Search

Earth’s global barometric pressure, currently 1 bar at sea level, may have changed over its 4.5-billion-year history. Proxy measurements, including N2/36Ar ratios in ~3.5 to 3.0 Ga hydrothermal quartz, ~2.7 Ga raindrop imprints, and ~2.7 Ga vesicle sizes in subaerial basalt lava flows indicate Archean air pressure could have been between 0.1 and 1.2 bar. However, some models argue air pressure in the Archean should have been much higher than now and could allow pressure broadening of greenhouse gas absorption lines to counteract the “Faint Young Sun”. Thus, additional paleobarometric measurements would be useful to further constrain Earth’s atmospheric evolution. We attempted to use vesicle sizes in lavas erupted near sea-level from the ~2.9 Ga Pongola Supergroup from Mahlangatsha and Mooihoek, eSwatini (formerly Swaziland) and the White Mfolozi River gorge of KwaZulu-Natal, South Africa to provide further Archean paleobarometric data. However, reliable results were unobtainable due to small and scarce amygdales, irregular vesicle morphologies and metamorphic mineralogical homogenization preventing the use of X-ray Computed Tomography for accurate vesicle size determination. Researchers attempting paleobarometric analysis using lava vesicle sizes should henceforth avoid these areas of the Pongola Supergroup and instead look at other subaerially emplaced lava flows. With this being only the second time this method has been used on Precambrian rocks, we provide a list of guidelines informed by this study to aid future attempts at vesicular paleobarometry.

Funding: This study was supported by National Aeronautics and Space Administration Exobiology Grant NNX16AI37G (R.B.) and by the MacArthur Professorship (J.D.B.) at the University of Michigan. M.A.K. acknowledges support from an Agouron Institute postdoctoral fellowship. Earth’s early atmosphere witnessed multiple transient episodes of oxygenation before the Great Oxidation Event 2.4 billion years ago (Ga) [e.g., A. D. Anbar et al., Science 317, 1903–1906 (2007); M. C. Koehler, R. Buick, M. E. Barley, Precambrian Res. 320, 281–290 (2019)], but the triggers for these short-lived events are so far unknown. Here, we use mercury (Hg) abundance and stable isotope composition to investigate atmospheric evolution and its driving mechanisms across the well-studied “whiff” of O2 recorded in the ∼2.5-Ga Mt. McRae Shale from the Pilbara Craton in Western Australia [A. D. Anbar et al., Science 317, 1903–1906 (2007)]. Our data from the oxygenated interval show strong Hg enrichment paired with slightly negative Δ199Hg and near-zero Δ200Hg, suggestive of increased oxidative weathering. In contrast, slightly older beds, which were evidently deposited under an anoxic atmosphere in ferruginous waters [C. T. Reinhard, R. Raiswell, C. Scott, A. D. Anbar, T. W. Lyons, Science 326, 713–716 (2009)], show Hg enrichment coupled with positive Δ199Hg and slightly negative Δ200Hg values. This pattern is consistent with photochemical reactions associated with subaerial volcanism under intense UV radiation. Our results therefore suggest that the whiff of O2 was preceded by subaerial volcanism. The transient interval of O2 accumulation may thus have been triggered by diminished volcanic O2 sinks, followed by enhanced nutrient supply to the ocean from weathering of volcanic rocks causing increased biological productivity. Postprint

Current evidence for oxygenated environments in the Mesoarchean is limited to the shallowest marine and fluvio-lacustrine settings. It is not until the Neoarchean that signs of oxygenated surface waters above outer shelf and basinal depositional environments become evident. In order to further explore the Mesoarchean redox landscape for signs of basinward surface water oxygenation, we present nitrogen and carbon isotope ratios from the turbiditic Mosquito Creek Formation of the Nullagine Group (∼2.9 Ga). The δ15N and δ13Corg values are invariant around −1.8‰ and −32‰ respectively throughout a 70 m section of drill-core, suggesting an ecosystem dominated by nitrogen fixers (anaerobic nitrogen cycling) and CO2 fixation by the Calvin Cycle. When compared with other Archean isotopic data, these results (i) provide further evidence that the Mosquito Creek Formation was deposited in a marine basin, and (ii) contain δ15N values that highlight the prevalence of nitrogen fixation by Mo-nitrogenase and the dearth of aerobic nitrogen metabolisms in the Mesoarchean.

This work was financially supported by a NASA Exobiology grant to RB (# NNX16AI37G), an NSF graduate student research fellowship to JZ, and a NASA postdoctoral fellowship to EES. The last decade has seen a steady rise in the number of publications on nitrogen isotopes in sedimentary rocks, which have become an established tool for investigating the evolution of life and environmental conditions. Nitrogen is contained in sedimentary rocks in two different phases: bound to kerogen or substituted in potassic minerals (mostly K-bearing phyllosilicates and feldspars). Isotopic measurements and interpretations typically focus either on kerogen extracts alone or on bulk rocks that include both phases. The community is split about which sample type more accurately captures the original composition of the biomass. To address this question, we combined nitrogen isotopes and carbon-to-nitrogen ratios with carbon-to-hydrogen ratios which act as an independent proxy for metamorphic alteration. Our results reveal that metamorphism drives kerogen-bound nitrogen isotopically lighter while silicate-bound nitrogen becomes heavier. For rocks up to greenschist facies, the isotopic effect of this internal partitioning (up to 3-4‰) is larger than the isotopic effect of metamorphic nitrogen loss from the system (up to 1-2‰). The opposite may be true for higher metamorphic grades. We conclude that for low-grade sedimentary rocks with more than 60% of their total nitrogen residing in the silicate phase the primary isotopic composition of the biomass is best approximated by the bulk rock measurement, whereas for high-grade rocks the kerogen extract may be the more accurate proxy. The isotopic difference between nitrogen phases can thus serve as a rough indicator of the degree of metamorphic alteration. Postprint

