Sedimentary pyrite sulfur isotope compositions preserve signatures of the surface microbial mat environment in sediments underlying low-oxygen cyanobacterial mats

The sedimentary pyrite sulfur isotope (δ³⁴S) record is an archive of ancient microbial sulfur cycling and environmental conditions. Interpretations of pyrite δ³⁴S signatures in sediments deposited in microbial mat ecosystems are based on studies of modern microbial mat porewater sulfide δ³⁴S geochemistry. Pyrite δ³⁴S values often capture δ³⁴S signatures of porewater sulfide at the location of pyrite formation. However, microbial mats are dynamic environments in which biogeochemical cycling shifts vertically on diurnal cycles. Therefore, there is a need to study how the location of pyrite formation impacts pyrite δ³⁴S patterns in these dynamic systems. Here, we present diurnal porewater sulfide δ³⁴S trends and δ³⁴S values of pyrite and iron monosulfides from Middle Island Sinkhole, Lake Huron. The sediment–water interface of this sinkhole hosts a low-oxygen cyanobacterial mat ecosystem, which serves as a useful location to explore preservation of sedimentary pyrite δ³⁴S signatures in early Earth environments. Porewater sulfide δ³⁴S values vary by up to ~25‰ throughout the day due to light-driven changes in surface microbial community activity that propagate downwards, affecting porewater geochemistry as deep as 7.5 cm in the sediment. Progressive consumption of the sulfate reservoir drives δ³⁴S variability, instead of variations in average cell-specific sulfate reduction rates and/or sulfide oxidation at different depths in the sediment. The δ³⁴S values of pyrite are similar to porewater sulfide δ³⁴S values near the mat surface. We suggest that oxidative sulfur cycling and other microbial activity promote pyrite formation in and immediately adjacent to the microbial mat and that iron geochemistry limits further pyrite formation with depth in the sediment. These results imply that primary δ³⁴S signatures of pyrite deposited in organic-rich, iron-poor microbial mat environments capture information about microbial sulfur cycling and environmental conditions at the mat surface and are only minimally affected by deeper sedimentary processes during early diagenesis.     

Environmental insights from high‐resolution (SIMS) sulfur isotope analyses of sulfides in Proterozoic microbialites with diverse mat textures

In modern microbial mats, hydrogen sulfide shows pronounced sulfur isotope (δ³⁴S) variability over small spatial scales (~50‰ over <4 mm), providing information about microbial sulfur cycling within different ecological niches in the mat. In the geological record, the location of pyrite formation, overprinting from mat accretion, and post‐depositional alteration also affect both fine‐scale δ³⁴S patterns and bulk δ³⁴S_pyrite values. We report μm‐scale δ³⁴S patterns in Proterozoic samples with well‐preserved microbial mat textures. We show a well‐defined relationship between δ³⁴S values and sulfide mineral grain size and type. Small pyrite grains (<25 μm) span a large range, tending toward high δ³⁴S values (?54.5‰ to 11.7‰, mean: ?14.4‰). Larger pyrite grains (>25 μm) have low but equally variable δ³⁴S values (?61.0‰ to ?10.5‰, mean: ?44.4‰). In one sample, larger sphalerite grains (>35 μm) have intermediate and essentially invariant δ³⁴S values (?22.6‰ to ?15.6‰, mean: ?19.4‰). We suggest that different sulfide mineral populations reflect separate stages of formation. In the first stage, small pyrite grains form near the mat surface along a redox boundary where high rates of sulfate reduction, partial closed‐system sulfate consumption in microenvironments, and/or sulfide oxidation lead to high δ³⁴S values. In another stage, large sphalerite grains with low δ³⁴S values grow along the edges of pore spaces formed from desiccation of the mat. Large pyrite grains form deeper in the mat at slower sulfate reduction rates, leading to low δ³⁴S_sulfide values. We do not see evidence for significant ³⁴S‐enrichment in bulk pore water sulfide at depth in the mat due to closed‐system Rayleigh fractionation effects. On a local scale, Rayleigh fractionation influences the range of δ³⁴S values measured for individual pyrite grains. Fine‐scale analyses of δ³⁴S_pyrite patterns can thus be used to extract environmental information from ancient microbial mats and aid in the interpretation of bulk δ³⁴S_pyrite records.     

