Selenium (Se) isotope ratios in marine carbonates are able to act as a novel geochemical proxy for reconstructing Earth's surface redox evolution, but our understanding of the Se cycling in shallow-water carbonates is currently limited. We thus explore the utility of carbonate-associated Se (CASe) as a paleoredox proxy by testing if modern carbonate 582/76Se values capture the coeval seawater 582/76Se value and evaluating the response of carbonate 582/76Se values to syndepositional processes. Our pilot study presents the first Se concentration and isotope ratio data for modern shallow-marine carbonates from open-marine environments on the Great Bahamas Bank (GBB). We find that the carbonate sediments deposited in fully oxygenated environments are characterized by authigenic Se enrichment with 0.149 to 0.224 mu g/g Se and fractionated carbonate 582/76Se values (deviation of 82Se/76Se relative to NIST SRM 3149) of 1.23 to 4.79 %o +/- 0.3 %o (2 s.d.) within the uppermost 50 cm. The extent of this authigenic Se enrichment is primarily driven by sulfidic conditions in the pore fluids due to ongoing microbial sulfate reduction in the carbonate sediments. The carbonate 582/76Se values identify two independent processes that record local processes specific to the shallow waters of the GBB. High biological productivity in the surface ocean produces two Se pools with distinct 582/76Se values, because the microbial reduction of Se oxyanions leads to isotopically light organic-bound Se and drives the remaining Se oxyanions to heavier 582/76Se values that is incorporated into the carbonates. The isotopically heavy CASe is further modified by syndepositional processes in the carbonate sediments. Our results thus suggests that Se in high-productivity surface oceans behaves as a non-conservative element, indicating that carbonate 582/76Se values in such environments do not necessarily record the seawater Se isotope composition. Instead, bulk carbonate 582/76Se values are potentially further modified during syndepositional diagenesis, which requires further assessment as a paleoredox proxy.

Capturing the loss of mass-independent sulphur isotope fractionation (MIF-S), the correlative South African Duitschland and Rooihoogte formations are widely held to bear the isotopic fingerprint of the first atmospheric oxygenation at the onset of the so-called Great Oxidation Event (GOE). Surprisingly, however, while the multiple sulphur isotope systematics of these formations remain central to our understanding of the GOE, until now, comparatively little work has been done to elucidate the repercussions within the marine realm. Here we present chemostratigraphic records from four drill cores covering a large area of the Transvaal Basin, transcending these crucial units and continuing into the overlying Timeball Hill Formation (TBH), that document the immediate, yet counterintuitive, marine response to atmospheric oxygenation. Specifically, irrespective of the interpretative framework employed, our basin-wide redox-sensitive trace element data document an environmental change from oxic/suboxic conditions within the lower and middle parts of the Duitschland and Rooihoogte formations to suboxic/anoxic conditions within their upper reaches. Interestingly, in concert with a similar to 35 parts per thousand negative delta S-34 excursion that implicates increased sulphate availability and bacterial sulphate reduction, delta Mo-98/95(3134+0.25) values increase by similar to 1.0 to 1.5 parts per thousand. Combining these observations with increased Fe/Mn ratios, elevated total sulphur and carbon contents and a trend towards lower delta C-13(org) values imply a shift toward less oxygenated conditions across the Transvaal Basin. The combined observations in the mentioned parameters expose a geobiological feedback-driven causality between the earliest oxygenation of the atmosphere and decreased redox potentials of medium to deep marine environments, at least within the Transvaal Basin.

