The Neoarchaean surficial sulphur cycle: An alternative hypothesis based on analogies with 20th‐century atmospheric lead

We revisit the S‐isotope systematics of sedimentary pyrite in a shaly limestone from the ca. 2.52 Ga Gamohaan Formation, Upper Campbellrand Subgroup, Transvaal, South Africa. The analysed rock is interpreted to have been deposited in a water depth of ca. 50–100 m, in a restricted sub‐basin on a drowning platform. A previous study discovered that the pyrites define a nonzero intercept δ³⁴S_V‐CDT–Δ³³S data array. The present study carried out further quadruple S‐isotope analyses of pyrite, confirming and expanding the linear δ³⁴S_V‐CDT–Δ³³S array with an δ³⁴S zero intercept at ?³³S ca. +5. This was previously interpreted to indicate mixing of unrelated S‐sources in the sediment environment, involving a combination of recycled sulphur from sulphides that had originally formed by sulphate‐reducing bacteria, along with elemental sulphur. Here, we advance an alternative explanation based on the recognition that the Archaean seawater sulphate concentration was likely very low, implying that the Archaean ocean could have been poorly mixed with respect to sulphur. Thus, modern oceanic sulphur systematics provide limited insight into the Archaean sulphur cycle. Instead, we propose that the 20th‐century atmospheric lead event may be a useful analogue. Similar to industrial lead, the main oceanic input of Archaean sulphur was through atmospheric raindown, with individual giant point sources capable of temporally dominating atmospheric input. Local atmospheric S‐isotope signals, of no global significance, could thus have been transmitted into the localised sediment record. Thus, the nonzero intercept δ³⁴S_V‐CDT–Δ³³S data array may alternatively represent a very localised S‐isotope signature in the Neoarchaean surface environment. Fallout from local volcanic eruptions could imprint recycled MIF‐S signals into pyrite of restricted depositional environments, thereby avoiding attenuation of the signal in the subdued, averaged global open ocean sulphur pool. Thus, the superposition of extreme local S‐isotope signals offers an alternative explanation for the large Neoarchaean MIF‐S excursions and asymmetry of the Δ³³S rock 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.     

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.     

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.     

Sulfur isotope's signal of nanopyrites enclosed in 2.7 Ga stromatolitic organic remains reveal microbial sulfate reduction

Microbial sulfate reduction (MSR) is thought to have operated very early on Earth and is often invoked to explain the occurrence of sedimentary sulfides in the rock record. Sedimentary sulfides can also form from sulfides produced abiotically during late diagenesis or metamorphism. As both biotic and abiotic processes contribute to the bulk of sedimentary sulfides, tracing back the original microbial signature from the earliest Earth record is challenging. We present in situ sulfur isotope data from nanopyrites occurring in carbonaceous remains lining the domical shape of stromatolite knobs of the 2.7‐Gyr‐old Tumbiana Formation (Western Australia). The analyzed nanopyrites show a large range of δ³⁴S values of about 84‰ (from ?33.7‰ to +50.4‰). The recognition that a large δ³⁴S range of 80‰ is found in individual carbonaceous‐rich layers support the interpretation that the nanopyrites were formed in microbial mats through MSR by a Rayleigh distillation process during early diagenesis. An active microbial cycling of sulfur during formation of the stromatolite may have facilitated the mixing of different sulfur pools (atmospheric and hydrothermal) and explain the weak mass independent signature (MIF‐S) recorded in the Tumbiana Formation. These results confirm that MSR participated actively to the biogeochemical cycling of sulfur during the Neoarchean and support previous models suggesting anaerobic oxidation of methane using sulfate in the Tumbiana environment.     

