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• Oxygen fugacity of neoarchean subduction-metasomatized peridotites is measured. • Neoarchean subduction zone mantle in the North China Craton was oxidized. • Neoarchean rise of mantle oxidation in the North China Craton was caused by plate subduction. The Archean mantle redox state played an important role in degassing of the Earth's interior and thus influenced atmospheric oxygen levels of the early Earth. But it is unclear if any parts of the uppermost mantle were significantly oxidized by a certain point in the Archean. Here, we investigate oxygen fugacity (f O 2) of Archean (> 2535–2517 Ma) peridotites in the North China Craton. Petrology and geochemistry reveal that they experienced strong Neoarchean subduction-related metasomatism. These Neoarchean subduction-metasomatized peridotites record f O 2 of ΔFMQ +1.3 ± 0.4 (SD) [relative to the fayalite-magnetite-quartz (FMQ) buffer], which are more oxidized than the Archean ambient mantle, but similar to the modern sub-arc mantle. We propose that this Neoarchean rise of mantle oxidation in the North China Craton was induced by plate subduction, during which the Neoarchean sub-arc mantle in the North China Craton could have been metasomatized and oxidized, and its oxygen fugacity was increased. This process may have had connections with the Great Oxidation Event in the Early Proterozoic. [ABSTRACT FROM AUTHOR]

• Repeated methane haze & glacial episodes occurred in Neoarchean & Paleoproterozoic. • These episodes are associated with increases in O 2 through the interval. • We investigated this using a dynamical model of microbial ecological interactions. • Results indicate interplay between resources and O 2 caused cyclic methane dynamics. Geochemical observations and models suggest methane was an important greenhouse gas in the Archean and early Proterozoic eons. Consequently, fluctuations in methane concentration in Earth's atmosphere could have contributed to climate change and influenced the flux of UV radiation reaching surface environments. If correct, understanding the factors (e.g., O 2 and/or resource concentration) that drove the biological methane cycle might shed light on the repetition of biological, atmospheric and climatic events preserved in the sedimentary rock record from ∼2.8 to 2.2 Ga. To explore these interdependencies, we developed a novel dynamical model of microbial ecological interactions to investigate the conditions under which methane is preferentially released to the atmosphere. We found that the interplay between resource and O 2 availability results in complex cyclic methane dynamics unrelated to the functional groups or efficiencies of microbial communities, to initial conditions, or to other model constraints. Based on these results, we propose that the cyclicity of methane haze events and glacial episodes in the Neoarchean and early Paleoproterozoic may have been linked to the progressive increase in oceanic and atmospheric O 2 through the interval. [ABSTRACT FROM AUTHOR]

• Primary Mn, Mo, Ce and Cr minerals evolved periodically after the GOE. • The evolutionary dynamics of high-valent minerals followed Mn>Ce>Mo>Cr. • All minerals kept continuous deposition and active evolution from 1.8 to 0.8 Ga. • There are two depositional discontinuities around the GOE and the NOE. Understanding the co-evolution of redox-sensitive minerals is crucial for unraveling Earth's oxygenation history. In this study, we used a global-scale mineralogical dataset encompassing 12,141 samples of both the primary and secondary minerals containing manganese (Mn), molybdenum (Mo), chromium (Cr) and cerium (Ce), and 18,777 samples of primary phosphorus (P) and carbon (C) minerals. By developing mathematical models of evolutionary dynamics, we quantified the time lag between changes in the atmospheric oxygen level (p O 2) and the corresponding response in the evolution of these minerals. The analysis revealed that the evolutionary time lag sequence of high-valent Mn, Ce, Mo and Cr minerals after 2.4 Ga, aligns with the thermodynamic sequence of oxidation reactions, with Mn and Ce minerals displaying the most rapid dynamic response, followed by Mo and Cr minerals (denoted as MnIV>CeIV>MoVI>CrVI). All minerals maintained active and continuous evolution throughout the period of orogenic quiescence from 1.8 to 0.8 Ga, but have two depositional discontinuities during the Great Oxidation Event (the GOE) and the Neoproterozoic Oxidation Event (the NOE) (C, P minerals had a depositional slowdown at around the NOE). The collective missing of mineralogical records pointed to the rapid evolution of the Earth's internal and external environment, and is possibly caused by a sharp decrease in material input (such as the tectono-magmatic lull or slowdown) or strong secondary alterations at around the GOE and the NOE. The quantitative analysis on the evolutionary dynamics of redox-sensitive minerals may contribute to refining proxies that constrain the evolving redox state of deep-time Earth. [ABSTRACT FROM AUTHOR]

Atmospheric O 2 and CO 2 levels inform us of the changes in chemical and biological environments, yet the history of atmospheric compositions, and p O 2 in particular, is not well-constrained. The triple oxygen isotope (16,17,18O) composition of marine SO 4 2 − has been proposed to directly record the ratio p O 2 / p CO 2 in the contemporaneous atmosphere. To resolve this atmospheric signal, both a precise measurement of the 17O composition of sulfate and a model with which to interpret the measurement are needed. Here we present precise measurements of the triple oxygen isotope composition of modern marine sulfate and then forward a novel sulfur cycle model that deconvolves the potential atmospheric and microbial inputs to this signal. Our interpretation of marine sulfate oxygen isotope composition provides a framework for calculating atmospheric composition, relative rates of biogeochemical activity, and can be applied to geologic records of marine sulfate to constrain the p O 2 / p CO 2 ratio over time. • New, precise measurements of the 17O composition of modern marine sulfate. • Interpretation of the sulfate signal with a novel sulfur cycle model. • Framework to calculate atmospheric composition and biogeochemical rates. [ABSTRACT FROM AUTHOR]

Reconstructing Earth's oxygenation history is key to deciphering environmental controls on early biospheric evolution. During the mid-Proterozoic Eon, low (but potentially variable) atmospheric p O 2 led to highly heterogeneous marine redox conditions, with most studies indicating a relatively shallow depth of oxygen penetration. The relative proportion of oxic, anoxic and iron-rich (ferruginous), and anoxic and sulfide-rich (euxinic) conditions on the global seafloor is difficult to quantify, however, due to a general reliance on local redox proxies applied to the temporally discontinuous black shale record. It is particularly important to constrain the global prevalence of euxinic bottom waters because sulfide toxicity has been implicated as a primary constraint on evolution in mid-Proterozoic oceans and limits the solubility of bioessential trace metals such as copper, zinc, and molybdenum. Here, we present a suite of new uranium (U) isotope data from marine carbonate rocks that span the entire mid-Proterozoic interval (∼1.8 to 0.8 Ga). U-isotopes in well-preserved carbonate rocks represent a powerful new proxy that can be used to quantitatively constrain the global extent of marine euxinia. The median δ 238 U value for mid-Proterozoic carbonate samples is −0.43‰, which is lower than the median value for modern post-depositional carbonate sediments from the Bahamas, yet significantly higher than carbonate δ 238 U values recorded during other times of expanded anoxia in Earth history, such as during the end-Permian mass extinction. When paired with a three-sink isotope mass balance model, our data indicate that no more than 7% of the global seafloor was euxinic during the mid-Proterozoic Eon, although transient pulses of expanded euxinia are recorded. Although these results challenge early expectations of a sulfide-rich mid-Proterozoic ocean, they are consistent with more recent inferences from the black shale trace metal record. Evidence therefore indicates that euxinic bottom waters were relatively limited in their spatial extent in the mid-Proterozoic oceans. • Uranium isotopes in marine carbonate constrain Proterozoic seafloor euxinia. • Median Proterozoic δ 238U (−0.43‰) lower than modern Bahamian carbonate. • Median Proterozoic δ 238U higher than end-Ediacaran and Permo-Triassic carbonate. • Mass balance model suggests that at maximum 7% of seafloor was euxinic. • Mid-Proterozoic euxinia was less prevalent than previously envisaged. [ABSTRACT FROM AUTHOR]

Atmospheric oxygen (O 2) concentrations likely remained below modern levels until the Silurian–Devonian, as indicated by several recent studies. Yet, the background redox state of early Paleozoic oceans remains poorly constrained, hampering our understanding of the relationship between early animal evolution and O 2. Here, we present a multi-proxy analysis of redox conditions in the Caledonian foreland basin to Baltica from the early to the mid-Silurian. Our results indicate that anoxic to severely hypoxic bottom waters dominated during deposition of the Silurian sediments cored in the Sommerodde-1 well (Bornholm, Denmark), and regional comparison suggests that these conditions persisted across the Baltoscandian foreland basin. Indeed, even during times of relative oxygenation, ichnological observations indicate that conditions were, at most, very weakly oxic. The results suggest that dissolved O 2 was generally scarce in the bottom waters of the extensive Silurian seaway between Baltica and Avalonia, even between Paleozoic "Anoxic Events". In light of delayed oxygenation of the atmosphere–hydrosphere system, it may be time to consider that early animals were adapted to "hypoxia" and thrived through ∼100 million years of low-O 2 conditions after the Cambrian. • New redox data from the Silurian stratigraphy of the Caledonian foreland basin. • Regional comparison indicates low-O 2 bottom waters dominated for ∼10 million years. • Discussion of conflicting redox terminology across scientific fields. • Delayed ocean oxygenation suggests low-O 2 was the norm for early Paleozoic animals. • This prompts us to rethink our assumptions about Paleozoic animals and O 2. [ABSTRACT FROM AUTHOR]