The stratigraphic correlation of the ca. 2353 +/- 18 Ma to 2316 +/- 7 Ma Duitschland and Rooihoogte formations, Transvaal Supergroup, South Africa, becomes critically important when viewed in the light of the Great Oxidation Event (GOE), since both formations record the transition from mass-independent (MIF-S) to mass -dependent fractionation of sulfur isotopes (MDF-S) indicative of the transition to an oxygenated atmosphere. At present, discrepancies exist as to whether the Duitschland and Rooihoogte formations are two distinct for-mations, formed in an older and younger depositional basin, respectively, or alternatively were deposited contemporaneously in the same basin at different positions to shorelines of the basin. A decoupled depositional history would indicate an oscillating oxygenation trajectory of the planet, whereas a contemporaneous deposi-tion of the two formations would point towards a sudden increase of atmospheric oxygen. To investigate the controversy of a contemporaneous versus a consecutive depositional relationship, this study presents a sedi-mentological investigation of 11 stratigraphic profiles intersecting the Duitschland to lower Timeball Hill and Rooihoogte to lower Timeball Hill formations. The 11 profiles are correlated between the Transvaal Supergroup in the Transvaal area of South Africa and the adjoining Kanye area in Botswana and show that the Duitschland and Rooihoogte formations are laterally correlative. The sedimentological observations are reinforced by similar major and trace element data as well as Sr-Nd isotope compositions measured in four scientific drill cores, intersecting both the Duitschland and the Rooihoogte formations. Consequently, the documented disappearance of MIF-S in these two formations should be regarded as a single-step atmospheric change, thereby removing the best supporting evidence for a protracted and dynamic oxygenation trajectory. Furthermore, bulk sediment TDM(Nd) of the Duitschland and Rooihoogte formations consistently display Archean crustal residence ages in the interval between 2.9 Ga and 3.3 Ga for most samples. Geochemical discrimination functions and zircon distri-bution patterns suggest a collisional tectonic setting and measured bulk sediment Nd isotope data combined with literature zircon Hf isotope data indicate a potential collision between a Rae-type craton and the Kaapvaal craton around 2.5 Ga.

Archaean supracrustal rocks carry a record of mass-independently fractionated S that is interpreted to be derived from UV-induced photochemical reactions in an oxygen-deficient atmosphere. Experiments with photochemical reactions of SO2 gas have provided some insight into these processes. However, reconciling experimental results with the multiple S isotopic composition of the Archaean sedimentary record has proven difficult and represents one of the outstanding issues in understanding the Archaean surface S-cycle. We present quadruple S isotope data (S-32, S-33, S-34, S-36) for pyrite from Mesoarchaean carbonaceous sediments of the Dominion Group, South Africa, deposited in an acidic volcanic lake, which help reconcile observations from the Archaean sedimentary record with the results of photochemical experiments. The data, which show low Delta S-433/delta S-34 ratios (mostly << 1) and very negative Delta S-36/Delta S-33 ratios (-4 and lower), contrast with the composition of most Archaean sedimentary sulfides and sulfates, having Delta S-36/Delta S-33 similar to -1 (the so-called 'Archaean reference array'), but match those of modern photochemical sulfate aerosols produced in the stratosphere, following super-large volcanic eruptions, and preserved in Antarctic ice. These data are also consistent with the results of UV-irradiation experiments of SO2 gas at variable gas pressure. The S isotope composition of the Dominion Group pyrite is here interpreted to reflect the products of photolysis in a low-oxygen-level atmosphere at high SO2 pressure during large volcanic eruptions, mixed with Archaean 'background' (having a composition broadly similar to the Archaean reference array) S pools. It is inferred that high sedimentation rates in a terrestrial basin resulted in an instantaneously trapped input of atmospheric S during short-lasted depositional intervals, which faithfully represents transient photochemical signals in comparison with marine sedimentary records. (C) 2022 The Author(s). Published by Elsevier B.V.

The later stages of Earth's transition to a permanently oxygenated atmosphere during the Great Oxidation Event (GOE; ∼2.43–2.06 Ga) is commonly linked with the suggestion of an “oxygen overshoot” during the ∼2.22–2.06 Ga Lomagundi Event (LE), which represents Earth's most pronounced and longest-lived positive carbon isotope excursion. However, the magnitude and extent of atmosphere-ocean oxygenation and implications for the biosphere during this critical period in Earth's history remain poorly constrained. Here, we present nitrogen (N), selenium (Se), and carbon (C) isotope data, as well as bio-essential element concentrations, for Paleoproterozoic marine shales deposited during the LE. The data provide evidence for a highly productive and well-oxygenated photic zone, with both inner and outer-shelf marine environments characterized by nitrate- and Se oxyanion-replete conditions. However, the redoxcline subsequently encroached back onto the inner shelf during global-scale deoxygenation of the atmosphere-ocean system at the end of the LE, leading to locally enhanced water column denitrification and quantitative reduction of selenium oxyanions. We propose that nitrate-replete conditions associated with fully oxygenated continental shelf settings were a common feature during the LE, but nitrification was not sufficiently widespread for the aerobic nitrogen cycle to impact the isotopic composition of the global ocean N inventory. Placed in the context of Earth's broader oxygenation history, our findings indicate that O2 levels in the atmosphere-ocean system were likely much lower than modern concentrations. Early Paleoproterozoic biogeochemical cycles were thus far less advanced than after Neoproterozoic oxygenation.