Multiple sulphur isotopic interpretations of biosynthetic pathways: implications for biological signatures in the sulphur isotope record

Isotopic fractionations produced by biosynthetic processes are the result of networks of individual biochemical reactions that operate at differing efficiencies and with distinct fractionation factors. These reaction networks determine the magnitude and direction of the net isotopic fractionation associated with a given process. Here we examine the ways that biological reaction networks control mass‐dependent isotopic fractionations of multiple sulphur isotopes. We describe how material‐flow through some networks can produce characteristic multiple‐sulphur‐isotope signatures that differ from those produced by their constituent steps and demonstrate that experimental results with Archaeaglobus fulgidus can be evaluated using multiple sulphur isotopes in the context of previously published models for dissimilatory sulphate reduction. Our evaluation of these data is consistent with the interpretation that the dependence of sulphur isotope fractionation on external sulphate concentration is rooted in differences between the forward and reverse  ? adenosine‐5′‐phosphosulphate (APS) ?  steps. The framework provided by our analysis has the potential to evaluate the biosynthetic pathways that produce the isotopic fractionations, to isolate the primary sources of isotopic fractionations (sulphate reduction or disproportionation reactions) and to establish criteria to identify the signature of specific sulphur metabolisms in the geological record. The results highlight the new types of information that can be obtained by including measurements of δ³³S {δ³³S = [(³³S/³²S)_sample/(³³S/³²S)_reference ? 1]*1000} with measurements of δ³⁴S.     

Biogeochemical probing of microbial communities in a basalt‐hosted hot spring at Kverkfjöll volcano,Iceland

We investigated bacterial and archaeal communities along an ice‐fed surficial hot spring at Kverkfjöll volcano—a partially ice‐covered basaltic volcano at Vatnajökull glacier, Iceland, using biomolecular (16S rRNA, apsA, mcrA, amoA, nifH genes) and stable isotope techniques. The hot spring environment is characterized by high temperatures and low dissolved oxygen concentrations at the source (68°C and <1 mg/L (±0.1%)) changing to lower temperatures and higher dissolved oxygen downstream (34.7°C and 5.9 mg/L), with sulfate the dominant anion (225 mg/L at the source). Sediments are comprised of detrital basalt, low‐temperature alteration phases and pyrite, with <0.4 wt. % total organic carbon (TOC). 16S rRNA gene profiles reveal that organisms affiliated with Hydrogenobaculum (54%–87% bacterial population) and Thermoproteales (35%–63% archaeal population) dominate the micro‐oxic hot spring source, while sulfur‐oxidizing archaea (Sulfolobales, 57%–82%), and putative sulfur‐oxidizing and heterotrophic bacterial groups dominate oxic downstream environments. The δ¹³C_org (‰ V‐PDB) values for sediment TOC and microbial biomass range from ?9.4‰ at the spring's source decreasing to ?12.6‰ downstream. A reverse effect isotope fractionation of ~3‰ between sediment sulfide (δ³⁴S ~0‰) and dissolved water sulfate (δ³⁴S +3.2‰), and δ¹⁸O values of ~ ?5.3‰ suggest pyrite forms abiogenically from volcanic sulfide, followed by abiogenic and microbial oxidation. These environments represent an unexplored surficial geothermal environment analogous to transient volcanogenic habitats during putative “snowball Earth” scenarios and volcano–ice geothermal environments on Mars.     

Micro-scale sulphur isotope evidence for sulphur cycling in the late Archean shallow ocean