Iron-rich micrometeorites from a 2.7 billion-year-old limestone could constrain atmospheric CO2 levels on the early Earth., Earth’s atmospheric composition during the Archean eon of 4 to 2.5 billion years ago has few constraints. However, the geochemistry of recently discovered iron-rich micrometeorites from 2.7 billion–year–old limestones could serve as a proxy for ancient gas concentrations. When micrometeorites entered the atmosphere, they melted and preserved a record of atmospheric interaction. We model the motion, evaporation, and kinetic oxidation by CO2 of micrometeorites entering a CO2-rich atmosphere. We consider a CO2-rich rather than an O2-rich atmosphere, as considered previously, because this better represents likely atmospheric conditions in the anoxic Archean. Our model reproduces the observed oxidation state of micrometeorites at 2.7 Ga for an estimated atmospheric CO2 concentration of >70% by volume. Even if the early atmosphere was thinner than today, the elevated CO2 level indicated by our model result would help resolve how the Late Archean Earth remained warm when the young Sun was ~20% fainter.

The ~2.0 Ga Zaonega Formation (ZF) holds one of the oldest phosphorites in the geologic record, reaching >15% P2O5. Understanding the depositional conditions that enabled sedimentary phosphorus enrichment in this unit will thus help us to interpret the significance of the temporal distribution of phosphorites in Earth’s early history. Here we use an array of major and trace element data to constrain the redox conditions in the water column and extent of basinal restriction during deposition of the ZF. We also present new selenium (Se) abundance and isotopic data to provide firmer constraints on fluctuations across high redox potentials, which might be critical for phosphogenesis. We find that Se isotope ratios shift over a range of ~3‰ in the ZF, with the earliest stratigraphically-resolved negative Se isotope excursion in the geologic record, implying at least temporarily suboxic waters in the basin. Furthermore, we find that redox-sensitive element (RSE) enrichments coincide with episodes of P enrichment, thereby implicating a common set of environmental controls on these processes. Together, our dataset implies deposition under a predominantly anoxic water column with periodic fluctuations to more oxidizing conditions because of connections to a large oxic reservoir containing Se oxyanions (and other RSE’s, as well as sulfate) in the open ocean. This is broadly consistent with the depositional setting of many modern and recent phosphorites, thereby tying these ancient deposits to a common depositional mechanism. In light of these data, we propose that the broader prevalence of phosphogenesis in the Paleoproterozoic Era was driven by growth of the seawater oxidant reservoir (namely sulfate), thus enabling diagenetic apatite precipitation in basins with high rates of export production, particularly by facilitating the activity of sulfide-oxidizing bacteria. This suggests that the muted authigenic P burial observed in marginal, marine siliciclastic sedimentary rocks during other intervals of the Precambrian was not merely a result of low dissolved P levels in the global deep ocean, but also was influenced by sulfate scarcity and strongly reducing bottom-water conditions.

This work was supported by an NSF Graduate Research Fellowship to MAK (DGE-1256082) and a NASA Exobiology grant to RB (NNX16AI37G). Mass-dependent variations in selenium stable isotope ratios have recently been developed as a paleo-redox proxy. Since the reduction of selenium oxyanions occurs at a relatively high redox potential, this system holds promise for probing conditions relevant to the evolution and diversification of eukaryotic and animal life, which required substantial dissolved oxygen levels. Although several studies have identified selenium isotopic variability during oxygenation events in Earth’s distant past, we still have only a broad understanding of the mechanisms controlling this isotopic variability. This currently limits the robust interpretation of selenium isotope variability to first-order mechanisms driving large-magnitude changes. Here, we explore selenium isotope variability within and among Paleozoic black shales deposited on the North American craton that have been well-studied using a variety of other paleo-environmental proxies. Using this combined dataset, we attempt to unravel the controls on selenium abundance and isotope ratios in organic-rich ancient marine sedimentary rocks. We find that in the Late Pennsylvanian units, an estuarine nutrient trap on the Midcontinent Shelf enabled vigorous selenium recycling, leading to very high concentrations in sediments and enrichment of heavy isotopes in the aqueous selenium reservoir. In contrast, we find that among the Late Devonian units, differences in local basinal hydrography led to a gradient in selenium abundance and isotopic fractionation, with the more restricted basins depleting their selenium reservoirs and causing enrichment of heavy isotopes in the residual aqueous reservoir. In both of these case studies, the additional context provided by complementary paleo-environmental proxies was critical for distinguishing between possible drivers of selenium isotopic variability. When extending such studies to other paleo-environmental settings, we suggest that the continued use of complementary datasets will enable the most robust use of the selenium paleo-redox proxy. Moreover, further development of techniques for high-precision and phase-specific selenium isotope measurements will greatly improve the ability to deduce subtle redox fluctuations with this proxy. Postprint

Funding for this work was provided by a University of Washington Royalty Research Fund Grant (R.B.), National Science Foundation Graduate Research Fellowship DGE‐1256082 (M.A.K.), and German Research Foundation (DFG) Fellowship GE2558/3‐1 (M.M.G). Cyanobiont collection was funded by grant no. 265‐605 of the Australian Biodiversity and Resources Programme (M.M.G). Molecular nitrogen (N2) constitutes the majority of Earth's modern atmosphere, contributing ~0.79 bar of partial pressure (pN2). However, fluctuations in pN2 may have occurred on 107–109 year timescales in Earth's past, perhaps altering the isotopic composition of atmospheric nitrogen. Here, we explore an archive that may record the isotopic composition of atmospheric N2 in deep time: the foliage of cycads. Cycads are ancient gymnosperms that host symbiotic N2‐fixing cyanobacteria in modified root structures known as coralloid roots. All extant species of cycads are known to host symbionts, suggesting that this N2‐fixing capacity is perhaps ancestral, reaching back to the early history of cycads in the late Paleozoic. Therefore, if the process of microbial N2 fixation records the δ15N value of atmospheric N2 in cycad foliage, the fossil record of cycads may provide an archive of atmospheric δ15N values. To explore this potential proxy, we conducted a survey of wild cycads growing in a range of modern environments to determine whether cycad foliage reliably records the isotopic composition of atmospheric N2. We find that neither biological nor environmental factors significantly influence the δ15N values of cycad foliage, suggesting that they provide a reasonably robust record of the δ15N of atmospheric N2. Application of this proxy to the record of carbonaceous cycad fossils may not only help to constrain changes in atmospheric nitrogen isotope ratios since the late Paleozoic, but also could shed light on the antiquity of the N2‐fixing symbiosis between cycads and cyanobacteria. Postprint

The composition of the Earth’s early atmosphere is uncertain. The morphology of vesicles in basalts suggests that the air pressure 2.7 billion years ago was less than half of modern levels.

