Rise Of Atmospheric Oxygen Research Articles

Earth’s oldest terrestrial red beds as direct evidence for the Great Oxidation Event ca. 2.3 Ga

Reconstructing the trajectory of Earth’s initial rise of atmospheric oxygen (i.e., the Great Oxidation Event, GOE) remains a significant but important challenge due to the intricate connections between oxygen and life. Further refinement in our understanding of the GOE requires establishing tighter links between geochemical and mineralogical oxygenation proxies specifically in terrestrial environments where signals reflect oxygen accumulation beyond realms of localized production. The appearance of terrestrial red beds in the Paleoproterozoic rock record is oft-cited evidence for the GOE; however, there is a lack of robust evidence that establishes Fe(III)-(oxy)(hydr)oxides (now hematite) as a primary clastic sedimentary feature, and often insufficient stratigraphic and geochronological context to link red beds to other oxygenation proxies. This study revisits the transition from the youngest detrital pyrite- and uraninite-hosting terrestrial (alluvial-fluvial) strata to the oldest reddened fluvial strata in the ca. 2.45–2.22 Ga Huronian Supergroup, with the aim to directly link the mineralogy of the latter deposits to environmental oxygenation and thus the GOE. Key fluvial sandstone units preserve hematite as rims of “dust” on detrital quartz encased by epitaxial quartz overgrowth cements, providing unequivocal evidence for Fe(III)-(oxy)(hydr)oxide adhesion to detrital quartz during early meteoric diagenesis, and thus indicating terrestrial Fe oxidation pathways were more widespread than oxidized paleosols formed at this time. Geochronological constraints place the appearance of these terrestrial red beds at ∼2.31 Ga, timing that closely matches with the S-isotope evidence for the GOE in correlative strata of the Transvaal Supergroup. The S-isotope and red bed proxy records show promise for a closely coupled oxygenation threshold, with the advantage that they are typically preserved in different depositional environments.

The dynamic ocean redox evolution during the late Cambrian SPICE: Evidence from the I/Ca proxy

The late Cambrian Steptoean positive carbon isotope excursion (SPICE) is a distinct chemostratigraphic feature of the Paleozoic, marked by a 4–5‰ shift in carbonate δ13C that has been recognized across the globe during the Paibian Stage. The SPICE may be related to enhanced burial of organic matter and pyrite during the expansion of marine euxinia, which as a source of O2 also results in a pulse of atmospheric oxygen. However, geochemical proxies have not clearly illustrated how the ocean redox evolved with atmospheric oxygen changes during the SPICE. This study presents new carbonate I/Ca data, a redox proxy for the upper ocean, from three basins. I/Ca values are low at Great Basin and South China from early into the peak of SPICE, indicating generally anoxic conditions in shallow waters. The overall increasing trend in I/Ca through the peak and recovery phase of the SPICE roughly correlates with the previously modeled rise in atmospheric oxygen. Spatially, the Georgina Basin (Mt. Whelan) might have recorded a relatively more oxic upper ocean compared to the Great Basin and South China. Earth system model simulations also demonstrate the importance of paleogeographic and oceanographic settings on local redox conditions, highlighting the redox heterogeneity during the SPICE.

Rare earth element and yttrium (REY) geochemistry of 3.46–2.45 Ga greenalite-bearing banded iron formations: New insights into iron deposition and ancient ocean chemistry

