oceanic paleoredox conditions just before atmospheric oxidation. Manganese enrichments of ~16 wt% in diagenetic kutnahorite horizons suggest that Mn2+ oxidation occurred, either by free O2 or by an ancient photosystem. Iron and molybdenum isotope trends also support the existence of a Mn4+ oxide sediment flux, suggesting that the Koegas basin may have been redox stratified. Evidence from detrital and authigenic pyrite with mass-independently fractionated sulfur isotopes, however, suggests that the atmosphere was devoid of oxygen. To resolve this contradiction, this paper presents new constraints on pathways of Mn2+ oxidation from field, petrographic, stable isotope, and rare earth element and yttrium (REYSN) analysis of stromatolitic carbonates from the upper Koegas Subgroup. Ferroan dolostones and limestones preserve marine REYSN arrays with positive CeSN anomalies. These differences are explained by a redox stratified basin, whereby Mn2+ and Ce3+ are oxidized at a redoxcline and Ce is adsorped onto sinking Mn oxide particles. Mn oxide particles and a negative Ce anomaly from the oxidized upper water column are transferred into carbonates accumulating above the redoxcline. Diagenetic fluids later reduce the Mn oxides to kutnahorite. Below the redoxcline, reduction of Mn oxides enriches carbonates in Mn and a positive Ce anomaly. This contribution adds evidence for the development of oxygen oases and redox-stratified basins before the GOE. Redox stratification was best developed during transgressions. During regressions, a deltaic system prograded into the Koegas Basin. High sedimentation rates likely allowed for preservation of detrital pyrite only in the deltaic sandstones, thus explaining the contradictory geochemical evidence. No previously unknown ancient photosystem of Mn oxidation is required to explain Mn oxidation.

In situ S-isotope compositions of sulfate and sulfide from the 3.2 Ga Moodies Group, South Africa: A record of oxidative sulfur cycling.

Sulfate minerals are rare in the Archean rock record and largely restricted to the occurrence of barite (BaSO4 ). The origin of this barite remains controversially debated. The mass-independent fractionation of sulfur isotopes in these and other Archean sedimentary rocks suggests that photolysis of volcanic aerosols in an oxygen-poor atmosphere played an important role in their formation. Here, we report on the multiple sulfur isotopic composition of sedimentary anhydrite in the ca. 3.22Ga Moodies Group of the Barberton Greenstone Belt, southern Africa. Anhydrite occurs, together with barite and pyrite, in regionally traceable beds that formed in fluvial settings. Variable abundances of barite versus anhydrite reflect changes in sulfate enrichment by evaporitic concentration across orders of magnitude in an arid, nearshore terrestrial environment, periodically replenished by influxes of seawater. The multiple S-isotope compositions of anhydrite and pyrite are consistent with microbial sulfate reduction. S-isotope signatures in barite suggest an additional oxidative sulfate source probably derived from continental weathering of sulfide possibly enhanced by microbial sulfur oxidation. Although depositional environments of Moodies sulfate minerals differ strongly from marine barite deposits, their sulfur isotopic composition is similar and most likely reflects a primary isotopic signature. The data indicate that a constant input of small portions of oxidized sulfur from the continents into the ocean may have contributed to the observed long-term increase in Δ33 Ssulfate values through the Paleoarchean.

Multiple Sulfur Isotope Records of the 3.22 Ga Moodies Group, Barberton Greenstone Belt

The Moodies Group, the uppermost unit in the Barberton Greenstone Belt (BGB) in South Africa, is a ~3.7-km-thick coarse clastic succession accumulated on terrestrial-to-shallow marine settings at around 3.22 Ga. The multiple sulfur isotopic composition of pyrite of Moodies intervals was newly obtained to examine the influence of these depositional settings on the sulfur isotope record. Conglomerate and sandstone rocks were collected from three synclines north of the Inyoka Fault of the central BGB, namely, the Eureka, Dycedale, and Saddleback synclines. The sulfur isotopic composition of pyrite was analyzed by Secondary Ion Mass Spectrometry (SIMS) for 6 samples from the three synclines and by Isotope Ratio Mass Spectrometry (IR-MS) for 17 samples from a stratigraphic section in the Saddleback Syncline. The present results show a signal of mass-independent fractionation of sulfur isotopes (S-MIF), although t-tests statistically demonstrated that the Moodies S-MIF signals (mostly 0‰ < ∆33S < +0.5‰) are significantly small compared to the signal of the older Paleoarchean (3.6–3.2 Ga) records. These peculiar signatures might be related to initial deposition of detrital pyrite of juvenile origin from the surrounding intrusive (tonalite–trondhjemite–granodiorite; TTG) and felsic volcanic rocks, and/or to secondary addition of hydrothermal sulfur during late metasomatism. Moreover, fast accumulation (~0.1–1 mm/year) of the Moodies sediments might have led to a reduced accumulation of sulfur derived from an atmospheric source during their deposition. As a result, the sulfur isotopic composition of the sediments may have become susceptible to the secondary addition of metasomatic sulfur on a mass balance point of view. The sulfur isotopic composition of Moodies pyrite is similar to the composition of sulfides from nearby gold mines. It suggests that, after the Moodies deposition, metasomatic pyrite formation commonly occurred north of the Inyoka Fault in the central BGB at 3.1–3.0 Ga.