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Zinc (Zn) is a bioessential element whose cycling in the marine environment is closely linked to the biological pump. As a vital micronutrient, Zn is depleted in marine surface waters and enriched in deep waters. This has given rise to the longstanding hope that Zn could serve as a robust paleoproductivity proxy. There has been a recent focus on how Zn isotope signatures can be used to track the evolution of the biological pump. However, the factors controlling variations in the sedimentary Zn isotope composition of core-top sediments are debated and these values are influenced by changes in the relative contributions of biological uptake and scavenging processes and by mineral fractionations. In order to provide a new perspective on the Zn isotopic signature of marine core-top sediments and their potential for tracing ocean biogeochemical processes, we developed a sample preparation procedure to extract organically bound Zn (excluding inorganic Zn) without a significant isotope fractionation. We tested five potential organic solvents and found that the isopropanol extraction procedure provided a reproducible and robust result that yielded the highest recovery rate of organically bound Zn without a significant associated isotopic fractionation and appeared to exclude common mineral bound forms of Zn. This extraction procedure was used to analyze the Zn concentrations and isotopic compositions (δ66Zn) of core-top sediment samples from Long Island Sound and the Peruvian continental margin. Our results suggest that the δ66Zn value of organic Zn increases with water depth, thus providing additional support for the finding that burial of isotopically light Zn occurs predominantly in shallow high-productivity sediments. However, we also found that the δ66Zn values of the estimated authigenic Zn component are consistently lighter than those of the organic Zn fraction, consistent with diagenetic (sediment pile) scavenging of organic derived Zn in sulfides imparting an isotope fractionation.

The evolution of the global carbon and silicon cycles is thought to have contributed to the long-term stability of Earth's climate1-3. Many questions remain, however, regarding the feedback mechanisms at play, and there are limited quantitative constraints on the sources and sinks of these elements in Earth's surface environments4-12. Here we argue that the lithium-isotope record can be used to track the processes controlling the long-term carbon and silicon cycles. By analysing more than 600 shallow-water marine carbonate samples from more than 100 stratigraphic units, we construct a new carbonate-based lithium-isotope record spanning the past 3 billion years. The data suggest an increase in the carbonate lithium-isotope values over time, which we propose was driven by long-term changes in the lithium-isotopic conditions of sea water, rather than by changes in the sedimentary alterations of older samples. Using a mass-balance modelling approach, we propose that the observed trend in lithium-isotope values reflects a transition from Precambrian carbon and silicon cycles to those characteristic of the modern. We speculate that this transition was linked to a gradual shift to a biologically controlled marine silicon cycle and the evolutionary radiation of land plants13,14.

Piecing together the history of carbon (C) perturbation events throughout Earth’s history has provided key insights into how the Earth system responds to abrupt warming. Previous studies, however, focused on short-term warming events that were superimposed on longer-term greenhouse climate states. Here, we present an integrated proxy (C and uranium [U] isotopes and paleo CO2) and multicomponent modeling approach to investigate an abrupt C perturbation and global warming event (∼304 Ma) that occurred during a paleo-glacial state. We report pronounced negative C and U isotopic excursions coincident with a doubling of atmospheric CO2 partial pressure and a biodiversity nadir. The isotopic excursions can be linked to an injection of ∼9,000 Gt of organic matter–derived C over ∼300 kyr and to near 20% of areal extent of seafloor anoxia. Earth system modeling indicates that widespread anoxic conditions can be linked to enhanced thermocline stratification and increased nutrient fluxes during this global warming within an icehouse.

The end-Permian mass extinction occurred alongside a large swath of environmental changes that are often invoked as extinction mechanisms, even when a direct link is lacking. One way to elucidate the cause(s) of a mass extinction is to investigate extinction selectivity, as it can reveal critical information on organismic traits as key determinants of extinction and survival. Here we show that machine learning algorithms, specifically gradient boosted decision trees, can be used to identify determinants of extinction as well as to predict extinction risk. To understand which factors led to the end-Permian mass extinction during an extreme global warming event, we quantified the ecological selectivity of marine extinctions in the well-studied South China region. We find that extinction selectivity varies between different groups of organisms and that a synergy of multiple environmental stressors best explains the overall end-Permian extinction selectivity pattern. Extinction risk was greater for genera that had a low species richness, narrow bathymetric ranges limited to deep-water habitats, a stationary mode of life, a siliceous skeleton, or, less critically, calcitic skeletons. These selective losses directly link the extinctions to the environmental effects of rapid injections of carbon dioxide into the ocean–atmosphere system, specifically the combined effects of expanded oxygen minimum zones, rapid warming, and potentially ocean acidification.

