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How atmospheric oxygen concentrations evolved from only small amounts for the early Earth to about 21 per cent today remains uncertain; here our latest understanding of the evolution of Earth’s oxygen levels is discussed. Recalibrating the rise and rise of Earth's oxygen Until about two billion years ago free oxygen was a scarce commodity in the Earth's atmosphere. For decades geochemists have been refining the story of how and when early life forms began to pump oxygen into the oceans and atmosphere and what then happened to it. But as this Review explains, there is still much detail to be filled in. Timothy Lyons et al . describe how recent work has made it possible to say with increasing confidence when and why the oxidation state of the ocean and atmosphere varied through time. In particular they focus on what has become known as the Great Oxidation Event, which they suggest was a more protracted and dynamic process than its name implies. The rapid increase of carbon dioxide concentration in Earth’s modern atmosphere is a matter of major concern. But for the atmosphere of roughly two-and-half billion years ago, interest centres on a different gas: free oxygen (O 2 ) spawned by early biological production. The initial increase of O 2 in the atmosphere, its delayed build-up in the ocean, its increase to near-modern levels in the sea and air two billion years later, and its cause-and-effect relationship with life are among the most compelling stories in Earth’s history.

Low phosphorus burial in shallow marine sedimentary rocks before about 750 million years ago implies a change in the global phosphorus cycle, coinciding with the end of what may have been a stable low-oxygen world. A history of phosphorus limitation It is thought that the nutrient phosphorus limits marine primary productivity on geological timescales, but it is not clear whether phosphorus limitation has persisted throughout Earth's history. On the basis of a compilation of phosphorus abundances in marine sedimentary rocks spanning the past 3.5 billion years, and a biogeochemical model, Christopher Reinhard, Noah Planavsky and colleagues suggest that a prolonged period of phosphorus biolimitation was followed by a fundamental shift in the phosphorus cycle during the late Proterozoic eon (between 800 million and 635 million years ago). This is coincident with a previously inferred shift in marine redox states, severe perturbations to Earth's climate system, and the emergence of animals. The macronutrient phosphorus is thought to limit primary productivity in the oceans on geological timescales 1 . Although there has been a sustained effort to reconstruct the dynamics of the phosphorus cycle over the past 3.5 billion years 2 , 3 , 4 , 5 , it remains uncertain whether phosphorus limitation persisted throughout Earth’s history and therefore whether the phosphorus cycle has consistently modulated biospheric productivity and ocean–atmosphere oxygen levels over time. Here we present a compilation of phosphorus abundances in marine sedimentary rocks spanning the past 3.5 billion years. We find evidence for relatively low authigenic phosphorus burial in shallow marine environments until about 800 to 700 million years ago. Our interpretation of the database leads us to propose that limited marginal phosphorus burial before that time was linked to phosphorus biolimitation, resulting in elemental stoichiometries in primary producers that diverged strongly from the Redfield ratio (the atomic ratio of carbon, nitrogen and phosphorus found in phytoplankton). We place our phosphorus record in a quantitative biogeochemical model framework and find that a combination of enhanced phosphorus scavenging in anoxic, iron-rich oceans 6 , 7 and a nutrient-based bistability in atmospheric oxygen levels could have resulted in a stable low-oxygen world. The combination of these factors may explain the protracted oxygenation of Earth’s surface over the last 3.5 billion years of Earth history 8 . However, our analysis also suggests that a fundamental shift in the phosphorus cycle may have occurred during the late Proterozoic eon (between 800 and 635 million years ago), coincident with a previously inferred shift in marine redox states 9 , severe perturbations to Earth’s climate system 10 , and the emergence of animals 11 , 12 .

The oxygenation of Earth’s surface fundamentally altered global biogeochemical cycles and ultimately paved the way for the rise of metazoans at the end of the Proterozoic. However, current estimates for atmospheric oxygen (O₂) levels during the billion years leading up to this time vary widely. On the basis of chromium (Cr) isotope data from a suite of Proterozoic sediments from China, Australia, and North America, interpreted in the context of data from similar depositional environments from Phanerozoic time, we find evidence for inhibited oxidation of Cr at Earth’s surface in the mid-Proterozoic (1.8 to 0.8 billion years ago). These data suggest that atmospheric O₂ levels were at most 0.1% of present atmospheric levels. Direct evidence for such low O₂ concentrations in the Proterozoic helps explain the late emergence and diversification of metazoans.