The origin (hydrothermal vs. continental Fe source) and environmental implications in particular for the atmospheric oxygen level of mid-Proterozoic ironstones have been widely studied, but remain highly debated. Here, we present Re-Os, Sr-Nd isotopic and PGE elemental data for ironstones from the ∼1.7 Ga Yunmengshan (YMS) Formation in south margin of the North China Craton to address the controversy. The initial 87Sr/86Sr ratios of the YMS ironstones are ∼0.7063, which are roughly comparable with seawater values around that period, suggesting a seawater signal with minor/minimal direct contributions from submarine hydrothermal fluids. The Nd isotopic compositions (ε Nd (t): −6.6 to −6.0) indicate a dominantly continental crustal signal for the formation of the ironstones. The PGE patterns of the YMS ironstones are distinctly similar to those of hydrogenous Fe-Mn crusts/nodules in modern ocean, also suggesting deposition from normal seawater conditions. Importantly, the YMS ironstones give initial 187Os/188Os ratios of ∼0.6 at ∼1.7 Ga, providing critical constraints for contemporaneous seawater values. This seawater 187Os/188Os value of ∼0.6 strongly suggests a result of oxidative weathering and transportation of Os in continental crust into the ocean, implying a relatively high atmospheric oxygen level at ∼1.7 Ga. • 87Sr/86Sr and PGE-Re patterns of the YMS ironstones suggest a normal seawater signal. • Nd isotopes suggest a primarily crustal source for the YMS ironstones. • YMS ironstones record elevated seawater 187Os/188Os values to ∼0.6 at ∼1.7 Ga. • Os isotopes in YMS ironstones imply a pulsed rise of atmospheric O 2 level at ∼1.7 Ga. • Ironstone is good candidate for Os isotope chemo-stratigraphic studies. [ABSTRACT FROM AUTHOR]

The concentration of atmospheric oxygen ( p O 2 ) is thought to have increased throughout Earth history, punctuated by rapid increases ca. 2.4 and 0.8 billion years ago near the beginning and end of the Proterozoic Eon. As photosynthesis is the largest source of free O 2 , the reigning paradigm of rising O 2 levels centres around biologic metabolism. Here we show that the phosphorus content of igneous rocks correlates, in a first-order sense, with secular increases in O 2 through time, suggesting that rising O 2 levels are affected by long-term mantle cooling and its effect on the continental phosphorus inventory. Because phosphorus is the limiting nutrient for primary productivity, its availability has fundamental control over the efficiency of oxygenic photosynthesis, pointing to a previously unrecognized role of the solid Earth in biologic and atmospheric evolution. Furthermore, as many bio-essential elements are effectively incompatible in the mantle, this relationship has implications for any terrestrial planet. All planets will cool, and those with efficient plate tectonic convection will cool more rapidly. We are left concluding that the speed of such cooling may affect pattern of biological evolution on any habitable planet. [ABSTRACT FROM AUTHOR]

The chemical response of the Precambrian oceans to rising atmospheric O 2 levels remains controversial. The iron isotope signature of sedimentary pyrite is widely used to trace the microbial and redox states of the ocean, yet the iron isotope fractionation accompanying pyrite formation in nature is difficult to constrain due to the complexity of the pyrite formation process, difficulties in translating the iron isotope systematics of experimental studies to natural settings, and insufficient iron isotope datasets for natural euxinic (i.e. anoxic and sulfidic) marine basins where pyrite formation occurs. Herein we demonstrate, that a large, permil-level shift in the isotope composition of dissolved iron occurs in the Black Sea euxinic water column during syngenetic pyrite formation. Specifically, iron removal to syngenetic pyrite gives rise to an iron isotope fractionation factor between Fe(II) and FeS 2 of 2.75 permil (‰), the largest yet reported for reactions under natural conditions that do not involve iron redox chemistry. These iron isotope systematics offer the potential to generate permil-level shifts in the sedimentary pyrite iron isotope record due to partial drawdown of the oceanic iron inventory. The implication is that the iron stable isotope signatures of sedimentary pyrites may record fundamental regime shifts between pyrite formation under sulfur-limited conditions and pyrite formation under iron-limited conditions. To this end, the iron isotope signatures of sedimentary pyrite may best represent the extent of euxinia in the past global ocean, rather than its oxygenation state. On this basis, the reinterpreted sedimentary pyrite Fe isotope record suggests a fundamental shift towards more sulfidic oceanic conditions coincident with the ‘Great Oxidation Event’ around 2.3 billion years ago. Importantly, this does not require the chemical state of the ocean to shift from mainly de-oxygenated to predominantly oxygenated in parallel with the permanent rise in atmospheric oxygen, contrary to other interpretations based on iron isotope systematics. [ABSTRACT FROM AUTHOR]

Constraining the timing of eukaryogenesis and the divergence of eukaryotic clades is a major challenge in evolutionary biology. Here, we present trace metal concentration and zinc isotope data for c. 2.1 billion-year-old Francevillian Group pyritized structures, previously described as putative remnants of the first colonial multicellular organisms, and their host black shales. Relative to the host rocks, pyritized structures are strongly enriched in zinc, cobalt and nickel, by at least one order of magnitude, with markedly lighter zinc isotope compositions. A metabolic demand for high concentrations of aqueous zinc, cobalt, and nickel combined with preferential uptake of lighter zinc isotopes may indicate metalloenzyme utilization by eukaryotes in marine habitats c. 2.1 billion years ago. Once confirmed, this would provide a critical calibration point for eukaryogenesis, suggesting that this major evolutionary innovation may have happened contemporaneously with elevated atmospheric oxygen levels during the latter part of the Great Oxidation Event, some 400 million years earlier than is currently widely accepted. • Biogenic origin of the c. 2.1-billion-year-old Francevillian Group biota. • Enrichment of light zinc isotopes in the Francevillian biota is consistent with eukaryotic metabolism. • Utilization of metalloenzymes by possible eukaryotes in the Paleoproterozoic oceans. • Possible eukaryotic biota preserved in the Paleoproterozoic Francevillian Group. • Inferred eukaryogenesis during the Great Oxidation Event. [ABSTRACT FROM AUTHOR]

Atmospheric oxygen concentration has increased over Earth history, from ∼0 before 2.5 billion years ago to its present-day concentration of 21%. The initial rise in p O 2 approximately 2.3 billion years ago required oxygenic photosynthesis, but the evolution of this key metabolic pathway was not sufficient to propel atmospheric oxygen to modern levels, which were not sustained until approximately two billion years later. The protracted lag between the origin of oxygenic photosynthesis and abundant O 2 in the surface environment has many implications for the evolution of animals, but the reasons for the delay remain unknown. Here we show that the history of sediment accumulation on continental crust covaries with the history of atmospheric oxygen concentration. A forward model based on the empirical record of net organic carbon burial and oxidative weathering of the crust predicts two significant rises in p O 2 separated by three comparatively stable plateaus, a pattern that reproduces major biological transitions and proxy-based p O 2 records. These results suggest that the two-phased oxygenation of Earth's surface environment, and the long delays between the origin of life, the evolution of metazoans, and their subsequent diversification during the Cambrian Explosion, was caused by step-wise shifts in the ability of the continents to accumulate and store sedimentary organic carbon. The geodynamic mechanisms that promote and inhibit sediment accumulation on continental crust have, therefore, exerted a first-order control on the evolution of Earth's life and environment. [ABSTRACT FROM AUTHOR]

The oxygenation state of the Mid-Proterozoic (1.8 – 0.8 Ga) ocean and atmosphere is heavily debated and has implications for the relationship between environmental O 2 and the emergence of complex life. While recent geochemical proxy studies of Mesoproterozoic marine sedimentary rocks inform a picture of deep water redox dominated by widespread ferruginous (iron-rich and anoxic) conditions, a growing number of datasets indicate that short-lived ocean oxygenation events were recurrent features of this time interval. The spatial extent of previously reported Mesoproterozoic oxygen pulses is often unclear as they are predominantly observed via localized environmental proxies. In this study, we use the vanadium (V) and thallium (Tl) isotope paleoredox proxies to provide a global marine redox perspective during a short interval of the Mesoproterozoic. We reconstructed seawater V (δ 51 V SW) and Tl (ε 205 Tl SW) isotopic compositions from shales from Unit 2 of the 1.38-1.39 Ga Xiamaling Formation (North China craton) that show an up-section shift in average δ 51 V SW values from 0.02‰ to 0.15‰ and a short-lived perturbation in ε 205 Tl SW from −2.5 to −4.2. Mass-balance models for both isotopic systems are consistent with these isotopic shifts representing an expansion of oxic water column and porewater conditions from a more anoxic (ferruginous) baseline ocean state. The short-lived nature of the ε 205 Tl SW perturbation versus the shift in δ 51 V SW suggests that environments sufficiently oxygenated to support widespread Mn oxide burial were only established for a fraction of this overall oxic expansion. The ocean residence time of V and Tl requires that this oxygenation event was global in extent and may reflect either a purely oceanic phenomenon such as global deepening of the oxycline, or alternatively, global ocean-atmosphere equilibration during a short-lived episode of elevated atmospheric oxygenation. • We report V and Tl isotopic systematics for Unit 2 of the Mesoproterozoic Xiamaling formation. • An up-section shift to higher δ 51 V and lower ε 205 Tl values is observed. • These shifts are interpreted as recording a change in global marine V and Tl mass balance. • Mass balance calculations suggest an expansion of oxic V and Tl sinks in the upper section. • We infer an episode of expanded global oxic V and Tl sinks in an otherwise anoxic regime. [ABSTRACT FROM AUTHOR]

