Archean Sediments Research Articles

Iron isotopes in an Archean ocean analogue

Iron isotopes have been extensively used to trace the history of microbial metabolisms and the redox evolution of the oceans. Archean sedimentary rocks display greater variability in iron isotope ratios and more markedly negative values than those deposited in the Proterozoic and Phanerozoic. This increased variability has been linked to changes in either water column iron cycling or the extent of benthic microbial iron reduction through time. We tested these contrasting scenarios through a detailed study of anoxic and ferruginous Lac Pavin (France), which can serve as a modern analogue of the Archean ocean. A depth-profile in the water column of Lac Pavin shows a remarkable increase in dissolved Fe concentration (0.1–1200μM) and δ56Fe values (−2.14‰ to +0.31‰) across the oxic–anoxic boundary to the lake bottom. The largest Fe isotope variability is found at the redox boundary and is related to partial oxidation of dissolved ferrous iron, leaving the residual Fe enriched in light isotopes. The analysis of four sediment cores collected along a lateral profile (one in the oxic layer, one at the redox boundary, one in the anoxic zone, and one at the bottom of the lake) indicates that bulk sediments, porewaters, and reactive Fe mostly have δ56Fe values near 0.0±0.2‰, similar to detrital iron. In contrast, pyrite δ56Fe values in sub-chemocline cores (60, 65, and 92m) are highly variable and show significant deviations from the detrital iron isotope composition (δ56Fepyrite between −1.51‰ and +0.09‰; average −0.93‰). Importantly, the pyrite δ56Fe values mirror the δ56Fe of dissolved iron at the redox boundary—where near quantitative sulfate and sulfide drawdown occurs—suggesting limited iron isotope fractionation during iron sulfide formation. This finding has important implications for the Archean environment. Specifically, this work suggests that in a ferruginous system, most of the Fe isotope variability observed in sedimentary pyrites can be tied to water column cycling—foremost to the oxidation of dissolved ferrous iron. This supports previous suggestions that enhanced iron isotope variability in the Archean may record a unique stage in Earth’s history where partial ferrous iron oxidation in upwelling water masses was a common process, probably linked to oxygenic or anoxygenic photosynthesis.

Nitrogen cycle in the Late Archean ferruginous ocean

The Hamersley Group comprises a Late Archean sedimentary succession, which is thought to record the prelude to the atmospheric oxygenation in the Paleoproterozoic, the so-called Great Oxidation Event (GOE), at ~2.4Ga. We studied drill-core samples of sedimentary rocks from the upper Mount McRae Shale and Brockman Iron Formation deposited before the GOE at ~2.5Ga in order to characterize the environments and ecosystems prevailing during their deposition. The rocks from the Mount McRae Shale and Brockman Iron Formation represent, respectively, proximal euxinic conditions and distal ferruginous depositional environments, thus providing an opportunity to examine lateral variability in the open-marine basin. We analyzed the concentration and isotopic composition of carbon in carbonate and organic matter, bulk nitrogen content and its isotopic composition as well as major element concentrations. The δ13Ccarb values and carbonate content range from −3.2 to −10.7‰ and 0.1 to 58wt.%, respectively. Organic carbon content also varies over a large range from 0.05 to 4.6wt.% with a near constant δ13Corg value of −28.7±0.8‰. Negative δ13Corg excursions (down to −31‰) are generally correlated with high organic matter content. Bulk nitrogen shows highly variable concentration, between 1.3 and 785ppm, and δ15N values between 0.4 and 13.4‰.The δ13Ccarb values reflect a diagenetic carbonate origin, with negative values typical for Fe-rich carbonates formed by organic matter mineralization with ferric oxyhydroxides. In contrast, δ13Corg and δ15N values record primary isotope signatures derived from ancient living organisms. The relatively constant δ13Corg values at around −28.7‰ are interpreted as reflecting photoautotrophs utilizing a large pool of dissolved inorganic carbon. Inverse stratigraphic co-variation between δ15N and δ13Ccarb values was observed for the Brockman Iron Formation. We propose that N and C biogeochemical cycles were coupled by Fe redox cycling in the water column and in sediments of the Late Archean ocean. Several models for biogeochemical N cycling linked to the redox structure of the water column are considered. Under fully anoxic conditions, the dominant N species available for assimilation by micro-organisms in the photic zone could be ammonium (NH4+). Highly positive δ15N values would reflect the assimilation of NH4+ enriched in 15N by partial oxidation to nitrite, followed by quantitative removal of the produced nitrite by denitrification or anamox processes. Ammonium oxidation could have been driven by (i) O2 produced locally via oxygenic photosynthesis, or (ii) microbial oxidation utilizing Fe(III)-oxyhydroxides formed in the water column. Under redox-stratified conditions, N assimilated by primary producers could have been in the form of NO3−, based on modern and Phanerozoic analogs. The positive δ15N values would have resulted in this case from partial denitrification of NO3− coupled to anaerobic microbial oxidation of Fe(II) to Fe(III). We conclude that similar positive δ15N signatures may record very different N biogeochemical cycles under anoxic, stratified and fully oxic conditions in the ocean. Interpretation of the N isotopes in terms of N biogeochemical cycle thus requires independent constraints on the redox structure of the ocean.

