Mass-independent Fractionation Of Sulfur Isotopes Research Articles

Evidence of magnetic isotope effects during thermochemical sulfate reduction

Thermochemical sulfate reduction experiments with simple amino acid and dilute concentrations of sulfate reveal significant degrees of mass-independent sulfur isotope fractionation. Enrichments of up to 13‰ for (33)S are attributed to a magnetic isotope effect (MIE) associated with the formation of thiol-disulfide, ion-radical pairs. Observed (36)S depletions in products are explained here by classical (mass-dependent) isotope effects and mixing processes. The experimental data contrasts strongly with multiple sulfur isotope trends in Archean samples, which exhibit significant (36)S anomalies. These results support an origin other than thermochemical sulfate reduction for the mass-independent signals observed for early Earth samples.

Iron and sulphur isotopes from the Carajás mining province (Pará, Brazil): Implications for the oxidation of the ocean and the atmosphere across the Archaean–Proterozoic transition

Mineralogical analyses and major and trace element geochemistry combined with iron and sulfur isotopes were applied to banded iron formations (BIFs) and pyrite-bearing sandstones from the Carajás mineral province, Brazil. The BIFs belong to the 2.7 Ga old Grão Pará Group and show a rather simple mineralogy (hematite and quartz or cryptocrystalline silica). Their REE + Y patterns suggest deposition under marine sub-oxic conditions. Fe isotopes show highly positive values (δ 57Fe IRMM-14 = 2.06 ± 0.34‰ in average) which can be explained by the partial oxidation of Fe 2+ aq in the surface ocean, subsequent to the upwelling of deep anoxic seawater. The resulting iron oxy-hydroxides likely formed in the shallow photic zone as a seasonal process and acquired highly positive iron isotope signature values consistent with a Rayleigh distillation model. The pyrite-bearing sandstones belong to the Aguas Claras Formation of (Palaeoproterozoic) younger age and were cored at the base of the giant Azul manganese deposit. Diagenetic pyrite is characterized by positive δ 57Fe IRMM-14 = 1.02 ± 0.2‰ and δ 34S VCTD (11.97 ± 0.12‰) average values, that are different from values of modern marine biogenic pyrite. Results likely indicate that dissolved sulfate was the limiting factor controlling biogenic pyrite formation at that time. Near-zero Δ 33S values (0.013 ± 0.003‰) indicate minimal mass-independent sulfur isotope fractionation (MIF-S), consistent with the 2.1 Ga age previously obtained from the Aguas Claras pyrites. This result suggests deposition at a time when the oxygen partial pressure in the atmosphere was already high enough (10 − 5 to 10 − 2 PAL- or Present Atmospheric Level-) to hinder MIF-S by photochemical reactions. Incidentally, this also confirms that the giant Mn deposit of Carajás is in the same age range (2.1 ± 0.2 Ga) as other Mn ores in Gabon and South Africa, pointing to the importance of Mn precipitation in Paleoproterozoic times.

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.

Impacts – The Key to Understand Earth

AbstractA significant factor, where are the major elements e.g. Mg, Fe, Si, Al, Ca, P, Ni that make up the asteroid or comet after the impact, has been ignored since the proposal of the impact events. In a striking contrast, there are many remaining unanswered or poorly answered questions regarding the formation and nature of banded iron formation (BIF), iron stone, phosphorite, bedded manganese ore, dolomite and carbonaceous black shales. The appearance of Lake Superior type banded ion formation associated with dolomite, quartzite, and black shale, the deposition of manganese ore and the occurrence of large phosphate repository during the Plaeoproterozoic; the onset of BIF with phosphate deposition and sedimentary manganese deposits and cap dolomite during the Neoproterozoic; the penecontemporary formation of oolitic iron stone taking place of the BIF, bedded manganese deposit, phosphorite, dolomite, and black shale during the Phanerozoic, which suggest a genetic link among them from the perspective of chemical elements. Actually, during hypervelocity impact cratering events, the main part of the asteroid or comet and target material are vaporizing, which forms a mushroom cloud of vapor in the atmosphere.The previous study shows that the calcite, magnetite, clay minerals and organic compounds condense from a solar gas at 10−4 atm with temperature less than 400 K, which provides an analog for the generation of the chemical sedimentary ore deposits. This also supported by table isotopes fractionation at small sacle (grain size, cm scale), all stable isotopes fractionation e.g. (Cr, Fe, S, Si, O,C) and mass-independent fractionation of sulphur isotopes following the rule of isotope fractionation during impacts: the earlier the condensed material, the more enriched in lighter isotopes. Thus, the impact of the asteroid or comet provides not only the elements source of chemical sedimentary ore deposits and the mechanism for their formation. The enrichment in iridium in BIFs and their associated impact spherule units are the other evidence to demonstrate that BIFs are the products of impacts. Impacts produce black shale, banded iron formation, limestone and dolomite. Based on the coeval sub-continental lithosphere mantle (SCLM) formation, juvenile crust generation, banded iron formations, mantle plumes and black shale, it seems that impacts maybe the driving force for Earth.

