A Zn isotope perspective on the rise of continents

Zinc isotope abundances are fairly constant in igneous rocks and shales and are left unfractionated by hydrothermal processes at pH < 5.5. For that reason, Zn isotopes in sediments can be used to trace the changing chemistry of the hydrosphere. Here, we report Zn isotope compositions in Fe oxides from banded iron formations (BIFs) and iron formations of different ages. Zinc from early Archean samples is isotopically indistinguishable from the igneous average (δ⁶⁶Zn ~0.3‰). At 2.9–2.7 Ga, δ⁶⁶Zn becomes isotopically light (δ⁶⁶Zn < 0‰) and then bounces back to values >1‰ during the ~2.35 Ga Great Oxygenation Event. By 1.8 Ga, BIF δ⁶⁶Zn has settled to the modern value of FeMn nodules and encrustations (~0.9‰). The Zn cycle is largely controlled by two different mechanisms: Zn makes strong complexes with phosphates, and phosphates in turn are strongly adsorbed by Fe hydroxides. We therefore review the evidence that the surface geochemical cycles of Zn and P are closely related. The Zn isotope record echoes Sr isotope evidence, suggesting that erosion starts with the very large continental masses appearing at ~2.7 Ga. The lack of Zn fractionation in pre‐2.9 Ga BIFs is argued to reflect the paucity of permanent subaerial continental exposure and consequently the insignificant phosphate input to the oceans and the small output of biochemical sediments. We link the early decline of δ⁶⁶Zn between 3.0 and 2.7 Ga with the low solubility of phosphate in alkaline groundwater. The development of photosynthetic activity at the surface of the newly exposed continents increased the oxygen level in the atmosphere, which in turn triggered acid drainage and stepped up P dissolution and liberation of heavy Zn into the runoff. Zinc isotopes provide a new perspective on the rise of continents, the volume of carbonates on continents, changing weathering conditions, and compositions of the ocean through time.     

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.     

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 diversity and iron oxidation at Okuoku‐hachikurou Onsen,a Japanese hot spring analog of Precambrian iron formations

Banded iron formations (BIFs) are rock deposits common in the Archean and Paleoproterozoic (and regionally Neoproterozoic) sedimentary successions. Multiple hypotheses for their deposition exist, principally invoking the precipitation of iron via the metabolic activities of oxygenic, photoferrotrophic, and/or aerobic iron‐oxidizing bacteria. Some isolated environments support chemistry and mineralogy analogous to processes involved in BIF deposition, and their study can aid in untangling the factors that lead to iron precipitation. One such process analog system occurs at Okuoku‐hachikurou (OHK) Onsen in Akita Prefecture, Japan. OHK is an iron‐ and CO₂‐rich, circumneutral hot spring that produces a range of precipitated mineral textures containing fine laminae of aragonite and iron oxides that resemble BIF fabrics. Here, we have performed 16S rRNA gene amplicon sequencing of microbial communities across the range of microenvironments in OHK to describe the microbial diversity present and to gain insight into the cycling of iron, oxygen, and carbon in this ecosystem. These analyses suggest that productivity at OHK is based on aerobic iron‐oxidizing Gallionellaceae. In contrast to other BIF analog sites, Cyanobacteria, anoxygenic phototrophs, and iron‐reducing micro‐organisms are present at only low abundances. These observations support a hypothesis where low growth yields and the high stoichiometry of iron oxidized per carbon fixed by aerobic iron‐oxidizing chemoautotrophs like Gallionellaceae result in accumulation of iron oxide phases without stoichiometric buildup of organic matter. This system supports little dissimilatory iron reduction, further setting OHK apart from other process analog sites where iron oxidation is primarily driven by phototrophic organisms. This positions OHK as a study area where the controls on primary productivity in iron‐rich environments can be further elucidated. When compared with geological data, the metabolisms and mineralogy at OHK are most similar to specific BIF occurrences deposited after the Great Oxygenation Event, and generally discordant with those that accumulated before it.     