Microbial production of isotopically light iron(II) in a modern chemically precipitated sediment and implications for isotopic variations in ancient rocks

The inventories and Fe isotope composition of aqueous Fe(II) and solid‐phase Fe compounds were quantified in neutral‐pH, chemically precipitated sediments downstream of the Iron Mountain acid mine drainage site in northern California, USA. The sediments contain high concentrations of amorphous Fe(III) oxyhydroxides [Fe(III)_am] that allow dissimilatory iron reduction (DIR) to predominate over Fe–S interactions in Fe redox transformation, as indicated by the very low abundance of Cr(II)‐extractable reduced inorganic sulfur compared with dilute HCl‐extractable Fe. δ⁵⁶Fe values for bulk HCl‐ and HF‐extractable Fe were ≈ 0. These near‐zero bulk δ⁵⁶Fe values, together with the very low abundance of dissolved Fe in the overlying water column, suggest that the pyrite Fe source had near‐zero δ⁵⁶Fe values, and that complete oxidation of Fe(II) took place prior to deposition of the Fe(III) oxide‐rich sediment. Sediment core analyses and incubation experiments demonstrated the production of millimolar quantities of isotopically light (δ⁵⁶Fe ≈ ?1.5 to ?0.5‰) aqueous Fe(II) coupled to partial reduction of Fe(III)_am by DIR. Trends in the Fe isotope composition of solid‐associated Fe(II) and residual Fe(III)_am are consistent with experiments with synthetic Fe(III) oxides, and collectively suggest an equilibrium Fe isotope fractionation between aqueous Fe(II) and Fe(III)_am of approximately ?2‰. These Fe(III) oxide‐rich sediments provide a model for early diagenetic processes that are likely to have taken place in Archean and Paleoproterozoic marine sediments that served as precursors for banded iron formations. Our results suggest pathways whereby DIR could have led to the formation of large quantities of low‐δ⁵⁶Fe minerals during BIF genesis.     

A new model of the formation of Pennsylvanian iron carbonate concretions hosting exceptional soft‐bodied fossils in Mazon Creek,Illinois

Preservation of Pennsylvanian‐aged (307 Ma) soft‐bodied fossils from Mazon Creek, Illinois, USA, is attributed to the formation of siderite concretions, which encapsulate the remains of terrestrial, freshwater, and marine flora and fauna. The narrow range of positive δ³⁴S values from pyrite in individual concretions suggests microenvironmentally limited ambient sulfate, which may have been rapidly exhausted by sulfate‐reducing bacteria. Tissue of the decaying carcass was rapidly encased by early diagenetic pyrite and siderite produced within the sulfate reduction and methanogenic zones of the sediment, with continuation of the latter resulting in concretion cementation. Cross‐sectional isotopic analyses (δ¹³C and δ¹⁸O) and mineralogical characterization of the concretions point to initiation of preservation in high porosity proto‐concretions during the early phases of microbially induced decay. The proto‐concretion was cemented prior to compaction of the sediments by siderite as a result of methanogenic production of ¹³C‐rich bicarbonate—which varies both between Essex and Braidwood concretions and between fossiliferous and unfossiliferous concretions. This work provides the first detailed geochemical study of the Mazon Creek siderite concretions and identifies the range of conditions allowing for exceptional soft‐tissue fossil formation as seen at Mazon Creek.     

Constraining the formation of authigenic carbonates in a seepage‐affected cold‐water coral mound by lipid biomarkers