In the wake of rapid CO

The geologic carbon cycle plays a fundamental role in controlling Earth's climate and habitability. For billions of years, stabilizing feedbacks inherent in the cycle have maintained a surface environment that could sustain life. Carbonation/decarbonation reactions are the primary mechanisms for transferring carbon between the solid Earth and the ocean–atmosphere system. These processes can be broadly represented by the reaction: CaSiO3 (wollastonite) + CO2 (gas) ↔ CaCO3 (calcite) + SiO2 (quartz). This class of reactions is therefore critical to Earth's past and future habitability. Here, we summarize their significance as part of the Deep Carbon Obsevatory's “Earth in Five Reactions” project. In the forward direction, carbonation reactions like the one above describe silicate weathering and carbonate formation on Earth's surface. Recent work aims to resolve the balance between silicate weathering in terrestrial and marine settings both in the modern Earth system and through Earth's history. Rocks may also undergo carbonation reactions at high temperatures in the ultramafic mantle wedge of a subduction zone or during retrograde regional metamorphism. In the reverse direction, the reaction above represents various prograde metamorphic decarbonation processes that can occur in continental collisions, rift zones, subduction zones, and in aureoles around magmatic systems. We summarize the fluxes and uncertainties of major carbonation/decarbonation reactions and review the key feedback mechanisms that are likely to have stabilized atmospheric CO2 levels. Future work on planetary habitability and Earth's past and future climate will rely on an enhanced understanding of the long-term carbon cycle.

Decision tree algorithms are rarely utilized in paleontological research, and here we show that machine learning algorithms can be used to identify determinants of extinction as well as predict extinction risk. This application of decision tree algorithms is important because the ecological selectivity of mass extinctions can reveal critical information on organismic traits as key determinants of extinction and hence the causes of extinction. To understand which factors led to the mass extinction of life during an extreme global warming event, we quantified the ecological selectivity of marine extinctions in the well-studied South China region during the end-Permian mass extinction using the categorized gradient boosting algorithm. We find that extinction selectivity varies between different groups of organisms and that a synergy of multiple environmental stressors best explains the overall end-Permian extinction selectivity pattern. Extinction risk was greater for genera that were limited to deep-water habitats, had a stationary mode of life, possessed a siliceous skeleton or, less critically, had calcitic skeletons. These selective losses directly link the extinction to the environmental effects of rapid injections of carbon dioxide into the ocean-atmosphere system, specifically the combined effects of expanded oxygen minimum zones, rapid warming, and ocean acidification.

Earth's ocean-atmosphere system has undergone a dramatic but protracted increase in oxygen (O2) abundance. This environmental transition ultimately paved the way for the rise of multicellular life and provides a blueprint for how a biosphere can transform a planetary surface. However, estimates of atmospheric oxygen levels for large intervals of Earth's history still vary by orders of magnitude-foremost for Earth's middle history. Historically, estimates of mid-Proterozoic (1.9-0.8 Ga) atmospheric oxygen levels are inferred based on the kinetics of reactions occurring in soils or in the oceans, rather than being directly tracked by atmospheric signatures. Rare oxygen isotope systematics-based on quantifying the rare oxygen isotope 17O in addition to the conventionally determined 16O and 18O-provide a means to track atmospheric isotopic signatures and thus potentially provide more direct estimates of atmospheric oxygen levels through time. Oxygen isotope signatures that deviate strongly from the expected mass-dependent relationship between 16O, 17O, and 18O develop during ozone formation, and these "mass-independent" signals can be transferred to the rock record during oxidation reactions in surface environments that involve atmospheric O2. The magnitude of these signals is dependent upon pO2, pCO2, and the overall extent of biospheric productivity. Here, we use a stochastic approach to invert the mid-Proterozoic Δ17O record for a new estimate of atmospheric pO2, leveraging explicit coupling of pO2 and biospheric productivity in a biogeochemical Earth system model to refine the range of atmospheric pO2 values that is consistent with a given observed Δ17O. Using this approach, we find new evidence that atmospheric oxygen levels were less than ∼1% of the present atmospheric level (PAL) for at least some intervals of the Proterozoic Eon.