Impacts – The Key to Understand Earth

A significant factor, where are the major elements e.g. Mg, Fe, Si, Al, Ca, P, Ni that make up the asteroid or comet after the impact, has been ignored since the proposal of the impact events. In a striking contrast, there are many remaining unanswered or poorly answered questions regarding the formation and nature of banded iron formation (BIF), iron stone, phosphorite, bedded manganese ore, dolomite and carbonaceous black shales. The appearance of Lake Superior type banded ion formation associated with dolomite, quartzite, and black shale, the deposition of manganese ore and the occurrence of large phosphate repository during the Plaeoproterozoic; the onset of BIF with phosphate deposition and sedimentary manganese deposits and cap dolomite during the Neoproterozoic; the penecontemporary formation of oolitic iron stone taking place of the BIF, bedded manganese deposit, phosphorite, dolomite, and black shale during the Phanerozoic, which suggest a genetic link among them from the perspective of chemical elements. Actually, during hypervelocity impact cratering events, the main part of the asteroid or comet and target material are vaporizing, which forms a mushroom cloud of vapor in the atmosphere.The previous study shows that the calcite, magnetite, clay minerals and organic compounds condense from a solar gas at 10−4 atm with temperature less than 400 K, which provides an analog for the generation of the chemical sedimentary ore deposits. This also supported by table isotopes fractionation at small sacle (grain size, cm scale), all stable isotopes fractionation e.g. (Cr, Fe, S, Si, O,C) and mass-independent fractionation of sulphur isotopes following the rule of isotope fractionation during impacts: the earlier the condensed material, the more enriched in lighter isotopes. Thus, the impact of the asteroid or comet provides not only the elements source of chemical sedimentary ore deposits and the mechanism for their formation. The enrichment in iridium in BIFs and their associated impact spherule units are the other evidence to demonstrate that BIFs are the products of impacts. Impacts produce black shale, banded iron formation, limestone and dolomite. Based on the coeval sub-continental lithosphere mantle (SCLM) formation, juvenile crust generation, banded iron formations, mantle plumes and black shale, it seems that impacts maybe the driving force for Earth.

Reconstructing Earth's surface oxidation across the Archean-Proterozoic transition

The Archean-Proterozoic transition is characterized by the widespread deposition of organic-rich shale, sedimentary iron formation, glacial diamictite, and marine carbonates recording profound carbon isotope anomalies, but notably lacks bedded evaporites. All deposits refl ect environmental changes in oceanic and atmospheric redox states, in part associated with Earth’s earliest ice ages. Time-series data for multiple sulfur isotopes from carbonateassociated sulfate as well as sulfi des in sediments of the Transvaal Supergroup, South Africa, capture the concomitant buildup of sulfate in the ocean and the loss of atmospheric massindependent sulfur isotope fractionation. In phase with sulfur is the earliest recorded positive carbon isotope anomaly, convincingly linking these environmental perturbations to the Great Oxidation Event (ca. 2.3 Ga).