We report in situ secondary ion mass spectrometer sulphur isotope data for sedimentary pyrite from the 2.52 Ga Upper Campbellrand Subgroup, Transvaal, South Africa. The analysed sedimentary rocks represent a transition in depositional environment from very shallow to deeper water, with strong sedimentological, facies distribution and geochemical evidence for the presence of a shallow redox chemocline. Data were obtained directly in thin section in order to preserve petrographic context. They reveal a very large extent of isotopic fractionation both in mass‐independent (MIF) and in mass‐dependent fractionation (MDF) on unprecedentedly small scale. In the shallow‐water microbical carbonates, three types of pyrite were identified. The texturally oldest pyrite is found as small, isotopically little fractionated grains in the microbial mats. Large (several mm) spheroidal pyrite concretions, which postdate the mat pyrite, record strong evidence for an origin by bacterial sulphate reduction. Rare pyrite surrounding late fenestral calcite is inferred to have formed from recycled bacterial pyrite on account of the slope of its correlated MIF and MDF array. This latter type of pyrite was also found in an interbedded black shale and a carbonate laminite. In a deeper water chert, pyrite with very heavy sulphur indicates partial to almost complete sulphate reduction across a chemocline whose existence has been inferred independently. The combined picture from all the studied samples is that of a sulphate availability‐limited environment, in which sulphur was cycled between reservoirs according to changing redox conditions established across the chemocline. Cycling apparently reduced the extent of recorded sulphur isotope fractionation relative to what is expected from projection in the correlated MIF and MDF arrays. This is consistent with regionally relatively high free oxygen concentrations in the shallow water, permitting locally strong MDF. Our new observations add to the growing evidence for a complex, fluctuating evolution of free atmospheric oxygen between c. 2.7 Ga and 2.3 Ga.     

Isotopic biosignatures in carbonate‐rich,cyanobacteria‐dominated microbial mats of the Cariboo Plateau,B.C.

Photosynthetic activity in carbonate‐rich benthic microbial mats located in saline, alkaline lakes on the Cariboo Plateau, B.C. resulted in pCO₂ below equilibrium and δ¹³C_DIC values up to +6.0‰ above predicted carbon dioxide (CO₂) equilibrium values, representing a biosignature of photosynthesis. Mat‐associated δ¹³C_carb values ranged from ~4 to 8‰ within any individual lake, with observations of both enrichments (up to 3.8‰) and depletions (up to 11.6‰) relative to the concurrent dissolved inorganic carbon (DIC). Seasonal and annual variations in δ¹³C values reflected the balance between photosynthetic ¹³C‐enrichment and heterotrophic inputs of ¹³C‐depleted DIC. Mat microelectrode profiles identified oxic zones where δ¹³C_carb was within 0.2‰ of surface DIC overlying anoxic zones associated with sulphate reduction where δ¹³C_carb was depleted by up to 5‰ relative to surface DIC reflecting inputs of ¹³C‐depleted DIC. δ¹³C values of sulphate reducing bacteria biomarker phospholipid fatty acids (PLFA) were depleted relative to the bulk organic matter by ~4‰, consistent with heterotrophic synthesis, while the majority of PLFA had larger offsets consistent with autotrophy. Mean δ¹³C_org values ranged from ?18.7 ± 0.1 to ?25.3 ± 1.0‰ with mean Δ¹³C_inorg‐org values ranging from 21.1 to 24.2‰, consistent with non‐CO₂‐limited photosynthesis, suggesting that Precambrian δ¹³C_org values of ~?26‰ do not necessitate higher atmospheric CO₂ concentrations. Rather, it is likely that the high DIC and carbonate content of these systems provide a non‐limiting carbon source allowing for expression of large photosynthetic offsets, in contrast to the smaller offsets observed in saline, organic‐rich and hot spring microbial mats.     

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).     

Investigation of diachronic dietary patterns on the islands of Ibiza and Formentera, Spain: Evidence from sulfur stable isotope ratio analysis