Cold‐water coral (CWC) mounds are build‐ups comprised of coral‐dominated intervals alternating with a mixed carbonate‐siliciclastic matrix. At some locations, CWC mounds are influenced by methane seepage, but the impact of methane on CWC mounds is poorly understood. To constrain the potential impact of methane on CWC mound growth, lipid biomarker investigations were combined with mineralogical and petrographic analyses to investigate the anaerobic oxidation of methane (AOM) and authigenic carbonate formation in sediment from a seep‐affected CWC mound in the Gulf of Cadiz. The occurrence of AOM was confirmed by characteristic lipids found within a semi‐lithified zone (SLZ) consisting of authigenic aragonite, high‐magnesium calcite and calcium‐excess dolomite. The formation of high‐Mg calcite is attributed to AOM, acting as a lithifying agent. Aragonite is only a minor phase. Ca‐excess dolomite in the SLZ and upper parts may be formed by organoclastic sulphate reduction, favouring precipitation by increased alkalinity. The AOM biomarkers in the SLZ include isoprenoid‐based archaeal membrane lipids, such as abundant glycerol dibiphytanyl glycerol tetraethers (GDGTs) dominated by GDGT‐2. The δ¹³C values of GDGT‐2, measured as ether‐cleaved monocyclic biphytanes, are as low as ?100‰ versus V‐PDB. Further, bacterial dialkyl glycerol diethers with two anteiso‐C₁₅ alkyl chains and δ¹³C values of ?81‰ are interpreted as biomarkers of sulphate‐reducing bacteria. The lipid biomarker signatures and mineralogical patterns suggest that anaerobic methane‐oxidizing archaea of the ANME‐1 group thrived in the subsurface at times of slow and diffusive methane seepage. Petrographic analyses revealed that the SLZ was exhumed at some point (e.g. signs of bioerosion of the semi‐lithified sediment), providing a hard substrate for CWC larval settlement. In addition, this work reveals that AOM‐induced semi‐lithification likely played a role in mound stabilization. Lipid biomarker analysis proves to be a powerful tool to disentangle early diagenetic processes induced by microbial metabolisms.     

In Situ Fe and S isotope analyses in pyrite from the 3.2 Ga Mendon Formation (Barberton Greenstone Belt,South Africa): Evidence for early microbial iron reduction

On the basis of phylogenetic studies and laboratory cultures, it has been proposed that the ability of microbes to metabolize iron has emerged prior to the Archaea/Bacteria split. However, no unambiguous geochemical data supporting this claim have been put forward in rocks older than 2.7–2.5 giga years (Gyr). In the present work, we report in situ Fe and S isotope composition of pyrite from 3.28‐ to 3.26‐Gyr‐old cherts from the upper Mendon Formation, South Africa. We identified three populations of microscopic pyrites showing a wide range of Fe isotope compositions, which cluster around two δ⁵⁶Fe values of ?1.8‰ and +1‰. These three pyrite groups can also be distinguished based on the pyrite crystallinity and the S isotope mass‐independent signatures. One pyrite group displays poorly crystallized pyrite minerals with positive Δ³³S values > +3‰, while the other groups display more variable and closer to 0‰ Δ³³S values with recrystallized pyrite rims. It is worth to note that all the pyrite groups display positive Δ³³S values in the pyrite core and similar trace element compositions. We therefore suggest that two of the pyrite groups have experienced late fluid circulations that have led to partial recrystallization and dilution of S isotope mass‐independent signature but not modification of the Fe isotope record. Considering the mineralogy and geochemistry of the pyrites and associated organic material, we conclude that this iron isotope systematic derives from microbial respiration of iron oxides during early diagenesis. Our data extend the geological record of dissimilatory iron reduction (DIR) back more than 560 million years (Myr) and confirm that micro‐organisms closely related to the last common ancestor had the ability to reduce Fe(III).     

Unprecedented 34S‐enrichment of pyrite formed following microbial sulfate reduction in fractured crystalline rocks

In the deep biosphere, microbial sulfate reduction (MSR) is exploited for energy. Here, we show that, in fractured continental crystalline bedrock in three areas in Sweden, this process produced sulfide that reacted with iron to form pyrite extremely enriched in ³⁴S relative to ³²S. As documented by secondary ion mass spectrometry (SIMS) microanalyses, the δ³⁴S_pyrite values are up to +132‰V‐CDT and with a total range of 186‰. The lightest δ³⁴S_pyrite values (?54‰) suggest very large fractionation during MSR from an initial sulfate with δ³⁴S values (δ³⁴S_sulfate,0) of +14 to +28‰. Fractionation of this magnitude requires a slow MSR rate, a feature we attribute to nutrient and electron donor shortage as well as initial sulfate abundance. The superheavy δ³⁴S_pyrite values were produced by Rayleigh fractionation effects in a diminishing sulfate pool. Large volumes of pyrite with superheavy values (+120 ± 15‰) within single fracture intercepts in the boreholes, associated heavy average values up to +75‰ and heavy minimum δ³⁴S_pyrite values, suggest isolation of significant amounts of isotopically light sulfide in other parts of the fracture system. Large fracture‐specific δ³⁴S_pyrite variability and overall average δ³⁴S_pyrite values (+11 to +16‰) lower than the anticipated δ³⁴S_sulfate,0 support this hypothesis. The superheavy pyrite found locally in the borehole intercepts thus represents a late stage in a much larger fracture system undergoing Rayleigh fractionation. Microscale Rb–Sr dating and U/Th–He dating of cogenetic minerals reveal that most pyrite formed in the early Paleozoic era, but crystal overgrowths may be significantly younger. The δ¹³C values in cogenetic calcite suggest that the superheavy δ³⁴S_pyrite values are related to organotrophic MSR, in contrast to findings from marine sediments where superheavy pyrite has been proposed to be linked to anaerobic oxidation of methane. The findings provide new insights into MSR‐related S‐isotope systematics, particularly regarding formation of large fractions of ³⁴S‐rich pyrite.     