Finely laminated cherts enclosing nanoparticles of greenalite and apatite are ubiquitous in Archean-Paleoproterozoic (3.46–2.45 Ga) ferruginous cherts, jaspilites and Banded Iron Formations (BIFs) and the greenalite and apatite are considered to be primary deposits. The cherts and BIFs are chemical sedimentary rocks interpreted to have been precipitated in marine settings prior to the first permanent rise in atmospheric oxygen at the Great Oxidation Event (GOE) ca. 2.45–2.32 M.y. ago. As chemical sediments, they are potential archives of the solutions from which they precipitated, incorporating signals from hydrothermal fluids and ambient seawater. Previous studies of rare earth elements and Y (REY) in pre-GOE BIFs have mostly found an “Archean seawater signature” with positive Eu anomalies, attributed to the influence of high-temperature hydrothermal processes, and positive anomalies for La, Gd and Y, ascribed to seawater. REY abundances determined by in situ LA-ICP-MS are presented for well-preserved, laminated greenalite-bearing cherts from ten formations of pre-GOE ferruginous cherts and BIFs from Western Australia. The samples come from a wide range of depositional environments, e.g., submarine proximal volcanic environments, basin floor, slope and deep marine shelf, and are between 3.46 Ga to 2.45 Ga in age. Five groups with different REY patterns are identified, namely (i) mafic-volcanic-influenced vent-proximal chert from the Marble Bar Chert Member of the Duffer Formation of the Warrawoona Group; (ii) felsic volcanic- and sediment-associated chert from the Kangaroo Caves Formation of the Sulphur Springs Group and the Wilgie Mia Formation of the Murchison Supergroup, (iii) ferruginous cherts in shelf sediments from the Bee Gorge Member of the Wittenoom Formation of the Hamersley Group, (iv) BIFs from the Nammuldi Member of the Marra Mamba Iron Formation and Joffre Member of the Brockman Iron Formation, Hamersley Group, and (v) BIFs from the Dales Gorge Member of the Brockman Iron Formation. Of these, only the Dales Gorge Member BIF has a typical Archean seawater signature while the others have REY patterns likely reflecting differing source fluids and environments of deposition but not necessarily global ocean chemistry. Analyses of chert containing sub-micron-sized particles of greenalite and apatite indicate that the likely hosts of the REEs are apatite, siderite and possibly greenalite. The REY patterns of the greenalite-bearing cherts may differ from those of bulk samples of the same formations, perhaps reflecting a diagenetic overprint in the bulk samples, whereas the greenalite-bearing cherts likely preserve their depositional compositions, locked in by early silicification.

Mechanisms for the rise of atmospheric oxygen: Bridging surface oxygenation processes and redox conditions of deep interiors

Mechanisms for the rise of atmospheric oxygen: Bridging surface oxygenation processes and redox conditions of deep interiors

Biogeochemical transformations after the emergence of oxygenic photosynthesis and conditions for the first rise of atmospheric oxygen.

The advent of oxygenic photosynthesis represents the most prominent biological innovation in the evolutionary history of the Earth. The exact timing of the evolution of oxygenic photoautotrophic bacteria remains elusive, yet these bacteria profoundly altered the redox state of the ocean-atmosphere-biosphere system, ultimately causing the first major rise in atmospheric oxygen (O2 )-the so-called Great Oxidation Event (GOE)-during the Paleoproterozoic (~2.5-2.2 Ga). However, it remains unclear how the coupled atmosphere-marine biosphere system behaved after the emergence of oxygenic photoautotrophs (OP), affected global biogeochemical cycles, and led to the GOE. Here, we employ a coupled atmospheric photochemistry and marine microbial ecosystem model to comprehensively explore the intimate links between the atmosphere and marine biosphere driven by the expansion of OP, and the biogeochemical conditions of the GOE. When the primary productivity of OP sufficiently increases in the ocean, OP suppresses the activity of the anaerobic microbial ecosystem by reducing the availability of electron donors (H2 and CO) in the biosphere and causes climate cooling by reducing the level of atmospheric methane (CH4 ). This can be attributed to the supply of OH radicals from biogenic O2 , which is a primary sink of biogenic CH4 and electron donors in the atmosphere. Our typical result also demonstrates that the GOE is triggered when the net primary production of OP exceeds >~5% of the present oceanic value. A globally frozen snowball Earth event could be triggered if the atmospheric CO2 level was sufficiently small (<~40 present atmospheric level; PAL) because the concentration of CH4 in the atmosphere would decrease faster than the climate mitigation by the carbonate-silicate geochemical cycle. These results support a prolonged anoxic atmosphere after the emergence of OP during the Archean and the occurrence of the GOE and snowball Earth event during the Paleoproterozoic.