Provenance of late Paleoproterozoic cover sequences in the central Gawler Craton: exploring stratigraphic correlations in eastern Proterozoic Australia using detrital zircon ages, Hf and Nd isotopic data

Provenance data from Paleoproterozoic and possible Archean sedimentary units in the central eastern Gawler Craton in southern Australia form part of a growing dataset suggesting that the Gawler Craton shares important basin formation and tectonic time lines with the adjacent Curnamona Province and the Isan Inlier in northern Australia. U–Pb dating of detrital zircons from the Eba Formation, previously mapped as the Paleoproterozoic Tarcoola Formation, yields exclusively Archean ages (ca 3300–2530 Ma), which are consistent with evolved whole-rock Nd and zircon Hf isotopic data. The absence of Paleoproterozoic detrital grains in a number of sequences (including the Eba Formation), despite the proximity of voluminous Paleoproterozoic rock units, suggests that the Eba Formation may be part of a Neoarchean or early Paleoproterozoic cover sequence derived from erosion of a multi-aged Archean source region. The ca 1715 Ma Labyrinth Formation, unconformably overlying the Eba Formation, shares similar depositional timing with other basin systems in the Gawler Craton and the adjacent Curnamona Province. Detrital zircon ages in the Labyrinth Formation range from Neoarchean to Paleoproterozoic, and are consistent with derivation from >1715 Ma components of the Gawler Craton. Zircon Hf and whole-rock Nd isotopic data also suggest a source region with a mixed crustal evolution (εNd –6 to –4.5), consistent with what is known about the Gawler Craton. Compared with the lower Willyama Supergroup in the adjacent Curnamona Province, the Labyrinth Formation has a source more obviously reconcilable with the Gawler Craton. Stratigraphically overlying the Eba and Labyrinth Formations is the 1656 Ma Tarcoola Formation. Zircon Hf and whole-rock Nd isotopic data indicate that the Tarcoola Formation was sourced from comparatively juvenile rocks (εNd –4.1 to + 0.5). The timing of Tarcoola Formation deposition is similar to the juvenile upper Willyama Supergroup, further strengthening the stratigraphic links between the Gawler and Curnamona domains. Additionally, the Tarcoola Formation is similar in age to extensive units in the Mt Isa and Georgetown regions in northern Australia, also shown to be isotopically juvenile. These juvenile sedimentary rocks contrast with the evolved underlying sequences and hint at the existence of a large-scale ca 1650 Ma juvenile basin system in eastern Proterozoic Australia.

Weathering of lower crustal rocks in the Kaveri river catchment, southern India: Implications to sediment geochemistry