Bacterial formation of phosphatic laminites off Peru

Authigenic phosphatic laminites enclosed in phosphorite crusts from the shelf off Peru (10°01′ S and 10°24′ S) consist of carbonate fluorapatite layers, which contain abundant sulfide minerals including pyrite (FeS₂) and sphalerite (ZnS). Low δ³⁴S_pyrite values (average ?28.8‰) agree with bacterial sulfate reduction and subsequent pyrite formation. Stable sulfur isotopic compositions of sulfate bound in carbonate fluorapatite are lower than that of sulfate from ambient sea water, suggesting bacterial reoxidation of sulfide by sulfide‐oxidizing bacteria. The release of phosphorus and subsequent formation of the autochthonous phosphatic laminites are apparently caused by the activity of sulfate‐reducing bacteria and associated sulfide‐oxidizing bacteria. Following an extraction–phosphorite dissolution–extraction procedure, molecular fossils of sulfate‐reducing bacteria (mono‐O‐alkyl glycerol ethers, di‐O‐alkyl glycerol ethers, as well as the short‐chain branched fatty acids i/ai‐C_15:0, i/ai‐C_17:0 and 10MeC_16:0) are found to be among the most abundant compounds. The fact that these molecular fossils of sulfate‐reducing bacteria are distinctly more abundant after dissolution of the phosphatic laminite reveals that the lipids are tightly bound to the mineral lattice of carbonate fluorapatite. Moreover, compared with the autochthonous laminite, molecular fossils of sulfate‐reducing bacteria are: (1) significantly less abundant and (2) not as tightly bound to the mineral lattice in the other, allochthonous facies of the Peruvian crusts consisting of phosphatic coated grains. These observations confirm the importance of sulfate‐reducing bacteria in the formation of the phosphatic laminite. Model calculations highlight that organic matter degradation by sulfate‐reducing bacteria has the potential to liberate sufficient phosphorus for phosphogenesis.     

Sulphur cycling in a Neoarchaean microbial mat

Multiple sulphur (S) isotope ratios are powerful proxies to understand the complexity of S biogeochemical cycling through Deep Time. The disappearance of a sulphur mass‐independent fractionation (S‐MIF) signal in rocks <~2.4 Ga has been used to date a dramatic rise in atmospheric oxygen levels. However, intricacies of the S‐cycle before the Great Oxidation Event remain poorly understood. For example, the isotope composition of coeval atmospherically derived sulphur species is still debated. Furthermore, variation in Archaean pyrite δ³⁴S values has been widely attributed to microbial sulphate reduction (MSR). While petrographic evidence for Archaean early‐diagenetic pyrite formation is common, textural evidence for the presence and distribution of MSR remains enigmatic. We combined detailed petrographic and in situ, high‐resolution multiple S‐isotope studies (δ³⁴S and Δ³³S) using secondary ion mass spectrometry (SIMS) to document the S‐isotope signatures of exceptionally well‐preserved, pyritised microbialites in shales from the ~2.65‐Ga Lokammona Formation, Ghaap Group, South Africa. The presence of MSR in this Neoarchaean microbial mat is supported by typical biogenic textures including wavy crinkled laminae, and early‐diagenetic pyrite containing <26‰ μm‐scale variations in δ³⁴S and Δ³³S = ?0.21 ± 0.65‰ (±1σ). These large variations in δ³⁴S values suggest Rayleigh distillation of a limited sulphate pool during high rates of MSR. Furthermore, we identified a second, morphologically distinct pyrite phase that precipitated after lithification, with δ³⁴S = 8.36 ± 1.16‰ and Δ³³S = 5.54 ± 1.53‰ (±1σ). We propose that the S‐MIF signature of this secondary pyrite does not reflect contemporaneous atmospheric processes at the time of deposition; instead, it formed by the influx of later‐stage sulphur‐bearing fluids containing an inherited atmospheric S‐MIF signal and/or from magnetic isotope effects during thermochemical sulphate reduction. These insights highlight the complementary nature of petrography and SIMS studies to resolve multigenerational pyrite formation pathways in the geological record.     