Did nutrient-rich oceans fuel Earth’s oxygenation?

Abstract Phosphorus (P) availability exerts a strong influence on primary productivity in global oceans. However, its abundance and role as a limiting nutrient prior to the start of the Great Oxygenation Event (GOE) 2.45–2.32 Ga is unclear. Low concentrations of seawater P have been proposed to explain the apparent delay between the early appearance of oxygen-producing Cyanobacteria and the onset of atmospheric oxygenation. We report evidence for seawater precipitation of Ca-phosphate nanoparticles in 2.46–2.40 Ga iron formations deposited on a marine shelf, including shallow-water facies, immediately prior to the onset of the GOE. Our modeling shows that the co-precipitation of Ca-phosphate and ferrous silicate (greenalite) required ferruginous seawater with dissolved P concentrations many orders of magnitude higher than in today’s photic zone. If correct, it follows that P availability is unlikely to have suppressed the expansion of Cyanobacteria prior to the GOE. A reservoir of P-rich surface water shortly before 2.40 Ga could ultimately have triggered a rapid rise in atmospheric oxygen by fueling a sharp increase in primary productivity and organic-carbon burial. We speculate that the enigmatic Lomagundi positive carbon-isotope excursion, recorded in 2.32–2.06 Ga shallow-water carbonates, may mark a key step in the transition toward a modern biosphere of high biological productivity controlled by nutrient availability.

Did the Neoproterozoic Revolution Extend to the Deep Mantle?

Did the Neoproterozoic Revolution Extend to the Deep Mantle?

Mesoarchean banded iron-formation from the northern Yangtze Craton, South China and its geological and paleoenvironmental implications

Banded iron formations (BIFs) are important sedimentary rocks used for reconstructing the geochemical and environmental evolution of Precambrian Earth surface conditions. Here, we describe the oldest BIFs of Mesoarchean age in the Yemadong Formation, and their implications for paleo-environmental evolution of the Mesoarchean Ocean in the northern Yangtze Craton, South China. U-Pb dating of metamorphic zircons obtained from metavolcanic rocks interbedded within the Yemadong Complex, record two major tecto-thermal events at 2901 ± 8 Ma, in agreement with a Mesoarchean origin of the Dujiagou BIFs, and as recently as 775 Ma. A high SiO2 + total Fe2O3 content of 90.95–94.62 wt%, moderate 0.54–1.42 wt% Al2O3 + TiO2, and broadly elevated levels of incompatible elements, point to a predominantly chemical origin of the BIF deposit, with a minor but influential terrigenous input. Rare Earth Element (REE) plus Yttrium patterns normalized against Post Archean Average Shale composition are characterized by the depletion of light REE relative to heavy REE. However, extremely weak Eu and Ce anomalies, together with low average chondrite Y/Ho ratios compared to average Mesoarchean BIF values, indicate that the primary REE composition of the rocks may have been diluted to some extent by terrigenous input, pointing to proximity or connection of the depositional setting to a continental margin environment. Despite potential terrigenous dilution of the syngenetic seawater-hydrothermal chemical composition, the preservation of an inherited hydrothermal REE signal in some samples, point to submarine hydrothermal provenance of the Dujiagou BIFs, as is the case for most Precambrian BIFs. Positive δ56Fe values, ranging from 0.25 to 0.45 ‰, characteristic of primary Archean BIFs, suggest partial oxidation of reduced Fe(II) to divalent (Fe(II)/Fe(III)) magnetite-rich BIFs in a broadly reducing Archean Ocean lacking a persistent and pervasive redoxcline before the rise of atmospheric oxygen.

