Archean Sediments Research Articles

Reply to the comments on “Late Archean sedimentary basins in the northeastern Superior Province, Canada: Plume-generated crustal tears or syn-convergent accretionary belts?” of Svojtka et al. (2024)

Reply to the comments on “Late Archean sedimentary basins in the northeastern Superior Province, Canada: Plume-generated crustal tears or syn-convergent accretionary belts?” of Svojtka et al. (2024)

Effects of CO2 on the Nitrogen Isotopic Composition of Marine Diazotrophic Cyanobacteria

AbstractBiological N2 fixation has been crucial for sustaining early life on Earth. Very negative δ15N values detected in Archean sediments, which are not observed in present‐day environments, have been attributed to the low efficiency of proto‐nitrogenases. Alternatively, variations in early atmospheric CO2 may also play a role. Here we examine the effects of CO2 concentrations on the biomass δ15N signatures of the diazotrophs Trichodesmium erythraeum and Crocosphaera watsonii, which utilize Mo‐Fe nitrogenase (the most common form of the enzyme). Our results show that these organisms produce biomass with δ15N values up to ∼3‰ lower under both decreased and elevated CO2 concentrations compared to modern levels (∼380 μatm). These deviations from modern CO2 levels reduce nitrogenase enzyme efficiency, leading to increased organismal isotopic fractionation during N2 fixation. This study offers an alternative explanation for the observed fluctuations in geological δ15N records and provides new insights into the past nitrogen cycle on Earth.

Mechanisms of nitrogen isotope fractionation at an ancient black smoker in the 2.7 Ga Abitibi greenstone belt, Canada

Abstract The biological nitrogen (N) cycle on early Earth is enigmatic because of limited data from Archean (meta-)sediments and the potential alteration of primary biotic signatures. Here we further investigate unusual 15N enrichments reported in 2.7 Ga meta-sediments from the Abitibi greenstone belt, Canada, purportedly related to a 15N-enriched Archean atmosphere. Given that sediments from this region are contemporaneous with large-scale volcanogenic massive sulfide deposits, we utilize Cu and Zn contents to trace the effects of hydrothermal circulation on N isotope fractionation. We show that high δ15Nbulk values as high as +23‰ are associated with Cu-Zn mineralization, whereas unmineralized organic-rich shales exhibit much lower δ15Nbulk and δ15Nkerogen values. Moreover, we find a large offset between δ15Nbulk and δ15Nkerogen of as much as 17‰ and relate this to the addition of organic-bound N during the late-stage emplacement of organic-rich veins. We conclude that the previously reported high δ15N values are most parsimoniously explained by biotic and abiotic mechanisms rather than a 15N-enriched atmosphere. Crucially, both mechanisms require the presence of NH4+ in hydrothermal fluids, supporting the hypothesis that hydrothermal discharge was an important nutrient source for Neoarchean marine life.

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 δ15 N 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 δ15 N 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 δ15 N signatures in Archean sediments.

Near-zero 33S and 36S anomalies in Pitcairn basalts suggest Proterozoic sediments in the EM-1 mantle plume

Volcanic rocks erupted among Pitcairn seamounts sample a mantle plume that exhibits an extreme Enriched Mantle-1 signature. The origin of this peculiar mantle endmember remains contentious, and could involve the recycling of marine sediments of Archean or Proterozoic ages, delaminated units from the lower continental crust, or metasomatized peridotites from a lithospheric mantle. Here, we report the sulfur multi-isotopic signature (32S, 33S, 34S, 36S) of 15 fresh submarine basaltic glasses from three Pitcairn seamounts. We observe evidence for magmatic degassing of sulfur from melts erupted ∼2,000 meters below seawater level (mbsl). Sulfur concentrations are correlated with eruption depth, and range between 1300 ppm S (collected ∼ 2,500 mbsl) and 600 ppm S (∼2,000 mbsl). The δ34S values can be accounted for under equilibrium isotope fractionation during degassing, with αgas-melt between 1.0020 and 1.0001 and starting δ34S values between −0.9‰ and +0.6‰. The δ34S estimates are similar or higher than MORB signatures, suggesting the contribution of recycled sulfur with a ∼ 1‰ 34S enrichment compared to the Pacific upper mantle. The Δ33S and Δ36S signatures average at +0.024±0.007‰ and +0.02±0.07‰ vs. CDT, respectively (all 1σ). Only Δ33S is statistically different from MORB, by +0.02‰. The Δ33S enrichment is invariant across degassing and sulfide segregation. We suggest it reflects a mantle source enrichment rather than a high-temperature fractionation of S in the basalts. Despite the small magnitude of the 33S-36S variations, our data require a substantial amount of recycled sulfur overwhelm the Pitcairn mantle source. We show that models involving metasomatized peridotites, lower crust units, or Archean sediments, may be viable, but are restricted to narrow sets of circumstances. Instead, scenarios involving the contribution of Proterozoic marine sediments appear to be the most parsimonious explanation for the EM-1 signature at Pitcairn.