Local paleoenvironmental controls on the carbon‐isotope record defining the Bitter Springs Anomaly

Large magnitude (>10‰) carbon‐isotope (δ¹³C) excursions recorded in carbonate‐bearing sediments are increasingly used to monitor environmental change and constrain the chronology of the critical interval in the Neoproterozoic stratigraphic record that is timed with the first appearance and radiation of metazoan life. The ~10‰ Bitter Springs Anomaly preserved in Tonian‐aged (1000–720 Ma) carbonate rocks in the Amadeus Basin of central Australia has been offered as one of the best preserved examples of a primary marine δ¹³C excursion because it is regionally reproducible and δ¹³C values covary in organic and carbonate carbon arguing against diagenetic exchange. However, here we show that δ¹³C values defining the excursion coincide with abrupt lithofacies changes between regularly cyclic grainstone and microbial carbonates, and desiccated red bed mudstones with interbedded evaporite and dolomite deposits, recording local environmental shifts from restricted marine conditions to alkaline lacustrine and playa settings that preserve negative (?4‰) and positive (+6‰) δ¹³C values, respectively. The stratigraphic δ¹³C pattern in both organic and carbonate carbon recurs within the basin in a similar way to associated sedimentary facies, reflecting the linkage of local paleoenvironmental conditions and δ¹³C values. These local excursions may be time transgressive or record a relative sea‐level influence manifest through exposure of sub‐basins isolated by sea‐level fall below shallow sills, but are independent of secular seawater variation. As the shallow intracratonic setting of the Bitter Springs Formation is typical of other Neoproterozoic carbonate successions used to construct the present δ¹³C seawater record, it identifies the potential for local influences on δ¹³C excursions that are neither diagenetic nor representative of the global exogenic cycle.     

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.     

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.     

Mass-independent fractionation of sulfur isotopes in sulfides from the pre-3770 Ma Isua Supracrustal Belt, West Greenland

Redox chemistry of the coupled atmosphere–hydrosphere system has coevolved with the biosphere, from global anoxia in the Archean to an oxygenated Proterozoic surface environment. However, to trace these changes to the very beginning of the rock record presents special challenges. All known Eoarchean (c. 3850–3600 Ma) volcanosedimentary successions (i.e. supracrustal rocks) are restricted to high‐grade gneissic terranes that seldom preserve original sedimentary structures and lack primary organic biomarkers. Although complicated by metamorphic overprinting, sulfur isotopes from Archean supracrustal rocks have the potential to preserve signatures of both atmospheric chemistry and metabolic fractionation from the original sediments. We present a synthesis of multiple sulfur isotope measurements (³²S, ³³S and ³⁴S) performed on sulfides from amphibolite facies banded iron‐formations (BIFs) and ferruginous garnet‐biotite (metapelitic) schists from the pre‐3770 Ma Isua Supracrustal Belt (ISB) in West Greenland. Because these data come from some of the oldest rocks of interpretable marine sedimentary origin, they provide the opportunity to (i) explore for possible biosignatures of sulfur metabolisms in early life; (ii) assess changes in atmospheric redox chemistry from ～3.8 Ga; and (iii) lay the groundwork to elucidate sulfur biogeochemical cycles on the early Earth. We find that sulfur isotope results from Isua do not unambiguously indicate microbially induced sulfur isotopic fractionation at that time. A significantly expanded data set of Δ³³S analyses for Isua dictates that the atmosphere was devoid of free oxygen at time of deposition and also shows that the effects of post‐depositional metamorphic remobilization and/or dilution can be traced in mass‐independently fractionated sulfur isotopes.     

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).     

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.     

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.     

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.     

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 abundance and composition in marine sediments beneath the Ross Ice Shelf,Antarctica