Regularly alternating reduction-oxidation (redox) patterns attributed to variations in the Earth's orbit and axis (Milankovitch cycles) are widely recorded in marine sediment successions of the Phanerozoic and attest to a dynamic history of biospheric oxygen in response to astronomically forced climate change. To date, however, such astronomical redox control remains elusive for much older, Precambrian intervals of the geological record that were characterized by a globally anoxic and iron-rich ocean, i.e., prior to Earth's atmospheric oxygenation (ca. 2.4–2.2 billion years ago). Here we report a detailed cyclostratigraphic and geochemical investigation of marine-sedimentary redox cycles identified in the ca. 2.46 billion-year-old Joffre Member of the Brockman Iron Formation, NW Australia, suggesting the imprint of Earth's climatic precession cycle. Around the base and top of regularly intercalated mudrock layers, we identify sharp enrichments in redox sensitive elements (Fe, S, Ca, P) that appear to represent chemical reaction fronts formed during nonsteady state diagenesis. Using a reactive transport model, we find that the formation of characteristic double S peaks required periods of increased organic matter deposition, coupled to strongly declining Fe2+ concentrations in the overlying water column. This combination, in turn, implies a periodic deepening of the iron chemocline due to enhanced oxygenic photosynthesis in marine surface waters, and is interpreted as the result of precession-induced changes in monsoonal intensity that drove variations in runoff and associated nutrient delivery. Our study results point to a dynamic redox evolution of Precambrian oceanic margin environments in response to Milankovitch forcing, and offer a temporal framework to investigate linkages between biological activity and the early build-up of oxygen in Earth's ocean-atmosphere system. • Precession-scale redox cycles are identified in banded iron formations from 2.46 Ga. • We present an early diagenetic model for characteristic patterns in iron and sulfur. • Cycles mark periodic increase in marine organic matter flux and redoxcline deepening. • Possible monsoonal forcing of enhanced runoff and oxygenic photosynthesis levels. • Astronomical pacing of dynamic redox evolution pre-GOE oceanic margin settings. [ABSTRACT FROM AUTHOR]

We use a one-dimensional atmospheric photochemical model to investigate the effect of temperature and humidity variations on the evolution of O 2 and O 3 across the Great Oxidation Event (GOE), and find that changes to the atmospheric temperature profile can change O 2 and O 3 levels by up to three orders of magnitude. Cold temperatures (<280 K) result in stable low O 2 levels (mixing ratio of 10 − 4), whereas temperate (280 K − 310 K) and hot (>310 K) temperatures result in stable high O 2 levels (mixing ratio of 10 − 2) after the GOE. Warm and wet climates lead to an increased atmospheric oxidation power through the increased production of hydrogen oxide (HO x) radicals, catalyzing the oxidation and depletion of the principal reduced species (CH 4 , CO and H 2). Consequently, warmer temperatures lead to less O 2 lost through the oxidation of reduced species, resulting in higher O 2 and O 3 levels relative to colder temperatures. The GOE occurred around the same period as the onset of the Snowball Earth Huronian glaciations. Cold climates during the glaciations were eventually terminated by the build-up of volcanic greenhouse gases in the atmosphere, leading to Hot-Moist Greenhouse climates. Temperature and humidity likely varied extensively from Snowball Earth climates during the glaciations to Hot-Moist Greenhouse climates. However, current photochemical models of the GOE do not consider the effect of this climate variability on the evolution of O 2 and O 3 across the GOE. Our results show that temperature and humidity exert a strong control on atmospheric chemistry through O 2 , O 3 , and HO x feedbacks, which has important implications for our understanding of the climate-redox evolution of the GOE. • We model the Great Oxidation Event (GOE) for different climates. • Temperature and humidity exert a strong control on O 2 and O 3 across the GOE. • Cold and dry climates result in scarce O 2 and O 3 after the GOE. • Warmer climates have more O 2 and O 3 , but O 3 decreases again above 340 K. • The ozone layer peaks at higher altitudes in warm climates. [ABSTRACT FROM AUTHOR]

The thermal state of the mantle impacts lithospheric dynamics and Earth's surface evolution. Geodynamic simulations predict that aggregated continents may act as an effective thermal insulator leading to the warming of the underlying mantle (mantle warming hypothesis). Mantle warming weakens the continental lithosphere, which should lead to more distributed strain at plate margins, and result in lower-relief orogens, with lower metamorphic peak pressures (peak- P), and slower cooling and exhumation of metamorphic rocks. To test the mantle warming hypothesis, we determine the secular change of metamorphic cooling and exhumation rates based on a new dataset compiled from Neoarchean and younger orogens. We find that sluggish cooling and exhumation during the mid-Proterozoic (1.85–0.85 Ga; aka the boring billion) correlate with lower peak- P and higher thermobaric (temperature/pressure, T / P) ratios of metamorphic rocks. This result is consistent with a weakened continental lithosphere and the development of hotter lower-relief orogens under warmer mantle conditions during the mid-Proterozoic. We posit that the long-lived Columbia–Rodinia supercontinents may have acted as a thermal insulator that led to the mantle warming beneath the continental lithosphere and thus a distinctive style of mid-Proterozoic orogenesis. Low-relief mid-Proterozoic orogens with sluggish exhumation, in turn, reduced continental erosion and restricted delivery of bio-essential nutrients to the oceans, and decreased rates of organic carbon burial and thereby hindered oxygen accumulation in the atmosphere. The persistent nutrient deficiency in the oceans and suppressed atmospheric oxygenation may have stalled the surface evolution during the boring billion. • A new dataset of metamorphic cooling and exhumation rates is built. • Mid-Proterozoic orogens are low-relief with reduced cooling/exhumation rates. • Mantle warming under Columbia–Rodinia caused mid-Proterozoic low-relief orogens. • Low-relief orogens may have stalled surface evolution during the boring billion. [ABSTRACT FROM AUTHOR]

Early animals experienced multiple-phase radiations and extinctions from the late Ediacaran to early Cambrian. Oxygen likely played an important role in these evolutionary events, but detailed marine redox evolution during this period remains highly debated. The emerging vanadium (V) isotope system can better capture short-term perturbations to global ocean redox conditions. In this study, we analyzed V isotope compositions (δ 51 V) of organic-rich cherts and black shales deposited from the late Ediacaran to early Cambrian (ca. 560–518 Ma) in the Yangtze Block, South China. The robust positive correlation between the δ 51 V values and previously reported δ 98 Mo values validates V isotope system as a paleo-oxybarometer. The continuously temporal open-ocean seawater δ 51 V variation is reconstructed from the sedimentary δ 51 V records. The results suggest that the ocean experienced a rapid transition from expansive euxinia at ca. 560–553 Ma to widespread oxygenation likely reaching the modern level at ca. 552–551 Ma, and kept extensively oxygenated approaching the modern oxygenation level from ca. 551 Ma to 521 Ma and reaching the modern level again at ca. 521–518 Ma. The prolonged and widespread oceanic oxygenation may have been beneficial to the ecological radiation of early animals from the late Ediacaran to early Cambrian, ultimately leading up to the "Cambrian Explosion". • Co-variation between the δ 51 V and δ 98 Mo validates V isotope system as a paleo-oxybarometer. • The ocean rapidly transited from euxinia to oxygenation at ca. 560–551 Ma. • The ocean kept extensively oxygenated since ca. 550 Ma. [ABSTRACT FROM AUTHOR]

• Source region of the Ordovician sandstones suffered stronger weathering than that of the Cambrian sandstones. • Baoshan block was located at Indian margin of East Gondwana in the early Paleozoic. • Subduction initiation of the Proto-Tethys Ocean was at 540-500 Ma. • A shift in the tectonic setting occurred at East Gondwana margin. • Subduction of the Proto-Tethys Ocean and formation of the peripheral orogens provided the driver for ecological evolution. The effect of plate interactions on surface processes, environmental changes of the earth and their contributions to biodiversification during Gondwana assembly has been hotly debated in the last decades. The late Neoproterozoic-early Paleozoic was a key period in the tectonic assembly of Gondwana, global climate change and metazoan evolution. During this interval, the amalgamation of Greater Gondwana was succeeded by the subduction initiation of oceanic slabs in the Proto-Tethys Ocean. Here, we use whole-rock geochemical data, detrital zircon U-Pb and Hf isotopic of Cambrian and Ordovician sedimentary rocks from the Baoshan block in the Southeastern Tibetan Plateau combined with existing data in the literature to constrain the timing of subduction initiation of the Proto-Tethys Ocean at 540-500 Ma. The results show that the Baoshan block, adjacent to the Pinjarra Orogen on the northern margin of India, witnessed the transition from passive to active continental margin in East Gondwana. After the subduction initiation, the source of the sedimentary rocks experienced more intense weathering, causing a higher sediment flux. Contemporaneously, the Cambrian Explosion and stepwise increase in atmospheric oxygen accompanied the Greater Gondwana assembly at the late Neoproterozoic-early Paleozoic boundary. Subduction of the Proto-Tethys Ocean and occurrence of peripheral orogens further increased atmospheric O 2 content, transported nutrients to the ocean and reduced the greenhouse effect, which provided major propulsion for biological evolution. [ABSTRACT FROM AUTHOR]