An estimate of the equilibrium speciation of sulfur vapor over solid sulfur and implications for planetary atmospheres

Sulfur allotropes have been observed in planetary atmospheres and are believed to have been present in the ancient Earth atmosphere. The vapor pressures of sulfur allotropes, especially S2, S3, and S4, are poorly known at typical atmospheric temperatures, but have generally been assumed to be high enough to represent significant gas phase abundances in model calculations. Here I present estimates of the speciation of the equilibrium vapor pressure over solid sulfur, which would seem to imply that the vapor pressures of most allotropes are too low at typical atmospheric temperatures for gas phase formation reactions to be important. However, consideration of the kinetics of condensation shows that gas phase sulfur allotrope reactions can be important under certain conditions. The implications for the mechanism of sulfur isotope mass-independent fractionation on early Earth are discussed. Implications for the presence of sulfur allotropes in Venusian clouds, the distribution of S2 in the atmosphere of Jupiter following the comet Shoemaker-Levy 9 impacts, and the observation of S2 in volcanic plumes on Io are also presented.

Organic haze, glaciations and multiple sulfur isotopes in the Mid-Archean Era

We use sulfur (S) isotope signatures within ancient sediments and a photochemical model of sulfur dioxide (SO 2) 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 (O 2) 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 O 2 and account for concurrent glacial deposits. Specifically, we suggest that prior to the rise of O 2 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 (CH 4) and carbon dioxide (CO 2) 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.

Evidence for a gradual rise of oxygen between 2.6 and 2.5 Ga from Mo isotopes and Re-PGE signatures in shales

Concerning the question of how and when free oxygen started to accumulate in the Earth’s atmosphere and hydrosphere, we report Mo concentrations and isotopic compositions as well as Platin group element and Rhenium (PGE-Re) data from black shales of the Transvaal Supergroup (Ghaap Group, 2.64–2.5 Ga, and Pretoria Group, 2.45–2.15 Ga). For comparison we also include new data from the 3.23 Ga Fig Tree Group, Barberton Greenstone Belt. This time range covers the period between the first robust evidence for cyanobacteria at 2.7 Ga and the disappearance of mass independent sulfur isotope fractionation (MIF) at 2.32 Ga. Due to the redox dependent solubility of Mo, Re and Os, such data are important proxies for changes in oxygen levels of the early atmosphere and oceans. In particular, Mo isotope fractionation can only occur when Mo is in solution as oxyanions, requiring free oxygen. In the Fig Tree Group samples, Mo is not fractionated relative to the continental crust, and PGE-Re abundance patterns reflect those of komatiites, indicating a purely detrital input of these element, a sign of no free oxygen. A general increase in Mo concentration and isotope fractionation, as well as an enrichment in Re, compared to the continental crust and Fig Tree Group, can be seen within the Ghaap Group indicating a gradual rise of oxygen between 2.64 and 2.5 Ga. However, Mo concentrations and δ 98/95Mo values vary strongly in this period. Local conditions of sedimentation (changes in redox and input conditions) and/or global oxic/anoxic fluctuations could have caused these Mo variations. Samples of the overlying Pretoria Group (2.45–2.15 Ga) show Mo concentrations and isotopic compositions somewhat below the continental input, while PGE-Re abundance patterns are quite similar to those for the Ghaap. The apparent contradiction between the decoupled Mo and PGE-Re values can be resolved assuming a stratified ocean with almost total scavenging of Mo. Such an increased scavenging can result from enhanced biological sedimentation between and especially after glacial events. This period coincides with the Lomagundi-Jatulian carbon isotope excursion, which indeed indicates very enhanced organic carbon burial.

Mass independent sulfur isotope fractionation discovered in aerosol of Beijing

Mass independent sulfur isotope fractionation discovered in aerosol of Beijing

Early evolution of atmospheric oxygen from multiple-sulfur and carbon isotope records of the 2.9 Ga Mozaan Group of the Pongola Supergroup, Southern Africa