In the upper catchment of the Kaveri river in the Sahyadri mountains of southern India, middle to lower crustal Archean granulite grade mafic and felsic rocks with similar structures and textures are exposed under the conditions of active tectonics, high rainfall and thick tropical vegetation. Occurrence of the two major rock types in close association under identical geological, geographical and biological conditions provides an uncommon situation for the study of weathering, elemental mobilization and sediment generation processes. Field observations, mineralogical and geochemical data including major, trace and rare earth elements (REE) of fresh rocks and variably weathered saprolite samples suggest that close association of mafic and felsic rocks accelerates the denudational processes by early weathering of mafic minerals in felsic rocks and mafic rocks in the terrain. Due to differential weathering of rocks, unweathered to less weathered felsic grains are likely transferred to the coarser fraction of fluvial sediments deposited on the floodplains of the river imposing an upper continental crust (UCC) geochemical signature. It is found that during chemical weathering, in addition to other factors, weatherability of host minerals of REE control the mobility of REE in the weathering profile. It is suggested from the observations on the weathering process and on the geochemistry of derivative sediments, that in a tectonically active system with a climate maximum, as in Sahyadris, an equilibrium could be dynamically maintained between weathering and erosional regimes. Also we infer from our study that there exist certain commonalities between surface denudational and mantle-magmatic geochemical differentiation processes. Similarity of these processes, therefore, may have implication to common UCC-like geochemistry of Post Archean sediments.

Metasomatic diamond growth: A multi-isotope study ( 13C, 15N, 33S, 34S) of sulphide inclusions and their host diamonds from Jwaneng (Botswana)

The formation of diamond through metasomatic events, from volatiles-enriched fluids/melts brought into pre-existing mantle rocks, has been suggested from a series of independent studies. However, the link between these hypothetical volatile-rich fluids and deep-seated mineral inclusions (mostly silicate and sulphides) entrapped in diamonds remains unclear, yet the relationship between these two species may provide the key to our understanding of diamond crystallization. In order to address the relationship between the origin and formation of diamonds and their mineral inclusions, we carried out the first coupled stable isotopic study ( δ 13C, δ 15N and multiple S-isotopes as δ 34S, δ 33S, Δ 33S) of diamonds containing sulphide inclusions, using samples from the Jwaneng kimberlite as our model system. Sulphides extracted from the present collection of 55 diamonds belong to either eclogitic (E-type, 52 diamonds) or peridotitic (P-type, 3 diamonds) paragenetic suites, as attested by their Cr and Ni content (Cr < 0.03 wt.% for E-types and Cr > 0.18 wt.% and Ni > 14 wt.% for P-types). Sulphur isotopic compositions have been measured in-situ by multicollector secondary ion mass spectrometry and reveal that peridotitic sulphide inclusions in diamonds ( n = 4) lie on the terrestrial fractionation line whereas Mass Independent sulphur isotope Fractionations (MIF) are preserved inside eclogitic sulphides inclusions (− 0.5‰ < Δ 33S < + 0.9‰, n = 33). Such large MIF values, extended here to the negative values recorded in Archean sediments, are thought to be produced through photochemical reactions in the Archean atmosphere, which implies that sulphides inclusions contain an Archean sedimentary sulphur component that was transferred to the Earth's mantle. In contrast to this isotopic dichotomy between E-type and P-type sulphides, the geochemical characteristics of their host diamonds ( δ 13C, δ 15N, N-content, N-speciation) are largely homogeneous throughout the population, and differ from silicate-bearing diamonds previously studied in the same locality. Sulphide-bearing diamonds appear to define a distinct population, which might reflect a different crystallization process characterized by carbon and nitrogen isotopic compositions falling within the range of unfractionated mantle values. The lack of a distinct isotopic signature of recycled sedimentary nitrogen and carbon in sulphide-bearing diamonds leads us to infer that sulphide-bearing Jwaneng diamonds were not derived from the same chemical reservoir as their inclusions. One possible formation mechanism that is compatible with our observations is diamond crystallization from a mantle-derived carbon-bearing fluid associated with pre-existing sedimentary sulphide minerals. This model has important implications for the interpretation of diamond ages obtained from sulphide inclusions using Re–Os systematics. Here we suggest that the incorporation of a significant amount of mantle-related sulphur during a diamond growth event into the original sulphide could lead to a re-equilibration of the Re–Os isotope composition in any inclusion, and accordingly, account for some scatter on Re–Os isochrons.

