Constraining the rise of oxygen with oxygen isotopes

After permanent atmospheric oxygenation, anomalous sulfur isotope compositions were lost from sedimentary rocks, demonstrating that atmospheric chemistry ceded its control of Earth’s surficial sulfur cycle to weathering. However, mixed signals of anoxia and oxygenation in the sulfur isotope record between 2.5 to 2.3 billion years (Ga) ago require independent clarification, for example via oxygen isotopes in sulfate. Here we show <2.31 Ga sedimentary barium sulfates (barites) from the Turee Creek Basin, W. Australia with positive sulfur isotope anomalies of ∆33S up to + 1.55‰ and low δ18O down to −19.5‰. The unequivocal origin of this combination of signals is sulfide oxidation in meteoric water. Geochemical and sedimentary evidence suggests that these S-isotope anomalies were transferred from the paleo-continent under an oxygenated atmosphere. Our findings indicate that incipient oxidative continental weathering, ca. 2.8–2.5 Ga or earlier, may be diagnosed with such a combination of low δ18O and high ∆33S in sulfates.

T oday, Earth's oxygen-rich (21% O 2 by volume) atmosphere drives a marine sulfur cycle dominated by oxidative weathering. Rivers supply the ocean with substantial loads of dissolved sulfate (SO 4 2− ) derived from the oxidation of pyrite and the dissolution of sulfate minerals in roughly equal proportions 1 . Within marine and terrestrial settings, microbial sulfate reduction (MSR) processes exert the most important controls on sulfur isotopic fractionation of SO 4 2− . During MSR, the partial reduction of SO 4 2− preferentially converts 32 S to sulfide that is mostly re-oxidised, whereas a fraction of the sulfide product is retained within iron sulfide minerals 2 . Because of mass-dependent sulfur fractionations from MSR, marine sulfide has a large, but relatively 34 S-depleted, range in δ 34 S (see Methods for isotope notations) and a Δ 36 S/Δ 33 S slope of approximately −9 3 . Simultaneously, marine SO 4 2− is concentrated at 28 mM, enriched in 34 S (δ 34 S = 21‰ 4 ;, and has a small 33 S enrichment (Δ 33 S < 0.1‰).
In contrast, before atmospheric oxygenation sufficiently drove the oxidation of sulfur at the Earth's surface, the surface sulfur cycle was first controlled by atmospheric inputs of sulfur that can be traced within Archaean age, 4.0-2.5 Ga, sedimentary rocks displaying strong sulfur mass independent fractionation (S-MIF) in their Δ 33 S values between +14‰ and −4 [5][6][7] (Fig. 1a). There are lingering geochemical 8 and quantitative 9 challenges still to be understood about the preservation and generation of Archaean S-MIF signals. However, it is generally accepted that S-MIF results from atmospheric photochemical reactions operating under low O 2 of <0.001% of the present atmospheric level (PAL) of oxygen, causing surface sulfur fluxes of insoluble S 0 with Δ 33 S > 0‰ and soluble SO 4 2− with Δ 33 S < 0‰ that were not homogenised during their transfer into sedimentary rocks 10,11 .
The disappearance of anomalous Δ 33 S signals from the sedimentary record is a crucial constraint on the rise of oxygen around the Archaean-Proterozoic boundary, but despite intensive efforts its exact timing remains unclear. This transition is constrained in continuous stratigraphic section only in S. Africa, within three different age-equivalent cores from the Transvaal Basin. There, Δ 33 S decreases below 0.4‰ between 2316 and 2326 ± 7 million years (Ma) ago, a horizon that has been interpreted as the shift to >0.001% PAL of oxygen and considered by some as the Great Oxidation Event (GOE) itself 6,12,13 . Alternatively, the slow disappearance of Δ 33 S signals from the rock record after 2.45 Ga may be attributable to the increasingly important oxidative weathering of an older, S-MIF-bearing, continental sulfide reservoir whose anomalous isotope compositions (i.e., Δ 33 S >   15,16 and challenged 17 this assertion. Because all of these studies interpret the record of S-MIF directly, independent tests are needed to delineate the importance of atmospheric and weathering controls on the early surficial sulfur cycle. The sparse sulfate record during the Archaean Eon, and into the early Palaeoproterozoic, has prevented a closer examination of the oxidative side of Earth's early sulfur cycle. Evaporites and bedded barites are discontinuous, while more continuous records are hindered by weakly concentrated carbonate-associated sulfates (CAS) that are vulnerable to contamination 18 , diagenetic alteration 19 and loss during metamorphic recrystallization 20 . Furthermore, as CAS concentrations should approximate ambient seawater SO 4 2− concentrations, low early Earth seawater sulfate can challenge the recovery of measurable CAS. For example, between 2.5 and 2.3 Ga, it can be difficult to obtain sufficient CAS from sediments for sulfur isotope measurements, where up to 1500 grams of carbonate may be necessary 6 . Such low CAS yields, being especially vulnerable to contamination and overprinting, may discourage the additional measurement of oxygen isotopes.