The “Cambrian Explosion” is known for rapid increases in the morphological disparity and taxonomic diversity of metazoans. It has been widely proposed that this biological event was a consequence of oxygenation of the global ocean, but this hypothesis is still under debate. Here, we present high-resolution Fe–S–C–Al-trace element geochemical records from the Jinsha (outer shelf) and Weng'an (outer shelf) sections of the early Cambrian Yangtze Platform, integrating these results with previously published data from six correlative sections representing a range of water depths (Xiaotan, Shatan, Dingtai, Yangjiaping, Songtao, and Longbizui). The integrated iron chemistry and redox-sensitive trace element data suggest that euxinic mid-depth waters dynamically coexisted with oxic surface waters and ferruginous deep waters during the earliest Cambrian, but that stepwise expansion of oxic waters commenced during Cambrian Stage 3 ( ∼ 521 – 514 Ma ). Combined with data from lower Cambrian sections elsewhere, including Oman, Iran and Canada, we infer that the global ocean exhibited a high degree of redox heterogeneity during the early Cambrian, consistent with low atmospheric oxygen levels ( ∼ 10 – 40 % of present atmospheric level, or PAL). A large spatial gradient in pyrite sulfur isotopic compositions ( δ 34 S py ), which vary from a mean of − 12.0 ‰ in nearshore areas to + 22.5 ‰ in distal deepwater sections in lower Cambrian marine units of South China imply low concentrations and spatial heterogeneity of seawater sulfate, which is consistent with a limited oceanic sulfate reservoir globally. By comparing our reconstructed redox chemistry with fossil records from the lower Cambrian of South China, we infer that a stepwise oxygenation of shelf and slope environments occurred concurrently with a gradual increase in ecosystem complexity. However, deep waters remained anoxic and ferruginous even as macrozooplankton and suspension-feeding mesozooplankton appeared during Cambrian Stage 3. These findings suggest that the “Cambrian Explosion” in South China may have been primarily a consequence of locally improved oxygenation of the ocean-surface layer rather than of the full global ocean. Our observations are inconsistent with predicted changes in ocean chemistry driven by early Cambrian animals, suggesting that the influence of early Cambrian animals on contemporaneous ocean chemistry, as proposed in previous studies, may be overly exaggerated. [ABSTRACT FROM AUTHOR]

It is recently proposed that early stellar luminosity evolution of M dwarfs leads to severe water loss and the buildup of massive O 2 atmospheres on rocky exoplanets in the habitable zone of these stars if interactions of such O 2 atmospheres with planetary surfaces are inefficient. Here we show that even without considering atmosphere–surface interactions, the existence of a massive O 2 atmosphere on such exoplanets is not an unavoidable consequence around M0–M3 stars and depends on stellar XUV properties, the mass of the exoplanets, and most importantly the initial planetary water inventories. In the case of inefficient atmosphere–surface interactions, the distribution of atmospheric O 2 contents on these exoplanets should be bi-modal and such a distribution could be verified by future surveys of rocky exoplanets. [ABSTRACT FROM AUTHOR]

The Great Oxidation Event (GOE) represents a crucial juncture in Earth history, signifying the rise in atmospheric oxygen from parts per million to percent levels at ∼2.45–2.32 billion-years-ago (Ga). Although planetary oxygenation undoubtedly led to the inception of the contemporary Earth system, the trigger(s) and mechanism(s) controlling this chemical reorganisation remain elusive. Quantitative estimates of the atmosphere's composition in the prelude to the GOE are central to understanding this oxygenation event. Previous analyses of 2.65–2.5 Ga sediments from the Griqualand Basin (South Africa) invoke a tantalising picture of an unusual Earth environment, alluding to an atmosphere periodically dominated by a layer of organic particles (“haze”) formed from methane photolysis. However, as yet this hypothesis has remained untested. Here we present four new coupled carbon and quadruple sulphur isotope records from distal, time equivalent (2.7–2.5 Ga), sedimentary successions from South Africa and Western Australia. These extended records reveal similar chemostratigraphic trends, supporting a dynamic terminal-Neoarchaean atmosphere, oscillating between a hazy state at elevated methane concentrations, and a haze-free anoxic background state. We suggest these atmospheric aberrations were related to heightened biogenic methane fluxes fuelled by enhanced nutrient delivery from climatically or weathering induced feedbacks. These data question the canonical view of a simple, unidirectional planetary oxygenation and signify that the overture to the GOE was governed by complex feedbacks within the Earth–biosphere system. [ABSTRACT FROM AUTHOR]

• Multiple S isotopes analysed in pyrite nodules from a Mesoarchaean volcanic lake. • 36S-depleted isotope compositions reflect mass-independent fractionation in a dense volcanic plume. • The terrestrial setting allowed instantaneous trapping and preservation of atmospheric S. Archaean supracrustal rocks carry a record of mass-independently fractionated S that is interpreted to be derived from UV-induced photochemical reactions in an oxygen-deficient atmosphere. Experiments with photochemical reactions of SO 2 gas have provided some insight into these processes. However, reconciling experimental results with the multiple S isotopic composition of the Archaean sedimentary record has proven difficult and represents one of the outstanding issues in understanding the Archaean surface S-cycle. We present quadruple S isotope data (32S, 33S, 34S, 36S) for pyrite from Mesoarchaean carbonaceous sediments of the Dominion Group, South Africa, deposited in an acidic volcanic lake, which help reconcile observations from the Archaean sedimentary record with the results of photochemical experiments. The data, which show low Δ 33 S/ δ 34 S ratios (mostly ≪ 1) and very negative Δ 36 S/ Δ 33 S ratios (−4 and lower), contrast with the composition of most Archaean sedimentary sulfides and sulfates, having Δ 36 S/ Δ 33 S ∼ − 1 (the so-called 'Archaean reference array'), but match those of modern photochemical sulfate aerosols produced in the stratosphere, following super-large volcanic eruptions, and preserved in Antarctic ice. These data are also consistent with the results of UV-irradiation experiments of SO 2 gas at variable gas pressure. The S isotope composition of the Dominion Group pyrite is here interpreted to reflect the products of photolysis in a low-oxygen-level atmosphere at high SO 2 pressure during large volcanic eruptions, mixed with Archaean 'background' (having a composition broadly similar to the Archaean reference array) S pools. It is inferred that high sedimentation rates in a terrestrial basin resulted in an instantaneously trapped input of atmospheric S during short-lasted depositional intervals, which faithfully represents transient photochemical signals in comparison with marine sedimentary records. [ABSTRACT FROM AUTHOR]

Marine ironstone is a Phanerozoic biochemical sedimentary rock that contains abundant primary iron. Although rare, ironstone is conspicuous in the Paleozoic sedimentary record. Its iron source remains contentious, with traditional models invoking a continentally derived source. Increasing sedimentologic evidence suggests that many Paleozoic ironstones formed along favourably oriented continental margins where coastal upwelling delivered ferruginous waters, with the postulated source of iron being deep-ocean hydrothermal fluids. Thus, deep-water redox conditions are potentially a critical control on the concentration of iron in early Phanerozoic seawater and the accumulation of ironstone. We report novel in situ strontium isotopic and europium anomaly data from five geographically and temporally distinct Ordovician-Silurian ironstones from the Rheic and Iapetus oceans to test the emerging ferruginous upwelling model. These data provide robust evidence that ironstone formed from marine fluids with a component of mid-ocean ridge hydrothermal waters that remained largely unmixed during transport to the locations of ironstone accumulation. These results imply that the deep oceans must have been oxygen deficient to allow local or regional enrichment in hydrothermal iron. Such conditions are interpreted to have been associated with a spatiotemporally variable redox-stratified global ocean with anoxia beneath an oxic surface layer. Thus, sedimentologic and geochemical data presented herein agree with other paleoredox proxies that suggest rising oxygen levels in the Neoproterozoic gave way to early Paleozoic oceans with dynamic redox conditions. The temporal record of Paleozoic ironstone suggests redox stratification, poor deep-ocean ventilation, and upwelling-related ironstone genesis persisted until the Devonian but not into the Carboniferous. • Paleozoic ironstone formed from hydrothermal mid-ocean ridge iron. • In situ analyses identify high-resolution chemical signatures lost in bulk analyses. • Data implies Ordovician-Silurian oceans were at least transiently redox stratified. • Ironstone is a novel proxy supporting widespread deep anoxia until the Devonian. [ABSTRACT FROM AUTHOR]

The later stages of Earth's transition to a permanently oxygenated atmosphere during the Great Oxidation Event (GOE; ∼2.43–2.06 Ga) is commonly linked with the suggestion of an "oxygen overshoot" during the ∼2.22–2.06 Ga Lomagundi Event (LE), which represents Earth's most pronounced and longest-lived positive carbon isotope excursion. However, the magnitude and extent of atmosphere-ocean oxygenation and implications for the biosphere during this critical period in Earth's history remain poorly constrained. Here, we present nitrogen (N), selenium (Se), and carbon (C) isotope data, as well as bio-essential element concentrations, for Paleoproterozoic marine shales deposited during the LE. The data provide evidence for a highly productive and well-oxygenated photic zone, with both inner and outer-shelf marine environments characterized by nitrate- and Se oxyanion-replete conditions. However, the redoxcline subsequently encroached back onto the inner shelf during global-scale deoxygenation of the atmosphere-ocean system at the end of the LE, leading to locally enhanced water column denitrification and quantitative reduction of selenium oxyanions. We propose that nitrate-replete conditions associated with fully oxygenated continental shelf settings were a common feature during the LE, but nitrification was not sufficiently widespread for the aerobic nitrogen cycle to impact the isotopic composition of the global ocean N inventory. Placed in the context of Earth's broader oxygenation history, our findings indicate that O 2 levels in the atmosphere-ocean system were likely much lower than modern concentrations. Early Paleoproterozoic biogeochemical cycles were thus far less advanced than after Neoproterozoic oxygenation. • Enhanced oxygenic photosynthesis during the ∼2.22–2.06 Ga Lomagundi carbon isotope excursion. • Fully oxygenated continental shelf seawater column during the Paleoproterozoic Lomagundi Event. • Coevolution of deep redoxcline in the Paleoproterozoic oceans and atmospheric oxygen overshoot during the Lomagundi Event. • Modern biogeochemical cycles were not achieved in the Paleoproterozoic oceans during the Great Oxidation Event. [ABSTRACT FROM AUTHOR]