Anoxic photochemical weathering of pyrite on Archean continents

Sulfur is an essential element of life that is assimilated by Earth’s biosphere through the chemical breakdown of pyrite. On the early Earth, pyrite weathering by atmospheric oxygen was severely limited, and low marine sulfate concentrations persisted for much of the Archean eon. Here, we show an anoxic photochemical mechanism of pyrite weathering that could have provided substantial amounts of sulfate to the oceans as continents formed in the late Archean. Pyrite grains suspended in anoxic ferrous iron solutions produced millimolar sulfate concentrations when irradiated with ultraviolet light. The Fe2+(aq) was photooxidized, which, in turn, led to the chemical oxidation of pyritic sulfur. Additional experiments conducted with 2.68 Ga shale demonstrated that photochemically derived ferric iron oxidizes and dissolves sedimentary pyrite during chemical weathering. The results suggest that before the rise of atmospheric oxygen, oxidative pyrite weathering on Archean continents was controlled by the exposure of land to sunlight.

Large igneous provinces track fluctuations in subaerial exposure of continents across the Archean–Proterozoic transition

AbstractGeological observations and numerical models imply that Archean continents were mostly submarine. In contrast, approximately one third of modern earth's surface area consists of subaerial continental crust. To temporally constrain changes in the subaerial exposure of continents, we evaluate the eruptive environment (submarine vs subaerial) of 3.4–2.0 Ga continental large igneous provinces (LIPs). Our results indicate that up until 2.4 Ga LIPs predominantly erupted onto submerged continents. This period of low freeboard was punctuated by local subaerial eruptions at 2.8–2.7 Ga and 2.5 Ga. From 2.4 Ga–2.2 Ga, extensive subaerial continental volcanism is recorded in six different cratons, supporting widespread subaerial continents at this time. An increase in exposed continental crust significantly impacts atmospheric and oceanic geochemical cycles and the supply of nutrients for marine bioproductivity. Thus, the 2.4–2.2 Ga high‐freeboard conditions may have triggered the earliest global glaciation event and the first significant rise of atmospheric oxygen.

Long-term evolution of terrestrial weathering and its link to Earth's oxygenation

Terrestrial weathering releases phosphorus and other essential nutrients that fuel life in Earth's surface environments and sustain oxygenic photosynthesis. Despite previous suggestions that major changes in terrestrial chemical weathering might have played a role in the global oxygen cycle in the geological past, little is known about the Earth's weathering history. To date, the cause-and-effect relationship between weathering and the long-term evolution of atmospheric oxygen, and whether chemical weathering became more efficient after the initial rise of atmospheric oxygen in the early Paleoproterozoic, remained largely elusive. Here we report a reconstruction of the intensity of terrestrial weathering for the last 2.7 billion years, based on coupled neodymium-hafnium isotope (ΔεHf(i)CLAY) and elemental analyses of the fine-grained clay-size fraction of shales. A pronounced shift towards higher ΔεHf(i)CLAY values and rubidium/aluminium (Rb/Al) ratios indicates that preferential dissolution of phosphate-bearing minerals and biotite intensified between ∼2.5 and 2.4 billion years ago (Ga), following the emergence of continental landmasses and coinciding with the initiation of the Great Oxidation Event. After a long time interval characterized by a constant degree of low-intensity chemical weathering, between ∼2.3 to 0.7 Ga, terrestrial weathering further accelerated after the Neoproterozoic glaciations at ∼0.6 Ga, as inferred from markedly decreased Rb/Al ratios, coincident with the second rise of atmospheric oxygen. These findings support a link between the long-term intensity of chemical weathering and atmospheric oxygen level since the late Archean. We further propose that the 100-million-year-long period of enhanced terrestrial weathering from ∼2.5 Ga played a major role, via crustal recycling of phosphorus and export to the surface ocean, in the early expansion of the aerobic biosphere that ultimately led to the Great Oxidation Event.