Abstract Serpentinization of ultramafic rocks in the sea and on land leads to the generation of alkaline fluids rich in molecular hydrogen (H2) and methane (CH4) that favour the formation of carbonate mineralization, such as veins in the sub-seafloor, seafloor carbonate chimneys and terrestrial hyperalkaline spring deposits. Examples of this type of seawater–rock interaction and the formation of serpentinization-derived carbonates in a shallow-marine environment are scarce, and almost entirely lacking in the geological record. Here we present evidence for serpentinization-induced fluid seepage in shallow-marine sedimentary rocks from the Upper Cretaceous (upper Campanian to lower Maastrichtian) Qahlah Formation at Jebel Huwayyah, United Arab Emirates. The research object is a metre-scale structure (the Jebel Huwayyah Mound) formed of calcite-cemented sand grains, which formed a positive seafloor feature. The Jebel Huwayyah Mound contains numerous vertically orientated fluid conduits containing two main phases of calcite cement. We use C and O stable isotopes and elemental composition to reconstruct the fluids from which these cements precipitated and infer that the fluids consisted of variable mixtures of seawater and fluids derived from serpentinization of the underlying Semail Ophiolite. Based on their negative δ13C values, hardgrounds in the same section as the Jebel Huwayyah Mound may also have had a similar origin. The Jebel Huwayyah Mound shows that serpentinization of the Semail Ophiolite by seawater occurred very soon after obduction and marine transgression, a process that continued through to the Miocene, and, with interaction of meteoric water, up to the present day.

Platinum-rich nuggets offer an opportunity for understanding how precious metals accumulate. We analyzed the selenium (Se) isotopic composition of Se-rich (10(2)-10(3) mu g g(-1)) platinum-palladium (Pt-Pd) nuggets from a recent placer deposit in Minas Gerais, Brazil, for which a biogenic origin has been inferred. We obtained Se isotopic values with a relatively narrow range (delta Se-82/76(NIST3149) =- 17.4 parts per thousand to -15.4 parts per thousand +/- 0.2 parts per thousand, two standard deviations [2 SD]). The Pt-Os age of the nuggets is 181 +/- 6 Ma (2 SD). The data indicate that the nuggets did not form in the recent placer deposit, but by replacement of hydrothermal vein minerals at similar to 70 degrees C and at least 800 m below the surface. The high abundance and extreme isotopic composition of Se as well as the presence of other biophilic elements like iodine, organic carbon, and nitrogen within the nugget matrix are consistent with a microbial origin. Although abiogenic reduction of Se oxyanions cannot be ruled out, the nuggets plausibly record Se-supported microbial activity in the deep biosphere.

The Loki's Castle vent field at the ultraslow-spreading Arctic Mid-Ocean Ridge (AMOR) hosts a low-temperature venting area, which is characterized by microbial mats and numerous up to 1 m tall barite chimneys. An activelyventing barite chimney yielded delta O-18(SO4) and delta S-34(SO4) values heavier than ambient seawater and sulfide-oxidizing bacteria in microbial mats identified microbial sulfate reduction and sulfide oxidation as the main processes. In order to investigate the chemical and microbial structure below the barite field, we obtained two gravity cores and present chemical composition (e.g., H2S, SO42-, NH4+, DIC) and stable isotope data for the pore fluids (delta S-34(SO4), delta C-13(DIC)) together with stable isotope (delta C-13(org), delta S-34, Delta S-33) and lipid biomarker data on bulk sediments. The gravity core more distant to the high-temperature vents shows seawater-like pore fluid profiles with only minor vent fluid contribution (< 1%), whereas the bulk sediments yield negative delta S-34 and positive.33S values indicative of sulfate reduction. In contrast, the pore fluid data in close proximity to the high-temperature vents (5-9% vent fluid contribution) record distinct horizons showing sulfate depletion, which coincide with delta S-34(SO4) values that are higher than those for ambient seawater sulfate. The sediments in these horizons record negative delta S-34 and positive Delta S-33 values, indicating that both the pore fluids and the sediments are influenced by active sulfate reduction. We also detected a greater abundance of archaeal mono- and dialkyl tetraether lipids (GMGTs, GDGTs) and bacterial fatty acids in the sediments at actively venting sites, pointing to a more diverse microbial community. Moreover, a positive correlation observed between GMGT abundance and sulfur concentration in the sediments indicates that the availability of sulfur is crucial for the presence of GMGT-producing archaea. Our multi-proxy approach suggests that sulfate reduction in the sub-seafloor sediments of the Loki's Castle barite field is largely driven by microbial processes.