The Ediacaran Period (∼635 to 541 Ma) witnessed the early diversification and radiation of metazoans, in the form of the Ediacaran Biota. This biological revolution, beginning at ∼575 Ma, has been widely attributed to a temporally restricted episode of deeper ocean oxygenation, potentially caused by a contemporaneous rise in atmospheric oxygen levels. However, quantitative geochemical-record-driven estimates of Ediacaran atmospheric and oceanic redox evolution are lacking, and hence possible links between oceanic and atmospheric oxygenation remain speculative. Here, after screening for possible post-depositional alteration, we utilize paleogeographically-diverse carbon and sulfur isotope records from South China, Oman and USA-Mexico, to develop a biogeochemical isotope mass balance model to quantify Ediacaran atmospheric oxygen and oceanic sulfate evolution. Model results from all three continents indicate that Ediacaran atmospheric oxygen levels rose monotonically between ∼630 Ma and ∼590 Ma, and subsequently remained relatively stable at around 0.6 present atmospheric level for the remainder of the Ediacaran. By contrast, the marine sulfate reservoir appears to have remained relatively stable before ∼575 Ma, with a subsequent large pulse where sulfate concentrations rose to ∼8 mM. These quantitative results indicate that Ediacaran oceanic and atmospheric oxygenation were decoupled, which is consistent with published geochemical records. We propose that the early Ediacaran rise of atmospheric oxygen levels, driven by increased net burial of organic carbon and pyrite, may not have established widespread deep-ocean oxygenation. Instead, later pulsed input of oxidizing power (mainly sulfate) from the continents drove transient episodes of seafloor oxygenation that accompanied radiations of the Ediacaran Biota. • Systematic quantification of Ediacaran atmospheric oxygen levels and oceanic sulfate concentrations. • An isotope mass balance technique forced by global C-S isotopic records. • Decoupled atmospheric and oceanic oxygenation. • Marine oxygenation lagged behind atmospheric O 2 rise. [ABSTRACT FROM AUTHOR]

Highlights: [•] Geomagnetic field reversal substantially weakens the protection for the atmosphere. [•] Solar wind energizes more oxygen ions to escape when geomagnetic field is weakened. [•] Oxygen escape may explain the drop of atmospheric level during mass extinction. [•] The causal relation between reversal and mass extinction should be “many-to-one”. [•] The simulated oxygen escape rate based on knowledge of Mars support our hypothesis. [Copyright &y& Elsevier]

Abstract: Many studies suggest that oxygen has remained near modern levels throughout the Phanerozoic, but was much less abundant from the “Great Oxygenation Event” around 2.4 Ga until the late Neoproterozoic around 600 Ma (Kump, 2008). Using a simple model, we show that the maintenance of atmospheric pO2 at ∼1% of present atmospheric levels (PAL) is inconsistent with modern biogeochemical cycling of carbon, sulfur and iron unless new feedbacks are included. Low oxygen conditions are stable in our model if the flux of phosphorus to the oceans was greatly reduced during the Proterozoic. We propose a mechanism to reduce this flux through the scavenging of phosphate ions with an “iron trap” driven by greater surface mobility of ferrous iron in a low pO2 world. Incorporating this feedback leads to two stable equilibria for atmospheric oxygen, the first quantitative hypothesis to explain both Proterozoic and Phanerozoic O2 concentrations. [Copyright &y& Elsevier]

Abstract: Recent observations of several planet-hosting M dwarfs show that most have FUV/NUV flux ratios 1000 times greater than that of the Sun. Here we show that the atmospheric oxygen contents (O2 and O3) of potentially habitable planets in this type of UV environment could be 2–3 orders of magnitude greater than those of their counterparts around Sun-like stars as a result of decreased photolysis of O3, H2O2, and HO2. Thus detectable levels of atmospheric oxygen, in combination with the existence of H2O and CO2, may not be the most promising biosignatures on planets around stars with high FUV/NUV ratios such as the observed M dwarfs. [Copyright &y& Elsevier]

Abstract: The rise in atmospheric oxygen between 2.45 and 2.2 Ga has been linked to the demise of a methane-rich atmosphere and the onset of multiple glaciations, culminating in a possible Snowball Earth. The glacial deposits represent possible global marker horizons, however, their correlation is uncertain because key sedimentary successions are undated, hampering paleoenvironmental reconstructions. Three potentially global glaciations have been proposed based on the correlation of glacial deposits in southern Africa with those in North America. However, it has also been suggested that there were four glaciations and that the youngest was a Snowball Earth event recorded only in South African successions. In situ U–Pb zircon ages for tuffs in southern African and North American successions establish the existence of four glaciations between 2.45 and 2.22 Ga. Geochronological and stratigraphic data demonstrate that the three oldest glaciations predate ∼2.31 Ga and that the final glaciation, which is only recognized in South Africa, occurred between 2.26 and 2.22 Ga. The new age-calibrated correlations show that a rise in atmospheric oxygen inferred from sulfur isotope data occurred between the second and third glaciations. At 2.31 Ga, the first marine sulfate evaporites were deposited contemporaneously with 13C-enriched carbonates, indicating a direct link between perturbations of the carbon and sulfur cycles and rising atmospheric oxygen. The appearance of “red beds” and oxidized palesols after the third glaciation signals a major increase in atmospheric oxygen levels, which culminated with the fourth glaciation between 2.26 and 2.22 Ga, after which the atmosphere remained irreversibly oxygenated. [Copyright &y& Elsevier]

Abstract: The Paleoproterozoic (2.5–2.0 Ga) is one of the most important periods in Earthʼs history, and was characterized by a rise in atmospheric oxygen levels and repeated (at least three) severe glaciations (the Huronian glaciations). In this study, we investigate redox conditions in the atmosphere and in shallow-marine environments immediately after the first Huronian glaciation based on the isotopic composition of Os, and the abundance of redox-sensitive elements (Os, Re, and Mo) in sedimentary rocks from the Huronian Supergroup, Canada. We found no significant authigenic enrichment of Os in the sedimentary rocks deposited during the first Huronian deglaciation. The initial isotopic composition of Os in the sediments was close to that of chondrite at the time of deposition ( ). These results suggest that atmospheric O2 levels were insufficient to mobilize radiogenic Os through continental weathering ( present atmospheric level (PAL)). In contrast, we found enrichment of Re in the sedimentary rocks, which suggests the occurrence of oxidative weathering of Re under mildly oxidizing conditions ( PAL). Despite the Re enrichment, low abundances of Mo imply possible non-sulfidic conditions in shallow-marine environments at the time of deposition. Together with the results of organic carbon and sulfur analyses, we suggest that atmospheric O2 remained at relatively low levels of around PAL after the first Huronian deglaciation, which contrasts with proposed dramatic increases in O2 after the second and third Huronian deglaciations. These results imply that the second and third Huronian glaciations may have been global events, associated with climatic jumps from severe glaciations to super-greenhouse conditions and the subsequent blooming of photosynthetic cyanobacteria in the glacial aftermath. [Copyright &y& Elsevier]

Abstract: Understanding the key factors influencing the global oceanic redox system is crucial to fully explaining the variations in oceanic chemical dynamics that have occurred throughout the Earth's history. In order to elucidate the mechanisms behind these variations on geological timescales, numerical sensitivity experiments were conducted with respect to the partial pressure of atmospheric molecular oxygen (pO2), the continental shelf area (A cs), and the riverine input rate of reactive phosphorus to the oceans (R P). The sensitivity experiment for atmospheric pO2 indicates that pervasive oceanic anoxia and euxinia appear when pO2<0.145atm and <0.125atm, respectively. These critical values of pO2 are higher than a previous estimate of ~50% PAL (present atmospheric level) due to redox-dependent phosphorus cycling. The sensitivity experiment regarding the shelf area demonstrates that changes in the shelf area during the Phanerozoic significantly affected oceanic oxygenation states by changing marine biogeochemical cycling; a large continental shelf acts as an efficient buffer against oceanic eutrophication and prevents the appearance of ocean anoxia/euxinia. We also found that an enhanced R P is an important mechanism for generation of widespread anoxia/euxinia via expansion of both the oxygen minimum zone and coastal deoxygenation, although the critical R P value depends significantly on pO2, A cs, and the redox-dependent burial efficiency of phosphorus at the sediment--water interface. Our systematic examination of the oceanic redox state under Cretaceous greenhouse climatic conditions also supports the above results. [Copyright &y& Elsevier]

Abstract: The atmosphere–ocean system experienced a progressive change from anoxic to more oxidizing conditions through time. This oxidation is traditionally envisaged to have occurred as two stepwise increases in atmospheric oxygen at the beginning and end of the Proterozoic Eon. Here, we present a study of the redox-sensitive element, uranium, in organic-rich shales to track the history of Earth's surface oxidation at an unprecedented temporal resolution. Fluctuations in the degree of uranium enrichment in organic-rich shales suggest that the initial rise of atmospheric oxygen ~2.4billionyr ago was followed by a decline to less oxidizing conditions during the mid-Proterozoic. This redox state persisted for almost 1billionyr, ending with a second oxygenation event in the latest Neoproterozoic. The U record tracks major fluctuations in surface oxygen level and challenges conventional models that suggest the Earth underwent a unidirectional rise in atmospheric oxygen during the Precambrian. [Copyright &y& Elsevier]

• Low-Lu zircons were used to document formation of supermountains through time. • Nuna and Gondwana were the two periods of supermountain formation. • These two periods are also the time when major advances in evolution occurred. • The Boring Billion was a period of few mountains and evolutionary tranquillity. • Rapid supermountain erosion may have increased atmospheric O 2 and nutrient levels. 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. [ABSTRACT FROM AUTHOR]