We present sulfur isotope ratio measurements of bone collagen from animals (n = 75) and humans (n = 120) from five sites dating to four chronological periods (Chalcolithic, Punic, Late Antiquity‐Early Byzantine, and Islamic) from the Balearic Islands of Ibiza and Formentera, Spain. This study is a follow up to previously published δ¹³C and δ¹⁵N values by [Fuller et al.: Am J Phys Anthropol 143 (2010) 512–522] and focuses on using δ³⁴S values to better understand the dietary patterns of these populations through time and to possibly identify immigrants to these islands. The range of δ³⁴S values (10.5–17.8‰) observed for the animals was relatively broad, which suggests that a significant sea spray effect has added marine sulfates to the soils of Formentera and Ibiza. The mean δ³⁴S values of the different human populations were found to be: Chalcolithic (16.5 ± 1.4‰), Punic rural (13.6 ± 1.7‰), Punic urban (12.9 ± 1.8‰), Late Antiquity‐Early Byzantine (12.3 ± 2.1‰), and Islamic (9.1 ± 2.7‰). These human δ³⁴S results are similar to the animal data, a finding that supports the notion that there was little marine protein consumption by these societies and that the diet was mainly based on terrestrial resources. During the Punic and Late Antiquity‐Early Byzantine periods the δ³⁴S values were used to identify individuals in the population who likely were not born or raised on the islands. In addition, 18 of the 20 individuals analyzed from the Islamic period have δ³⁴S values that indicate that they were immigrants to Ibiza who died before acquiring the new local sulfur isotopic signature. Am J Phys Anthropol 2012. © 2012 Wiley Periodicals, Inc.     

A new occurrence of ambient inclusion trails from the ~1900‐million‐year‐old Gunflint Formation,Ontario: nanocharacterization and testing of potential formation mechanisms

Ambient inclusion trails (AITs) are tubular microstructures thought to form when a microscopic mineral crystal is propelled through a fine‐grained rock matrix. Here, we report a new occurrence of AITs from a fossilized microbial mat within the 1878‐Ma Gunflint Formation, at Current River, Ontario. The AITs are 1–15 μm in diameter, have pyrite as the propelled crystal, are infilled with chlorite and have been propelled through a microquartz (chert) or chlorite matrix. AITs most commonly originate at the boundary between pyrite‐ and chlorite‐rich laminae and chert‐filled fenestrae, with pyrite crystals propelled into the fenestrae. A subset of AITs originate within the fenestrae, rooted either within the chert or within patches of chlorite. Sulphur isotope data (³⁴S/³²S) obtained in situ from AIT pyrite have a δ³⁴S of ?8.5 to +8.0 ‰, indicating a maximum of ~30 ‰ fractionation from Palaeoproterozoic seawater sulphate (δ³⁴S ≈ +20 ‰). Organic carbon is common both at the outer margins of the fenestrae and in patches of chlorite where most AITs originate, and can be found in smaller quantities further along some AITs towards the terminal pyrite grain. We infer that pyrite crystals now found within the AITs formed via the action of heterotrophic sulphate‐reducing bacteria during early diagenesis within the microbial mat, as pore waters were becoming depleted in seawater sulphate. Gases derived from this process such as CO₂ and H₂S were partially trapped within the microbial mat, helping produce birds‐eye fenestrae, while rapid microquartz precipitation closed porosity. We propose that propulsion of the pyrite crystals to form AITs was driven by two complementary mechanisms during burial and low‐grade metamorphism: firstly, thermal decomposition of residual organic material providing CO₂, and potentially CH₄, as propulsive gases, plus organic acids to locally dissolve the microquartz matrix; and secondly, reactions involving clay minerals that potentially led to enhanced quartz solubility, plus increases in fluid and/or gas pressure during chlorite formation, with chlorite then infilling the AITs. This latter mechanism is novel and represents a possible way to generate AITs in environments lacking organic material.     

Pigment production and isotopic fractionations in continuous culture: okenone producing purple sulfur bacteria Part II