Measurements of triple oxygen isotope ratios in barite horizons within post-Marinoan cap carbonates have provided some of the most compelling evidence that the Marinoan glaciation was a Snowball Earth event. However, the origin of these barite horizons remains unresolved. To constrain the Ba sources, and thus formation mechanisms of these horizons, we analyzed the Ba isotope composition of post-Marinoan barite deposits from Northwest Canada, Northern Norway, Brazil and South China. We augment these analyses with a Ba isotope survey of almost 100 modern and ancient additional barite measurements, including samples from pelagic (or 'marine'), hydrothermal, terrestrial, Proterozoic stratiform and cold seep environments. Unlike modern cold seep or terrestrial barites, we find that globally-distributed post-Marinoan barites exhibit a relatively narrow isotopic range, suggesting a well-mixed, effectively limitless Ba source. Moreover, post-Marinoan deposits exhibit a similar mean Ba isotope composition to modern marine barites, which we interpret as evidence of a marine Ba source. Considered alongside existing geochemical, geological, and new Ba isotope survey data, we conclude that Ba in barite horizons was sourced from a well-mixed, Ba-replete but SO4-poor reservoir that accumulated during the Marinoan Snowball Earth interval. This deep Ba reservoir was then transported upward-either by ocean circulation or dolomitization of underlying cap carbonates-and was brought into contact with continental weathering-derived sulfate in a post-glacial meltwater surface layer. Thus, in addition to providing a plausible mechanism for generating globally-synchronous deposition of post-Marinoan barite horizons that reconciles all existing geochemical and geological data, our results demonstrate the utility of Ba isotopes to interrogate the origin of enigmatic barite deposits throughout the sedimentary record. (C) 2019 Elsevier B.V. All rights reserved.

The Paleoproterozoic sandstones of the ca. 2.1 Ga Francevillian Group, southeast Gabon, host the oldest known redox-controlled high-grade uranium (U) deposits. Uranium-bearing detrital minerals (e.g., monazite) in the Francevillian Group sediments were derived by erosion of Archean basement rocks from the East Gabonian block of the northwestern Congo craton surrounding the Francevillian Basin. They are viewed as primary sources for U, but the nature of these sources remains poorly constrained. Here, U-Pb ages and Hf isotope data for detrital zircon grains are combined with literature geochemical data for igneous rocks from the Archean basement, to characterize the nature, age, and origin of the provenance for the Francevillian Basin and the ultimate source of U for the high-grade uranium deposits.Detrital zircon ages range from ca. 3.1 to 2.6 Ga with a dominant population between ca. 2.90 and 2.80 Ga, suggesting a major contribution from ca. 2.88 to 2.81 Ga high-K granitoids with a limited contribution from ca. 3.1 to 2.84 Ga TTGs and 2.75 to 2.7 Ga granitoids of the East Gabonian block. Most detrital zircon grains share similar depleted-mantle Hf model ages between ca. 3.08 and 3.30 Ga, indicating significant recycling of a Paleoarchean to Mesoarchean crust. We suggest that ca. 2.9 to 2.8 Ga high K, U, and Th granitoids, which reflect hybrid melts of both mantle and crustal parentage were the major detrital sources of U to the Francevillian Basin and, ultimately, for the U deposits. We propose that ca. 2.9 to 2.8 Ga detrital zircons with Hf TDM ages between ca. 3.1 and 3.3 Ga may thus allow for vectoring U deposits in sedimentary basins across the Congo craton.