Stable W and Mo isotopic evidence for increasing redox-potentials from the Paleoarchean towards the Paleoproterozoic deep ocean

The scavenging of dissolved trace metals such as chromium (Cr), molybdenum (Mo) and tungsten (W) and their authigenic enrichment in sedimentary archives mainly occur by particle shuttling that cause resolvable isotope fractionation. Because their scavenging is also dependent on the local marine redox potential and the overall marine chemical environment, the stable isotope composition of these elements in Archean sediments is widely used as paleoredox proxy. Tungsten, a new element in this tool box, is dissolved as the oxyanion tungstate (WO42−) at extremely low marine redox potentials and can thus help to reconstruct the earliest changes in the marine redox state.We tested the applicability of stable W isotopes as a new paleoredox proxy of the atmosphere–ocean system. We analyzed the δ186/184W of Precambrian igneous rocks to investigate the detrital background δ186/184W signature. This Precambrian igneous inventory (PII) shows δ186/184W values between −0.007 and +0.097‰, identical to the range of modern igneous crustal rocks, which indicates that the average Earth’s crust δ186/184W remained constant over billions of years. Furthermore, we present δ186/184W values of euxinic sediments from the Black Sea as a modern sedimentary analog, which we then compare with δ186/184W data of Archean and Proterozoic black shales (3.47–2.3 Ga). Modern Black Sea sapropels reveal crustal-like δ186/184W values between +0.050 and +0.071‰ suggesting limited authigenic enrichment of W in euxinic environments. Similarly, post-Great Oxidation Event shales (2.3 Ga) show crustal- or PII-like δ186/184W values indicating the deposition in euxinic or oxic environment. However, the Archean shale sample suites show elevated W concentrations and fractionated δ186/184W values up to +0.246‰, which we attribute to authigenic enrichment of heavy seawater W in a ferruginous setting. Thus, black shales δ186/184W values can help to distinguish between anoxic-ferruginous and euxinic depositional environments.Furthermore, we combine previously published Cr and Mo isotope data with new Cr and W isotope data to quantify the evolution of the marine redox state during the Archean. By combining the Eh-pH stability fields of W and Mo with the δ186/184W and δ98/95Mo values of Precambrian sediments, we suggest a 3-step evolution of the Precambrian ocean. (1) From 3.47 to 3.0 Ga, the Eh of a dominantly ferruginous ocean was between −0.4 V and −0.25 V, enabling the persistence of soluble WO42− but not MoO42− and thus only the authigenic enrichment of isotopically fractionated W. (2) Starting from 3.0 Ga, the Eh of some shallow-marine environments increased above −0.25 V, as indicated by δ98/95Mo values above the detrital background in shallow-marine sediments, while the Eh of the deep ocean still remained below −0.25 V indicated by the lack of authigenic Mo enrichment and fractionated δ98/95Mo values in deep-sea shales. (3) After 2.7 Ga elevated δ98/95Mo and δ186/184W values in deep-marine sediments suggest that the Eh of the ferruginous open-ocean now also increased above −0.25 V. We therefore suggest that the combination of δ98/95Mo and δ186/184W values of shales is a very promising tool to investigate earliest changes in marine redox conditions during the Archean.