MAK acknowledges support from NSF Graduate Research Fellowship DGE-1256082. AB acknowledges funding from NSERC Discovery and Accelerator grants. Funding for isotopic analyses was provided by the UW Department of Earth & Space Sciences to MAK and by NASA Exobiology grant NNX16AI37G to RB. Nitrogen isotope ratios in marine sedimentary rocks have become a widely used biogeochemical proxy that records information about nutrient cycling and redox conditions in Earth's distant past. While the past two decades have seen considerable progress in our understanding of the Precambrian sedimentary nitrogen isotope record, it is still compromised by substantial temporal gaps. Furthermore, quantitative links between nitrogen isotope data, marine redox conditions, and nutrient availability are largely lacking in a Precambrian context. Here we present new nitrogen isotope data from a suite of marine sedimentary rocks with ca. 2.4 to 1.8 Ga ages, spanning the Great Oxidation Event in the Paleoproterozoic, to better constrain the response of the nitrogen cycle to the first major redox transition in Earth's history. We further construct a simple box model to describe the major pathways that influenced the nitrogen isotope mass balance of the Precambrian ocean and use this as a platform to evaluate the Precambrian nitrogen isotope record. Within this framework, we find that consistently positive nitrogen isotope values, ranging from +1.1 to +7.7‰, across the early Paleoproterozoic are strong evidence for an expansion of oxygenated surface waters. Since the isotopic signature of aerobic nitrogen cycling is recorded in the biomass of nitrate-assimilating organisms, this implicates widespread nitrate bioavailability in this time interval. The decline in offshore nitrogen isotope ratios in the Mesoproterozoic is consistent with the contraction of oxic waters, which could have inhibited the expansion of nitrate-fueled ecosystems to pelagic waters until the widespread oxygenation of the ocean in the latest Neoproterozoic to early Phanerozoic. Postprint

We acknowledge the financial support from the UnivEarths Labex program of Sorbonne Paris Cite (ANR 11-IDEX-00005-02). CH and AL acknowledge support from the Max Planck Society. EES and RB were funded by the NASA Exobiology grant NNX16AI37G and the Virtual Planetary Laboratory of the NASA Astrobiology Institute and were technically assisted by Andy Schauer. Interpreting the organic carbon content (TOC) and stable carbon isotopic composition (δ13C) of organic matter in the sedimentary rock record depends on our capability to accurately measure them, while excluding sources of contamination. This however becomes increasingly problematic as we analyze samples with ever-lower organic carbon content. Accordingly, organic solvents are sometimes used to remove contaminating traces of modern organic matter from ancient rock samples. However, especially for very low TOC samples, traces of solvents or their impurities remaining in the sample may contribute a significant organic contamination that can impact the bulk measurements of both TOC and δ13C values. This study, including three independent investigations performed in different laboratories, is the first detailed examination of the effect of cleaning treatments on the reliability of TOC and δ13C values in a range of natural rock samples and synthetic materials with low TOC content from below detection limit to 3330 ppm. We investigated the four most common organic solvents used to remove modern organic matter: dichloromethane (DCM), n-hexane, methanol and ethanol, and evaluated the effect of grain size and mineralogy. We find that (i) cleaning treatments with methanol, n-hexane and dichloromethane contaminate rock samples when used directly on sample powder, regardless of the grain size; (ii) this pollution buffers the natural variability and homogenizes the δ13C values of samples around the isotopic composition of the solvent, i.e. between −27 and −29‰; (iii) the extent of contamination depends on the solvent used, DCM being the most contaminating (up to 6000 ppm) and ethanol the only solvent that does not seem to contaminate rock samples above our detection limit; (iv) sample mineralogy also exerts an influence on the extent of contamination, clay minerals being more prone to adsorb contaminants. We conclude that the response of carbon concentrations and the stable carbon isotopic composition of organic matter in geological samples to cleaning treatments is neither negligible nor systematic when investigating samples with low carbon content. Postprint

The causes of the greatest mass extinction in Earth's history, in the latest Permian, remain actively debated. Here we use Se isotopes and abundances in marine sediments from an outer-shelf environment to test one of the most common hypotheses for the collapse of the biosphere, i.e. widespread euxinia in the open ocean. Our data show a small positive excursion in δ 82/78 Se prior to the extinction, consistent with local euxinia. However, this is followed by a significant negative excursion with a minimum of −1.8‰ (relative to NIST SRM 3149), immediately preceding the principal extinction horizon. A net fractionation of this magnitude likely resulted from partial reduction of Se oxyanions dissolved in the water column. Due to their low abundance, Se oxyanions are rapidly scavenged in anoxic basins or regions of high biological productivity with little net isotopic fractionation. We therefore interpret the uniquely negative fractionations in this section as an indicator for relatively oxygenated conditions in this marine basin at the time when biological productivity declined. The offset between the peak excursion and the major extinction horizon possibly reflects a slow-down in ocean circulation leading to nutrient limitation, which may thus have prohibited a rapid recovery of the local biosphere in the early Triassic. Although we are unable to extrapolate to the global ocean due to the short residence time of Se in seawater, our data are consistent with the newly emerging view that euxinia developed along ocean margins and in oxygen minimum zones before the extinction, but was probably replaced by (sub-)oxic conditions during the ~ 1 kyr peak productivity decline and was thus not solely responsible for the extinction event.