Carbon and sulfur isotopic signatures of ancient life and environment at the microbial scale: Neoarchean shales and carbonates

An approach to coordinated, spatially resolved, in situ carbon isotope analysis of organic matter and carbonate minerals, and sulfur three‐ and four‐isotope analysis of pyrite with an unprecedented combination of spatial resolution, precision, and accuracy is described. Organic matter and pyrite from eleven rock samples of Neoarchean drill core express nearly the entire range of δ¹³C, δ³⁴S, Δ³³S, and Δ³⁶S known from the geologic record, commonly in correlation with morphology, mineralogy, and elemental composition. A new analytical approach (including a set of organic calibration standards) to account for a strong correlation between H/C and instrumental bias in SIMS δ¹³C measurement of organic matter is identified. Small (2–3 μm) organic domains in carbonate matrices are analyzed with sub‐permil accuracy and precision. Separate 20‐ to 50‐μm domains of kerogen in a single ~0.5 cm³ sample of the ~2.7 Ga Tumbiana Formation have δ¹³C = ?52.3 ± 0.1‰ and ?34.4 ± 0.1‰, likely preserving distinct signatures of methanotrophy and photoautotrophy. Pyrobitumen in the ~2.6 Ga Jeerinah Formation and the ~2.5 Ga Mount McRae Shale is systematically ¹³C‐enriched relative to co‐occurring kerogen, and associations with uraniferous mineral grains suggest radiolytic alteration. A large range in sulfur isotopic compositions (including higher Δ³³S and more extreme spatial gradients in Δ³³S and Δ³⁶S than any previously reported) are observed in correlation with morphology and associated mineralogy. Changing systematics of δ³⁴S, Δ³³S, and Δ³⁶S, previously investigated at the millimeter to centimeter scale using bulk analysis, are shown to occur at the micrometer scale of individual pyrite grains. These results support the emerging view that the dampened signature of mass‐independent sulfur isotope fractionation (S‐MIF) associated with the Mesoarchean continued into the early Neoarchean, and that the connections between methane and sulfur metabolism affected the production and preservation of S‐MIF during the first half of the planet's history.     

Anaerobic oxidation of methane by sulfate in hypersaline groundwater of the Dead Sea aquifer

Geochemical and microbial evidence points to anaerobic oxidation of methane (AOM) likely coupled with bacterial sulfate reduction in the hypersaline groundwater of the Dead Sea (DS) alluvial aquifer. Groundwater was sampled from nine boreholes drilled along the Arugot alluvial fan next to the DS. The groundwater samples were highly saline (up to 6300 mm chlorine), anoxic, and contained methane. A mass balance calculation demonstrates that the very low δ¹³C_DIC in this groundwater is due to anaerobic methane oxidation. Sulfate depletion coincident with isotope enrichment of sulfur and oxygen isotopes in the sulfate suggests that sulfate reduction is associated with this AOM. DNA extraction and 16S amplicon sequencing were used to explore the microbial community present and were found to be microbial composition indicative of bacterial sulfate reducers associated with anaerobic methanotrophic archaea (ANME) driving AOM. The net sulfate reduction seems to be primarily controlled by the salinity and the available methane and is substantially lower as salinity increases (2.5 mm sulfate removal at 3000 mm chlorine but only 0.5 mm sulfate removal at 6300 mm chlorine). Low overall sulfur isotope fractionation observed (³⁴ε = 17 ± 3.5‰) hints at high rates of sulfate reduction, as has been previously suggested for sulfate reduction coupled with methane oxidation. The new results demonstrate the presence of sulfate‐driven AOM in terrestrial hypersaline systems and expand our understanding of how microbial life is sustained under the challenging conditions of an extremely hypersaline environment.     