• A supercontinent-like cycle is observed in sedimentary record for the past 3.0 Gyr. • Continent-derived sediment flux into oceans is in phase with subduction flux. • Sediment subduction modulated the operation of Wilson cycle plate tectonics. • Feedbacks between surface and deep processes contributed to the atmospheric oxygenation. 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 ε Hf and δ 18 O 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. [ABSTRACT FROM AUTHOR]

• Messinian sulfate evaporites are offset from contemporaneous seawater sulfate composition toward higher δ 18 O and lower Δ'17O values. • Normal distributions of sulfate δ 18 O and Δ'17O reflect a well-mixed sulfate reservoir. • Evaporite sulfate reflects both original seawater and post-restriction alteration. The triple oxygen isotope composition of seawater sulfate, as recorded in marine sulfate evaporites and barites, is commonly used to interpret past changes in atmospheric p O 2 / p CO 2 and gross primary production (GPP). In practice, the most-negative measured triple oxygen isotope value (Δ'17O) of sulfate from a marine evaporite deposit is thought to most closely represent contemporaneous seawater sulfate and is used to calculate atmospheric composition. However, a range of triple oxygen isotope compositions are typically measured within a single marine evaporite basin. Here, we characterize in detail the variability in the triple oxygen isotope composition of the sulfate in gypsum sampled from three Messinian (5-6 Ma) marine evaporite sub-basins from the Western Mediterranean Basin. Evaporite sulfate is offset from contemporaneous seawater sulfate and reflects mixing between two end-member sulfate populations: the original seawater sulfate and sulfate that has been isotopically reset after basin restriction. The combined Δ'17O and δ 18 O compositions of sulfate within a stratigraphic context offer the opportunity to better constrain the degree to which marine sulfate evaporites preserve the original isotopic composition of open ocean seawater sulfate. This study encompasses an exploration of mass-dependent fractionation, isotope equilibrium with water, and various scenarios of mixing. Our results calibrate the utility of marine sulfate evaporites in constraining the contemporaneous, open ocean triple oxygen isotope composition of seawater sulfate. [ABSTRACT FROM AUTHOR]

• Snowpack emissions due to photolysis dominate atmospheric NO 3 − production in summer. • Stratospheric inputs contribute 55±21% of atmospheric NO 3 − budget in winter. • Atmospheric NO 3 − is mainly produced via reactions of O 3 and H 2 O/OH with NO x. • Oxygen isotopes of snowfall NO 3 − are close to those for the atmosphere year-round. • Differences in δ 15 N of NO 3 − between snowfall and the atmosphere vary between seasons. Atmospheric samples and snowfall collected in coastal East Antarctica over two years are used to investigate the sources, production of atmospheric nitrate (NO 3 −) and its link with snowfall NO 3 − based upon the isotopic composition of NO 3 − (δ 15 N, δ 18 O, and Δ 17 O). Snowfall and the atmosphere show similar seasonal trends in concentrations and isotopic composition of NO 3 −. In summer, atmospheric NO 3 − is closely associated with snowpack emissions of NO x from photolysis of snow NO 3 −. In winter, linear relationships between δ 15 N and δ 18 O (or Δ 17 O) of NO 3 − in both snowfall and the atmosphere indicate mixing between stratospheric inputs and tropospheric sources contributing to NO 3 − , with stratospheric inputs contributing 55±21% of the atmospheric NO 3 − budget. The linear relationships suggest that the lower limits of δ 15 N, δ 18 O, and Δ 17 O of stratospheric-sourced NO 3 − are close to ∼18, ∼120, and ∼45‰, respectively. Concentration correlates well with the isotopic composition of NO 3 − in winter, indicating less variable contribution of tropospheric sources. A significant linear correlation between δ 18 O and Δ 17 O of NO 3 − suggests a mix of oxidation processes by O 3 and H 2 O/OH which can influence NO x cycling and the production of NO 3 −. Lower values of Δ 17 O of atmospheric NO 3 − were observed during O 3 depletion events in September, suggesting that oxygen isotopes of NO 3 − could be more sensitive to the changes in surface O 3 compared to BrO concentrations. Oxygen isotopic composition of NO 3 − in snowfall is close to that of the atmosphere throughout the year, suggesting that snowfall NO 3 − can relay information on oxidative chemistry of NO x in the atmosphere. Snowfall δ 15 N is close in value to that in the atmosphere during winter, but ∼20‰ higher than that in the atmosphere during summer, possibly associated with seasonal changes in the gas-aerosol partitioning of atmospheric NO 3 −. This suggests that the interpretation of δ 15 N in snow needs to consider seasonal changes in sources and chemistry. [ABSTRACT FROM AUTHOR]

Abstract: Mass-dependently fractionated (MDF) sulfur isotopes in sedimentary sulfides and sulfates can provide information on the past activity of microbial metabolisms and serve as a proxy for the concentration of seawater sulfate. In conjunction with these proxies, mass-independently fractionated (MIF) sulfur isotopes can be used to track the rise of atmospheric oxygen. The anoxic–oxic transition of the surface environment in the Paleoproterozoic has been constrained by MIF sulfur in 2.47 Ga sulfides from Western Australian banded iron-formations, and the absence of MIF in diagenetic sulfides from 2.32 Ga black shales in the Transvaal Supergroup (South Africa). We report new multicollector ion microprobe data for individual sulfides from water-lain sedimentary units in the 2.45–2.22 Ga Huronian Supergroup (Ontario, Canada). The authigenecity of sulfides in these units is interpreted on the basis of host lithology, mode of distribution, sulfide chemistry and MDF sulfur isotopes. Sulfides interpreted to have an authigenic-sedimentary origin from the McKim Fm. have a range of 0.7‰ in Δ33S values and those in the Pecors Fm. preserve small magnitude MIF Δ33S values up to +0.88‰, which we propose represents the last remnants of an anoxic atmosphere. High δ 34S values (up to +31.2‰) and near-zero Δ33S values in authigenic sulfides from the stratigraphically higher Espanola and Gordon Lake Fms. suggest local variations of seawater sulfate concentrations and/or δ 34Ssulfate, consistent with increased atmospheric O2 levels. MIF sulfur is absent from the marine record after the second Huronian glaciation, which is analogous to the record of the Transvaal Supergroup where MIF is absent from the upper part of the interglacial sequence. Our results suggest that the possible simultaneous demise of MIF sulfur isotopes at these two localities may serve as a useful global geochemical marker to correlate Paleoproterozoic supergroups. We propose a new biogeochemical model where enhanced weathering rates during Paleoproterozoic post-glacial thawing served as a critical stimulus for interglacial blooms of oxygenic photosynthesis, the demise of methane, and ultimately to the irreversible rise in atmospheric oxygen. [Copyright &y& Elsevier]

Re–Os dating of synsedimentary to early diagenetic pyrite from carbonaceous shale that straddles the boundary between the Rooihoogte and Timeball Hill formations, Transvaal Supergroup, South Africa, provides a precise isochron age of 2316±7 Ma (±4 statistical uncertainty if error on decay constant is excluded) and a chondritic initial 187Os/188Os ratio of 0.1121±0.0012. These units were deposited between what are interpreted as the second and third of three Paleoproterozoic global glacial events, and thus provide minimum and maximum ages, respectively, for these events. The Rooihoogte Formation is correlative with the Duitschland Formation, which records previously undated carbon isotope excursions. Because the pyrite samples show no evidence of mass independent fractionation of sulfur and have highly negative δ34S values, the rise of atmospheric oxygen most likely began prior to 2.32 Ga. The chondritic initial 187Os/188Os ratio requires that primitive hydrothermal sources of Os dominated the Os budget of seawater at 2.32 Ga. Os is mobile only under oxidizing conditions. Therefore, the chondritic initial 187Os/188Os ratio may reflect atmospheric oxygen levels too low to introduce sufficient riverine flux of dissolved radiogenic Os to offset the primitive hydrothermal/magmatic flux. Even if atmospheric oxygen levels had increased significantly by 2.32 Ga, anoxic conditions throughout the Archean most likely limited Os enrichment in Archean marine shales, so that their subsequent exposure and weathering was unable to provide a significant amount of radiogenic Os to Paleoproterozoic seawater. [Copyright &y& Elsevier]

• The bulk continental volcanic Ni supply gradually decreased during the Archean. • The decreasing volcanic Ni fluxes resulted in diminishing Ni in Archean oceans. • The increasing pressure of melting maintained a stable oceanic Ni after 2.0 Ga. 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. [ABSTRACT FROM AUTHOR]