Here, to investigate the influence of early Earth sulfide weathering, we focus on barite (BaSO 4 ), a highly insoluble and diagenesis-resistant mineral that offers a robust record of sulfate S and O isotope compositions 21 . The majority (70-92% 22 ) of oxygen atoms incorporated in sulfide-derived SO 4 2− are contributed by ambient water at the locus of oxidation (e.g., δ 18 O values of present-day seawater ≈ 0‰, meteoric waters ≈ 0‰ to −20‰, snow and ice ≈ −20‰ to −60‰ 23 ), with a lesser contribution from other oxidants such as O 2 , followed by negligible isotopic exchange 24 . For example, sulfates in sediments are normally enriched in 18 O versus their water oxygen source, where seawater at δ 18 O ≈ 0‰, or lower 25 , results in marine SO 4 2− being ≥6‰ through time (Fig. 1b) Geological and geochemical context. The analysed barites are from the Kazput Formation of the Turee Creek Group, an overallshallowing succession with Mn-enriched units and decreasing iron contents that have been interpreted in favor of increasing atmospheric oxygen 30 . Recently reported nitrogen isotope and iron speciation evidence from the Kazput Formation further indicates free oxygen availability during its deposition 31 . An age of 2.25 Ga was assigned, as based on detrital zircon U-Pb dating and estimated sedimentation rates 28 . While this age may be   23 . The significant S-MIF of the barite that falls within the Archaean Δ 36 S/Δ 33 S array (ARA, Fig. 3a) suggests that the sulfur source was originally fractionated in an atmosphere containing <0.001% PAL oxygen. Importantly, the barite S isotope compositions closely overlap with those of previously reported early diagenetic pyrites in the same TCDP3 core (Fig. 2), which implies that the sulfur precursor was mostly unmodified during its conversion to reduced (sulfide) or oxidised (sulfate) mineral phases, or that one phase was derived from the other without isotopic fractionation. Because, under various conditions, it is possible for low temperature redox reactions to proceed without S-isotope fractionation between sulfate and reduced sulfur phases 34 , by itself, the S-isotope match between Kazput barite and pyrite cannot be used to diagnose a specific process. Broadly, however, the S-MIF in Kazput barite requires an atmospheric sulfur source that originated under an atmosphere with <0.001% PAL O 2 , occurring within one of two scenarios: either the capture of such atmospheric sulfur under an essentially oxygen-free atmosphere existing at <2.31 Ga, or the reworking/recycling of an archival atmospheric sulfur source that was generated any time before 2.31 Ga. Understanding how and when these barites inherited their atmospherically derived sulfur is of fundamental importance to understanding Earth's oxygenation and its imprints on the minor sulfur isotope record.