The Earth’s surface has undergone major transitions in its redox state over the past three billion years, which have affected the mobility and distribution of many elements. Here we use Se isotopic and abundance measurements of marine and non-marine mudrocks to reconstruct the evolution of the biogeochemical Se cycle from ∼3.2 Gyr onwards. The six stable isotopes of Se are predominantly fractionated during redox reactions under suboxic conditions, which makes Se a potentially valuable new tool for identifying intermediate stages from an anoxic to a fully oxygenated world. δ 82/78 Se shows small fractionations of mostly less than 2‰ throughout Earth’s history and all are mass-dependent within error. In the Archean, especially after 2.7 Gyr, we find an isotopic enrichment in marine (+0.37 ± 0.27‰) relative to non-marine samples (−0.28 ± 0.67‰), paired with increasing Se abundances. Student t -tests show that these trends are statistically significant. Although we cannot completely rule out the possibility of volcanic Se addition, these trends may indicate the onset of oxidative weathering on land, followed by non-quantitative reduction of Se oxyanions during fluvial transport. The Paleoproterozoic Great Oxidation Event (GOE) is not reflected in the marine δ 82/78 Se record. However, we find a major inflection in the secular δ 82/78 Se trend during the Neoproterozoic, from a Precambrian mean of +0.42 ± 0.45‰ to a Phanerozoic mean of −0.19 ± 0.59‰. This drop probably reflects the oxygenation of the deep ocean at this time, stabilizing Se oxyanions throughout the water column. Since then, reduction of Se oxyanions has likely been restricted to anoxic basins and diagenetic environments in sediments. In light of recent Cr isotope data, it is likely that oxidative weathering before the Neoproterozoic produced Se oxyanions in the intermediate redox state Se IV , whereas the fully oxidized species Se VI became more abundant after the Neoproterozoic rise of atmospheric oxygen.

Paleomagnetism has provided key constraints on the evolution of Earth's climate, geomagnetic field, and continental geography through Phanerozoic and Proterozoic time. Extending these constraints into the Archean eon has been particularly challenging due to the paucity of the ancient rock record. Here we report paleomagnetic measurements on the NASA Astrobiology Drilling Project (ABDP)-8 core drilled through one of the world's least deformed and least metamorphosed Paleoarchean [3200–3600 million year old (Ma)] rock successions located in the East Strelley Belt of the eastern Pilbara Craton, Australia. Our results show that the ∼3350 Ma Euro Basalt preserves a shallow magnetic inclination that appears to have formed as a result of early seafloor hydrothermal alteration, suggesting that the evaporitic carbonate platform of the conformably underlying Strelley Pool Formation was deposited in a near-equatorial location. This is consistent with (although does not require) late Paleoarchean climatic zoning, low orbital obliquity, and a geocentric axial dipole (GAD) field geometry similar to that of the Phanerozoic. The Euro Basalt paleopole overlaps with previously published Paleoarchean poles from the East Pilbara craton and with time-equivalent poles reported from the Barberton Greenstone Belt of the Kaapvaal craton, supporting the existence of a Paleoarchean Vaalbara continental aggregation.

Hopanes and steranes found in Archean rocks have been presented as key evidence supporting the early rise of oxygenic photosynthesis and eukaryotes, but the syngeneity of these hydrocarbon biomarkers is controversial. To resolve this debate, we performed a multilaboratory study of new cores from the Pilbara Craton, Australia, that were drilled and sampled using unprecedented hydrocarbon-clean protocols. Hopanes and steranes in rock extracts and hydropyrolysates from these new cores were typically at or below our femtogram detection limit, but when they were detectable, they had total hopane (

Organic and inorganic carbon isotope records reflect the burial of organic carbon over geological timescales. Permanent burial of organic carbon in the crust or mantle oxidizes the surface environment (atmosphere, ocean and biosphere) by removing reduced carbon. It has been claimed that both organic and inorganic carbon isotope ratios have remained approximately constant throughout Earth9s history, thereby implying that the flux of organic carbon burial relative to the total carbon input has remained fixed and cannot be invoked to explain the rise of atmospheric oxygen (Schidlowski, 1988; Catling and others, 2001; Holland, 2002; Holland, 2009; Kump and others, 2009; Rothman, 2015). However, the opposite conclusion has been drawn from the same carbon isotope record (Des Marais and others, 1992; Bjerrum and Canfield, 2004). To test these opposing claims, we compiled an updated carbon isotope database and applied both parametric and non-parametric statistical models to the data to quantify trends and mean-level changes in fractional organic carbon burial with associated uncertainties and confidence levels. We first consider a conventional mass-balance model where carbon input to surficial reservoirs is balanced by burial of sedimentary carbonates and organic carbon. For this model, statistical analysis implies fractional organic burial has increased over Earth history by a factor of 1.5 relative to organic burial at 3.6 Ga, with the 95 percent confidence interval ranging from factors of 1.2 to 2.0. An increase in organic burial by a factor of 1.2 cannot explain the rise of oxygen, whereas an increase by a factor of 2 could conceivably explain the rise of oxygen. There is, however, a highly significant and well constrained increase in organic burial from the Proterozoic to the Phanerozoic. We also analyze changes in the difference between carbonate and organic carbon isotopic ratios over Earth history. There is a statistically significant increase in this difference from the early to late Archean, possibly caused by increased biological fractionation due to methanotrophic recycling. This transition is consistent with the evolution of oxygenic photosynthesis at 2.8 Ga or earlier. Finally, we explore how these conclusions change if we modify the traditional mass balance model to include other carbon cycle fluxes, specifically ocean crust carbonatization and authigenic carbonates. Because the size of these fluxes has a large, poorly constrained range, our statistical analysis with this uncertainty implies that the carbon isotope record does not constrain the history of organic burial at all. However, it remains possible that the magnitude of these additional processes has been inconsequential throughout geologic time, in which case conclusions from the conventional model would be valid.