Paleozoic and Precambrian sedimentary successions frequently contain massive dolomicrite [CaMg(CO₃)₂] units despite kinetic inhibitions to nucleation and precipitation of dolomite at Earth surface temperatures (<60 °C). This paradoxical observation is known as the “dolomite problem.” Accordingly, the genesis of these dolostones is usually attributed to burial–hydrothermal dolomitization of primary limestones (CaCO₃) at temperatures of >100 °C, thus raising doubt about the validity of these deposits as archives of Earth surface environments. We present a high-resolution, >63-My-long clumped-isotope temperature (TΔ47) record of shallow-marine dolomicrites from two drillcores of the Ediacaran (635 to 541 Ma) Doushantuo Formation in South China. Our TΔ47 record indicates that a majority (87%) of these dolostones formed at temperatures of <100 °C. When considering the regional thermal history, modeling of the influence of solid-state reordering on our TΔ47 record further suggests that most of the studied dolostones formed at temperatures of <60 °C, providing direct evidence of a low-temperature origin of these dolostones. Furthermore, calculated δ18O values of diagenetic fluids, rare earth element plus yttrium compositions, and petrographic observations of these dolostones are consistent with an early diagenetic origin in a rock-buffered environment. We thus propose that a precursor precipitate from seawater was subsequently dolomitized during early diagenesis in a near-surface setting to produce the large volume of dolostones in the Doushantuo Formation. Our findings suggest that the preponderance of dolomite in Paleozoic and Precambrian deposits likely reflects oceanic conditions specific to those eras and that dolostones can be faithful recorders of environmental conditions in the early oceans.

Reconstructing historical atmospheric oxygen (O 2 ) levels at finer temporal resolution is a top priority for exploring the evolution of life on Earth. This goal, however, is challenged by gaps in traditionally employed sediment-hosted geochemical proxy data. Here, we propose an independent strategy—machine learning with global mafic igneous geochemistry big data to explore atmospheric oxygenation over the last 4.0 billion years. We observe an overall two-step rise of atmospheric O 2 similar to the published curves derived from independent sediment-hosted paleo-oxybarometers but with a more detailed fabric of O 2 fluctuations superimposed. These additional, shorter-term fluctuations are also consistent with previous but less well-established suggestions of O 2 variability. We conclude from this agreement that Earth’s oxygenated atmosphere may therefore be at least partly a natural consequence of mantle cooling and specifically that evolving mantle melts collectively have helped modulate the balance of early O 2 sources and sinks. Earth’s oxygenation history can be reconstructed using machine learning and mafic igneous geochemical data. Agreement with independent proxy predictions for surface conditions implies that interior processes are critical in atmospheric oxygenation.

The emergence and expansion of complex eukaryotic life on Earth is linked at a basic level to the secular evolution of surface oxygen levels. However, the role that planetary redox evolution has played in controlling the timing of metazoan (animal) emergence and diversification, if any, has been intensely debated. Discussion has gravitated toward threshold levels of environmental free oxygen (O₂) necessary for early evolving animals to survive under controlled conditions. However, defining such thresholds in practice is not straightforward, and environmental O₂ levels can potentially constrain animal life in ways distinct from threshold O₂ tolerance. Herein, we quantitatively explore one aspect of the evolutionary coupling between animal life and Earth’s oxygen cycle—the influence of spatial and temporal variability in surface ocean O₂ levels on the ecology of early metazoan organisms. Through the application of a series of quantitative biogeochemical models, we find that large spatiotemporal variations in surface ocean O₂ levels and pervasive benthic anoxia are expected in a world with much lower atmospheric pO₂ than at present, resulting in severe ecological constraints and a challenging evolutionary landscape for early metazoan life. We argue that these effects, when considered in the light of synergistic interactions with other environmental parameters and variable O₂ demand throughout an organism’s life history, would have resulted in long-term evolutionary and ecological inhibition of animal life on Earth for much of Middle Proterozoic time (∼1.8–0.8 billion years ago).

Pervasive anoxia in the subsurface ocean during the Proterozoic may have allowed large fluxes of biogenic CH₄ to the atmosphere, enhancing the climatic significance of CH₄ early in Earth’s history. Indeed, the assumption of elevated pCH₄ during the Proterozoic underlies most models for both anomalous climatic stasis during the mid-Proterozoic and extreme climate perturbation during the Neoproterozoic; however, the geologic record cannot directly constrain atmospheric CH₄ levels and attendant radiative forcing. Here, we revisit the role of CH₄ in Earth’s climate system during Proterozoic time. We use an Earth system model to quantify CH₄ fluxes from the marine biosphere and to examine the capacity of biogenic CH₄ to compensate for the faint young Sun during the “boring billion” years before the emergence of metazoan life. Our calculations demonstrate that anaerobic oxidation of CH₄ coupled to SO₄2− reduction is a highly effective obstacle to CH₄ accumulation in the atmosphere, possibly limiting atmospheric pCH₄ to less than 10 ppm by volume for the second half of Earth history regardless of atmospheric pO₂. If recent pO₂ constraints from Cr isotopes are correct, we predict that reduced UV shielding by O₃ should further limit pCH₄ to very low levels similar to those seen today. Thus, our model results likely limit the potential climate warming by CH₄ for the majority of Earth history—possibly reviving the faint young Sun paradox during Proterozoic time and challenging existing models for the initiation of low-latitude glaciation that depend on the oxidative collapse of a steady-state CH₄ greenhouse.