High- and low-temperature platinum nuggets from two historical localities, Choco in Colombia and Corrego Born Sucesso in Brazil, are compared with respect to trace elements. Supergene platinum nuggets from Corrego Bom Sucesso are enriched in Sc and Hg, but depleted in siderophile and chalcophile elements, and have fractionated platinum group element patterns, in comparison with magmatic platinum nuggets from Choco. In particular, Se concentrations over similar to 100 mu g/g Se and S/Se ratios above unity indicate Se recycling in a supergene environment with abundant organic matter.

The Barberton Greenstone Belt hosts abundant structurally controlled gold mineralisation of Mesoarchaean age. More than 300 gold occurrences have been reported, although most of the gold production so far (>350 tonnes Au) has come from a handful of deposits located along the northern margin of the greenstone belt. Most deposits are hosted by greenschist-facies metasedimentary and metamafic rocks, with the notable exception of the amphibolite-facies rocks at New Consort mine. Mineralisation is associated with quartz-carbonate veins that truncate major compressional structures at the greenstone belt scale. The age of mineralisation is loosely constrained at circa 3080-3030 Ma, based on U-Pb dating of hydrothermal rutile and titanite. In greenschist-facies deposits, the ore assemblage is dominated by pyrite and arsenopyrite, which contain up to thousands of ppm of 'invisible' gold, Ni-As-Sb sulphides and native gold. At New Consort mine, mineralisation includes massive replacement-style ore and vein-hosted or disseminated types. Both structural studies in the field and microstructural observation point to a multistage ore deposition process, which is reflected in the re-activation of brittle to ductile structures and the overprinting of sulphide assemblages. The presence of mass-independently fractionated S isotopes (Delta S-33 = -0.6 to +1.0 parts per thousand) in pyrite from Sheba and Fairview mines suggests that hydrothermal fluids mobilised S from volcanic and sedimentary rocks of the greenstone belt and places constraints on the origin of the Au itself.

The Archean Eon was a time of predominantly anoxic Earth surface conditions, where anaerobic processes controlled bioessential element cycles. In contrast to "oxygen oases" well documented for the Neoarchean [2.8 to 2.5 billion years ago (Ga)], the magnitude, spatial extent, and underlying causes of possible Mesoarchean (3.2 to 2.8 Ga) surface-ocean oxygenation remain controversial. Here, we report δ15N and δ13C values coupled with local seawater redox data for Mesoarchean shales of the Mozaan Group (Pongola Supergroup, South Africa) that were deposited during an episode of enhanced Mn (oxyhydr)oxide precipitation between ∼2.95 and 2.85 Ga. Iron and Mn redox systematics are consistent with an oxygen oasis in the Mesoarchean anoxic ocean, but δ15N data indicate a Mo-based diazotrophic biosphere with no compelling evidence for a significant aerobic nitrogen cycle. We propose that in contrast to the Neoarchean, dissolved O2 levels were either too low or too limited in extent to develop a large and stable nitrate reservoir in the Mesoarchean ocean. Since biological N2 fixation was evidently active in this environment, the growth and proliferation of O2-producing organisms were likely suppressed by nutrients other than nitrogen (e.g., phosphorus), which would have limited the expansion of oxygenated conditions during the Mesoarchean.