Earth’s Great Oxidation Event facilitated by the rise of sedimentary phosphorus recycling

The rise of atmospheric oxygen during the Great Oxidation Event some 2.4 billion years ago was a defining transition in the evolution of global biogeochemical cycles and life on Earth. However, mild oxidative continental weathering and the development of ocean oxygen oases occurred several hundred million years before the Great Oxidation Event. The Great Oxidation Event thus represents a tipping point, whereby primary productivity and O2 production overwhelmed the input of reduced species that consume O2, and its timing is determined by the input of phosphate, the major limiting nutrient, and the dynamics of the solid Earth. Here, we determine the phase partitioning of phosphorus in 2.65 to 2.43 billion year old drill core samples from the Transvaal Supergroup, South Africa, to investigate the sequence of events that facilitated persistent atmospheric oxygenation. On the basis of the elevated C/P ratios found within sulfidic sediments, relative to the Redfield ratio, we suggest that, as oxidative continental weathering increased the influx of dissolved sulfate and hence dissolved sulfide in the oceans, bioavailable phosphorus became more abundant due to anoxic recycling of sedimentary phosphorus phases. Biogeochemical modelling indicates that this initiated a positive feedback on primary productivity and shows that the evolution of phosphorus recycling may have been a critical step that enabled Earth’s transition to a persistently oxygenated atmosphere. Recycling of sedimentary phosphorus driven by increasing oceanic sulfide availability contributed to the persistent oxygenation of Earth’s atmosphere, according to analysis of Archean drill-core samples and biogeochemical modelling

The temporal distribution of Earth's supermountains and their potential link to the rise of atmospheric oxygen and biological evolution

We use the distribution of low-Lu and low-Lu/Dy zircons, derived from the eroded roots of mountains, where trace amounts of zircon compete with abundant garnets for heavy rare earth elements, to identify periods of extensive high mountain (supermountain) formation. The data reveal that Earth has two, and they correlate with periods when the average metamorphic pressure of orogenic belts exceeded 1.2 GPa, the pressure at which metamorphic garnet becomes abundant. The first supermountains formed at 2.0-1.8 Ga, during the assembly of Nuna, and the second at 650-500 Ma, during the amalgamation of Gondwana. The 650-500 Ma event has been previously recognized and named the Transgondwanan Supermountain, but the 2.0-1.8 Ga Nuna Supermountains have not. Our data also show that mountain building was limited during the Boring Billion, between 1.8 and 0.8 Ga.Mountain building results in high rates of erosion and sedimentation, and the two periods of supermountain formation are associated with voluminous sedimentation, whereas sediment production during the Boring Billion was more limited. Enhanced erosion would have increased the supply of bio-limiting nutrients such as phosphorous to the oceans, potentially increasing primary productivity and the flow of energy through ecosystems. Enhanced carbon production and sedimentation, both driven by supermountain erosion, are also expected to lead to increases in atmospheric oxygen. During Transgondwanan Supermountain erosion, increasing oxygen and nutrient levels may be connected to the proliferation of chlorophyte algae after 650 Ma and the emergence of large, animal-like organisms 75 Myr later. Targeted research of Nuna-aged sediments is needed to evaluate whether rapid erosion of supermountains is linked to geochemical and biotic events, such as the disappearance of banded iron formations at 1.85 Ga, the emergence of the first macroscopic organisms (Grypania) at 1.9 Ga, and the radiation of early eukaryotes, which become visible in the fossil record at 1.65 Ga.