Assessing the origin of Kazput barite. Given that pyrite-derived SO 4 2− can closely match the sulfur isotope composition of its pyrite precursor under abiotic or biologically mediated oxidation 34,35 , the nearly identical δ 34 S (and Δ 33 S) compositions  16 . The ARA describes quadruple sulfur isotope correlations during the Archaean attributed to mass independent processes, where Δ 33 S = 0.9 × δ 34 S, and Δ 36 S = −0.9 or −1.5 × Δ 33 S 68 . See Methods for isotope notation definitions and the Source Data file for data of Kazput barite and pyrite could be attributed to low temperature pyrite oxidation during sample preparation in the laboratory. While pyrite oxidation produces dissolved SO 4 2− as its immediate product, barite formation further requires the capture of SO 4 2− with barium, which makes it more difficult to accidentally produce in the lab during sample handling. This requirement is important because fluids can be concentrated in barium or SO 4 2− , but not both, due to barite's low solubility 36 . We performed extensive tests of pyrite oxidation during barite extraction, as well as during attempted extractions of CAS, where pyrite oxidation during sample processing was excluded (see Methods).
A metamorphic origin for the barite must also be considered, as it could explain the occurrence of their S-MIF signals, irrespective of the host rock's age. For example, postdepositional metamorphic processes have been implicated in the remobilization and dilution of S-MIF signals in sulfur from N. American Palaeoproterozoic Huronian sections 37 . The metamorphism of the Turee Creek Group is constrained to prehnitepumpellyite-epidote facies up to 300°C 38 . Additional constraint from a SIMS study on the Turee Creek Group yields a maximum metamorphic temperature of 240°C that was estimated from sharp sulfur isotope gradients of δ 34 S ranging over 30‰ across a <4 µm transect in pyrite 39 . More importantly, previous work on trace element abundances in pyrites from TCDP drill cores has demonstrated the early diagenetic origin of their sulfur 16 . Considering the S isotope match between the Kazput barite and pyrite, we conclude that the barites are, likewise, of low temperature origin without significant metamorphic alteration.
A detrital origin for Kazput barites could provide another explanation for the unexpected young age, <2.31 Ga, of their sulfur isotope anomalies. Evidence for an anoxic atmosphere that is found in sedimentary rocks are rounded detrital grains of pyrite and uraninite that were not dissolved during riverine transport due to low oxygen availability 40 . The pyrite in the Kazput Formation is not detrital, and instead appears to have grown in situ. Kazput pyrites occur as microcrystalline aggregates, euhedral crystals, and elongated layer-parallel concretions in a range of sizes (Supplementary Figs. 5 and 6). In contrast, sedimentary reworking of Kazput barites is difficult to ascertain due to the barite size, <10 μm, and disseminated occurrence (Supplementary Figs. 3 and 4), but they share a tight S-isotopic match with the co-occurring diagenetic pyrites. Therefore, the mutual sulfur source for the barite and pyrite, despite different mode of occurrence, also excludes a detrital origin for the barite.
Secondary oxidation of pyrite to SO 4 2− could occur at low temperature within the rock during its burial history, for example, via pyrite oxidation by pore waters. Firstly, it is feasible to oxidize pyrite under anoxic conditions via radiolysis of water that is accompanied by an enrichment of 1.5-3.4‰ in the δ 34 S values of SO 4 2− versus pyrite 41 , but this scenario is ruled out because Kazput barite does not show such S-isotope enrichment versus the previously reported pyrite (Fig. 2). Meanwhile, if late oxidation by infiltrating fluids occurred, then carbonate, which is very susceptible to diagenetic alteration of its oxygen isotope composition 42 , should register alteration by the same 18 33 that is typical for marine carbonates of similar age, but 10‰ lower than present-day marine carbonates (Fig. 1b). As reviewed by Gomes and Johnston 34 , the oxygen isotope fractionation between pyrite-derived SO 4 2− and water spans a range of δ 18 O values between 0‰ and +20‰ for naturally relevant conditions. Oxidation experiments using waters with δ 18 O compositions >15‰ have produced sulfide-derived sulfates that have very low δ 18 O compositions versus their source waters, as exemplified by an extreme case of −65‰ fractionation between pyrite-derived sulfate (+71‰) oxidised in isotopically labeled water (+127‰) 43 . Although the topic of sulfate oxygen isotope fractionation demands clarification, we contend that Kazput barites are well within the range of δ 18 O values, below 15‰, of natural sulfates that are 18 O-enriched versus their water oxidant sources. Regardless of assumptions concerning sulfatewater oxygen isotope fractionations, distinct water-oxygen sources are required to produce sulfate δ 18 O compositions that are appreciably lower than those of coeval carbonates. These low sulfate compositions are apparent in non-marine sulfates as compared to concurrent marine carbonates at ca. 0.0, 0.6 and 1.4 Ga (Fig. 1b). Similarly, relative to coeval Kazput carbonates, the low δ 18 O values of our barites require a meteoric-water-oxygen source.