Atmospherically-derived mass-independent sulfur isotope signatures, and incorporation into sediments

The discovery of sulfur mass-independent fractionation (S-MIF) in Archean sediments has motivated new work on atmospheric sulfur chemistry. Laboratory experiments showed that SO 2 photolysis produces S-MIF at wavelengths both shortward of and longward of the SO 2 photodissociation wavelength of 220 nm. It has been argued that the underlying S-MIF mechanism at wavelengths < 220 nm is SO 2 self-shielding. Additional S-MIF signatures associated with SO 2 photolysis are possible due to isotopologue-dependent variations in absorption intensity and dissociation probability, which must be evaluated through new spectral measurements. Here, I claim that SO 2 photoexcitation, near-UV CS 2 photolysis, OCS photolysis, non-statistical sulfur allotrope reactions, and surface reactions during thermochemical sulfate reduction are all unlikely sources of the largest Archean S-MIF, signatures with arguments presented for each proposed source. A potential problem with the theory proposed here is that large mass-dependent fractionation (MDF) accompanies S-MIF during SO 2 photolysis. The range of δ 34S values is ∼ 100‰ in photochemically produced elemental sulfur, which exceeds the δ 34S range observed in Archean rocks by a factor of ∼ 3–5, and represents a weakness of the photochemical theory for the origin of Archean S-MIF. A combination of chemical and biogenic MDF processes may have acted to reduce the δ 34S range of SO 2 photolysis products. MDF during reactions that form elemental sulfur in the atmosphere and during aqueous phase reaction of HS − with Fe 2+ and FeS to form FeS 2 may have reduced δ 34S values by ∼ 40‰ relative to atmospheric SO. A simple mixing model suggests that a mixture of FeS 2 in sediments from both elemental sulfur (yielding pyrite with δ 34S > 0 and Δ 33S > 0) and from bacterial sulfate reduction (BSR) of BaSO 4 (yielding pyrite with δ 34S < 0 and Δ 33S < 0) may contribute to reducing δ 34S from photochemical values to the observed range in Archean pyrites.

Was There a Late Archean Biospheric Explosion?

There is a growing body of evidence which suggests that the evolution of the planet drives the evolution of the biosphere. There have been 2 significant stages in Earth history when atmospheric oxygen levels rose rapidly, and both appear to be associated with supercontinent cycles. The earlier biospheric event, which extends across the Archean-Proterozoic boundary (ca. 3.0-2.2 Ga), has received little attention and is the focus of this study. Recent work on the Pilbara Craton of Western Australia has shown that concretion formed by microbial activity during the diagenesis of these sediments are absent from early Archean sediments but abundant in late Archean and early Paleoproterozoic successions of the Hamersley Basin, appearing abruptly in sedimentary rocks younger than 2.7 Ga. This study suggests that their internal architecture may have been defined by the diffusion of humic acids and the formation of polymer gels during diagenesis. The data imply that the biosphere expanded suddenly shortly after 3.0 Ga and may have begun to raise the oxygen levels of the oceanic water column earlier than thought-possibly as much as 300 my earlier.

Analysis of unresolved complex mixtures of hydrocarbons extracted from Late Archean sediments by comprehensive two-dimensional gas chromatography (GC×GC)

Hydrocarbon mixtures too complex to resolve by traditional capillary gas chromatography display gas chromatograms with dramatically rising baselines or “humps” of coeluting compounds that are termed unresolved complex mixtures (UCMs). Because the constituents of UCMs are not ordinarily identified, a large amount of geochemical information is never explored. Gas chromatograms of saturated/unsaturated hydrocarbons extracted from Late Archean argillites and greywackes of the southern Abitibi Province of Ontario, Canada contain UCMs with different appearances or “topologies” relating to the intensity and retention time of the compounds comprising the UCMs. These topologies appear to have some level of stratigraphic organization, such that samples collected at any stratigraphic formation collectively are dominated by UCMs that either elute early- (within a window of C 15–C 20 n-alkanes), early- to mid- (C 15–C 30 n-alkanes), or have a broad UCM that extends through the entire retention time of the sample (from C 15–C 42 n-alkanes). Comprehensive two-dimensional gas chromatography time-of-flight mass spectrometry (GC×GC–MS) was used to resolve the constituents forming these various UCMs. Early- to mid-eluting UCMs are dominated by configurational isomers of alkyl-substituted and non-substituted polycyclic compounds that contain up to six rings. Late eluting UCMs are composed of C 36–C 40 mono-, bi-, and tricyclic archaeal isoprenoid diastereomers. Broad UCMs spanning the retention time of compound elution contain nearly the same compounds observed in the early-, mid-, and late-retention time UCMs. Although the origin of the polycyclic compounds is unclear, the variations in the UCM topology appear to depend on the concentration of initial compound classes that have the potential to become isomerized. Isomerization of these constituents may have resulted from hydrothermal alteration of organic matter.