Sulfur isotope mass-independent fractionation (S-MIF) is a unique geologic record of Archean atmospheric chemistry that provides important constraints on the evolution of the early Earth’s atmosphere and its impact on early life. In this contribution, we report multiple-sulfur (33S/32S, 34S/32S, and 36S/32S) isotope ratios of sulfide minerals and carbon (13C/12C) isotope ratios of organic carbon for shale in the ~2.96 to ~2.84 Ga Mozaan Group of the Pongola Supergroup, Southern Africa. The δ13C of organic carbon shows two populations: one with δ13C of ~ −26 ‰ and another with δ13C of −32 ‰. The Δ33S values from nine samples ranges from −0.49 to +0.36 ‰, which is considerably smaller than what was measured for the sulfide and sulfate minerals from other Archean intervals but outside the range of Δ33S values measured for post-2.0 Ga sulfide and sulfate minerals. Moreover, some samples from the Mozaan Group yield Δ36S/Δ33S ratios that are different from Phanerozoic sulfides, suggesting sulfide sulfur from the Mozaan Group carries mass-independent isotope fractionation originated from atmospheric photochemistry. The relatively small Δ33S values for the Mozaan Group may suggest that the atmosphere became slightly oxidized at ~2.9 Ga with oxygen level above 10−5 but below 10−2 times present atmospheric level. This intermediate oxygen level would allow production of S-MIF in atmospheric chemistry but prohibit preservation of large S-MIF signatures in surface deposits. Our hypothesis implies the evolution of oxygenic photosynthesis as early as ~2.9 Ga. Such an ephemeral oxidation event could have triggered the Mozaan-Witwatersrand glaciation by destabilizing an existing methane-rich Archean atmosphere.

Multiple sulfur isotopes of sulfides from sediments in the aftermath of Paleoproterozoic glaciations

Geochemical evidence reported from Paleoproterozoic sediments has long been used to evaluate the transition from the anoxic Archean atmosphere to an oxygenated atmosphere. Sulfur isotopes ( 32S, 33S, 34S and 36S) in sedimentary sulfides and sulfates are an especially sensitive means to monitor this transition, such that the timing of the Paleoproterozoic “Great Oxidation Event” can be investigated using mass-independently fractionated (MIF) sulfur isotope systematics expressed as Δ 33S. Here we report data from 83 individual analyses of pyrite, pyrrhotite and chalcopyrite on a new suite of 30 different samples from Finland, South Africa, Wyoming and Ontario that span ∼600 My and follow one or several “Snowball Earth” events in the Paleoproterozoic. The samples were measured using a high-resolution secondary ion mass spectrometry technique in multicollection mode that investigates multiple sulfur isotopes in microdomains (<30 μm) within individual sulfide grains while preserving petrographic context. We focused on sediments deposited in the aftermath of the Paleoproterozoic glaciations (between 1.9 and 2.2 Ga) to trace fluctuations in atmospheric O 2 concentrations that were likely affected by an interplay of O 2 sinks in the atmosphere and the upper ocean and continental crust, and by the emergence and diversification of aerobic organisms. Our results demonstrate that MIF sulfur isotopes are absent in sediments deposited after the period of protracted global cooling in the Paleoproterozoic and independently confirm observations that MIF ceased during this time. We interpret our results by integrating Δ 33S and δ 34S data in sulfides, δ 13C data in carbonates and the estimated timing of glaciation events in the Paleoproterozoic. Data strongly hint at the presence of microbial sulfate reduction and fluctuations in the concentration of dissolved seawater sulfate and/or in δ 34S sulfate in the aftermath of glaciations and likely were affected by changing erosion rates and nutrient delivery to the oceans. These changes modulated the population of primary producers, especially oxygenic photosynthesizers, and led to fluctuations in the abundance of atmospheric O 2, CO 2 and CH 4. Our results support the interpretation that the world-wide δ 13C carb excursion observed between ∼2.25 and 2.05 Ga ( Karhu and Holland, 1996) was a period of significant accumulation of O 2 in the atmosphere.