Okenone is a carotenoid pigment unique to certain members of Chromatiaceae, the dominant family of purple sulfur bacteria (PSB) found in euxinic photic zones. Diagenetic alteration of okenone produces okenane, the only recognized molecular fossil unique to PSB. The in vivo concentrations of okenone and bacteriochlorophyll a (Bchl a) on a per cell basis were monitored and quantified as a function of light intensity in continuous cultures of the purple sulfur bacterium Marichromatium purpuratum (Mpurp1591). We show that okenone‐producing PSB have constant bacteriochlorophyll to carotenoid ratios in light‐harvesting antenna complexes. The in vivo concentrations of Bchl a, 0.151 ± 0.012 fmol cell^?1, and okenone, 0.103 ± 0.012 fmol cell^?1, were not dependent on average light intensity (10–225 Lux) at both steady and non‐steady states. This observation revealed that in autotrophic continuous cultures of Mpurp1591, there was a constant ratio for okenone to Bchl a of 1:1.5. Okenone was therefore constitutively produced in planktonic cultures of PSB, regardless of light intensity. This confirms the legitimacy of okenone as a signature for autotrophic planktonic PSB and by extrapolation water column euxinia. We measured the δ¹³C, δ¹⁵N, and δ³⁴S bulk biomass values from cells collected daily and determined the isotopic fractionations of Mpurp1591. There was no statistical relationship in the bulk isotope measurements or stable isotope fractionations to light intensity or cell density under steady and non‐steady‐state conditions. The carbon isotope fractionation between okenone and Bchl a with respect to overall bulk biomass (¹³ε_{pigment – biomass}) was 2.2 ± 0.4‰ and ?4.1 ± 0.9‰, respectively. The carbon isotopic fractionation () for the production of pigments in PSB is more variable than previously thought with our reported values for okenone at ?15.5 ± 1.2‰ and ?21.8 ± 1.7‰ for Bchl a.     

Multiple sulphur isotope record of Paleoarchean sedimentary rocks across the Onverwacht Group,Barberton Greenstone Belt,South Africa

This study presents multiple sulphur isotope (³²S, ³³S, ³⁴S, ³⁶S) data on pyrites from silicified volcano-sedimentary rocks of the Paleoarchean Onverwacht Group of the Barberton greenstone belt, South Africa. These rocks include seafloor cherts and felsic conglomerates that were deposited in shallow marine environments preserving a record of atmospheric and biogeochemical conditions on the early Earth. A strong variation in mass independent sulphur isotope fractionation (MIF-S) anomalies is found in the cherts, with Δ³³S ranging between −0.26‰ and 3.42‰. We explore possible depositional and preservational factors that could explain some of this variation seen in MIF-S. Evidence for microbial activity is recorded by the c. 3.45 Ga Hooggenoeg Formation Chert (HC4) preserving a contribution of microbial sulphate reduction (−Δ³³S and –δ³⁴S), and a c. 3.33 Ga Kromberg Formation Chert (KC5) recording a possible contribution of microbial elemental sulphur disproportionation (+Δ³³S and –δ³⁴S). Pyrites from a rhyo-dacitic conglomerate of the Noisy Formation do not plot along a previously proposed global Felsic Volcanic Array, and this excludes short-lived pulses of intense felsic volcanic gas emissions as the dominant control on Archean MIF-S. Rather, we suggest that the MIF-S signals measured reflect dilution during marine deposition, early diagenetic modification, and mixing with volcanic/hydrothermal S sources. Given the expanded stratigraphic interval (3.47–3.22 Ga) now sampled from across the Barberton Supergroup, we conclude that large MIF-S exceeding >4‰ is atypical of Paleoarchean near-surface environments on the Kaapvaal Craton.     

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.     

Chromium‐isotope signatures in scleractinian corals from the Rocas Atoll,Tropical South Atlantic