Data are presented that support the idea of an oxygenation event in the immediate aftermath of the Marinoan glaciation, pre-dating previous estimates for post-Marinoan oxygenation by more than 50 million years. A breath of oxygen for the early metazoans Macroscopic metazoans first appeared in the fossil record shortly after the termination of the late Cryogenian (Marinoan) glaciation about 635 million years ago. It has been suggested that an oxygenation event at about this time was the driving factor behind the rise of the metazoans, but current estimates suggest that oxygenation occurred between 580 million and 550 million years ago, well after initial animal diversification. New geochemical data from early Ediacaran organic-rich black shales of the basal Doushantuo Formation in South China now suggest that the oxidation event occurred more than 50 million years earlier, in the immediate aftermath of the Marinoan glaciation. The data provide evidence for a significant postglacial oxygenation and support a link between the most severe glaciations in Earth's history, the oxygenation of Earth's surface and the earliest emergence of complex animals. Metazoans are likely to have their roots in the Cryogenian period 1 , 2 , 3 , but there is a marked increase in the appearance of novel animal and algae fossils shortly after the termination of the late Cryogenian (Marinoan) glaciation about 635 million years ago 4 , 5 , 6 . It has been suggested that an oxygenation event in the wake of the severe Marinoan glaciation was the driving factor behind this early diversification of metazoans and the shift in ecosystem complexity 7 , 8 . But there is little evidence for an increase in oceanic or atmospheric oxygen following the Marinoan glaciation, or for a direct link between early animal evolution and redox conditions in general 9 . Models linking trends in early biological evolution to shifts in Earth system processes thus remain controversial 10 . Here we report geochemical data from early Ediacaran organic-rich black shales (∼635–630 million years old) of the basal Doushantuo Formation in South China. High enrichments of molybdenum and vanadium and low pyrite sulphur isotope values (Δ 34 S values ≥65 per mil) in these shales record expansion of the oceanic inventory of redox-sensitive metals and the growth of the marine sulphate reservoir in response to a widely oxygenated ocean. The data provide evidence for an early Ediacaran oxygenation event, which pre-dates the previous estimates for post-Marinoan oxygenation 11 , 12 , 13 by more than 50 million years. Our findings seem to support a link between the most severe glaciations in Earth’s history, the oxygenation of the Earth’s surface environments, and the earliest diversification of animals.

To understand the evolution of the biosphere, we need to know how much oxygen was present in Earth's atmosphere during most of the past 2.5 billion years. However, there are few proxies sensitive enough to quantify O 2 at the low levels present until slightly less than 1 billion years ago. Lu et al. measured iodine/calcium ratios in marine carbonates, which are a proxy for dissolved oxygen concentrations in the upper ocean. They found that a major, but temporary, rise in atmospheric O 2 occurred at around 400 million years ago and that O 2 levels underwent a step change to near-modern values around 200 million years ago. Science , this issue p. 174 The I/Ca ratio in marine carbonates tracks atmospheric oxygen levels for the past 2.5 billion years. Rising oceanic and atmospheric oxygen levels through time have been crucial to enhanced habitability of surface Earth environments. Few redox proxies can track secular variations in dissolved oxygen concentrations around threshold levels for metazoan survival in the upper ocean. We present an extensive compilation of iodine-to-calcium ratios (I/Ca) in marine carbonates. Our record supports a major rise in the partial pressure of oxygen in the atmosphere at ~400 million years (Ma) ago and reveals a step change in the oxygenation of the upper ocean to relatively sustainable near-modern conditions at ~200 Ma ago. An Earth system model demonstrates that a shift in organic matter remineralization to greater depths, which may have been due to increasing size and biomineralization of eukaryotic plankton, likely drove the I/Ca signals at ~200 Ma ago.

The partial pressure of oxygen in Earth’s atmosphere has increased dramatically through time, and this increase is thought to have occurred in two rapid steps at both ends of the Proterozoic Eon (∼2.5–0.543 Ga). However, the trajectory and mechanisms of Earth’s oxygenation are still poorly constrained, and little is known regarding attendant changes in ocean ventilation and seafloor redox. We have a particularly poor understanding of ocean chemistry during the mid-Proterozoic (∼1.8–0.8 Ga). Given the coupling between redox-sensitive trace element cycles and planktonic productivity, various models for mid-Proterozoic ocean chemistry imply different effects on the biogeochemical cycling of major and trace nutrients, with potential ecological constraints on emerging eukaryotic life. Here, we exploit the differing redox behavior of molybdenum and chromium to provide constraints on seafloor redox evolution by coupling a large database of sedimentary metal enrichments to a mass balance model that includes spatially variant metal burial rates. We find that the metal enrichment record implies a Proterozoic deep ocean characterized by pervasive anoxia relative to the Phanerozoic (at least ∼30–40% of modern seafloor area) but a relatively small extent of euxinic (anoxic and sulfidic) seafloor (less than ∼1–10% of modern seafloor area). Our model suggests that the oceanic Mo reservoir is extremely sensitive to perturbations in the extent of sulfidic seafloor and that the record of Mo and chromium enrichments through time is consistent with the possibility of a Mo–N colimited marine biosphere during many periods of Earth’s history.