Marine sediments of the Ross Sea, Antarctica, harbor microbial communities that play a significant role in the decomposition, mineralization, and recycling of organic carbon (OC). In this study, the cell densities within a 153‐cm sediment core from the Ross Sea were estimated based on microbial phospholipid fatty acid (PLFA) concentrations and acridine orange direct cell counts. The resulting densities were as high as 1.7 × 10⁷ cells mL^?1 in the top ten centimeters of sediments. These densities are lower than those calculated for most near‐shore sites but consistent with deep‐sea locations with comparable sedimentation rates. The δ¹³C measurements of PLFAs and sedimentary and dissolved carbon sources, in combination with ribosomal RNA (SSU rRNA) gene pyrosequencing, were used to infer microbial metabolic pathways. The δ¹³C values of dissolved inorganic carbon (DIC) in porewaters ranged downcore from ?2.5‰ to ?3.7‰, while δ¹³C values for the corresponding sedimentary particulate OC (POC) varied from ?26.2‰ to ?23.1‰. The δ¹³C values of PLFAs ranged between ?29‰ and ?35‰ throughout the sediment core, consistent with a microbial community dominated by heterotrophs. The SSU rRNA gene pyrosequencing revealed that members of this microbial community were dominated by β‐, δ‐, and γ‐Proteobacteria, Actinobacteria, Chloroflexi and Bacteroidetes. Among the sequenced organisms, many appear to be related to known heterotrophs that utilize OC sources such as amino acids, oligosaccharides, and lactose, consistent with our interpretation from δ¹³C_PLFA analysis. Integrating phospholipids analyses with porewater chemistry, δ¹³C_DIC and δ¹³C_POC values and SSU rRNA gene sequences provides a more comprehensive understanding of microbial communities and carbon cycling in marine sediments, including those of this unique ice shelf environment.     

The role of microaerophilic Fe‐oxidizing micro‐organisms in producing banded iron formations

Despite the historical and economic significance of banded iron formations (BIFs), we have yet to resolve the formation mechanisms. On modern Earth, neutrophilic microaerophilic Fe‐oxidizing micro‐organisms (FeOM) produce copious amounts of Fe oxyhydroxides, leading us to wonder whether similar organisms played a role in producing BIFs. To evaluate this, we review the current knowledge of modern microaerophilic FeOM in the context of BIF paleoenvironmental studies. In modern environments wherever Fe(II) and O₂ co‐exist, microaerophilic FeOM proliferate. These organisms grow in a variety of environments, including the marine water column redoxcline, which is where BIF precursor minerals likely formed. FeOM can grow across a range of O₂ concentrations, measured as low as 2 μm to date, although lower concentrations have not been tested. While some extant FeOM can tolerate high O₂ concentrations, many FeOM appear to prefer and thrive at low O₂ concentrations (~3–25 μm ). These are similar to the estimated dissolved O₂ concentrations in the few hundred million years prior to the ‘Great Oxidation Event’ (GOE). We compare biotic and abiotic Fe oxidation kinetics in the presence of varying levels of O₂ and show that microaerophilic FeOM contribute substantially to Fe oxidation, at rates fast enough to account for BIF deposition. Based on this synthesis, we propose that microaerophilic FeOM were capable of playing a significant role in depositing the largest, most well‐known BIFs associated with the GOE, as well as afterward when global O₂ levels increased.     

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.     

Dissolved silica affects the bulk iron redox state and recrystallization of minerals generated by photoferrotrophy in a simulated Archean ocean

Chemical sedimentary deposits called Banded Iron Formations (BIFs) are one of the best surviving records of ancient marine (bio)geochemistry. Many BIF precursor sediments precipitated from ferruginous, silica-rich waters prior to the Great Oxidation Event at ~2.43 Ga. Reconstructing the mineralogy of BIF precursor phases is key to understanding the coevolution of seawater chemistry and early life. Many models of BIF deposition invoke the activity of Fe(II)-oxidizing photoautotrophic bacteria as a mechanism for precipitating mixed-valence Fe(II,III) and/or fully oxidized Fe(III) minerals in the absence of molecular oxygen. Although the identity of phases produced by ancient photoferrotrophs remains debated, laboratory experiments provide a means to explore what their mineral byproducts might have been. Few studies have thoroughly characterized precipitates produced by photoferrotrophs in settings representative of Archean oceans, including investigating how residual Fe(II)_aq can affect the mineralogy of expected solid phases. The concentration of dissolved silica (Si) is also an important variable to consider, as silicate species may influence the identity and reactivity of Fe(III)-bearing phases. To address these uncertainties, we cultured Rhodopseudomonas palustris TIE-1 as a photoferrotroph in synthetic Archean seawater with an initial [Fe(II)_aq] of 1 mM and [Si] spanning 0–1.5 mM. Ferrihydrite was the dominant precipitate across all Si concentrations, even with substantial Fe(II) remaining in solution. Consistent with other studies of microbial iron oxidation, no Fe-silicates were observed across the silica gradient, although Si coprecipitated with ferrihydrite via surface adsorption. More crystalline phases such as lepidocrocite and goethite were only detected at low [Si] and are likely products of Fe(II)-catalyzed ferrihydrite transformation. Finally, we observed a substantial fraction of Fe(II) in precipitates, with the proportion of Fe(II) increasing as a function of [Si]. These experimental results suggest that photoferrotrophy in a Fe(II)-buffered ocean may have exported Fe(II,III)-oxide/silica admixtures to BIF sediments, providing a more chemically diverse substrate than previously hypothesized.     