Geochemistry of the Neoproterozoic Mbondo-Ngazi Tina Metasediments, Adamawa Area, Central Cameroon: Source Provenance and Tectonic Setting

The Mbondo-Ngazi Tina area belongs to the Adamawa-Yade domain within the Pan-African Central Africa Fold Belt in Cameroon (CAFB). The basement of this area is dominated by metasedimentary rocks composed of sericite schist, chlorite schist and muscovite schist. Whole-rock geochemical compositions of these rocks were investigated in order to determine their provenance and tectonic setting. The studied metasedimentary rocks have SiO2 and Al2O3 contents comparable to the average composition of the Neoproterozoic upper continental crust (UCC). These rocks are strongly depleted in CaO, MgO, and enriched in K2O, Ba and Rb with respect to UCC, reflecting K addition during diagenesis. The CIA, CIW, PIA and the SiO2/Al2O3 and Th/U ratios indicated that these rocks had suffered varying degrees of weathering as the source rocks underwent mild to moderate chemical weathering. The PAAS-normalized REE patterns are almost flat with slightly LREE depletion with respect to HREE and null to weakly positive Eu anomalies. Their chondrite-normalized REE patterns are parallel to sub-parallel, LREE-enriched, and display distinct negative Eu anomalies and weakly fractionated HREE segments. Overall, they are geochemically mature and have suffered sedimentary recycling. They derived mainy from felsic to intermediate rocks with minor contamination of mafic rocks. The Mbondo-Ngazi Tina metasedimentary rocks show REE and trace element compositions similar to those of Archean sediments, suggesting that the continental crust of the study area during the early Proterozoic had chemical compositions similar to those of the Archean crust and were probably deposited in active to passive continental margin settings.

Tracing the Early Emergence of Microbial Sulfur Metabolisms

Hydrogen sulfide (nH2S) and sulfur oxide (SO n ; n = 1, 2, 3) gases in early Earth’s globally anoxic atmosphere were subjected to gas-phase chemical transformations by UV light. A principal photolysis pathway at that time produced elemental sulfur aerosols with mass-independently fractionated (MIF) isotopic values carrying variable minor isotope (33S, 36S) compositions. These rained into the sulfate-deficient Archean (ca. 3.85–2.5 Ga) oceans to react with [Fe2+]aq and form sedimentary sulfides. The MIF-bearing sulfides were incorporated into Archean sediments, including banded iron formations (BIF). Such aerosols may also have fueled microbial sulfur metabolisms, and thus are traceable by the MIF sulfur isotopes. Yet, data show that before ∼3.5 Ga mass-dependent34S/32S values in Early Archean sediments tend to fall within a narrow (±0.1%) range even as they carry mass-independent values. By about 3.5 Ga, 34S/32S values show much greater changes (>1%) in range congruent with microbial metabolic processing. Here, we trace probable pathways of elemental sulfur aerosols into Archean sediments, and couple our study with analysis of the evolutionary relationships of enzymes involved in sulfur metabolism to explain the observed trends. Our model explains why elemental sulfur aerosols were apparently not utilized by the Eoarchean (pre-3.65 Ga) biosphere even though an immediate precursor to the required enzyme may have already been present. Highlights Evolution of microbial sulfur metabolisms is tracked by multiple sulfur isotopes Alkaline hydrothermal vents were an abode for early microbial life Sulfite detoxification prompted anaerobic respiration Reversal of respiratory electron transport chain (ETC) stimulated photothiotrophy Surplus e- acceptors permitted the emergence of elemental sulfur reduction

Sulfur isotope behavior during metamorphism and anatexis of Archean sedimentary rocks: A case study from the Ghost Lake batholith, Ontario, Canada