Feedback between surface and deep processes: Insight from time series analysis of sedimentary record

Sediment accumulation and subduction at continental margins are hypothesized to be crucial in the operation of Wilson cycle style plate tectonics on Earth. To test this hypothesis, we explore the interaction between continental sedimentation and plate tectonics by utilizing a 3.5 Gyr continent-scale sediment abundance record from the Macrostrat database. Wavelet analysis of a time series of sedimentary rock volume identified a supercontinent-like cycle of ∼500 Myr across the past 3.0 Gyr. Intriguingly, this long-term fluctuation in net sedimentation record that indicates continent-derived sediment flux into oceanic crust is in phase with the vigor of subduction activity (as characterized by zircon εHf and δ18O isotopes) through time. These agreements provide independent evidence for the hypothesis that the operation of Wilson cycle style plate tectonics was facilitated by sediment accumulation and subduction (lubrication) at continental margins. Moreover, wavelet cross-correlation analysis of sediment abundance and subduction flux suggests an increasingly efficient sediment cycling via subduction since ∼3.0 Ga, notably coinciding with transition periods of atmospheric oxygen levels at the beginning and end of the Proterozoic. The results suggest that the co-evolving surface process and plate tectonics that increased long-term net sequestration of organic carbon could have contributed to the initial rise of atmospheric oxygen at the beginning of the Proterozoic and the climb to sustained modern levels in the early Phanerozoic.

Temporal variation of planetary iron as a driver of evolution

Iron is an irreplaceable component of proteins and enzyme systems required for life. This need for iron is a well-characterized evolutionary mechanism for genetic selection. However, there is limited consideration of how iron bioavailability, initially determined by planetary accretion but fluctuating considerably at global scale over geological time frames, has shaped the biosphere. We describe influences of iron on planetary habitability from formation events >4 Gya and initiation of biochemistry from geochemistry through oxygenation of the atmosphere to current host–pathogen dynamics. By determining the iron and transition element distribution within the terrestrial planets, planetary core formation is a constraint on both the crustal composition and the longevity of surface water, hence a planet’s habitability. As such, stellar compositions, combined with metallic core-mass fraction, may be an observable characteristic of exoplanets that relates to their ability to support life. On Earth, the stepwise rise of atmospheric oxygen effectively removed gigatons of soluble ferrous iron from habitats, generating evolutionary pressures. Phagocytic, infectious, and symbiotic behaviors, dating from around the Great Oxygenation Event, refocused iron acquisition onto biotic sources, while eukaryotic multicellularity allows iron recycling within an organism. These developments allow life to more efficiently utilize a scarce but vital nutrient. Initiation of terrestrial life benefitted from the biochemical properties of abundant mantle/crustal iron, but the subsequent loss of iron bioavailability may have been an equally important driver of compensatory diversity. This latter concept may have relevance for the predicted future increase in iron deficiency across the food chain caused by elevated atmospheric CO2.

Origin, tectonic environment and age of the Bibole banded iron formations, northwestern Congo Craton, Cameroon: geochemical and geochronological constraints

AbstractThe newly discovered Bibole banded iron formations are located within the Nyong Group at the northwest of the Congo Craton in Cameroon. The Bibole banded iron formations comprise oxide (quartz-magnetite) and mixed oxide-silicate (chlorite-magnetite) facies banded iron formations, which are interbedded with felsic gneiss, phyllite and quartz-chlorite schist. Geochemical studies of the quartz-magnetite banded iron formations and chlorite-magnetite banded iron formations reveal that they are composed of &gt;95 wt % Fe2O3 plus SiO2 and have low concentrations of Al2O3, TiO2 and high field strength elements. This indicates that the Bibole banded iron formations were not significantly contaminated by detrital materials. Post-Archaean Australian Shale–normalized rare earth element and yttrium patterns are characterized by positive La and Y anomalies, a relative depletion of light rare earth elements compared to heavy rare earth elements and positive Eu anomalies (average of 1.86 and 1.15 for the quartz-magnetite banded iron formations and chlorite-magnetite banded iron formations, respectively), suggesting the influence of low-temperature hydrothermal fluids and seawater. The quartz-magnetite banded iron formations display true negative Ce anomalies, while the chlorite-magnetite banded iron formations lack Ce anomalies. Combined with their distinct Eu anomalies consistent with Algoma- and Superior-type banded iron formations, we suggest that the Bibole banded iron formations were deposited under oxic to suboxic conditions in an extensional basin. SIMS U–Pb data indicate that the Bibole banded iron formations were deposited at 2466 Ma and experienced metamorphism and metasomatism at 2078 Ma during the Eburnean/Trans-Amazonian orogeny. Overall, these findings suggest that the studied banded iron formations probably marked the onset of the rise of atmospheric oxygen, also known as the Great Oxidation Event in the Congo Craton.