Mineralized microbialites as archives of environmental evolution,Laguna Negra,Catamarca Province,Argentina

Environmental fluctuations are recorded in a variety of sedimentary archives of lacustrine depositional systems. Geochemical signals recovered from bottom sediments in closed‐basin lakes are among the most sensitive paleoenvironmental indicators and are commonly used in reconstructing lake evolution. Microbialites (i.e., organosedimentary deposits accreted through microbial trapping and binding of detrital sediment or in situ mineral precipitation on organics [Palaios, 2, 1987, 241]), however, have been largely overlooked as paleoenvironmental repositories. Here, we investigate concentrically laminated mineralized microbialites from Laguna Negra, a high‐altitude (4,100 m above sea level) hypersaline, closed‐basin lake in northwestern Argentina, and explore the potential for recovery of environmental signals from these unique sedimentary archives. Spatial heterogeneity in hydrological regime helps define zones inside Laguna Negra, each with their own morphologically distinct microbialite type. Most notably, platey microbialites (in Zone 3A) are precipitated by evaporative concentration processes, while discoidal oncolites (in Zone 3C) are interpreted result from fluid mixing and biologically mediated nucleation. This spatial heterogeneity is reflected in petrographically distinct carbonate fabrics: micritic, botryoidal, and isopachous. Fabric type is interpreted to reflect a combination of physical and biological influences during mineralization, and paired C‐isotope measurement of carbonate and organic matter supports ecological differences as a dominant control on C‐isotopic evolution between zones. Laminae of Laguna Negra microbialites preserve a range of δ¹³C_carb from +5.75‰ to +18.25‰ and δ¹⁸O_carb from ?2.04‰ to +9.28‰. Temporal trends of lower carbon and oxygen isotopic compositions suggest that the influence of CO₂ degassing associated with evaporation has decreased over time. Combined, these results indicate that microbialite archives can provide data that aid in interpretation of both lake paleohydrology and paleoenvironmental change.     

Iron-mediated anaerobic ammonium oxidation recorded in the early Archean ferruginous ocean

The nitrogen isotopic composition of organic matter is controlled by metabolic activity and redox speciation and has therefore largely been used to uncover the early evolution of life and ocean oxygenation. Specifically, positive δ¹⁵N values found in well-preserved sedimentary rocks are often interpreted as reflecting the stability of a nitrate pool sustained by water column partial oxygenation. This study adds much-needed data to the sparse Paleoarchean record, providing carbon and nitrogen concentrations and isotopic compositions for more than fifty samples from the 3.4 Ga Buck Reef Chert sedimentary deposit (BRC, Barberton Greenstone Belt). In the overall anoxic and ferruginous conditions of the BRC depositional environment, these samples yield positive δ¹⁵N values up to +6.1‰. We argue that without a stable pool of nitrates, these values are best explained by non-quantitative oxidation of ammonium via the Feammox pathway, a metabolic co-cycling between iron and nitrogen through the oxidation of ammonium in the presence of iron oxides. Our data contribute to the understanding of how the nitrogen cycle operated under reducing, anoxic, and ferruginous conditions, which are relevant to most of the Archean. Most importantly, they invite to carefully consider the meaning of positive δ¹⁵N signatures in Archean sediments.     

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.