Ashes of ancient meteors recovered from a 2.7-billion-year-old lake bed imply that the upper atmosphere was rich in oxygen at a time when all other evidence implies that the atmosphere was oxygen-free. See Letter p.235

We thank the NASA postdoctoral program (EES) and the NASA Exobiology Program grant number NNX16AI37G (RB) for funding. Multiple lines of evidence have revealed a thriving marine biosphere capable of diverse metabolic strategies back to at least 3.5 billion years ago (Ga). However, little is known about microbial ecosystems in lakes and rivers during the Mesoarchean and their role in the evolution of the biosphere. Here we report new carbon and nitrogen isotopic data from the fluvio-lacustrine Lalla Rookh Sandstone in Western Australia (∼3.0 Ga) – one of the oldest known non-marine sedimentary deposits. Organic δ13C values (-30‰ to -38‰) are best interpreted as recording carbon fixation by methanogens using the acetyl CoA pathway mixed with organisms using the Calvin cycle, while δ15N data (0‰ to -1‰) likely reflect biological N2 fixation using FeMo-nitrogenase. When compared with data from the literature, we show that lacustrine habitats of Mesoarchean age (3.2-2.8 Ga) are systematically depleted in δ13C (-37 ± 5‰) relative to marginal marine (-32 ± 7‰) and open marine settings (-27 ± 3‰), suggesting that methanogenesis was relatively more important in lacustrine communities. Our findings highlight: (a) the widespread use of biological N2 fixation by the Mesoarchean biosphere, (b) the potential importance of continental habitats for methane production and perhaps for the formation of hydrocarbon haze, and (c) the possible role of land masses in driving microbial diversification on the early Earth. Postprint

Selenium (Se) undergoes redox transformations and isotopic fractionations at relatively high redox potentials and could therefore provide insight into changes in oceanic and atmospheric O2 levels over Earth’s history. We test this idea with Se data from the 2.5 Ga Mount McRae Shale (Hamersley Basin, Australia), which records temporary enrichments in abundances and isotopes of other redox-sensitive elements indicating a “whiff of oxygen” in Earth’s atmosphere before the Great Oxidation Event. Se isotopic ratios expressed as δ82/78Se and abundances relative to crustal background show significant positive excursions of up to 1.1‰ and an enrichment 13 times above background, respectively, overlapping with excursions in Mo and N isotopes and abundances. Because Se has a relatively high redox potential and photosynthetic oxidation pathways are unknown, our data thus suggest that Se was mobilized by free O2 during this interval. The isotopic fractionation likely occurred during transport of Se oxyanions from the site of weathering to the outer shelf. Although O2 could have been produced locally on land and may not necessarily have increased in the global atmosphere, our results strengthen the inference of an early origin of oxygenic photosynthesis long before the Paleoproterozoic Great Oxidation Event. This is the first report of a Se isotope excursion in the Precambrian rock record, and it confirms that Se isotopes can serve as a useful redox proxy in deep time.

Nitrogen isotope ratios in ancient sedimentary rocks are generally interpreted as a proxy for metabolic nitrogen pathways and the redox state of the water column. Fractionation processes occurring under anoxic, alkaline conditions during the dissociation of NH 4 + to H+ and volatile NH3 are frequently overlooked, although this mechanism imparts large isotopic fractionations. Here we propose that NH3 volatilization is largely responsible for δ15N values of up to + 50 ‰ at high C/N ratios in the late Archean Tumbiana Formation. This sequence of sedimentary rocks represents a system of lakes that formed on subaerial flood basalts and were partly filled by basaltic volcanic ash. Aqueous alteration of volcanic glass followed by evaporative concentration of ions should have led to the development of high alkalinity with a pH of 9 or higher, as in modern analogues. In this sedimentologically unusual setting, nitrogen isotope ratios thus provide indirect evidence for the oldest alkaline lake system in the rock record. These very heavy lacustrine δ15N values contrast markedly with those of Archean marine sedimentary rocks, making a Precambrian “soda ocean” unlikely. Today, alkaline lakes are among the most productive ecosystems on Earth. Some nutrients, in particular molybdenum, are more soluble at high pH, and certain prebiotic reactions would likely have been favored under alkaline conditions in similar settings earlier in Earth's history. Hence alkaline lakes in the Archean could have been significant for the origin and early evolution of life.

Funding was provided by the NASA postdoctoral program (EES) and by the NASA Astrobiology Institute through the Virtual Planetary Laboratory (RB, Cooperative Agreement No. NNA13AA93A) and the Alternative Earths Center (TWL, Cooperative Agreement No. NNA15BB03A). The Earth-Life Transitions Program of the NSF provided additional funds to TWL. The Callanna Group was deposited around 800 million years ago (Ma) during an interval in Earth’s history that saw a transition towards a more oxygenated atmosphere, increasing biodiversity among eukaryotic microfossils and climatic perturbations culminating in low-latitude glaciations. Previous researchers proposed that the Callanna basin was lacustrine and highly alkaline, which could provide important new insights into environmental cause-effect relationships at this time. To further interrogate these records, we examined standard biogeochemical proxies, including organic carbon and nitrogen isotopes, iron speciation, metal abundances and carbonate-associated sulfate. Much of the primary information has been lost because the rocks of the Callanna Group have experienced extensive metamorphism up to amphibolite facies and are altered by modern weathering. However, relics of these proxies, combined with sedimentological features, preserve evidence of redox stratification within this basin. Furthermore, our observations, in particular weakly fractionated nitrogen isotopes and abundant gypsum pseudomorphs, are incompatible with the interpretation of high alkalinity. The high salt content and occurrences of tidal indicators are most parsimoniously explained by frequent incursions of seawater. Thus, the Callanna Group cannot speak straightforwardly to environmental conditions in non-marine habitats at this time. Lastly, the absence of a large carbon isotope anomaly indicates that these rocks do not correlate with the Bitter Springs Formation. Postprint