Global continental volcanism controlled the evolution of the oceanic nickel reservoir

Wanning nickel (Ni) abundances in the Neoarchean may have led to a decline in marine methanogenic bacteria due to the loss of a key micronutrient, causing a collapse in atmospheric methane levels and the subsequent rise of atmospheric oxygen – the Great Oxidation Event ∼2.5 Ga. However, this hypothesis currently lacks geological evidence that corroborates the processes that led to a decline in the supply of Ni to the oceans at that time. Here we investigate temporal variations of Ni concentrations in the likely source rocks – various volcanic lithologies - spanning the past 3.5 Byr. By applying a spatially gridded resampling to geochemical data from a global compilation of ∼96,000 continental volcanic rocks, we show that komatiites and basaltic-andesitic rocks largely controlled Ni fluxes to the Earth's surface during the Archean but had a waning influence thereafter due to the secular cooling of the mantle. A new compilation of marine shales (397 samples) and an updated compilation of banded iron formations (2037 samples) further confirms a declining clastic and dissolved supply of terrestrial Ni, respectively. Interestingly, we further observed a stable marine Ni concentration for the last 2 Byr. To explain this unexpected trend, we provide a computational model of mantle melting demonstrating that increasing melting pressures likely inhibited a further decline of Ni concentration in volcanic rocks and led to this relative consistency in Ni supply.

The grandest of them all: the Lomagundi–Jatuli Event and Earth's oxygenation

The Paleoproterozoic Lomagundi–Jatuli Event (LJE) is generally considered the largest, in both amplitude and duration, positive carbonate C-isotope ( δ 13 C carb ) excursion in Earth history. Conventional thinking is that it represents a global perturbation of the carbon cycle between 2.3–2.1 Ga linked directly with, and in part causing, the postulated rise in atmospheric oxygen during the Great Oxidation Event. In addition to new high-resolution δ 13 C carb measurements from LJE-bearing successions of NW Russia, we compiled 14 943 δ 13 C carb values obtained from marine carbonate rocks 3.0–1.0 Ga in age and from selected Phanerozoic time intervals as a comparator of the LJE. Those data integrated with sedimentology show that, contra to consensus, the δ 13 C carb trend of the LJE is facies (i.e. palaeoenvironment) dependent. Throughout the LJE interval, the C-isotope composition of open and deeper marine settings maintained a mean δ 13 C carb value of +1.5 ± 2.4‰, comparable to those settings for most of Earth history. In contrast, the 13 C-rich values that are the hallmark of the LJE are limited largely to nearshore-marine and coastal-evaporitic settings with mean δ 13 C carb values of +6.2 ± 2.0‰ and +8.1 ± 3.8‰, respectively. Our findings confirm that changes in δ 13 C carb are linked directly to facies changes and archive contemporaneous dissolved inorganic carbon pools having variable C-isotopic compositions in laterally adjacent depositional settings. The implications are that the LJE cannot be construed a priori as representative of the global carbon cycle or a planetary-scale disturbance to that cycle, nor as direct evidence for oxygenation of the ocean–atmosphere system. This requires rethinking models relying on those concepts and framing new ideas in the search for understanding the genesis of the grandest of all positive C-isotope excursions, its timing and its hypothesized linkage to oxygenation of the atmosphere. Supplementary material : C–O isotope data, figure S1, tables S1–S5 and the dataset for δ 13 C carb values are available at https://doi.org/10.6084/m9.figshare.c.5471815

Evolution of Superoxide Dismutases and Catalases in Cyanobacteria: Occurrence of the Antioxidant Enzyme Genes before the Rise of Atmospheric Oxygen.