• Carbonate Cr isotopes altered during both meteoric diagenesis and dolomitization. • Diagenetic carbonates under rock-buffered conditions retain primary Cr isotopes. • Fractionated Cr isotopes could form under reducing atmospheric conditions. Stable chromium (Cr) isotopes have emerged as a new tool for tracking broad-scale changes in Earth's surface oxygen levels. Carbonates are one proposed sedimentary Cr isotope archive. In order to contribute to the development of a robust framework for interpreting carbonate Cr isotopic compositions and evaluating their ability to record the global redox state, we explored Cr isotope systematics of modern and Archean carbonate successions—representing end member oxic and anoxic Earth system states. We generated new data from carbonate platform sediments from the Great Bahama Bank with variable post-depositional histories to better understand the effects of diagenetic alteration on carbonate-bound Cr and its isotopic composition in a modern well-oxygenated ocean-atmosphere system. More specifically, we investigated the effects of dolomitization and aragonite-to-calcite neomorphism in marine and meteoric fluids. We also present δ 53Cr values from three carbonate successions, the ∼3.0 Ga Chobeni Formation (South Africa), ∼2.8 Ga Mosher Carbonate Formation (Canada), and ∼2.65 Ga Cheshire Formation (Zimbabwe), that were deposited under an anoxic atmosphere. We find that modern Bahamian carbonates have a large range of almost exclusively positive δ 53Cr values (from −0.04 to 2.88‰). The δ 53Cr values appear to be altered during both meteoric diagenesis and dolomitization but there may also be instances of rock-buffered conditions that may best preserve depositional δ 53Cr values. These observations provide a baseline for carbonate Cr isotope behavior in a well-oxygenated ocean-atmosphere system. Our Archean carbonate successions contain both positively fractionated and crustal δ 53Cr values, ranging between −0.37 and 0.89‰. The Cr isotope fractionation observed in Archean strata is most plausibly linked to either non-redox dependent Cr isotope fractionation, local Cr redox cycling, late-stage diagenetic alteration, or some combination thereof. Given the significant effects of post-depositional alteration on the isotopic composition of carbonate-bound Cr, we suggest that Cr isotope values from ancient carbonate sediments should be generally interpreted with caution and cannot be straightforwardly linked to changes in atmospheric oxygen abundance. [ABSTRACT FROM AUTHOR]

Various proxies and numerical models have been used to constrain O 2 levels over geological time, but considerable uncertainty remains. Previous investigations using 1-D photochemical models have predicted how O 3 concentrations vary with assumed ground-level O 2 concentrations, and indicate how the ozone layer might have developed over Earth history. These classic models have utilised the numerical simplification of fixed mixing ratio boundary conditions. Critically, this modelling assumption requires verification that predicted fluxes of biogenic and volcanic gases are realistic, but also that the resulting steady states are in fact stable equilibrium solutions against trivial changes in flux. Here, we use a 1-D photochemical model with fixed flux boundary conditions to simulate the effects on O 3 and O 2 concentrations as O 2 (and CH 4) fluxes are systematically varied. Our results suggest that stable equilibrium solutions exist for trace- and high-O 2 /O 3 cases, separated by a region of instability. In particular, the model produces few stable solutions with ground O 2 mixing ratios between 6 × 10 − 7 and 2 × 10 − 3 (3 × 10 − 6 and 1% of present atmospheric levels). A fully UV-shielding ozone layer only exists in the high-O 2 states. Our atmospheric modelling supports prior work suggesting a rapid bimodal transition between reducing and oxidising conditions and proposes Proterozoic oxygen levels higher than some recent proxies suggest. We show that the boundary conditions of photochemical models matter, and should be chosen and explained with care. • Lower boundary conditions are critical in interpreting photochemical model results. • Flux-driven models generate atmospheres with p O 2 (atm) < 6 × 10 − 7 or p O 2 > 2 × 10 − 3. • We suggest a lower limit on Proterozoic oxygen concentration of ∼1% PAL. [ABSTRACT FROM AUTHOR]

The δ 18 O value of seawater is presently buffered by high-temperature hydrothermal alteration and low-temperature weathering of lithospheric rocks. It is much debated whether the δ 18 O of seawater has been buffered to this steady-state value throughout Earth's history or, alternatively, whether it gradually increased towards the present value since the Archean. A third possibility is that the δ 18 O of seawater was buffered at a higher value before continents emerged, and has been buffered at its current value (δ 18 O ∼ −1‰) since the emergence of continents. In this contribution, we reconstruct the δ 18 O of Archean seawater from triple oxygen isotope (δ 17 O, δ 18 O) variations in >2.7 Ga ultramafic rocks that reacted with fluids that, in turn, had been derived from seawater in the Archean. The samples that were studied are peridotites from the Ivnartivaq complex, an ultramafic lens in the Archean Kuummiut terrane (Rae craton, southeast Greenland) that have unusually low δ 18 O values for peridotites (in olivine, 1.7 ‰ ≤ δ 18 O ≤ 4.6 ‰ , relative to VSMOW). Bulk rock trace element concentrations, mineral compositions and U-Pb dating of zircon grains in the peridotites indicate that these samples are metamorphic peridotites, that formed by the dehydration of serpentinites ∼2.7 Ga ago. The serpentinite protoliths, in turn, had formed by alteration of ultramafic cumulate rocks at high temperatures (250–450 °C) by fluids that were derived from seawater. Triple oxygen isotope variations in olivine from the peridotites indicate that the fluids could not have been derived from seawater with a significantly higher or lower δ 18 O value than that of seawater in the Phanerozoic, but could have been derived from seawater with a value of δ 18 O ∼ −1‰, i.e., the modern (ice-free) seawater composition. We conclude that the δ 18 O of seawater had reached its current steady state value by 2.7 Ga ago. • Archean peridotites from southeast Greenland contain olivine with 1.7‰ ≤ δ 18 O ≤ 4.6‰. • They are crystal cumulates that were altered by seawater to low- δ 18 O serpentinites. • The serpentinites became dehydrated during metamorphism ∼2.7 Ga ago. • We reconstruct the δ 18 O of >2.7 Ga seawater from triple O isotope variations in olivine. • Seawater had reached its current steady state value (δ 18 O ∼ −1‰) by 2.7 Ga ago. [ABSTRACT FROM AUTHOR]

• Laminated lake sediments identify hydroclimate variability in the Guatemala highlands. • The record is based on >1000 stable isotope values, dated with 4114C ages and 210Pb. • The middle Holocene was drier, followed by a wetter late Holocene. • The pattern of hydroclimate variability is inconsistent with direct insolation forcing. • Precipitation changes were likely driven by regional-scale ocean-atmospheric forcing. Long-term perspectives on past hydroclimate variability provide context for evaluating the potential future impacts of changes in Northern Hemisphere temperatures and oceanic-atmospheric circulation on the timing and distribution of precipitation in Central America. Here we use the isotopic composition of fine-grained (< 63 μm), endogenic CaCO 3 (δ 18 O calcite and δ 13 C calcite) from Lake Kail, located in the western highlands of Guatemala, to infer multi-decadal to multi-centennial-scale variability in the balance between precipitation and evaporation (P/E) over the last ∼6000 years. The sediment age model is based on 210Pb and 41 14C ages, with chronological uncertainty estimated using Bayesian methods. A model that couples lake hydrology and isotope mass balance is applied to characterize the isotopic responses of lake water and calcite to changes in drought-controlling climate variables such as precipitation, temperature and relative humidity. The δ 18 O calcite variations indicate intermediate-to-dry P/E conditions during the late-middle Holocene from ∼6000 to ∼4100 cal yr BP. There was then a shift to drier conditions at the start of the late Holocene until ∼3050 cal yr BP, followed by a trend to wetter conditions until ∼1500 cal yr BP. The most recent ∼1500 years is characterized by high and relatively stable P/E, with particularly wet intervals from ∼1500 to 1170 cal yr BP and from 470 to 260 cal yr BP. Comparisons of the Lake Kail δ 18 O calcite record with Central American/circum-Caribbean proxy datasets spanning the late-middle and late Holocene indicate northern tropical hydroclimate changes were highly variable across space and time and were likely driven by regional oceanic-atmospheric responses with a secondary influence by insolation forcing. [ABSTRACT FROM AUTHOR]

The pyrite sulfur isotope record of the 1.98 Ga Zaonega Formation in the Onega Basin, NW Russia, has played a central role in understanding ocean-atmosphere composition and inferring worldwide fluctuations of the seawater sulfate reservoir during the pivotal times of the Paleoproterozoic Era. That, in turn, has led to a concept that Earth's atmospheric oxygen levels underwent global-scale changes. Here we present a steady-state isotope mass-balance model to gain insight into the mechanisms governing the sulfur cycle and sulfate reservoir during deposition of the organic-rich Zaonega Formation. We demonstrate that coupling between high microbial sulfate reduction rates and effective sulfate removal by pyrite precipitation can lead to Rayleigh distillation of the basinal sulfate reservoir and development of high amplitude positive δ 34 S excursions. This modelling approach illustrates that secular changes in sedimentary pyrite isotope trends can be explained by processes that reflect local (basin-scale) fluctuations in sulfur cycling rather than global mechanisms. • An isotope mass-balance model for the Zaonega Formation sulfur cycle is presented. • Zaonega Formation pyrite S isotope trends reflect local environmental conditions. • Measured pyrite Δ 33 S – δ 34 S – Δ 36 S data are explained by basin-specific processes. • Covarying Δ 33 S – δ 34 S data reflect high microbial sulfate reduction and pyrite burial. [ABSTRACT FROM AUTHOR]

• Glacial diamictites record changes in chemical weathering of the crust. • Diamictites formed before the GOE have similar δ 98 Mo values as igneous rocks. • Diamictites that formed after the GOE retain isotopically light Mo. • The onset of the GOE drove Mo isotope fractionation during crustal weathering. • The weathered continental crust is an isotopically light Mo reservoir. Molybdenum isotopes in twenty-four composites of glacial diamictites spanning depositional ages of 2900 to 300 Ma show a systematic shift to lighter compositions and a decrease in Mo concentration over time. The diamictites fall into three age groups relative to the Great Oxidation Event (GOE): pre-GOE (2.43 – 2.90 Ga), syn-GOE (2.20 – 2.39 Ga), and post-GOE (0.33 – 0.75 Ga). Pre-GOE composites have an average δ 98 Mo NIST3134 of +0.03‰ (± 0.18‰), syn-GOE composites average − 0.29 ‰ (± 0.60‰), and post-GOE composites average − 0.45 ‰ (± 0.51‰). These groups are statistically different at p=0.05. We use the pre-GOE data to estimate the average Archean upper continental crust (UCC) δ 98 Mo signature as +0.03 ± 0.18‰ (2 σ), which falls within the range of previous estimates of modern igneous rocks. As the diamictites represent a mixture of igneous and weathered crust, the shift to lighter Mo values over time likely reflects Mo isotope fractionation during oxidative weathering and increased retention of light Mo isotopes in weathered regolith and soils. We hypothesize that this fractionation is due to the mobilization of oxidized Mo following the GOE, and subsequent adsorption of light Mo onto Fe-Mn oxides and/or organic matter in weathered regolith. We conclude that Mo isotopes in continental weathering products record the rise of atmospheric oxygen and onset of oxidative weathering. As the regolith formed under oxidative conditions is isotopically lighter than average continental igneous rocks, mass balance dictates that Mo isotope fractionation during oxidative weathering should result in isotopically heavy groundwater and river water, which is observed in modern systems. [ABSTRACT FROM AUTHOR]