Phosphorus is an essential element for life, and the phosphorous cycle is widely believed to be a key factor limiting the extent of Earth's biosphere and its impact on remotely detectable features of Earth's atmospheric chemistry. Continental weathering is conventionally considered to be the only source of bioavailable phosphorus to the marine biosphere, with submarine hydrothermal processes acting as a phosphorus sink. Here, we use a novel 29Si tracer technique to demonstrate that alteration of submarine basalt under anoxic conditions leads to significant soluble phosphorus release, with an estimated ratio between phosphorus release and CO2 consumption (P/CO2) of 3.99+/-1.03 umol/mmol. This ratio is comparable to that of modern rivers, suggesting that submarine weathering under anoxic conditions is potentially a significant source of bioavailable phosphorus to planetary oceans and that volatile-rich Earth-like planets lacking exposed continents could develop robust biospheres capable of sustaining remotely detectable atmospheric biosignatures., Manuscript under consideration in Geophysical Research Letters

The protracted oxygenation of the ocean-atmosphere system is one of the most fundamental changes to the Earth system through its history. The uranium isotopic composition (238U/235U, denoted as δ238U) of marine carbonates has been developed as a proxy to quantitatively track the timing, duration, and extent of global marine redox chemistry changes. This proxy has been applied to many critical evolutionary intervals in the last decade, significantly advancing our understanding of how life on Earth and its environment have co-evolved through geological history. Successful application of the uranium isotope paleoredox proxy requires a thorough understanding of the marine uranium budget, the processes by which seawater U-isotope signatures are recorded in marine carbonates, and the potential for alteration of these primary signatures by syn- and post-depositional diagenetic processes. Here, we provide a critical review of the U isotope proxy in marine carbonates with a focus on the current problems and areas where future work is needed to further develop this proxy. We also use a recently developed global C-P-U cycle model to illustrate that when the carbon cycle is perturbed by volcanic carbon injections, the ensuing transient relationship between seafloor anoxic area and δ238U can be complex and sometimes counter-intuitive.

Iron speciation and trace metal proxies are commonly applied together in efforts to identify anoxic settings marked by the presence of free sulfide (euxinia) or dissolved iron (ferruginous) in the water column. Here, we use a literature compilation from modern localities to provide a new empirical evaluation of coupled Fe speciation and Mo concentrations as a proxy for pore water sulfide accumulation at non-euxinic localities. We also present new Fe speciation, Mo concentration, and S isotope data from the Friends of Anoxic Mud (FOAM) site in Long Island Sound, which is marked by pore water sulfide accumulation of up to 3 mM beneath oxygen-containing bottom waters. For the operationally defined Fe speciation scheme, ‘highly reactive’ Fe (FeHR) is the sum of pyritized Fe (Fepy) and Fe dominantly present in oxide phases that is available to react with pore water sulfide to form pyrite. Observations from FOAM and elsewhere confirm that Fepy/FeHR from non-euxinic sites is a generally reliable indicator of pore fluid redox, particularly the presence of pore water sulfide. Molybdenum (Mo) concentration data for anoxic continental margin sediments underlying oxic waters but with sulfidic pore fluids typically show authigenic Mo enrichments (2–25 ppm) that are elevated relative to the upper crust (1–2 ppm). However, compilations of Mo concentrations comparing sediments with and without sulfidic pore fluids underlying oxic and low oxygen (non-euxinic) water columns expose non-unique ranges for each, exposing false positives and false negatives. False positives are most frequently found in sediments from low oxygen water columns (for example, Peru Margin), where Mo concentration ranges can also overlap with values commonly found in modern euxinic settings. FOAM represents an example of a false negative, where, despite elevated pore water sulfide concentrations and evidence for active Fe and Mn redox cycling in FOAM sediments, sedimentary Mo concentrations show a homogenous vertical profile across 50 cm depth at 1 to 2 ppm. A diagenetic model for Mo provides evidence that muted authigenic enrichments are derived from elevated sedimentation rates. Consideration of a range of additional parameters, most prominently pore water Mo concentration, can replicate the ranges of most sedimentary Mo concentrations observed in modern non-euxinic settings. Together, the modern Mo and Fe data compilations and diagenetic model provide a framework for identifying paleo-pore water sulfide accumulation in ancient settings and linked processes regulating seawater Mo and sulfate concentrations and delivery to sediments. Among other utilities, identifying ancient accumulation of sulfide in pore waters, particularly beneath oxic bottom waters, constrains the likelihood that those settings could have hosted organisms and ecosystems with thiotrophy at their foundations.