Environmental context for the terminal Ediacaran biomineralization of animals

In terminal Ediacaran strata of South China, the onset of calcareous biomineralization is preserved in the paleontological transition from Conotubus to Cloudina in repetitious limestone facies of the Dengying Formation. Both fossils have similar size, funnel‐in‐funnel construction, and epibenthic lifestyle, but Cloudina is biomineralized, whereas Conotubus is not. To provide environmental context for this evolutionary milestone, we conducted a high‐resolution elemental and stable isotope study of the richly fossiliferous Gaojiashan Member. Coincident with the first appearance of Cloudina is a significant positive carbonate carbon isotope excursion (up to +6‰) and an increase in the abundance and ³⁴S composition of pyrite. In contrast, δ³⁴S values of carbonate‐associated sulfate remain steady throughout the succession, resulting in anomalously large (>70‰) sulfur isotope fractionations in the lower half of the member. The fractionation trend likely relates to changes in microbial communities, with sulfur disproportionation involved in the lower interval, whereas microbial sulfate reduction was the principal metabolic pathway in the upper. We speculate that the coupled paleontological and biogeochemical anomalies may have coincided with an increase in terrestrial weathering fluxes of sulfate, alkalinity, and nutrients to the depositional basin, which stimulated primary productivity, the spread of an oxygen minimum zone, and the development of euxinic conditions in subtidal and basinal environments. Enhanced production and burial of organic matter is thus directly connected to the carbon isotope anomaly, and likely promoted pyritization as the main taphonomic pathway for Conotubus and other soft‐bodied Ediacara biotas. Our studies suggest that the Ediacaran confluence of ecological pressures from predation and environmental pressures from an increase in seawater alkalinity set the stage for an unprecedented geobiological response: the evolutionary novelty of animal biomineralization.     

Diversity decoupled from sulfur isotope fractionation in a sulfate‐reducing microbial community

The extent of fractionation of sulfur isotopes by sulfate‐reducing microbes is dictated by genomic and environmental factors. A greater understanding of species‐specific fractionations may better inform interpretation of sulfur isotopes preserved in the rock record. To examine whether gene diversity influences net isotopic fractionation in situ, we assessed environmental chemistry, sulfate reduction rates, diversity of putative sulfur‐metabolizing organisms by 16S rRNA and dissimilatory sulfite reductase (dsrB) gene amplicon sequencing, and net fractionation of sulfur isotopes along a sediment transect of a hypersaline Arctic spring. In situ sulfate reduction rates yielded minimum cell‐specific sulfate reduction rates < 0.3 × 10^?15 moles cell^?1 day^?1. Neither 16S rRNA nor dsrB diversity indices correlated with relatively constant (38‰–45‰) net isotope fractionation (ε³⁴S_{sulfide‐sulfate}). Measured ε³⁴S values could be reproduced in a mechanistic fractionation model if 1%–2% of the microbial community (10%–60% of Deltaproteobacteria) were engaged in sulfate respiration, indicating heterogeneous respiratory activity within sulfate‐reducing populations. This model indicated enzymatic kinetic diversity of Apr was more likely to correlate with sulfur fractionation than DsrB. We propose that, above a threshold Shannon diversity value of 0.8 for dsrB, the influence of the specific composition of the microbial community responsible for generating an isotope signal is overprinted by the control exerted by environmental variables on microbial physiology.     

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

Sulfate minerals are rare in the Archean rock record and largely restricted to the occurrence of barite (BaSO₄). The origin of this barite remains controversially debated. The mass‐independent fractionation of sulfur isotopes in these and other Archean sedimentary rocks suggests that photolysis of volcanic aerosols in an oxygen‐poor atmosphere played an important role in their formation. Here, we report on the multiple sulfur isotopic composition of sedimentary anhydrite in the ca. 3.22 Ga Moodies Group of the Barberton Greenstone Belt, southern Africa. Anhydrite occurs, together with barite and pyrite, in regionally traceable beds that formed in fluvial settings. Variable abundances of barite versus anhydrite reflect changes in sulfate enrichment by evaporitic concentration across orders of magnitude in an arid, nearshore terrestrial environment, periodically replenished by influxes of seawater. The multiple S‐isotope compositions of anhydrite and pyrite are consistent with microbial sulfate reduction. S‐isotope signatures in barite suggest an additional oxidative sulfate source probably derived from continental weathering of sulfide possibly enhanced by microbial sulfur oxidation. Although depositional environments of Moodies sulfate minerals differ strongly from marine barite deposits, their sulfur isotopic composition is similar and most likely reflects a primary isotopic signature. The data indicate that a constant input of small portions of oxidized sulfur from the continents into the ocean may have contributed to the observed long‐term increase in Δ³³S_sulfate values through the Paleoarchean.     