The effect of sulfate‐δ18O upon on‐line sulfate‐δ34S analysis, and implications for measurements of δ33S and Δ33S

On-line delta34S analysis of sulfate using an elemental analyzer has a number of advantages vs. conventional off-line techniques, such as ease of operation, rapidity, and the requirement for small amounts of material. Although the analyses are performed by converting sulfate into SO2 gas, the effect of sulfate-delta18O composition upon the SO2-delta18O composition and the value of delta66 during elemental analysis, and ultimately the calculated sulfate-delta34S composition, has rarely been addressed. Three BaSO4 samples were prepared with known identical delta34S compositions, but with a wide range of delta18O compositions. delta18O values were shown to range over 40 per thousand, but conventional on-line delta34S analyses verified that the sulfate-delta34S compositions were identical. These results indicate that conventional on-line analysis of sulfate-delta34S is unaffected by the value of sulfate-delta18O, and suggest that sulfide-delta34S standards can be used to calibrate sulfate-delta34S analyses (and vice versa). Moreover, these results suggest that it may be possible to use on-line sulfur isotope analysis of SO2 to measure delta33S and Delta33S in addition to delta34S, as a faster and safer alternative to the SF6 technique currently utilized, and hence promote further study of mass-independent sulfur isotope fractionation effects.

New insights into Archean sulfur cycle from mass-independent sulfur isotope records from the Hamersley Basin, Australia

We have measured multiple sulfur isotope ratios (34S/33S/32S) for sulfide sulfur in shale and carbonate lithofacies from the Hamersley Basin, Western Australia. The Δ33S values (Δ33S≈δ33S−0.515×δ34S) shift from −1.9 to +6.9‰ over a 22-m core section of the lower Mount McRae Shale (∼2.5 Ga). Likewise, sulfide sulfur analyses of the Jeerinah Formation (∼2.7 Ga) yield Δ33S values of −0.1 to +8.1‰ over a 50-m section of core. Despite wide variations in Δ33S and δ34S, these two shale units yield a similar positive correlation between Δ33S and δ34S. In contrast, pyrite sulfur analyses of the Carawine Dolomite (∼2.6 Ga) yield a broad range in δ34S (+3.2 to +16.2‰) but a relatively small variation and negative values in Δ33S (−2.5 to −1.1‰). The stratigraphic distribution of δ33S, δ34S, and Δ33S in Western Australia allows us to speculate on the sulfur isotopic composition of Archean sulfur reservoirs and to trace pathways in the Archean sulfur cycle. Our data are explained by a combination of mass-independent fractionation (MIF) in the atmosphere and biological mass-dependent fractionation in the ocean. In the Archean, volcanic, sulfur-bearing gas species were photolysed by solar ultraviolet (UV) radiation in an oxygen-free atmosphere, resulting in MIF of sulfur isotopes. Aerosols of S8 (with Δ33S>0) and sulfuric acid (with Δ33S<0) formed from the products of UV photolysis and carried mass-independently fractionated sulfur into the hydrosphere. The signatures of atmospheric photolysis were preserved by precipitation of pyrite in sediments. Pyrite precipitation was mediated by microbial enzymatic catalysis that superimposed mass-dependent fractionation on mass-independent atmospheric effects. Multiple sulfur isotope analyses provide new insights into the early evolution of the atmosphere and the evolution and distribution of early sulfur-metabolizing organisms.

Mass-independent fractionation of sulfur isotopes in Archean sediments: strong evidence for an anoxic Archean atmosphere.

Mass-independent fractionation (MIF) of sulfur isotopes has been reported in sediments of Archean and Early Proterozoic Age (> 2.3 Ga) but not in younger rocks. The only fractionation mechanism that is consistent with the data on all four sulfur isotopes involves atmospheric photochemical reactions such as SO2 photolysis. We have used a one-dimensional photochemical model to investigate how the isotopic fractionation produced during SO2 photolysis would have been transferred to other gaseous and particulate sulfur-bearing species in both low-O2 and high-O2 atmospheres. We show that in atmospheres with O2 concentrations < 10(-5) times the present atmospheric level (PAL), sulfur would have been removed from the atmosphere in a variety of different oxidation states, each of which would have had its own distinct isotopic signature. By contrast, in atmospheres with O2 concentrations > or = 10(-5) PAL, all sulfur-bearing species would have passed through the oceanic sulfate reservoir before being incorporated into sediments, so any signature of MIF would have been lost. We conclude that the atmospheric O2 concentration must have been < 10(-5) PAL prior to 2.3 Ga.

A review of the geology of the dolomite-dominated carbonatite complexes of the Kalahari Craton, southern Africa

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.