Natural evidence for rapid abiogenic hydrothermal generation of CH 4

Although recent hydrothermal experiments imply that abiogenic methane (CH 4) generation from hydrothermal reduction of CO 2 can occur, evidence from natural systems was still lacking. Based on the chemical and isotopic equilibrium signatures of low-temperature fumarolic gas discharges, we are able to provide hard evidence for its natural occurrence, namely in three subduction-related bi-phase hydrothermal systems of the Mediterranean, whose temperatures range from 260 to 470 °C. The attainment of equilibrium and the time spans of recent volcanic dormancy allowed us to calculate minimum rates for chemical and isotopic equilibration. These are significantly higher than those previously reported and might be due to the presence of a saturated water vapor phase in the investigated systems. The fact that nature provides conditions enabling relatively fast production of hydrocarbons from CO 2 strongly supports the concerns that were recently raised from laboratory experiments. These address the use of the carbon isotope composition of reduced carbon in Archean sediments as a tracer of early life and the occurrence of CH 4 on extraterrestrial planets as a bioindicator. In view of the potential role of abiogenic CH 4 as a precursor of life, we also present an estimate of abiogenic hydrothermal CH 4 fluxes throughout the Archean. It is not expected that these fluxes exceeded 80 Mt/yr during the past 4.0 Ga. This, however, would have been enough to facilitate HCN production on the prebiotic Earth.

Kitty's Gap始生代堆積岩中の窒素同位体比組成

Kitty's Gap始生代堆積岩中の窒素同位体比組成

Rare sulfur and triple oxygen isotope geochemistry of volcanogenic sulfate aerosols

We present analyses of stable isotopic ratios 17O/ 16O, 18O/ 16O, 34S/ 32S, and 33S/ 32S, 36S/ 32S in sulfate leached from volcanic ash of a series of well known, large and small volcanic eruptions. We consider eruptions of Mt. St. Helens (Washington, 1980, ∼1 km 3), Mt. Spurr (Alaska, 1953, <1 km 3), Gjalp (Iceland, 1996, 1998, <1 km 3), Pinatubo (Phillipines, 1991, 10 km 3), Bishop tuff (Long Valley, California, 0.76 Ma, 750 km 3), Lower Bandelier tuff (Toledo Caldera, New Mexico, 1.61 Ma, 600 km 3), and Lava Creek and Huckleberry Ridge tuffs (Yellowstone, Wyoming, 0.64 Ma, 1000 km 3 and 2.04 Ma 2500 km 3, respectively). This list covers much of the diversity of sizes and the character of silicic volcanic eruptions. Particular emphasis is paid to the Lava Creek tuff for which we present wide geographic sample coverage. This global dataset spans a significant range in δ 34S, δ 18O, and Δ 17O of sulfate (29‰, 30‰, and 3.3‰, respectively) with oxygen isotopes recording mass-independent ( Δ 17O > 0.2‰) and sulfur isotopes exhibiting mass-dependent behavior. Products of large eruptions account for most of‘ these isotopic ranges. Sulfate with Δ 17O > 0.2‰ is present as 1–10 μm gypsum crystals on distal ash particles and records the isotopic signature of stratospheric photochemical reactions. Sediments that embed ash layers do not contain sulfate or contain little sulfate with Δ 17O near 0‰, suggesting that the observed sulfate in ash is of volcanic origin. Mass-dependent fractionation of sulfur isotopic ratios suggests that sulfate-forming reactions did not involve photolysis of SO 2, like that inferred for pre-2.3 Ga sulfates from Archean sediments or Antarctic ice-core sulfate associated with few dated eruptions. Even though the sulfate sulfur isotopic compositions reflect mass-dependent processes, the products of caldera-forming eruptions display a large δ 34S range and exhibit fractionation relationships that do not follow the expected equilibrium slopes of 0.515 and 1.90 for 33S/ 32S vs. 34S/ 32S and 36S/ 32S vs. 34S/ 32S, respectively. The data presented here are consistent with modification of a chemical mass-dependent fractionation of sulfur isotopes in the volcanic plume by either a kinetic gas phase reaction of volcanic SO 2 with OH and/or a Rayleigh processes involving a residual Rayleigh reactant—volcanic SO 2 gas, rather than a Rayleigh product. These results may also imply at least two removal pathways for SO 2 in volcanic plumes. Above-zero Δ 17O values and their positive correlation with δ 18O in sulfate can be explained by oxidation by high- δ 18O and high- Δ 17O compounds such as ozone and radicals such as OH that result from ozone break down. Large caldera-forming eruptions have the highest Δ 17O values, and the largest range of δ 18O, which can be explained by stratospheric reaction with ozone-derived OH radicals. These results suggest that massive eruptions are capable of causing a temporary depletion of the ozone layer. Such depletion may be many times that of the measured 3–8% depletion following 1991 Pinatubo eruption, if the amount of sulfur dioxide released scales with the amount of ozone depletion.