A unique combination of environmental conditions likely contributed to the isotopic signatures of Kazput barite, but perhaps they were also preserved due to the sulfuretum, or consortium of sulfur-metabolising microbiota, active in the Turee Creek Basin during the Palaeoproterozoic. Besides prevalent microbial mats in the Kazput carbonate, it also contains microfossils that have been interpreted as sulfur-oxidizing filamentous bacteria 44 . What is remarkable about such bacteria from an isotopic perspective is that a sulfuretum of SO 4 2− -reducing microbes and sulfur-oxidizing bacteria can produce sulfide that gets re-oxidized to SO 4 2− , via elemental sulfur, with no net S isotope fractionation 45 . Sulfur-oxidizing bacteria may have contributed to the Kazput barites' oxygen isotope signals, perhaps by introducing Turee Creek Basinal water-oxygen to the SO 4 2− produced during re-oxidation of MSR-produced sulfide, or by enhancing rates of sulfide oxidation on land. An MSR influence can be indicated by positive δ 18 Oδ 34 S correlation in sulfate 46 , however, the negative correlations between Kazput barite δ 18 O and its sulfur isotope parameters (Fig. 3c, d) imply that source mixing, with sulfide weathering in meteoric waters, was much more important than MSR for setting these S and O isotope signatures. As the source-controlled barite sulfur and oxygen isotope variations mask microbial influence, further speculation about the biological role in Turee Creek sulfur cycling remains open.
A continental source of isotopically light oxygen. We suggest, given the previous evidence that has ruled out post-depositional processes and a detrital origin while pointing to a meteoric water oxygen source, that the O isotope signals preserved in Kazput barite are best explained by pyrite oxidation on land. Kazput barite δ 18 O, extending to −19.5‰, are among the lowest values in the sedimentary record, including other sulfate occurrences going down to −20.3‰ from a Neoproterozoic snowball Earth glacial diamictite deposited at 635 Ma 47 , and in relatively more recent terrestrial deposits from within the Arctic Circle that reach down to −19.7‰ 22 (Fig. 1b). Similarly, the range of oxygen isotope compositions of Kazput barite may require a glacial meltwater oxidant source. However, in contrast to those other examples of very low δ 18 O in sulfate, Kazput barite δ 18 O may not necessitate glacial or ice meltwater oxygen sources, and instead could reflect a hydrosphere anchored to seawater with a lower δ 18 O than today. Meteoric waters evolve lower δ 18 O compositions than seawater due to evaporation from seawater followed by Rayleigh distillation in cooling air masses 23 . The most evolved meteoric waters (and snow and ice melt) that can be found at high latitudes, high elevations and away from coastlines are marked by the lowest δ 18 O compositions relative to seawater. This relative relationship between seawater and more 18 Odepleted meteoric waters is significant here, as it has been suggested that the δ 18 O of seawater (0‰ at present 23 ) may have changed through time, perhaps being as low as −10‰ in the Archaean 25 . Newly reported iron oxide δ 18 O records, which are insensitive to temperature, convincingly support that seawater was lower in the past, having a δ 18 O at near −8‰ by around 2.0 Ga 48 . Palaeoproterozoic seawater appears to be faithfully recorded in Kazput carbonate that is, as mentioned previously, on the order of 10‰ lower than modern carbonates but comparable to carbonates from around 2.3 Ga. Therefore, the Kazput barites recording the lowest δ 18 O indicate their precursor sulfate was oxidised in the most evolved meteoric waters, placing this water-oxygen source for this sulfate firmly on land, as compared to a seawater δ 18 O that was likely around −10‰. Although sulfate-water oxygen isotope fractionation during sulfide oxidation requires further study, we take a median sulfide-derived sulfate-water δ 18 O fractionation value of +10‰ 34 , as compared to the barite, to roughly estimate that the water sources for Kazput sulfate may have ranged between −30‰ and −8‰. A model of Kazput barite formation. Taken together, the S-and O-isotope systematics suggest a singular scenario for the origin of the Kazput barite. The barite and pyrite Δ 33 S values reach a maximum in the studied drill