Financial support for this study came from the NASA postdoctoral program (EES, REA), the NSF-FESD program (RB, TWL), the NASA Astrobiology Institute (TWL, NJP, and RB), and the NASA Exobiology program (grant NNX16AI37G to RB). The diversification of macro-organisms over the last 500 million years often coincided with the development of new environmental niches. Microbial diversification over the last 4 billion years likely followed similar patterns. However, linkages between environmental settings and microbial ecology have so far not been described from the ancient rock record. In this study, we investigated carbon, nitrogen, and molybdenum isotopes, and iron speciation in five non-marine stratigraphic units of the Neoarchean Fortescue Group, Western Australia, that are similar in age (2.78–2.72 Ga) but differ in their hydro-geologic setting. Our data suggest that the felsic-dominated and hydrologically open lakes of the Bellary and Hardey formations were probably dominated by methanogenesis (δ13Corg = −38.7 ± 4.2‰) and biologic N2 fixation (δ15Nbulk =−0.6 ± 1.0‰), whereas the Mt. Roe, Tumbiana and Kylena Formations, with more mafic siliciclastic sediments, preserve evidence of methanotrophy (δ13Corg as low as −57.4‰, δ13Ccarb as low as −9.2‰) and NH3 loss under alkaline conditions. Evidence of oxygenic photosynthesis is recorded only in the closed evaporitic Tumbiana lakes marked by abundant stromatolites, limited evidence of Fe and S cycling, fractionated Mo isotopes (δ98/95Mo = +0.4 ± 0.4‰), and the widest range in δ13Corg (−57‰ to −15‰), suggesting oxidative processes and multiple carbon fixation pathways. Methanotrophy in the three mafic settings was probably coupled to a combination of oxidants, including O2 and SO42-. Overall, our results may indicate that early microbial evolution on the Precambrian Earth was in part influenced by geological parameters. We speculate that expanding habitats, such as those linked to continental growth, may have been an important factor in the evolution of life. Postprint

It has been proposed that an “oxygen overshoot” occurred during the early Paleoproterozoic Great Oxidation Event (GOE) in association with the extreme positive carbon isotopic excursion known as the Lomagundi Event. Moreover, it has also been suggested that environmental oxygen levels then crashed to very low levels during the subsequent extremely negative Shunga–Francevillian carbon isotopic anomaly. These redox fluctuations could have profoundly influenced the course of eukaryotic evolution, as eukaryotes have several metabolic processes that are obligately aerobic. Here we investigate the magnitude of these proposed oxygen perturbations using selenium (Se) geochemistry, which is sensitive to redox transitions across suboxic conditions. We find that δ82/78Se values in offshore shales show a positive excursion from 2.32 Ga until 2.1 Ga (mean +1.03 ± 0.67‰). Selenium abundances and Se/TOC (total organic carbon) ratios similarly show a peak during this interval. Together these data suggest that during the GOE there was pervasive suboxia in near-shore environments, allowing nonquantitative Se reduction to drive the residual Se oxyanions isotopically heavy. This implies O2 levels of >0.4 μM in these settings. Unlike in the late Neoproterozoic and Phanerozoic, when negative δ82/78Se values are observed in offshore environments, only a single formation, evidently the shallowest, shows evidence of negative δ82/78Se. This suggests that there was no upwelling of Se oxyanions from an oxic deep-ocean reservoir, which is consistent with previous estimates that the deep ocean remained anoxic throughout the GOE. The abrupt decline in δ82/78Se and Se/TOC values during the subsequent Shunga–Francevillian anomaly indicates a widespread decrease in surface oxygenation.

Nitrogen is a major nutrient for all life on Earth and could plausibly play a similar role in extraterrestrial biospheres. The major reservoir of nitrogen at Earth's surface is atmospheric N2, but recent studies have proposed that the size of this reservoir may have fluctuated significantly over the course of Earth's history with particularly low levels in the Neoarchean - presumably as a result of biological activity. We used a biogeochemical box model to test which conditions are necessary to cause large swings in atmospheric N2 pressure. Parameters for our model are constrained by observations of modern Earth and reconstructions of biomass burial and oxidative weathering in deep time. A 1-D climate model was used to model potential effects on atmospheric climate. In a second set of tests, we perturbed our box model to investigate which parameters have the greatest impact on the evolution of atmospheric pN2 and consider possible implications for nitrogen cycling on other planets. Our results suggest that (a) a high rate of biomass burial would have been needed in the Archean to draw down atmospheric pN2 to less than half modern levels, (b) the resulting effect on temperature could probably have been compensated by increasing solar luminosity and a mild increase in pCO2, and (c) atmospheric oxygenation could have initiated a stepwise pN2 rebound through oxidative weathering. In general, life appears to be necessary for significant atmospheric pN2 swings on Earth-like planets. Our results further support the idea that an exoplanetary atmosphere rich in both N2 and O2 is a signature of an oxygen-producing biosphere., 33 pages, 11 figures, 2 tables (includes appendix), published in Astrobiology