Chemical Mapping of Proterozoic Organic Matter at Submicron Spatial Resolution

A NanoSIMS ion microprobe was used to map the submicron-scale distributions of carbon, nitrogen, sulfur, silicon, and oxygen in organic microfossils and laminae in a thin section of the approximately 0.85 billion year old Bitter Springs Formation of Australia. The data provide clues about the original chemistry of the microfossils, the silicification process, and the biosignatures of specific microorganisms and microbial communities. Chemical maps of fossil unicells and filaments revealed distinct wall- and sheath-like structures enriched in C, N, and S, consistent with their accepted biological origin. Surprisingly, organic laminae, previously considered to be amorphous, also exhibited filamentous and apparently compressed spheroidal structures defined by strong enrichments in C, N, and S. By analogy to NanoSIMS data from the well-preserved microfossils, these structures were interpreted as being of biological origin, most likely representing densely packed remnants of microbial mats. Given that the preponderance of organic matter in Precambrian sediments is similarly "amorphous," our findings indicate that a re-evaluation of ancient specimens via in situ structural, chemical, and isotopic study is warranted. Our analyses have led us to propose new criteria for assessing the biogenicity of problematic kerogenous materials, and, thus, these criteria can be applied to assessments of poorly preserved or fragmentary organic residues in early Archean sediments and any that might occur in meteorites or other extraterrestrial samples.

Carbonate Minerals in the Warrawoona Group, Pilbara Craton: Implications for Continental Crust, Life, and Global Carbon Cycle in the Early Archean

Abstract: The occurrences of the Early Archean carbonate minerals are compiled and their precipitation processes are investigated for the Warrawoona Group, Pilbara Craton. Sedimentary carbonate rocks such as limestone and dolostone are very rare, and only a small amount of sedimentary carbonate minerals are sometimes contained in the hydrothermal bedded chert, implying that a sink of CO2 was minor in the Early Archean sediments. Moreover, it is very likely that the activity of cyanobacteria forming stromatolites was considerably low in the Early Archean. Microfossils and carbonaceous matter in the hydrothermal cherts are probably derived from a non‐photosynthetic microorganisms related to the seafloor hydrothermal activity. Their preservation in sediments may play a very minor role in carbon sink of the Earth's surface.On the other hand, carbonatized volcanic rocks subjected to seafloor hydrothermal alteration occur ubiquitously in the Early Archean greenstone belts such as the Warrawoona Group, suggesting that the hydrothermally altered oceanic crust had large amounts of CO2 as carbonate minerals. Global carbon cycle in the Early Archean is considered to have been controlled by the intense seafloor hydrothermal alteration. Large amounts of CO2 were sunk into the oceanic crust by the alteration. The carbonatized oceanic crust was partly accreted to the continents and/or island–arcs, and partly subducted into the mantle without decomposition. Significant amounts of carbonate minerals in the carbonatized oceanic crust were very likely to store in the accretionary prisms and mantle, consequently giving rise to a decrease of atmospheric and oceanic CO2.

Mass-independent fractionation of sulfur isotopes in Archean sediments: strong evidence for an anoxic Archean atmosphere.