Chromium‐isotope compositions (expressed as δ⁵³Cr) of recent and ancient skeletal and non‐skeletal carbonates are currently explored as a (paleo‐) redox‐proxy for shallow seawater. The idea behind this approach is that biogenic and non‐biogenic carbonates could potentially be used as archives recording the Cr‐isotope composition of seawater in which they formed, and with this contribute to the reconstruction of past paleo‐environmental changes in the marine realm, and potentially to climate changes on land. However, investigations addressing the behavior and uptake mechanism of Cr, and the potential isotope fractionations between seawater and biogenic carbonates are scarce. Here, we present a study of Cr‐isotope variations in three species of corals and contemporary seawater from the Rocas Atoll, NE, Brazil. Cr‐isotope values of the studied coral species (Siderastrea stellata, Porites sp., and Montastrea cavernosa) vary from ?0.5 to +0.33‰ and point to significant isotopic disequilibrium with coexisting seawater characterized by a Cr‐isotope value of +0.92 ± 0.2‰. This isotopic offset requires reduction of hexavalent Cr(VI) in the sequestration process of all the studied coral species. Cr‐isotope values in a profile across an S. stellata colony returned homogeneous, slightly positively fractioned δ⁵³Cr values of +0.07 ± 0.08‰ (n = 8, 2σ), which we interpret to reflect a constant reductive uptake during the 20‐year growth period recorded in this coral. In contrast, samples across a 12‐year growth profile from Porites sp. display rather heterogeneous Cr‐isotope values with δ⁵³Cr varying from ?0.50 to +0.10‰, indicating Cr incorporation under changing redox processes during its growth intervals. We propose a mechanism whereby initial photoreduction of isotopically heavy Cr(VI) to isotopically lighter Cr(III) in the endodermal layer of corals must be followed by efficient and effective re‐oxidation of reduced Cr species to favor subsequent chromate () substitution during the calcifying processes ultimately leading to the formation of the coral skeleton.     

Biogeochemical cycling of sulphur in karst and transfer into speleothem archives at Grotta di Ernesto, Italy

Trace amounts of sulphur in speleothems suggest that stalagmites may act as archives of sulphur deposition, thereby recording aspects of atmospheric variability in sulphur content. Accurate interpretation of this novel sulphur archive depends upon understanding how biogeochemical cycling in the soil and epikarst above the cave may modify the precursor atmospheric values of sulphur concentration and isotopic composition prior to incorporation into the speleothem record. Dual isotope analysis of δ³⁴S-SO₄ and δ¹⁸O-SO₄ is used to trace biogeochemical transformations of atmospheric sulphur through the cave system at Grotta di Ernesto in the Italian Alps and builds towards a framework for interpretation of speleothem sulphur archives which depends on overlying ecosystem dynamics and karst hydrological properties. A three component model of atmospheric sulphate signal modification is proposed to be driven by (1). vegetation and soil cycling, (2). the degree of groundwater mixing in the karst aquifer; and (3). redox status. The relative influence of each process is specific to individual drip flow sites and associated stalagmites, rendering each sulphur archive a unique signal of environmental conditions. Under conditions found in the soil and epikarst above Grotta di Ernesto, the dual isotope signatures of sulphate sulphur and oxygen incorporated into speleothem carbonate, closely reflect past conditions of industrial sulphur loading to the atmosphere and the extent of signal modification through biogeochemical cycling and aquifer mixing.     

Isotopic discrimination and kinetic parameters of RubisCO from the marine bloom‐forming diatom,Skeletonema costatum

The cosmopolitan, bloom‐forming diatom, Skeletonema costatum, is a prominent primary producer in coastal oceans, fixing CO₂ with ribulose 1,5‐bisphosphate carboxylase/oxygenase (RubisCO) that is phylogenetically distinct from terrestrial plant RubisCO. RubisCOs are subdivided into groups based on sequence similarity of their large subunits (IA–ID, II, and III). ID is present in several major oceanic primary producers, including diatoms such as S. costatum, coccolithophores, and some dinoflagellates, and differs substantially in amino acid sequence from the well‐studied IB enzymes present in most cyanobacteria and in green algae and plants. Despite this sequence divergence, and differences in isotopic discrimination apparent in other RubisCO enzymes, stable carbon isotope compositions of diatoms and other marine phytoplankton are generally interpreted assuming enzymatic isotopic discrimination similar to spinach RubisCO (IB). To interpret phytoplankton δ¹³C values, S. costatum RubisCO was characterized via sequence analysis, and measurement of its K_CO2 and V_max, and degree of isotopic discrimination. The sequence of this enzyme placed it among other diatom ID RubisCOs. Michaelis‐Menten parameters were similar to other ID enzymes (K_CO2 = 48.9 ± 2.8 μm ; V_max = 165.1 ± 6.3 nmol min^?1 mg^?1). However, isotopic discrimination (ε = [¹²k/¹³k ? 1] × 1000) was low (18.5‰; 17.0–19.9, 95% CI) when compared to IA and IB RubisCOs (22–29‰), though not as low as ID from coccolithophore, Emiliania huxleyi (11.1‰). Variability in ε‐values among RubisCOs from primary producers is likely reflected in δ¹³C values of oceanic biomass. Currently, δ¹³C variability is ascribed to physical or chemical factors (e.g. illumination, nutrient availability) and physiological responses to these factors (e.g. carbon‐concentrating mechanisms). Estimating the importance of these factors from δ¹³C measurements requires an accurate ε‐value, and a mass‐balance model using the ε‐value for S. costatum RubisCO is presented. Clearly, appropriate ε‐values must be included in interpreting δ¹³C values of environmental samples.     