core as compared to the two other Turee Creek Group drill cores in older underlying sediments 16 . The strongest expression of this sulfur source appears with maxima of Δ 33 S and δ 34 S coincident with the δ 18 O minimum in barite, where negative correlations between δ 18 O and Δ 33 S and δ 34 S data exist between 112 and 135 metre depth within the center of the carbonate unit (Fig. 3c, d). Such S-O isotope correlation may indicate the mixing of sulfide-derived SO 4 2− sources with distinct S-isotope compositions but overlapping δ 18 O ranges due to their oxidation in meteoric waters under variable humidity, as is observed in major river systems today 49,50 . The preservation of such a strongly riverine sulfate signal in marine sedimentary rocks is unlikely today due to buffering by high contemporary seawater sulfate concentrations. A previously identified source of sulfur in the Turee Creek Basin with a monotonous Δ 33 S ≈ 0.9‰ is well expressed in the Meteorite Bore Member diamictite 16 . Mixing between this monotonous sulfur source and a different, continental-weathering, source with a higher Δ 33 S ≈ 1.6‰ is observed in S-O isotope space (Fig. 3c). The first-order controls on the mix of sulfur sources are relative sea level and the exposure of sedimentary rocks to weathering. Minimal overprinting resulted in a largely faithful transfer of Sand O-isotope signatures as sulfate was captured as barite or reduced to form pyrite (0.32 weight % on average, Supplementary Table 2). Anoxic sediment porewaters favoring MSR would provide a flux of barium in sufficient concentration to precipitate SO 4 2− as barite at the sediment-water interface 51 . Remaining porewater SO 4 2− would be quantitatively reduced to sulfide, via closed system MSR, with capture by iron to form iron sulfides (pyrites) during early diagenesis in the sediment 16 . This is consistent with the lack of CAS-sulfate from the Kazput carbonate that supports near-quantitative reduction of SO 4 2− in porewaters. Previously reported Fe/Al mass ratios in core TCDP3 and the Kazput carbonate are 0.64 on average, excluding enriched intervals >1 (Fig. 2, Supplementary Table 3). The Fe/Al and total sulfur enrichments imply either transient water column anoxia, or movement of a chemocline during transgression-regression cycles that could in turn represent enhanced sulfate fluxes from land and their drawdown into anoxic sediments via microbial sulfate reduction into sulfide. The enrichments in Fe/Al and total sulfur observed at 164 and 182 meters depth in two muddy intervals below the carbonate are also likely pyrite (FeS 2 ) enrichments, though they were not directly quantified here.
Atmospheric and sedimentary controls on anomalous sulfur. Barites isolated from Kazput carbonate show the Δ 36 S/Δ 33 S slope of −0.9 (Fig. 3a) that typifies much of the Archaean sulfur isotope record. This slope, however, steepens to near −1.5 within the upper and lower portion of the carbonate (Fig. 2). Close links between low δ 13 C values and steeper Δ 36 S/Δ 33 S slopes (−1.5) in Neoarchaean sedimentary rocks 52 have been interpreted as evidence for changes in atmospheric chemistry catalysed by the biosphere 53 . In comparison to these Neoarchaean records, however, Kazput barite show relatively small Δ 36 S and Δ 33 S ranges. Furthermore, recent iron-speciation and nitrogen isotope evidence imply that suboxic-to-oxic water column conditions prevailed beneath an atmosphere containing appreciable oxygen 31 . The apparent availability of significant free O 2 (>0.001% PAL), concurrent with mass independent sulfur isotope anomalies, strongly supports the latter being due to a memory effect carried by sulfate derived from oxidative weathering of S-MIF-bearing rocks 15 . It follows that the attendant Δ 36 S/Δ 33 S variations in the barites may reflect control from oxidative weathering of pyrite in continental source rocks of various ages, whereas the S-MIF signal is simply tracking the integrated sulfur isotope composition of the rocks being weathered. As interpreted previously, the S-MIF systematics and their limited range in the Turee Creek Group may represent homogenisation during the weathering of older, more isotopically variable sulfur generated under earlier atmospheric states, together with dilution by addition of sulfate lacking S-MIF (Δ 33 S ≈ 0‰) 16 .