Mass-independent fractionation (MIF) of sulfur isotopes has been reported in sediments of Archean and Early Proterozoic Age (> 2.3 Ga) but not in younger rocks. The only fractionation mechanism that is consistent with the data on all four sulfur isotopes involves atmospheric photochemical reactions such as SO2 photolysis. We have used a one-dimensional photochemical model to investigate how the isotopic fractionation produced during SO2 photolysis would have been transferred to other gaseous and particulate sulfur-bearing species in both low-O2 and high-O2 atmospheres. We show that in atmospheres with O2 concentrations < 10(-5) times the present atmospheric level (PAL), sulfur would have been removed from the atmosphere in a variety of different oxidation states, each of which would have had its own distinct isotopic signature. By contrast, in atmospheres with O2 concentrations > or = 10(-5) PAL, all sulfur-bearing species would have passed through the oceanic sulfate reservoir before being incorporated into sediments, so any signature of MIF would have been lost. We conclude that the atmospheric O2 concentration must have been < 10(-5) PAL prior to 2.3 Ga.

Tectonic setting and provenance of the Neoproterozoic Uinta Mountain and Big Cottonwood groups, northern Utah: constraints from geochemistry, Nd isotopes, and detrital modes

The Neoproterozoic Uinta Mountain Group was deposited in an east-trending intracratonic rift bounded on the north by an active fault system and opening into a shallow sea on the west where the Big Cottonwood Group was deposited in an estuary. Although this rift may have been associated with the early stages in the breakup of Rodinia, it was not an aulacogen. Geochemical, Nd isotope, and detrital mode studies indicate that Uinta Mountain Group sediments were derived from mixed Archean and Paleoproterozoic sources with the former dominating. Big Cottonwood Group sediments appear to have been derived predominantly from Paleoproterozoic sources. The Archean sediment source is the Wyoming craton, and source rocks comprised dominantly granites enriched in Th, U, Y, Zr, Hf, and REE. The relative abundance of enriched granite implied by sedimentary rocks of the Uinta Mountain Group indicates that the Wyoming craton is anomalous compared to other Archean cratons. CIA values and A–CN–K relationships in shales of the Uinta Mountain and Big Cottonwood groups indicate high degrees of weathering of sources, probably in subtropical to tropical climates supporting a near-equatorial location for southwestern Laurentia at about 800 Ma. Differences in the Nd isotopic composition between the Big Cottonwood Group and Neoproterozoic sedimentary rocks in western Utah and northeast Nevada suggest a northwest-striking uplift in northwest Utah, possibly ancestral to the Paleozoic Toole–Uinta arch.

Organic haze in Earth's early atmosphere: Source of low-13C Late Archean kerogens?

High concentrations of greenhouse gases would have been required to offset low solar luminosity early in Earth9s history. Enhanced CO 2 levels are probably at least part of the solution, but CH 4 may have played a significant role as well, particularly during the Late Archean era, 2.5–3.0 Ga, when methanogenic bacteria were almost certainly present. Indeed, biological CH 4 production should have led to CO 2 drawdown by way of a negative feedback loop involving the carbonate-silicate geochemical cycle. We suggest here that the atmospheric CH 4 /CO 2 ratio approached the value of ∼1 needed to trigger formation of Titan-like organic haze. This haze was strongly depleted in 13 C relative to 12 C and was produced at a rate comparable to the modern rate of organic carbon burial in marine sediments. Therefore, it could provide a novel explanation for the presence of extremely low- 13 C kerogens in Late Archean sediments.

A Search for Extraterrestrial Eukaryotes: Biological and Planetary Science Aspects