Recycling of surface-derived sulfur into the deep earth can impart distinct sulfur isotope signatures to magmas. The details of sulfur transfer from sedimentary rocks to magmas (and ultimately igneous rocks) through metamorphism and devolatilization and/or partial melting, however, is difficult to trace. To understand this process in detail we studied multiple-sulfur isotope compositions of sulfides in the Archean (c. 2685 Ma) Ghost Lake batholith (GLB) and its surrounding host metasedimentary rocks of the Superior Craton (Ontario, Canada) by high spatial resolution secondary ion mass spectrometry, complemented by high-precision gas source isotope ratio mass spectrometry measurements. The GLB comprises strongly peraluminous biotite+cordierite, biotite+muscovite, and muscovite+garnet+tourmaline granites to leucogranites, which are thought to represent the partial melts of surrounding metagreywackes and metapelites. The metasedimentary rocks display a range of metamorphic grades increasing from biotite-chlorite (280-380°C) at ∼5 km away from the GLB to sillimanite-K-feldspar grade (∼660°C) immediately adjacent to the batholith, thus providing a natural experiment to understand sulfur isotope variations from low- to high-grade Archean sedimentary rocks, as well as granites representative of their partial melts.We find that metasedimentary sulfide δ34S values increase with progressive metamorphism at most 2-3‰ (from −1‰ up to +1 to +2‰). An increase in δ34S values in pyrrhotite during prograde metamorphism can be explained through Rayleigh fractionation during pyrite desulfidation reactions. Pyrite from all but one of the granite samples preserve δ34S values similar to that of the high-grade metasedimentary rocks, indicating that partial melting did not result in significant fractionation of δ34S. The exception to this is one granite sample from a part of the batholith characterized by abundant metasedimentary inclusions. This sample contains pyrite with heterogeneous and low δ34S values (down to −16‰) which likely resulted from incomplete homogenization of sulfur between the granitic melt and metasedimentary inclusions. Small (several tenths of a permil), mostly positive Δ33S are observed in both the metasedimentary rocks and granites.Our results suggest that Archean strongly peraluminous granites could be a high-fidelity archive to quantify the bulk sulfur isotope composition of the Archean siliciclastic sediments. Further, our findings indicate that subduction of reduced sulfur-bearing sediments in the Archean with δ34S at or near 0‰ should result in release of sulfur-bearing fluids in the mantle wedge with similar values (within a few permil). S-MIF (if initially present in Archean surface material) may be preserved during this process. However, the absence of S-MIF in igneous rocks does not preclude assimilation of Archean sedimentary material as either S-MIF may not be originally present in the Archean sedimentary sulfur and/or homogenization or dilution could obscure any S-MIF originally present in assimilated Archean sediments.

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. 3.22Ga 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 Δ33 Ssulfate values through the Paleoarchean.

Role of APS reductase in biogeochemical sulfur isotope fractionation

Sulfur isotope fractionation resulting from microbial sulfate reduction (MSR) provides some of the earliest evidence of life, and secular variations in fractionation values reflect changes in biogeochemical cycles. Here we determine the sulfur isotope effect of the enzyme adenosine phosphosulfate reductase (Apr), which is present in all known organisms conducting MSR and catalyzes the first reductive step in the pathway and reinterpret the sedimentary sulfur isotope record over geological time. Small fractionations may be attributed to low sulfate concentrations and/or high respiration rates, whereas fractionations greater than that of Apr require a low chemical potential at that metabolic step. Since Archean sediments lack fractionation exceeding the Apr value of 20‰, they are indicative of sulfate reducers having had access to ample electron donors to drive their metabolisms. Large fractionations in post-Archean sediments are congruent with a decline of favorable electron donors as aerobic and other high potential metabolic competitors evolved.

An appeal to magic? The discovery of a non-enzymatic metabolism and its role in the origins of life.

Until recently, prebiotic precursors to metabolic pathways were not known. In parallel, chemistry achieved the synthesis of amino acids and nucleotides only in reaction sequences that do not resemble metabolic pathways, and by using condition step changes, incompatible with enzyme evolution. As a consequence, it was frequently assumed that the topological organisation of the metabolic pathway has formed in a Darwinian process. The situation changed with the discovery of a non-enzymatic glycolysis and pentose phosphate pathway. The suite of metabolism-like reactions is promoted by a metal cation, (Fe(II)), abundant in Archean sediment, and requires no condition step changes. Knowledge about metabolism-like reaction topologies has accumulated since, and supports non-enzymatic origins of gluconeogenesis, the S-adenosylmethionine pathway, the Krebs cycle, as well as CO2 fixation. It now feels that it is only a question of time until essential parts of metabolism can be replicated non-enzymatically. Here, I review the ‘accidents’ that led to the discovery of the non-enzymatic glycolysis, and on the example of a chemical network based on hydrogen cyanide, I provide reasoning why metabolism-like non-enzymatic reaction topologies may have been missed for a long time. Finally, I discuss that, on the basis of non-enzymatic metabolism-like networks, one can elaborate stepwise scenarios for the origin of metabolic pathways, a situation that increasingly renders the origins of metabolism a tangible problem.