A diagenetic control on the Early Triassic Smithian–Spathian carbon isotopic excursions recorded in the marine settings of the Thaynes Group (Utah,USA)

In the aftermath of the end‐Permian mass extinction, Early Triassic sediments record some of the largest Phanerozoic carbon isotopic excursions. Among them, a global Smithian‐negative carbonate carbon isotope excursion has been identified, followed by an abrupt increase across the Smithian–Spathian boundary (SSB; ~250.8 Myr ago). This chemostratigraphic evolution is associated with palaeontological evidence that indicate a major collapse of terrestrial and marine ecosystems during the Late Smithian. It is commonly assumed that Smithian and Spathian isotopic variations are intimately linked to major perturbations in the exogenic carbon reservoir. We present paired carbon isotopes measurements from the Thaynes Group (Utah, USA) to evaluate the extent to which the Early Triassic isotopic perturbations reflect changes in the exogenic carbon cycle. The δ¹³C_carb variations obtained here reproduce the known Smithian δ¹³C_carb‐negative excursion. However, the δ¹³C signal of the bulk organic matter is invariant across the SSB and variations in the δ³⁴S signal of sedimentary sulphides are interpreted here to reflect the intensity of sediment remobilization. We argue that Middle to Late Smithian δ¹³C_carb signal in the shallow marine environments of the Thaynes Group does not reflect secular evolution of the exogenic carbon cycle but rather physicochemical conditions at the sediment–water interface leading to authigenic carbonate formation during early diagenetic processes.     

Formation of large carbonate concretions in black cherts in the Gufeng Formation (Guadalupian) at Enshi,South China

The formation of carbonate concretions is a cementation process which passively infills the pore spaces within sediments. They record the original environments of deposition and diagenetic conditions of the host rocks. Little is known about the precise mechanisms responsible for the precipitation of carbonate concretions. The most common host rocks are mudstones/shales, sandstones, and limestones. This study presents an example of large carbonate concretions from an unusual host rock, the black bedded cherts of the Gufeng Formation (Guadalupian) at Enshi on the northern Yangtze Platform, South China. Petrographic observations (X‐ray diffraction, optical microscopy, scanning electron microscopy) and multiple geochemical analyses (pyrite‐ and carbonate‐associated‐sulfate (CAS)‐sulfur isotopes, carbon isotopes) indicate that (a) the studied carbonate concretion are mainly composed of micritic calcite with subordinate dolomite; (b) the concretions may have been mainly formed in the bacterial sulfate reduction (BSR) zone during very early diagenesis near the sediment–water surface; (c) the paleo‐bottom water overlying the sediments during formation of the concretions was mainly euxinic; and (d) the growth of the studied concretions proceeded via a pervasive model, where later cementation phase initiated in the lower part of the concretions and progressed upward.     

Sulfur stable isotope signature identifies the source of reduced sulfur in benthic communities in macrophyte zones of Lake Biwa, Japan

Carbon, nitrogen and sulfur stable isotope ratios (δ¹³C, δ¹⁵N and δ³⁴S) were measured from water column sulfate, sediments, particulate organic matter (POM), macrophytes, periphyton, macroinvertebrates and fish, sampled from the littoral, open water and macrophyte zones of Lake Biwa. In most of the littoral zones, the δ¹³C and δ¹⁵N values of organisms indicated that POM and periphyton support the consumers. However, in the dysoxic interior macrophyte (IM) zone, the δ¹³C values of Sinotaia quadrata histrica, Propsilocerus akamusi and Anodonta woodiana were lower than that of all resources. The δ¹⁵N values of S. quadrata histrica were lower than those of P. akamusi and A. woodiana. δ¹³C and δ¹⁵N values thus failed to distinguish the foods of these consumers. The δ³⁴S values of sediment and S. quadrata histrica were lower than those of water column sulfate, suggesting that this consumer incorporated reduced sulfur derived from sulfate reduction in the sediment by ingesting detritus. In contrast, the δ³⁴S values of P. akamusi and A. woodiana were higher than that of S. quadrata histrica, suggesting that they incorporated sulfur derived from water column sulfate by ingesting POM. Consequently, δ¹³C, δ¹⁵N and δ³⁴S signatures provide complementary estimates of foods for consumers in this freshwater lake.     