AbstractOne pressing question in bioastronomy is to attempt to devise assays in the search for extraterrestrial eukaryotes (SETE). In the positive case of encountering a fossil of a certain microorganism, it is not yet clear how to identify it in an unequivocal manner, even if found arrested and preserved in the midst of biological activity, as in some known Archean sediments on Earth. On the other hand, for living microorganisms the present approach may have some advantages over the straightforward probe of the morphological features of the putative eukaryote, as we argue that we are not constrained to base identification on morphological properties, such as the presence of organelles, which are known to be missing in some higher taxa. We sketch a possible experimental set-up required for a robot to investigate whether eukaryogenesis has already taken place in Mars or Europa. Besides, we also discuss the biological and astronomical implications of a SETE program. Previous observation and theoretical work have suggested that underneath the ice crust of the Jovian satellite Europa there may be an ocean of liquid water. We therefore argue that as hot springs may be present at the bottom of that ocean, eukaryotes should be searched for, not only deep in the Martian permafrost, as discussed by others, but a SETE program should be extended to the aquatic environment of the Jovian satellite as well. Our current work emphasising gene silencing, may suggest how to decide on the degree evolution which microbiota may have reached in the Europa ocean, a topic which deserves our attention, as eukaryogenesis is the process which introduced the basic cellular plan of those Earth-bound organisms that have been raised to the level of intelligent beings. We make a specific suggestion for a robotic investigation of eukaryogenesis in solar system exploration.

Evidence of heterogeneous crustal sources: the Harney Peak Granite, South Dakota, U.S.A.

ABSTRACT:The Early Proterozoic (1715 Ma) Harney Peak Granite (Black Hills, SD, U.S.A.) is a complex of hundreds of dykes and sills. Earlier studies of Nd, O and Pb isotope variations demonstrated that the complex was not derived from a single source, or even different sources of a single age. Instead, the granites can be divided into a group with sources probably dominated by Early Proterozoic sediments and a group with sources probably dominated by Archean sediments. New results on the Nd isotopic variations of many additional samples indicate that there is considerable overlap between Nd isotopic compositions within the complex. Values of εNd (1715 Ma) of the Harney Peak Granite suite (n = 20) range from −2·0, indicating an Early Proterozoic (2300-2200 Ma) crustal source, to −13·4, indicating a Middle to Late Archean (3200-3100 Ma) protolith. These results suggest that the Early Proterozoic source may have included rocks such as the c. 2200-1900 Ma metasedimentary rocks that occur in the southern Black Hills. The Archean sources might have included rocks such as those exposed on the periphery of the Black Hills. The range in Nd model ages negates the usefulness of the concept of the ‘average’ age of the crust in this part of the craton. Because such heterogeneity is present in the magmatic compositions of the Harney Peak Granite, it can be inferred that at least as much heterogeneity was present in the sources. In this granite system, melts were evidently derived from isolated, heterogeneous zones and did not have the opportunity to coalesce into large magma bodies. In systems where coalescence does occur, the evidence for such highly heterogeneous sources may be lost. These results emphasise that inferences drawn from a few samples of plutonic rocks in which magma mixing and homogenisation occurred can lead to erroneous conclusions about the age and nature of protoliths and, consequently, the development of continental crust.

Confocal study of microaggregates in 2.5 billion year-old sediments

Light microscopy conducted on samples of Archean sediments reveals microaggregates (Fig. 1) which are suggestive of a biotic origin. These aggregates, typically 15 μm wide and 50 μm long, are thought to be the mineral remains of colonies of microorganisms that lived 2.5 billion years ago. Confocal microscopy is used to study the structures of these microaggregates in 3-D.Samples under study are from the lowest section drill core taken from the Dales Gorge Member of the Brockman Iron-Formation (Hamersley Basin) in Western Australia. These sediments are well preserved and escaped metamorphic conditions typically experienced by older rocks of this type. Two types of samples were prepared for study under the microscope: thin sections (~40 μm) for transmitted light microscopy to study the general rock texture and to locate the features of interest, and thick sections (3 mm) for confocal microscopy to determine the 3-D structure of the aggregates in their undisturbed state.

A much warmer Earth surface for most of geologic time: Implications to biotic weathering

The authors present two scenarios for the temperature history of Earth. One scenario is conventional, the other relies on a warmer history. Both scenarios include surface cooling determined by the evolution of the biosphere and are similar until the Proterozoic period. The warmer scenario requires a higher plant/lichen terrestrial biota to increase weathering intensity. Justification for a warmer surface includes period temperatures from the oxygen isotope record of coexisting phosphates and cherts, an upper limit of 58 degrees C from primary gypsum precipitation, and the lack of fractionation of sulfur isotopes between sulfide and sulfates in Archean sediments.