1H-NMR as implemented in several origin of life studies artificially implies the absence of metabolism-like non-enzymatic reactions by being signal-suppressed

Background. Life depends on small subsets of chemically possible reactions. A chemical process can hence be prebiotically plausible, yet be unrelated to the origins of life. An example is the synthesis of nucleotides from hydrogen cyanide, considered prebiotically plausible, but incompatible with metabolic evolution. In contrast, only few metabolism-compatible prebiotic reactions were known until recently. Here, we question whether technical limitations may have contributed to the situation. Methods: Enzymes evolved to accelerate and control biochemical reactions. This situation dictates that compared to modern metabolic pathways, precursors to enzymatic reactions have been slower and less efficient, yielding lower metabolite quantities. This situation demands for the application of highly sensitive analytical techniques for studying ‘proto-metabolism’. We noticed that a set of proto-metabolism studies derive conclusions from the absence of metabolism-like signals, yet do not report detection limits. We here benchmark the respective 1H-NMR implementation for the ability to detect Krebs cycle intermediates, considered examples of plausible metabolic precursors. Results: Compared to metabolomics ‘gold-standard’ methods, 1H-NMR as implemented is i) at least one hundred- to thousand-fold less sensitive, ii) prone to selective metabolite loss, and iii) subject to signal suppression by Fe(II) concentrations as extrapolated from Archean sediment. In sum these restrictions mount to huge sensitivity deficits, so that even highly concentrated Krebs cycle intermediates are rendered undetectable unless the method is altered to boost sensitivity. Conclusions 1H-NMR as implemented in several origin of life studies does not achieve the sensitivity to detect cellular metabolite concentrations, let alone evolutionary precursors at even lower concentration. These studies can hence not serve as proof-of-absence for metabolism-like reactions. Origin of life theories that essentially depend on this assumption, i.e. those that consider proto-metabolism to consist of non-metabolism-like reactions derived from non-metabolic precursors like hydrogen cyanide, may have been derived from a misinterpretation of negative analytical results.

Multiple sulfur isotopes monitor fluid evolution of an Archean orogenic gold deposit

The evolution of a gold-bearing hydrothermal fluid from its source to the locus of gold deposition is complex as it experiences rapid changes in thermochemical conditions during ascent through the crust. Although it is well established that orogenic gold deposits are generated during time periods of abundant crustal growth and/or reworking, the source of fluid and the thermochemical processes that control gold precipitation remain poorly understood. In situ analyses of multiple sulfur isotopes offer a new window into the relationship between source reservoirs of Au-bearing fluids and the thermochemical processes that occur along their pathway to the final site of mineralisation. Whereas δ34S is able to track changes in the evolution of the thermodynamic conditions of ore-forming fluids, Δ33S is virtually indelible and can uniquely fingerprint an Archean sedimentary reservoir that has undergone mass independent fractionation of sulfur (MIF-S). We combine these two tracers (δ34S and Δ33S) to characterise a gold-bearing laminated quartz breccia ore zone and its sulfide-bearing alteration halo in the (+6 Moz Au) structurally-controlled Archean Waroonga deposit located in the Eastern Goldfields Superterrane of the Yilgarn Craton, Western Australia. Over 250 analyses of gold-associated sulfides yield a δ34S shift from what is interpreted as an early pre-mineralisation phase, with chalcopyrite-pyrrhotite (δ34S = +0.7‰ to +2.9‰) and arsenopyrite cores (δ34S = ∼−0.5‰), to a syn-mineralisation stage, reflected in Au-bearing arsenopyrite rims (δ34S = −7.6‰ to +1.5‰). This shift coincides with an unchanging Δ33S value (Δ33S = +0.3‰), both temporally throughout the Au-hosting hydrothermal sulfide paragenesis and spatially across the Au ore zone. These results indicate that sulfur is at least partially recycled from MIF-S-bearing Archean sediments. Further, the invariant nature of the observed MIF-S signature demonstrates that sulfur is derived from a homogeneous MIF-S-bearing fluid reservoir at depth, rather than being locally sourced at the site of Au precipitation. Finally, by constraining the MIF-S-bearing sulfur source to a fixed reservoir, we are able to display the thermochemical evolution of the ore fluid in δ34S space and capture the abrupt change in oxidation state that causes Au precipitation. Our results highlight the importance of constraining multiple sulfur isotopes in space and time in order to elucidate the source and evolution of any given Au-bearing fluid.