DIFFERENCES IN FORAGING LOCATION OF MEXICAN AND CALIFORNIA ELEPHANT SEALS: EVIDENCE FROM STABLE ISOTOPES IN PUPS

Female northern elephant seals, Mirounga angustirostris, from Año Nuevo (AN) in central California feed offshore in mid‐latitude waters (40°–55°N). Migratory patterns and foraging locations of seals from Mexico are unknown. Rookeries on San Benitos (SB) islands in Baja California Sur, Mexico, are ～1,170 km south of AN. Although the colonies are similar in size, seals from SB begin breeding earlier and have an earlier breeding birthing peak than seals from AN. To determine if the foraging location of seals from Mexico was similar to that of seals from California, we measured δ¹³C and δ¹⁵N values in the hair of 48 suckling pups at SB and 37 from AN, assuming that their isotopic signatures reflected those of mothers' milk, their exclusive diet. The mean δ¹³C and δ¹⁵N values for SB pups (?16.1‰± 0.9‰ and 17.7‰± 0.9‰, respectively) were significantly higher than those for AN pups (?17.6‰± 0.4‰ and 15.6‰± 1.0‰, respectively). From data on environmental isotope gradients and known behavior of SB and AN populations, we hypothesize that the isotope differences are due to females in the SB colony foraging ～8° south of seals from AN. This hypothesis can be tested by deployment of satellite tags on adult females from the SB colony.     

Large 13C/12C and small 15N/14N isotope fractionation in an experimental detrital foodweb (litter–fungi–collembolans)

Correctly estimating the trophic fractionation factors (Δ¹⁵N and Δ¹³C) in controlled laboratory conditions is essential for the application of stable isotope analysis in studies on the trophic structure of soil communities. Laboratory experiments usually suggest large ¹⁵N/¹⁴N and small ¹³C/¹²C trophic fractionation, but in field studies litter-dwelling microarthropods and other invertebrates are consistently enriched in ¹³C relative to plant litter. In the present study, we report data from two laboratory experiments investigating both fungi–collembolans and litter–fungi–collembolans systems. In the fungi–collembolans system, Δ¹⁵N and Δ¹³C averaged 1.4 ± 0.1 and 1.0 ± 0.2 ‰, respectively. In microcosms with fungi-inoculated litter, the difference in δ¹⁵N between collembolans and plant litter averaged 1.5 ± 0.2 ‰, confirming the relatively small ¹⁵N/¹⁴N trophic fractionation at the basal level of detrital foodwebs reported in numerous field studies. In full agreement with field observations, the difference in δ¹³C between bulk litter and collembolans in laboratory microcosms averaged 3.6 ± 0.1 ‰ and only little depended on collembolan species identities or the presence of water-soluble compounds in the litter. We conclude that increased δ¹³C values typical of litter-dwelling decomposers are largely determined by an increased ¹³C content in saprotrophic microorganisms.