Despite being deposited in relative proximity, within similar equatorial intracratonic basins 7,12,30 , W. Australian and S. African records display stark sulfur isotopic differences between 2.45 and 2.2 Ga. These differences may partly reflect spatial and temporal variations of SO 4 2− concentrations. Geochemical constraints from the first basin-scale bedded evaporites at 2 Ga indicate a significant marine SO 4 2− concentration of at least 10 mM 54 , however, between 2.25 and 2.1 Ga, the δ 34 S variability in extant CAS records imply a range of seawater SO 4 2− concentrations of 5-20 mM 55 . Meanwhile, modern analog environments suggest late Archaean marine SO 4 2− concentrations of <2.5 μM 56 . As a result of such sulfate concentrations that could span four orders of magnitude between 2.5 and 2.1 Ga, sulfur isotope variations would be very sensitive to local environmental controls. We suggest that such local control extends to the record of sulfur isotope anomalies. As compared to S. Africa, the W. Australian Turee Creek Group succession seems to require within-sediment closed system MSR to preserve its persistent S-MIF signals. Frequency distributions of δ 34 S data support this assertion, with W. Australian sulfur δ 34 S data displaying a broadly unimodal distribution, centered near 5‰ (Fig. 4a); while S. African data show a more bimodal δ 34 S distribution (Fig. 4b), reflecting the preferential incorporation of 32 S into sulfide at the expense of sulfate. These different behaviors are also manifested in their respective Δ 33 S distributions. Despite similar ranges of Δ 33 S values, an important source of mass-dependent sulfur (Δ 33 S ≈ 0‰, Fig. 4d) is indicated for S. Africa, while two sources of S-MIF-bearing sulfur (Δ 33 S =+0.9‰ and +1.6‰, Fig. 4c) are observed from W. Australia. Closed system MSR is also suggested by the δ 34 S-Δ 33 S systematics. Similar to previous observations from microbial carbonates 57 , the 34 S-enrichment versus the ARA seen in Kazput pyrites and barites (Fig. 3b) could reflect the isolation of the sediment from the water column due to carbonate precipitation. Indeed, stromatolites and carbonate are prevalent throughout the Turee Creek Group, especially in the Kazput Formation. Finally, to help elucidate the possibility that the Turee Creek Basin may have been relatively more isolated from the global ocean as compared to contemporaneous S. African basins, the δ 18 O compositions of CAS from S. Africa may be useful but are yet to be measured. As suggested by the S-isotope evidence for open system MSR, it is likely that sulfate from S. Africa is relatively enriched in 18 O as compared to the Kazput barite.