U-Pb age of zircon from paragneisses in granulite terrane of the Sharyzhalgai uplift (southwest of the Siberian craton): Evidence for the Archean sedimentation and evolution of continental crust from Eoarchean to Mesoarchean

The Archean stage of sedimentation has been first substantiated for the granulite-gneiss terranes of the Sharyzhalgai uplift (southwest of the Siberian craton). High-alumina paragneisses contain detrital zircons varying in age from 3.7 to 2.74 Ga and corresponding in REE patterns to magmatic zircons. The Paleoproterozoic (~1.86 Ga) metamorphic zircons are strongly depleted in HREE and Y as a result of their formation in equilibrium with garnet. Zircons with an age of ~2.5 Ga also show geochemical signs of alteration during metamorphism. The formation of terrigenous sedimentary rocks preceded the Neoarchean stage of magmatism (~2.7-2.6 Ga), and their metamorphism occurred at the Neoarchean-Paleoproterozoic boundary and in the Late Paleoproterozoic. The sources of detrital zircons were mainly Mesoarchean rocks, such as magmatic protoliths of granulites and intermediate-felsic magmatic rocks. The single Eoarchean and Paleoarchean detrital zircon grains in the paragneisses are the first direct evidence for the oldest crust (up to 3.7 Ga) in the granulite terranes of the Sharyzhalgai uplift. The set of geochronological data for granulites and paragneisses suggests the following sequence of geologic events for the granulite-gneiss terranes: ~3.7—the beginning of crustal formation, 3.4-3.2 Ga—intermediate-felsic magmatism, including the recycling of the more ancient crust, and ~3.0 Ga—coeval magmatic and metamorphic processes and differentiation of the continental crust. Thus, the whole cycle from the beginning of crustal growth to the crustal differentiation and turn into continental crust proceeded from 3.7 to 3.0 Ga.

Sulfate radicals enable a non-enzymatic Krebs cycle precursor.

The evolutionary origins of the tricarboxylic acid cycle (TCA), or Krebs cycle, are so far unclear. Despite a few years ago, the existence of a simple non-enzymatic Krebs-cycle catalyst has been dismissed ‘as an appeal to magic’, citrate and other intermediates have meanwhile been discovered on a carbonaceous meteorite and do interconvert non-enzymatically. To identify the non-enzymatic Krebs cycle catalyst, we used combinatorial, quantitative high-throughput metabolomics to systematically screen iron and sulfate reaction milieus that orient on Archean sediment constituents. TCA cycle intermediates are found stable in water and in the presence of most iron and sulfate species, including simple iron-sulfate minerals. However, we report that TCA intermediates undergo 24 interconversion reactions in the presence of sulfate radicals that form from peroxydisulfate. The non-enzymatic reactions critically cover a topology as present in the Krebs cycle, the glyoxylate shunt and the succinic semialdehyde pathways. Assembled in a chemical network, the reactions achieve more than ninety percent carbon recovery. Our results show that a non-enzymatic precursor for the Krebs cycle is biologically sensible, efficient, and forms spontaneously in the presence of sulfate radicals.