The ca. 2.1 Ga Francevillian Group of Gabon was deposited in the aftermath of the Great Oxidation Event (GOE) and records the Lomagundi Event (LE), which is the most pronounced and long-lived carbon isotope excursion in the geologic record. Moreover, the sedimentary succession contains putative evidence for the earliest appearance of macro-eukaryotes. An emerging paradigm is that the end of the LE was accompanied by a deoxygenation event that preceded the apparent stability of environmental and redox conditions as well as the carbon cycle characteristic of the Mesoproterozoic. However, the processes that led to deoxygenation some 300 to 200 Ma after the beginning of the GOE are not well understood. Here we present a multi-proxy stable isotope (δ34S, Δ33S, Δ36S, δ98Mo, δ13Corg, δ13Ccarb, and δ18Ocarb) study of the Francevillian Group. We suggest that sedimentation of the lower part of the Francevillian Group took place during the LE in oxygenated shallow waters with elevated sulfate concentrations. Two episodes of anoxic water shoaling during deposition of the upper Francevillian Group correspond with broader marine deoxygenation and a contraction of the seawater sulfate reservoir. This shoaling of anoxic conditions may be linked to intense submarine hydrothermal and volcanic activity that led to sedimentary manganese deposits. We propose that increased concentrations of aqueous, hydrothermally sourced reductants drove oxygen consumption during the first deoxygenation event and established a sulfidic oxygen-minimum zone at the margin of the shallow shelf. Carbonates with positive δ13Ccarb values characteristic of the LE precipitated during this first stage of deoxygenation. The second deoxygenation, separated from the previous event by a period of well-oxygenated conditions, was marked by a stronger contraction of the seawater sulfate reservoir and coincided with the end of the LE. During this time, widespread euxinic conditions were established in shallow (above storm wave base) marine environments. The presence of a shallow-water redoxcline points to a generally low-oxygen atmosphere–ocean system. Further, the negative co-variation between δ34S and δ13C values in sediments of the Francevillian Group and other sedimentary successions of similar age worldwide suggests that the inferred two-step deoxygenation corresponding to the end of the LE reflects global rather than local events that likely occurred between ∼2.1 and 2.05 Ga ago.

This study presents the first combined S and Se isotope investigation of sulfide suited to explore differences in fractionation between these two redox sensitive isotope systematics as recorded in the same mineral. A case study of Cretaceous Navajún pyrite from the Mesozoic Cameros Basin, Spain, with known petrogenesis and geological context shows systematic decoupling at the microscale: Variable S isotope values within the analyzed pyrite coincide with rather constant Se isotope values and vice versa. These signatures were not generated during pyrite growth but record previous redox induced fractionations in fluids that each contributed both elements from two sources. It is likely that both S and Se isotope fractionation occurred during strong reduction from one fully oxidized source whereas only S but no Se isotope fractionation occurred during minor reduction following sulfide dissolution via H2O from another source. Subsequent mixing of these two H2S-H2Se fluids at different elemental S-Se ratios during incorporation into the pyrite can then explain the S-Se isotope variations in the investigated specimen. These inferences are in accordance with a larger range in the redox potential Eh of Se relative to S, resulting in coupled or decoupled Se and S isotope fractionation depending on the oxygen fugacity during the reduction process. If extended to other sulfides of diverse origin, for a given pH, combined Se and S isotope studies may allow to investigate the magnitude of redox variations and place more robust constraints on minimum and maximum oxygen concentrations in the source. We therefore suggest combined S-Se isotope analyses in sulfide as a new powerful proxy for studying Earth’s redox evolution beyond the bulk rock scale.

Mass-independent fractionation of sulfur isotopes (MIF-S) in Archaean sediments results from photochemical processing of atmospheric sulfur species in an oxygen-depleted atmosphere. Geological preservation of MIF-S provides evidence for microbial sulfate reduction (MSR) in low-sulfate Paleoarchaean (3.8–3.2 billion years ago (Ga)) and Neoarchaean (2.8–2.5 Ga) oceans, but the significance of MSR in Mesoarchaean (3.2–2.8 Ga) oceans is less clear. Here we present multiple sulfur and iron isotope data of early diagenetic pyrites from 2.97-Gyr-old stromatolitic dolomites deposited in a tidal flat environment of the Nsuze Group, Pongola Supergroup, South Africa. We identified consistently negative Δ 33 S values in pyrite, which indicates photochemical reactions under anoxic atmospheric conditions, but large mass-dependent sulfur isotope fractionations of ~30‰ in δ 34 S, identifying active MSR. Negative pyrite δ 56 Fe values (−1.31 to −0.88‰) record Fe oxidation in oxygen-bearing shallow oceans coupled with biogenic Fe reduction during diagenesis, consistent with the onset of local Fe cycling in oxygen oases ~3.0 Ga. We therefore suggest the presence of oxygenated near-shore shallow-marine environments with ≥5 μM sulfate at this time, in spite of the clear presence of an overall reduced Mesoarchaean atmosphere.