Financial support during the compilation of this manuscript was provided by the NASA postdoctoral program (EES), the NSF Graduate Research Fellowship Program (MAK), the Agouron Institute (MCK, RB) and the NSF FESD program (grant number 1338810, subcontract to RB). Nitrogen is an essential nutrient for all life on Earth and it acts as a major control on biological productivity in the modern ocean. Accurate reconstructions of the evolution of life over the course of the last four billion years therefore demand a better understanding of nitrogen bioavailability and speciation through time. The biogeochemical nitrogen cycle has evidently been closely tied to the redox state of the ocean and atmosphere. Multiple lines of evidence indicate that the Earth’s surface has passed in a non-linear fashion from an anoxic state in the Hadean to an oxic state in the later Phanerozoic. It is therefore likely that the nitrogen cycle has changed markedly over time, with potentially severe implications for the productivity and evolution of the biosphere. Here we compile nitrogen isotope data from the literature and review our current understanding of the evolution of the nitrogen cycle, with particular emphasis on the Precambrian. Combined with recent work on redox conditions, trace metal availability, sulfur and iron cycling on the early Earth, we then use the nitrogen isotope record as a platform to test existing and new hypotheses about biogeochemical pathways that may have controlled nitrogen availability through time. Among other things, we conclude that (a) abiotic nitrogen sources were likely insufficient to sustain a large biosphere, thus favoring an early origin of biological N2 fixation, (b) evidence of nitrate in the Neoarchean and Paleoproterozoic confirm current views of increasing surface oxygen levels at those times, (c) abundant ferrous iron and sulfide in the mid-Precambrian ocean may have affected the speciation and size of the fixed nitrogen reservoir, and (d) nitrate availability alone was not a major driver of eukaryotic evolution. Postprint

Life on land dates back at least 2.7 billion years, but the effects of this early terrestrial biosphere on biogeochemical cycling are poorly constrained. Marine sulphur data and geochemical modelling suggest that microbial pyrite weathering has transferred a substantial amount of sulphur to the oceans for at least 2.5 billion years.

Experiments dropping raindrops onto ash combined with raindrop fossil imprints yield an upper limit for air density in the Archaean. About 2.7 billion years ago, towards the end of the Archaean period, a rain shower left its mark on ash deposits from a volcanic eruption in what is now the South African veldt. As the ash hardened to form tuff rock, the crater-like imprints of the individual raindrops were fossilized. An analysis of these imprints, aided by comparison with similar prints formed during the 2010 Eyjafjallajokull eruption in Iceland, suggests that air density during the Archaean was no more than twice modern levels. At that time, the Sun was dimmer than it is today, but the climate was warm. Most theories to explain this 'Faint Young Sun' paradox have assumed that the atmosphere was denser in the Archaean than it is now, and that the greenhouse effect was stronger, but this latest work seems to rule out higher carbon dioxide levels; nitrogen-pressure broadening remains unlikely, but possible. According to the ‘Faint Young Sun’ paradox, during the late Archaean eon a Sun approximately 20% dimmer warmed the early Earth such that it had liquid water and a clement climate1. Explanations for this phenomenon have invoked a denser atmosphere that provided warmth by nitrogen pressure broadening1 or enhanced greenhouse gas concentrations2. Such solutions are allowed by geochemical studies and numerical investigations that place approximate concentration limits on Archaean atmospheric gases, including methane, carbon dioxide and oxygen2,3,4,5,6,7. But no field data constraining ground-level air density and barometric pressure have been reported, leaving the plausibility of these various hypotheses in doubt. Here we show that raindrop imprints in tuffs of the Ventersdorp Supergroup, South Africa, constrain surface air density 2.7 billion years ago to less than twice modern levels. We interpret the raindrop fossils using experiments in which water droplets of known size fall at terminal velocity into fresh and weathered volcanic ash, thus defining a relationship between imprint size and raindrop impact momentum. Fragmentation following raindrop flattening limits raindrop size to a maximum value independent of air density, whereas raindrop terminal velocity varies as the inverse of the square root of air density. If the Archaean raindrops reached the modern maximum measured size, air density must have been less than 2.3 kg m−3, compared to today’s 1.2 kg m−3, but because such drops rarely occur, air density was more probably below 1.3 kg m−3. The upper estimate for air density renders the pressure broadening explanation1 possible, but it is improbable under the likely lower estimates. Our results also disallow the extreme CO2 levels required for hot Archaean climates8.

Microscopic pyrites with low 34 S/ 32 S ratios in ~ 3.47-Gyr-old sedimentary barites from North Pole, Australia have been interpreted as evidence for microbial sulfate reduction and/or sulfur disproportionation in the early Archean. We show that these microscopic sulfides have similar to slightly less negative Δ 33 S and slightly more negative Δ 36 S values compared to the enclosing sulfate crystals. This finding is consistent with a primary mass-independent signature overprinted by biological sulfate reduction, as calibrated by previous experimental laboratory culture studies. However, it is inconsistent with an overprint by abiological sulfate reduction or sulfur disproportionation, as predicted by isotope exchange theory and laboratory culture studies. Thus, our multiple sulfur isotope measurements support the contention that sulfate-reducing microbes had evolved by ~ 3.47 billion years ago.

The observation of oil inclusions trapped prior to 2.0 Ga in Palaeoproterozoic rocks and the ability to obtain detailed molecular geochemical information from them provide a robust way for understanding the early biogeochemical evolution of the Earth. Oil-bearing fluid inclusions (FI) in ca. 2.45 Ga fluvial metaconglomerate of the Matinenda Formation at Elliot Lake, Canada were trapped in quartz and feld-spar during diagenesis and early metamorphism of the host rock, probably before ca. 2.2 Ga. The 2.1 Ga FA Formation sandstone of the Franceville Basin in Gabon that hosts the Oklo natural fission reactors has also been discovered to contain abundant Palaeoproterozoic oil-bearing FIs. This oil occurs within H2O and CO2-dominated inclusions trapped in syntaxial quartz overgrowths and intragranular and transgranular microfractures in detrital quartz, and was most likely trapped 2.1–1.98 Ga. Molecular geochemical analyses of both FI oils reveal a wide range of compounds, including n-alkanes, isoprenoids, monomethylalkanes, aromatic hydrocarbons, and trace amounts of complex multi-ring biomarkers including terpanes, hopanes, methylhopanes, steranes and diasteranes. To ensure a reliable interpretation of oil inclusions, a comprehensive series of outside-rinse blanks and procedural system blanks was analysed by gas chromatography-mass spectrometry; quantitative amounts of the hydrocarbons in these blanks were compared to the FI extracts, so as to provide confidence limits on the experimental integrity of each compound class. Maturity ratios based on reliably detected compound classes show that the FI oils were generated in the oil window, with no evidence of extensive thermal cracking. The presence of biomarkers for cyanobacteria and eukaryotes derived from and trapped in rocks deposited prior to 2.0 Ga is consistent with early evolution of oxygenic photosynthesis and suggests that some aquatic settings had become sufficiently oxygenated for sterol biosynthesis by this time. The extraction of biomarker molecules from Palaeoproterozoic oil-bearing FIs thus establishes a new method, using low detection limits and system blank levels, to trace evolution through Earth’s early history that avoids the potential contamination problems affecting shale-hosted hydrocarbons.