Pyrite formation in marshes during early diagenesis

Pyrite was removed from peat cores by draining the sediments and allowing the pyrite to oxidize. Then the peat cores were placed back into intertidal salt marsh sediments to incubate. Pyrite accumulated rapidly in peat incubated in situ. A greater accumulation of pyrite was observed in peat that contained living grass than peat in which the grass had been killed.

Resin‐imbedded samples of peat from nearby sediments showed that small single crystals of pyrite were abundant, supporting the idea that pyrite in marshes forms rapidly through direct precipitation. Pyrite was also observed filling vascular channels in roots. It had been proposed that pyrite fills root channels in freshwater environments where the primary sulfur source used by sulfate‐reducing bacteria is organic sulfur rather than sulfate. The widespread occurrence of pyrite filling vascular channels in salt marsh peat makes it unlikely that pyrite morphology can be used to infer the salinity of the overlying water.

Marsh sediments are characterized by higher carbon/sulfur ratios and pyritization (Fe‐pyritel(Fe‐pyrite + Fe‐HCl)) indices than marine subtidal sediments. Within wide ranges these indices do not seem to be very sensitive to salinity of flooding water or carbon concentrations in sediments. Oxidation and iron availability appear to be the major controls on pyrite accumulation in marshes. While pyrite concentrations in submerged sediments can be used as indicators of relative rates of sulfate reduction, sulfur storage in intertidal marsh sediments is not as tightly linked to this microbial process.     

Stable isotope and trace element geochemistry of Upper Cretaceous carbonates and belemnite rostra (Middle Campanian, north Germany)

Trace element contents and stable isotopic composition of Middle Campanian marl-limestone rhythmites and belemnite rostra of Belemnitella mucronata were investigated. High strontium and low iron as well as manganese and magnesium contents of belemnite calcite and bulk rock samples suggest no diagenetic overprint. However, the orange-coloured cathodoluminescence of coccolith-rich sediments indicates diagenetic cementation and/or recrystallization. The non-luminescent belemnite rostra reveal an extraordinary preservation of the microstructures that is interpreted to have been favoured by a silification of the outer rim of the belemnite rostra. Carbon isotope ratios of the coccolith limestones and belemnite rostra are comparable, with higher δ¹³C variations observed for belemnite calcite. The 1.5-2‰ depletion in δ¹⁸O of the marl-limestone rhythmites relative to belemnite calcite is explained by diagenetic alteration of the sediments. Palaeotemperatures, calculated from the δ¹⁸O values of the well-preserved belemnite rostra, are around 12.5 ± 2 °C and suggest rather low sea-surface temperatures for the Middle Campanian epicontinental sea of north Germany assuming a water depth of less than 100 m.     

Oxygen isotopic composition of sulfate in deep sea pore fluid: evidence for rapid sulfur cycling

We present new data of oxygen isotopes in marine sulfate (δ¹⁸O_SO4) in pore fluid profiles through organic‐rich deep‐sea sediments from 11 ODP sites around the world. In almost all sites studied sulfate is depleted with depth, through both organic matter oxidation and anaerobic methane oxidation. The δ¹⁸O_SO4 increases rapidly near the top of the sediments, from seawater values of 9 to maxima between 22 and 25, and remains isotopically heavy and constant at these values with depth. The δ¹⁸O_SO4 in these pore fluid profiles is decoupled from variations in sulfur isotopes measured on the same sulfate samples (δ³⁴S_SO4); the δ³⁴S_SO4 increases continuously with depth and exhibits a shallower isotopic increase. This isotopic decoupling between the δ³⁴S_SO4 and the δ¹⁸O_SO4 is hard to reconcile with the traditional understanding of bacterial sulfate reduction in sediments. Our data support the idea that sulfate or sulfite and water isotopically exchange during sulfate reduction and that some of the isotopically altered sulfur pool returns to the environment. We calculate that the rapid increase in the δ¹⁸O_SO4 in the upper part of these sediments requires rates of this oxygen isotope exchange that are several orders of magnitude higher than the rates of net sulfate reduction calculated from the sulfate concentration profiles and supported by the δ³⁴S_SO4. We suggest several mechanisms by which this may occur, including ‘net‐zero’ sulfur cycling, as well as further experiments through which we can test and resolve these processes.