The distinct triple oxygen isotope composition of tropospheric O 2 relative to seawater is the result of biogeochemical reactions (e.g. primary productivity, respiration), exchange with the stratosphere, and the relative size of different oxygen-containing reservoirs, namely O 2 , O 3 , and CO 2. This difference in isotopic composition gives tropospheric O 2 utility as a record of biogeochemical and atmospheric processes and may also be used for determining where in the rock record isotopic fingerprints of tropospheric oxygen may be preserved. The isotopic record of tropospheric oxygen in previous studies is largely limited to analyses of gas trapped in continental glaciers and a patchwork of other proxies, most notably the triple oxygen signature of sulfate. Here we show the uppermost layers of hydrogenetic, deep-ocean ferromanganese crusts from each of the major ocean basins have a triple oxygen isotope composition consistent with the direct incorporation of dissolved oxygen. The range of δ 18O and Δ ′ 17O in ferromanganese crusts suggests the Mn oxide endmember contains a near 50:50 mixture of oxygen from water and dissolved O 2. Our data indicate this signal also persists into older layers of the crusts, potentially preserving near 75 million years of the oxygen isotopic composition of the lower troposphere and subsequent deep-ocean respiration. Our analysis of oxygen isotope values, bulk chemistry, and estimated local dissolved oxygen for crust top samples reveals that variations in bulk chemistry ultimately exhibit more influence on the oxygen mass balance than changes in dissolved oxygen, presenting a challenge for unambiguous determination of local dissolved oxygen. Although analytical challenges remain, these widespread, layered deposits of ferromanganese crust may offer a viable path for future interrogation of the history or relative history of the oxygen cycle of the troposphere and deep ocean millions of years into the past. • Mn oxides in ferromanganese crusts capture and retain atoms from dissolved oxygen. • Triple oxygen signature of dissolved oxygen persists for >30 million-year-old crust. • Ferromanganese crusts may be useful for marine dissolved oxygen reconstruction. [ABSTRACT FROM AUTHOR]

• We present the first U isotope data across the Late Devonian Hangenberg Crisis. • Our δ 238 U data show three negative shifts punctuated by two short positive events. • The positive δ 238 U events were associated with sulfidic porewater redox conditions. • Marine anoxia expanded to cover >5% seafloor area in the latest Devonian oceans. • Marine anoxia was likely an important kill mechanism for the Hangenberg Crisis. The global Hangenberg Crisis near the Devonian-Carboniferous boundary (DCB) represents one of the major Phanerozoic mass extinction events, which shaped the roots of modern vertebrate biodiversity. Marine anoxia has been cited as the proximate kill mechanism for this event. However, the detailed timing, duration, and extent of global marine redox chemistry changes across this critical interval remain controversial because most of the studies to date only constrain changes in local or regional redox chemistry. Thus, opinions on the significance of anoxia as a kill mechanism are variable—from anoxia being a primary driver to being relatively unimportant. In this study, we explore the evolution of global marine redox chemistry using U isotopes of marine limestones. The δ 238 U trends at Long'an section in South China document systematic oscillations with three negative shifts punctuated by two positive events in between. The magnitude of the δ 238 U oscillations implies that the sediments do not record contemporaneous seawater with a constant offset at all times. The lack of covariation between δ 238 U data and diagenetic indicators (e.g., Mn and Sr contents, Mn/Sr ratio, δ 18 O) suggests that the δ 238 U trends are not produced by the same post-depositional diagenetic processes. Instead, trace-metal enrichments suggest that more reducing conditions prevailed during the deposition of the two positive events. We present plausible model scenarios that fit the observed δ 238 U trends in the context of redox-sensitive trace metal data suggesting marine anoxia expanded in the latest Devonian oceans to cover >5% of the continental shelf seafloor area. The rapid expansion of marine anoxia coincident with the onset of the Hangenberg Crisis supports marine anoxia as an important kill mechanism. Biogeochemical modeling of the coupled C-P-U cycles suggests that intensified continental weathering, for example, assisted by the spread of seed plants with deeper root systems at this time, could have triggered expansion of marine anoxia and other global changes (e.g., positive excursion in δ 13 C carb and decrease in sea surface temperature) in the latest Devonian. The anoxic event is inferred to have been transient as climatic cooling would have reduced weathering fluxes. [ABSTRACT FROM AUTHOR]

Ordovician atmospheric CO 2 level was an important influence on climate and life. However, modelled estimates of Ordovician CO 2 differ widely and, in contrast to much of the subsequent Phanerozoic, they lack fossil proxy constraints. In vivo cyanobacterial sheath calcification, promoted by carbon dioxide concentrating mechanisms (CCMs), creates distinctive microfossils. These provide a direct ecophysiological link to ambient concentrations of CO 2 and, to a lesser extent, O 2. Cyanobacteria do not calcify at high CO 2 concentrations. Experiments show that CCMs can be induced in present-day cyanobacteria at CO 2 levels below ∼10 times present atmospheric level (PAL) and that this promotes sheath calcification. We compiled a global database of cyanobacterial calcification in marine environments throughout the Ordovician (485-443 Myr ago) and compared it with modelled estimates of atmospheric CO 2 and O 2. These data show that many genera of marine calcified cyanobacteria reappeared from Cambrian Series 2 or first appeared globally during the late middle to late Upper Ordovician (late Darriwilian to late Katian) in carbonate platform facies, resulting in a previously unrecognized ten-fold increase in global diversity. Such a large increase in calcification suggests widespread induction of CCM expression in cyanobacteria, consistent with sustained decline in external CO 2 concentrations below ∼10× PAL, together with increase in dissolved O 2 level, during the late Darriwilian, Sandbian and Katian stages, ∼460-445 Myr ago. These cyanobacterial calcification fossil proxy data provide a first order constraint on modelled estimates of atmospheric CO 2 for the Ordovician. Estimated CO 2 outputs from COPSE and recent GEOCARB-based models are broadly consistent with this cyanobacterial calcification proxy for the Ordovician. In view of the long history of cyanobacteria, the calcified cyanobacterial proxy offers potential to assist interpretation of CO 2 deeper into geological time. • Diversification curve of Ordovician calcified cyanobacteria was revealed firstly. • Cyanobacterial calcification substantially increased in the mid- to late Ordovician. • Induction of cyanobacterial CCM indicating sustained decline in CO 2 below ∼10× PAL. • Providing a constraint on differing published modeled estimates of atmospheric CO 2. • Calcified cyanobacteria proxy is effective to interpret CO 2 in deep geological time. [ABSTRACT FROM AUTHOR]

The relationship between oxygen and evolution of early eukaryotes including algae and primitive animals in geological history has been debated, partly due to the varying estimates of oxygen levels in the mid-Proterozoic (ca. 1.8–0.8 Ga) ocean and atmosphere. The upper part of the Gaoyuzhuang Formation (ca. 1.60–1.54 Ga) in North China hosts decimeter-scale multicellular eukaryotic fossils and is documented with a decrease in cerium anomaly indicative of ocean oxygenation. However, the atmospheric oxygen level across this interval and its subsequent oxidation state require further investigation using additional redox proxies. Here we report I/(Ca+Mg) ratios, carbonate/organic carbon isotopes (δ 13 C carb and δ 13 C org), and phosphorous (P) contents across the ca. 1.57–1.56 Ga fossil-bearing interval in the North China Platform. High I/(Ca+Mg) ratios (≥2.6 μmol/mol; up to 3.8 μmol/mol) from shallow-water carbonates of the Gaoyuzhuang Formation suggest an episode of significant oxygen increase up to ≥4% PAL (present atmospheric level). The I/(Ca+Mg) ratios return back to ≤0.5 μmol/mol shortly after the peak values without evidence for increasing water depth or diagenetic alteration, implying a short-lived oxidation event. The increase of I/(Ca+Mg) ratios is associated with a −3.5‰ negative δ 13 C carb and δ 13 C org anomaly and an increase in P/Al ratios that are best explained by oxidation of dissolved organic carbon (DOC) in the ocean. Oxygen consumption through oxidation of DOC may have quickly lowered marine and atmospheric O 2 levels to the early mid-Proterozoic (1.8–1.4 Ga) background oxygen concentration of ≤0.1–1% PAL. Short-lived oxidation events in an overall anoxic mid-Proterozoic ocean and atmosphere best explain the existing geochemical data and evolutionary stasis of eukaryotes during the "Boring Billion". • A short-lived oxidation event (∼1.57–1.56 Ga) is identified in North China. • This event is recorded by a prominent increase in I/(Ca+Mg) and P/Al ratios. • The oxidation event is accompanied by a −3.5‰ excursion in both δ 13 C carb and δ 13 C org. • The peak O 2 concentration of the oxidation event may have been ≥4% PAL. • Temporal O 2 instability may have been the barrier for eukaryotic evolution. [ABSTRACT FROM AUTHOR]