Implications for constraining atmospheric oxygenation. Beyond its production in an essentially oxygen-free atmosphere, the preservation of S-MIF is susceptible to second-order biotic and abiotic controls. Consequently, several hypotheses have been proposed to explain the temporal structure of the S-MIF record (Fig. 1a). For example, the enhanced preservation of large magnitude Δ 33 S compositions in the lead up to 2.5 Ga have been explained by a shift in the locus of MSR from euxinic water columns to sediment porewaters due to oxygenation of shallow surface oceans 58 , or by the changing oxidation state of gas influxes 59 . Conversely, contraction of Δ 33 S signals, such as observed in pyrite between 2.5 and 2.3 Ga, could be caused by dilution of sulfur isotope anomalies with sulfide biologically produced from mass-dependent sulfur sources 58 . The ca. 2.5-2.3 Ga weak signals of S-MIF could reflect millions of years of weathering out and dilution of anomalous sulfur that had previously been stockpiled in more reduced, pre-GOE (i.e., older), sediments 14,15 , as is likely the case for the Turee Creek Group 16 . Another possibility is that oscillating atmospheric oxygen levels after 2.5 Ga 7 permitted intervals of primary atmospheric S-MIF production and preservation as oxygen intermittently dipped below 0.001% PAL. In any case, it remains difficult to clearly recognize these various scenarios because the signals they leave behind in sediments can be ambiguous. Perhaps more detailed examination of linked δ 13 C and Δ 36 S/Δ 33 S slope variations from ca. 2.5 to 2.3 Ga will reveal more about the possibly of oscillating levels of atmospheric gases in that interval. That oxidative weathering should be the main control on marine sulfate after atmospheric oxygenation seems intuitive, but as mentioned, even under the high atmospheric oxygen concentration of the modern Earth, surface weathering does not directly control marine sulfate S-isotope values in lieu of MSR. The dominance of positive Δ 33 S values in the Kazput barites implies the SO 4 2− originated from the oxidative weathering of pre-GOE sedimentary rocks. Our results are at odds with the conclusion, based on contemporary weathering of old exposures, that the weathering of Archaean continents would yield sulfate with Δ 33 S summing to nearly 0‰ 17 . Perhaps this discrepancy is because present-day rock exposures simply are not comparable in composition and weathering susceptibility to the exposed sedimentary rocks at Earth's surface in the Palaeoproterozoic. Furthermore, small positive Δ 33 S have indeed been measured in riverine sulfate from catchments in S. Africa and Ontario today 60 , which suggests that recycling of S-MIF signals at the Earth surface could occur at least locally. From the Archaean to the the earliest Palaeoproterozoic ca. 2.5-2.3 Ga, when sulfur was being delivered by incipient oxidative weathering and/or atmospheric inputs, the marine SO 4 2− reservoir was smaller and its fraction buried as pyrite approached unity 2 . Under these conditions, it is possible that the S-isotope compositions of marine sulfate and sulfide records could approach those of S-MIF-bearing riverine SO 4 2− sources. In turn, early Palaeoproterozoic sulfur records, especially from basins with an unknown degree of connection to the global ocean, can be expected to show substantial variability. As spatial and temporal coverage of Palaeoproterozoic sulfide and sulfate records increases, a clearer composite portrait should emerge that will allow for more clearly differentiated local versus global symptoms of atmospheric oxygenation being recognized in the sulfur cycle at different scales.
Our findings suggest that the oxygen isotope composition of sulfate may prove as insightful as multiple sulfur isotope data in evaluating the imprint of atmospheric oxygenation within the sedimentary record. The combined negative δ 18 O and positive Δ 33 S compositions of Kazput barite imply weathering, as opposed to atmospheric, control on sulfur input fluxes by~2.31 Ga in the Turee Creek Basin. It follows that the onset of atmospheric oxygenation and its nascent imprint on the sulfur record must be older. It has been suggested that SO 4 2− flux to the oceans increased in advance of the GOE due to oxygen production and its consumption by sulfide weathering on land 61,62 (Fig. 1c). The conspicuous absence of negative Δ 33 S anomalies from rocks younger than 2.42 Ga (Fig. 1a) may mark the turning point when weathering derived SO 4 2− fluxes, with predominantly positive Δ 33 S anomalies, first dominated the surficial sulfur cycle. By using oxygen isotope compositions to identify sulfide-derived sulfates of continental weathering origin from the rock record, it may be possible to directly constrain the timing of early weathering fluxes  Barite extraction. Our barite extraction technique utilises a chelating agent that at once enables dissolution of barite while inhibiting the oxidation of pyrite. Again, the barite