Redox conditions in the marine realm prior to the Great Oxidation Event (GOE; similar to 2.46-232 Ga ago), during which the atmospheric oxygen level rose dramatically for the first time, are still debated. Here, we present C, O , Fe, and Mo stable isotope systematics of Fe-, Mn-, and carbonate-rich shales, deposited at different water depths in association with iron formations (IFs) of the Mesoarchean Mozaan Group, Pongola Supergroup, South Africa. delta(13C) values between-22.3 and -13.5%(0) VPDB, and delta O-18 values between -21.1 and -8.6%(0) VPDB for Fe-Mn-rich carbonate minerals indicate their precipitation out of equilibrium with seawater. Instead, early diagenetic reduction of Fe-Mn-oxyhydroxide precursor minerals, along with microbially induced oxidation of organic matter (OM), formed these carbonates. delta(56)Feinrm-014 values between -1.27 and 0.14%o and delta(MONIST)-M-98 (3134+0.25) values between -0.46 and 0.56%o co-vary with Mn concentrations and inferred water depth of deposition. This suggests that, despite the diagenetic origin of the Fe-Mn carbonates, the primary light Fe and Mo isotopic signature of Fe-Mn-oxyhydroxides that originally precipitated from seawater is still preserved. While isotopically light Mo implies that Mn(II) was oxidized to Mn(IV) due to the availability of free, photosynthetically produced 02, Mn enrichment suggests that the water column was redox stratified with a Mn-redoxcline situated at a depth below the storm wave base. A trend to highly negative delta Fe-56 values with increasing Mn/Fe ratios and decreasing depositional depth suggests progressive oxidation of Fe(II) as deep-waters upwelled across a redoxcline towards shallow, locally oxygenated waters where Mn(IV) oxyhydroxides precipitated. Combined delta Fe-56 and delta Mo-98 data indicate pervasive oxygenation of seawater with the O-2 content in the photic zone likely reaching levels higher than the maximum value of 10 mu M proposed for Archean oxygen oases. Since abiotic Mn(II) oxidation is kinetically very slow in marine environments, it is likely that Mn-oxidizing microorganisms catalyzed Mn-oxidation in the oxygenated Pongola surface waters during deposition of IFs. This implies that aerobic metabolism had evolved before the GOE in shallow, aquatic habitats, where it exerted a first-order control on the deposition of shallow-marine, Mn-rich iron formations. (C) 2018 Elsevier B.V. All rights reserved.

The Barberton Greenstone Belt of southern Africa hosts several Mesoarchaean gold deposits. The ores were mostly formed in greenschist facies conditions, and occur as hydrothermal alteration zones around extensional faults that truncate and post-date the main compressional structures of the greenstone belt. Ore deposition was accompanied by the intrusion of porphyries, which has led to the hypothesis that gold may have been sourced from magmas. Because the transport of Au in the hydrothermal fluids is widely believed to have involved S complexes, tracing the origin of S may place strong constraints on the origin of Au. We measured multiple S isotopes in sulfide ore from Sheba and Fairview mines of the Barberton Greenstone Belt to distinguish "deep" S sources (e.g. magmas) from "surface" S sources (i.e. rocks of the volcano-sedimentary succession that contain S processed in the atmosphere preserved as sulfide and sulfate minerals). Ion probe (SIMS) analyses of pyrite from ore zones indicate mass-independent fractionation of S isotopes (Delta S-33 = 0.6%o to +1.0%O) and the distribution of the analyses in the Delta S-33-delta S-34 space matches the distribution peak of previously published analyses of pyrite from the entire volcano-sedimentary succession. Notwithstanding that the H2O-CO2 components of the fluids may have been introduced from a deep source external to the greenstone belt rocks, the fact that S bears an atmospheric signature suggests the hypothesis that the source of Au should also be identified in the supracrustal succession of the greenstone belt. Our findings differ from conclusions of previous studies of other Archaean shear-hosted Au deposits based on mineralogical and isotopic evidence, which suggested a magmatic or mantle source for Au, and imply that there is no single model that can be applied to this type of mineralisation in the Archaean. (C) 2016 Elsevier B.V. All rights reserved.