The atmosphere has apparently been oxygenated since the ‘Great Oxidation Event’ ca 2.4 Ga ago, but when the photosynthetic oxygen production began is debatable. However, geological and geochemical evidence from older sedimentary rocks indicates that oxygenic photosynthesis evolved well before this oxygenation event. Fluid-inclusion oils in ca 2.45 Ga sandstones contain hydrocarbon biomarkers evidently sourced from similarly ancient kerogen, preserved without subsequent contamination, and derived from organisms producing and requiring molecular oxygen. Mo and Re abundances and sulphur isotope systematics of slightly older (2.5 Ga) kerogenous shales record a transient pulse of atmospheric oxygen. As early as ca 2.7 Ga, stromatolites and biomarkers from evaporative lake sediments deficient in exogenous reducing power strongly imply that oxygen-producing cyanobacteria had already evolved. Even at ca 3.2 Ga, thick and widespread kerogenous shales are consistent with aerobic photoautrophic marine plankton, and U–Pb data from ca 3.8 Ga metasediments suggest that this metabolism could have arisen by the start of the geological record. Hence, the hypothesis that oxygenic photosynthesis evolved well before the atmosphere became permanently oxygenated seems well supported.

High-resolution chemostratigraphy reveals an episode of enrichment of the redox-sensitive transition metals molybdenum and rhenium in the late Archean Mount McRae Shale in Western Australia. Correlations with organic carbon indicate that these metals were derived from contemporaneous seawater. Rhenium/osmium geochronology demonstrates that the enrichment is a primary sedimentary feature dating to 2501 ± 8 million years ago (Ma). Molybdenum and rhenium were probably supplied to Archean oceans by oxidative weathering of crustal sulfide minerals. These findings point to the presence of small amounts of O 2 in the environment more than 50 million years before the start of the Great Oxidation Event.

New lithologic and organic carbon-isotope data are presented for the Triassic–Jurassic boundary section at Kennecott Point, Queen Charlotte Islands, British Columbia, Canada. The previously reported Late Norian to earliest Hettangian record is extended by over 130 m, and three new isotopic features are revealed. The record now shows a negative offset in baseline carbon isotope values from approximately − 29‰ in the Late Norian to − 31‰ in the Hettangian. This offset is accompanied by the previously reported 2‰ negative excursion at the Triassic–Jurassic boundary and a 5‰ positive excursion in the early Hettangian. There is a significant long-term negative isotopic trend in the Hettangian interval of the section, which may be due to CAMP volcanism. The positive excursion is attributed to a decline in bio-calcification as well as changes in microbial ecology, both related to the mass extinction at the Triassic–Jurassic boundary.

The Karoo basin of South Africa exposes a succession of Upper Permian to Lower Triassic terrestrial strata containing abundant terrestrial vertebrate fossils. Paleomagnetic/magnetostratigraphic and carbon-isotope data allow sections to be correlated across the basin. With this stratigraphy, the vertebrate fossil data show a gradual extinction in the Upper Permian punctuated by an enhanced extinction pulse at the Permian-Triassic boundary interval, particularly among the dicynodont therapsids, coinciding with negative carbon-isotope anomalies.

Eleven high precision (±2–5 million years) SHRIMP zircon U–Pb ages have been obtained from felsic rocks within a single stratigraphic section of late Archaean volcanic and sedimentary rocks in the east Pilbara of Western Australia. The stratigraphic succession (Nullagine and Mount Jope Supersequences in sequence-stratigraphic terminology, Fortescue Group in lithostratigraphic terminology) is interpreted to be the rock record of three major geotectonic cycles that formed in an extensional, rift-related environment between about 2772 and 2715 Ma. The geochronology is constrained by a detailed stratigraphic framework based on unconformities and supported by a preliminary magnetostratigraphy. Field mapping, geochemical and petrographic studies have shown that previously unrecognised thin felsic tuff bands are interbedded in subaerial flood basalt piles and mafic tuffs. While flood basalts and proximal felsic volcanic rocks comprise by volume most of the volcanogenic components of the succession, felsic volcanism is now known to have been active periodically through each geotectonic cycle. The succession covers a time period of about 57 million years. The lower ∼1400 m of a thick (∼1700 m) clastic sedimentary succession from the oldest geotectonic cycle was deposited at a rate of about 100 m per million years over a mean time period of 14 million years. In contrast, a younger ∼150 m thick cogenetic tuff-basalt unit accumulated in less than 3 million years, and others probably accumulated at similar rates, comparable to those of Phanerozoic flood basalts. Unconformities in the succession are shown to be of variable duration and one unconformity marking the boundary between the first and second geotectonic cycles may represent a time-gap of more than 10 million years. The unconformity-based stratigraphic framework, the new geochronology and palaeomagnetic studies [J. Geophys. Res. 108 (2003) B12, 2551, EMP 2-1 to 2-21] have been combined to determine a possible late Archaean continental drift rate for one part of the succession, implying a period of motion as fast as or up to five times faster than any known from the Phanerozoic.