Sulfur and lead isotopic evidence of relic Archean sediments in the Pitcairn mantle plume

The isotopic diversity of oceanic island basalts (OIB) is usually attributed to the influence, in their sources, of ancient material recycled into the mantle, although the nature, age, and quantities of this material remain controversial. The unradiogenic Pb isotope signature of the enriched mantle I (EM I) source of basalts from, for example, Pitcairn or Walvis Ridge has been variously attributed to recycled pelagic sediments, lower continental crust, or recycled subcontinental lithosphere. Our study helps resolve this debate by showing that Pitcairn lavas contain sulfides whose sulfur isotopic compositions are affected by mass-independent fractionation (S-MIF down to Δ33S = -0.8), something which is thought to have occurred on Earth only before 2.45 Ga, constraining the youngest possible age of the EM I source component. With this independent age constraint and a Monte Carlo refinement modeling of lead isotopes, we place the likely Pitcairn source age at 2.5 Ga to 2.6 Ga. The Pb, Sr, Nd, and Hf isotopic mixing arrays show that the Archean EM I material was poor in trace elements, resembling Archean sediment. After subduction, this Archean sediment apparently remained stored in the deep Earth for billions of years before returning to the surface as Pitcairn´s characteristic EM I signature. The presence of negative S-MIF in the deep mantle may also help resolve the problem of an apparent deficit of negative Δ33S anomalies so far found in surface reservoirs.

Dissecting ancient microbial sulfur cycling

Early Earth Before the rise of oxygen, life on Earth depended on the marine sulfur cycle. The fractionation of different sulfur isotopes provides clues to which biogeochemical cycles were active long ago (see the Perspective by Ueno). Zhelezinskaia et al. found negative isotope anomalies in Archean rocks from Brazil and posit that metabolic fluxes from sulfate-reducing microorganisms influenced the global sulfur cycle, including sulfur in the atmosphere. In contrast, Paris et al. found positive isotope anomalies in Archean sediments from South Africa, implying that the marine sulfate pool was more disconnected from atmospheric sulfur. As an analog for the Archean ocean, Crowe et al. measured sulfur isotope signatures in modern Lake Matano, Indonesia, and suggest that low seawater sulfate concentrations restricted early microbial activity. Science , this issue p. [703][1], p. [742][2], p. [739][3]; see also p. [735][4] [1]: /lookup/doi/10.1126/science.1261676 [2]: /lookup/doi/10.1126/science.1256211 [3]: /lookup/doi/10.1126/science.1258211 [4]: /lookup/doi/10.1126/science.1258966

Large sulfur isotope fractionations associated with Neoarchean microbial sulfate reduction.

The minor extent of sulfur isotope fractionation preserved in many Neoarchean sedimentary successions suggests that sulfate-reducing microorganisms played an insignificant role in ancient marine environments, despite evidence that these organisms evolved much earlier. We present bulk, microdrilled, and ion probe sulfur isotope data from carbonate-associated pyrite in the ~2.5-billion-year-old Batatal Formation of Brazil, revealing large mass-dependent fractionations (approaching 50 per mil) associated with microbial sulfate reduction, as well as consistently negative Δ(33)S values (~ -2 per mil) indicative of atmospheric photochemical reactions. Persistent (33)S depletion through ~60 meters of shallow marine carbonate implies long-term stability of seawater sulfate abundance and isotope composition. In contrast, a negative Δ(33)S excursion in lower Batatal strata indicates a response time of ~40,000 to 150,000 years, suggesting Neoarchean sulfate concentrations between ~1 and 10 μM.

Neoarchean carbonate–associated sulfate records positive Δ 33 S anomalies

Mass-independent fractionation of sulfur isotopes (reported as Δ(33)S) recorded in Archean sedimentary rocks helps to constrain the composition of Earth's early atmosphere and the timing of the rise of oxygen ~2.4 billion years ago. Although current hypotheses predict uniformly negative Δ(33)S for Archean seawater sulfate, this remains untested through the vast majority of Archean time. We applied x-ray absorption spectroscopy to investigate the low sulfate content of particularly well-preserved Neoarchean carbonates and mass spectrometry to measure their Δ(33)S signatures. We report unexpected, large, widespread positive Δ(33)S values from stratigraphic sections capturing over 70 million years and diverse depositional environments. Combined with the pyrite record, these results show that sulfate does not carry the expected negative Δ(33)S from sulfur mass-independent fractionation in the Neoarchean atmosphere.