extractions were completed at the Laboratoire Géosciences Océan at IUEM in Plouzané, France. As a test for pyrite oxidation in our extraction technique, finely ground pyrite was stirred in the chelating solution for a week followed by attempted recovery of pyrite-derived SO 4 2− from the filtered solution, with no resulting sulfate yield, likely because the chelation of iron and other metal cations also served to prevent pyrite oxidation. In addition, we intentionally oxidized pyrite in the same laboratory-distilled water that is used in the barite extracting solution followed by measurement of the oxygen isotope composition of the produced sulfate, which was isotopically distinct from that of the measured samples ( Supplementary Fig. 7). Barite in TCDP3 was only observed by scanning electron microscopy, where barites <10 μm were observed in both sample residues (Supplementary Figs. 3 and 4) and in thin sections. Barite was not observed during conventional optical petrographic observations of thin sections. Barite extractions were performed oñ 100 g of powdered drill core rock sample using a modified barite purification technique (the DDARP method) from Bao 2006 64 that was originally developed to purify sulfate samples for triple oxygen isotope measurements. Sample powders were decarbonated in HCl-acidified solution, rinsed in distilled water, and then treated for 3 days with constant stirring in a 0.05 M Diethylenetriaminepentaacetic acid (DTPA) and 1 M NaOH solution to dissolve barium sulfate. The supernatant was then separated from the sample residue by vacuum filtration. After acidification (pH < 2) with HCl, the barite-housed sulfate was precipitated out of the supernatant as BaSO 4 via addition of supersaturated BaCl 2 solution. Coprecipitation of silicates dissolved in the basic DTPA solution renders the precipitated BaSO 4 impure, requiring an additional purification step. Here, residual DTPA must first be removed by triplicate washing with distilled water, centrifugation, and decanting of the supernatant. After heated overnight at 70°C in a 2 M NaOH solution, the sample was again centrifuged and the supernatant decanted. The sample was once again redissolved in 0.05 M DTPA and 1 M NaOH solution by agitating overnight then re-precipitated by acidification with HCl (to pH < 2) and addition of BaCl 2 solution. The BaSO 4 precipitate is finally washed in triplicate and dried at 70°C overnight, then ready for isotope analysis.
Oxygen isotope (δ 18 O) measurements. Purified barite samples were measured in duplicate on an Elementar vario PYRO cube elemental analyzer in-line with an Isoprime 100 mass spectrometer in continuous flow mode at the University of Burgundy in Dijon, France. Oxygen isotope data are expressed in delta notation, δ ≡ R sample /R standard − 1, where R is the mole ratio of 18 Table 4), which was used for data correction via standard-sample-standard bracketing.
Quadruple sulfur isotope (δ 34 S, Δ 33 S and Δ 36 S) measurements. Purified barite samples required additional wet chemistry for quadruple sulfur isotope analysis, all done at the Institut de Physique du Globe de Paris in Paris, France. Approximately 3 mg of each barite sample was converted to Ag 2 S by heating in an anoxic distillation apparatus in a sub-boiling solution of HCl, HI, and H 3 PO 2 after Thode, Monster and Dunford 65 and Pepkowitz and Shirley 66 . Barite was thus converted to H 2 S that was then flushed by N 2 gas to an awaiting trap of silver nitrate solution where the H 2 S was precipitated as Ag 2 S. The sample Ag 2 S precipitate was washed in triplicate in distilled water, oven-dried overnight, then ready for sulfur isotope analysis.
Quadruple sulfur isotope analyses were made by first heating~3 mg sample Ag 2 S under an excess of F 2 gas (~300 torr) at 350°C in nickel bombs overnight. The produced SF 6 gas was then purified via cryogenic trapping and separation by gas chromatograph before introduction to the mass spectrometer, a Thermo Finnigan MAT 253 running in dual inlet mode, for determination of quadruple sulfur isotope composition. Delta notation is used to report 34 S, with δ ≡ R sample /R standard − 1, where R is the mole ratio of 34 S/ 32 S and reported in units per mille (‰). The minor isotopes 33 S and 36 S are reported in capital delta notation, where Δ 33 S = δ 33 S -((δ 34 S/1000 + 1) 0.515 -1) × 1000‰ and Δ 36 S = δ 36 S -((δ 34 S/1000 + 1) 1.89 -1) × 1000‰. The δ 34 S, Δ 33 S and Δ 36 S data are reported with respect to the Vienna Canyon Diablo Troilite (VCDT) international standard, with analytical errors of ±0.1‰, ±0.01‰ and ±0.2‰ (2σ), respectively, as based on the long-term reproducibility of analyses of the international standard IAEA S-1 (Supplementary Table 5).
Reporting summary. Further information on research design is available in the Nature Research Reporting Summary linked to this article.

Data availability
All data generated or analysed during this study are included with this published article in its Supplementary