Knowledge on the evolution of antioxidant systems in cyanobacteria is crucial for elucidating the cause and consequence of the rise of atmospheric oxygen in the Earth's history. In this study, to elucidate the origin and evolution of cyanobacterial antioxidant enzymes, we analyzed the occurrence of genes encoding four types of superoxide dismutases and three types of catalases in 85 complete cyanobacterial genomes, followed by phylogenetic analyses. We found that Fe superoxide dismutase (FeSOD), Mn superoxide dismutase (MnSOD), and Mn catalase (MnCat) are widely distributed among modern cyanobacteria, whereas CuZn superoxide dismutase (CuZnSOD), bifunctional catalase (KatG), and monofunctional catalase (KatE) are less common. Ni superoxide dismutase (NiSOD) is distributed among marine Prochlorococcus and Synechococcus species. Phylogenetic analyses suggested that bacterial MnSOD evolved from cambialistic Fe/MnSOD before the diversification of major bacterial lineages. The analyses suggested that FeSOD evolved from MnSOD before the origin of cyanobacteria. MnCat also evolved in the early stages of bacterial evolution, predating the emergence of cyanobacteria. KatG, KatE, and NiSOD appeared 2.3-2.5 billion years ago. Thus, almost all cyanobacterial antioxidant enzymes emerged before or during the rise of atmospheric oxygen. The loss and appearance of these enzymes in marine cyanobacteria may be also related to the change in the metal concentration induced by the increased oxygen concentration in the ocean.

Evolution of seawater continentally-sourced Nd isotopic composition prior to and during the Great Oxidation Event

An ongoing debate concerns what initiated the Great Oxidation Event (GOE) and associated glaciation between ~ 2.45 and 2.2 Ga. One possibility is the emergence of continental landmasses and the increase of subaerial igneous province weathering during the Late Archean. We test this hypothesis in the Hamersley Basin by reporting Nd-isotope data from a succession of iron formations (IFs), mudstone/siltstones and glacial diamictites from the Boolgeeda Iron Formation and overlying Turee Creek Group deposited during the GOE. In a 147Sm/144Nd - ɛNd(t) diagram, the data define a negative trend indicating the contribution of a high ɛNd(t) ~ +3 hydrothermal component and a strongly negative ɛNd(t) ~ -9 crustal component, which is compatible with the Nd-isotope composition of the upper continental crust but also of the underlying felsic volcanics of the Woongarra Rhyolite and crustally-contaminated mafic volcanics of the Fortescue Group. A less pronounced negative trend originating from the same hydrothermal source but correlated with non-contaminated ultramafic Fortescue volcanics (ɛNd(t) ~ -2) is observed for the older Joffre, Dales Gorge and Marra Mamba IFs. As Nd-isotopes are not sensitive to redox conditions, the major shift of Nd-isotopic compositions at ~ 2.45 Ga cannot be linked to a change in the weathering regime, rather to a change in the nature of the continental surface exposed to weathering. One explanation is that the Sm-Nd sources for sediments deposited before and during the GOE were locally derived from the underlying subaerial LIPs, reflecting a change in the geodynamic context of deposition and/or hydrographic network and catchment areas. Another explanation could be a significant change in the nature of fluid–rock interactions due to the increase role of weathering processes associated with the emergence of continental landmasses. Additional Nd isotope data from different cratons worldwide are needed, however, to infer as to whether or not the marked shift in Nd isotope compositions recorded in the Turee Creek Group reflect a change in the global hydrological cycle. Our data support the role of large subaerial magmatic provinces as triggers of the rise of atmospheric oxygen and the onset of glaciations at the beginning of the Proterozoic.