Over the last two decades, popular opinion about prevailing conditions in the mid-Proterozoic deep ocean has evolved from fully oxygenated to globally euxinic (sulfidic) to a more heterogeneous, stratified water column with localized pockets of euxinia existing in predominantly iron-rich (ferruginous) deep waters. The Animikie Basin in the Lake Superior region has been essential in shaping our view of marine redox evolution over this time period. In this study, we present a multi-proxy paleoredox investigation of previously unanalyzed strata of the late Paleoproterozoic Animikie Basin using drill cores through the ~1.85 Ga Stambaugh Formation (Paint River Group) in the Iron River-Crystal Falls district of the Upper Peninsula of Michigan, USA. Based on previous tectonic reconstructions and analysis of sedimentary regimes, the Iron River-Crystal Falls section captures strata from among the deepest-water facies of the Animikie Basin. In contrast to previous work on sedimentary rocks in this basin, we find evidence from iron speciation, trace metal, and Mo isotope data for episodes of at least local deep-water oxygenation within a basin otherwise dominated by ferruginous and euxinic conditions. While trace-metal enrichments and iron speciation data suggest predominantly anoxic conditions, the occurrence of Mn-rich intervals (up to 12.3 wt% MnO) containing abundant Mn-Fe carbonate, and a wide range of Mo isotope data with extremely negative values (δ98/95Mo = −1.0 to +1.1‰), record the shuttling of Mn-oxides from surface waters through oxic or suboxic waters to the sediment-water interface. We propose that such conditions are analogous to those of locally restricted modern and Holocene basins in the Baltic Sea, which receive episodic inflow of oxygenated water, producing similar geochemical signatures to those observed for the Animikie Basin. We argue that the mid-Proterozoic was characterized by a lack of a strong redox buffer (low sulfide, ferrous iron, and oxygen contents), and thus was vulnerable to dramatic, and at least local, redox shifts—including briefly oxygenated bottom waters. A refined view of the mid-Proterozoic ocean is emerging: one that was still predominantly anoxic, but marked by regional heterogeneities and short-term redox variability that may, in part, reflect a transitional state between prevailingly anoxic Archean and predominantly oxic Phanerozoic oceans.

Snowball Earth glaciations are the most extreme climate perturbations recorded in Earth’s history. It has been argued that the termination of these events was characterized by a single rapid transition from near-global ice coverage to an ice-free greenhouse climate state. Notably, this deviates with more extended transition periods of ice sheet waxing and waning typical of Phanerozoic glaciations. Using a coupled mineralogical and Mg and Li isotopic approach, we explore the role that authigenic clay formation within the seafloor may have played on Earth’s climate during deglaciation of the Marinoan Snowball Earth event. Marine authigenic clay formation—a process referred to as reverse weathering—recycles carbon within the ocean–atmosphere system and acts to elevate atmospheric CO2 levels. The results indicate a shift towards more extensive reverse weathering within the uppermost portion of the glaciogenic Nantuo Formation in South China. Carbon cycle modeling indicates that widespread reverse weathering could have driven a protracted (millions of years) carbon dioxide drawdown following high carbon dioxide levels expected during deglaciation.

The oxygenation of the atmosphere — one of the most fundamental transformations in Earth's history — dramatically altered the chemical composition of the oceans and provides a compelling example of how life can reshape planetary surface environments. Furthermore, it is commonly proposed that surface oxygen levels played a key role in controlling the timing and tempo of the origin and early diversification of animals. Although oxygen levels were likely more dynamic than previously imagined, we make a case here that emerging records provide evidence for low atmospheric oxygen levels for the majority of Earth's history. Specifically, we review records and present a conceptual framework that suggest that background oxygen levels were below 1% of the present atmospheric level during the billon years leading up to the diversification of early animals. Evidence for low background oxygen levels through much of the Proterozoic bolsters the case that environmental conditions were a critical factor in controlling the structure of ecosystems through Earth's history.

For the first four billion years of Earth’s history, climate was marked by apparent stability and warmth despite the Sun having lower luminosity1. Proposed mechanisms for maintaining an elevated partial pressure of carbon dioxide in the atmosphere ( $${p}_{{{\rm{CO}}}_{{\rm{2}}}}$$ ) centre on a reduction in the weatherability of Earth’s crust and therefore in the efficiency of carbon dioxide removal from the atmosphere2. However, the effectiveness of these mechanisms remains debated2,3. Here we use a global carbon cycle model to explore the evolution of processes that govern marine pH, a factor that regulates the partitioning of carbon between the ocean and the atmosphere. We find that elevated rates of ‘reverse weathering’—that is, the consumption of alkalinity and generation of acidity during marine authigenic clay formation4–7—enhanced the retention of carbon within the ocean–atmosphere system, leading to an elevated $${p}_{{{\rm{CO}}}_{{\rm{2}}}}$$ baseline. Although this process is dampened by sluggish kinetics today, we propose that more prolific rates of reverse weathering would have persisted under the pervasively silica-rich conditions8,9 that dominated Earth’s early oceans. This distinct ocean and coupled carbon–silicon cycle state would have successfully maintained the equable and ice-free environment that characterized most of the Precambrian period. Further, we propose that during this time, the establishment of a strong negative feedback between marine pH and authigenic clay formation would have also enhanced climate stability by mitigating large swings in $${p}_{{{\rm{CO}}}_{{\rm{2}}}}$$ —a critical component of Earth’s natural thermostat that would have been dominant for most of Earth’s history. We speculate that the late ecological rise of siliceous organisms8 and a resulting decline in silica-rich conditions dampened the reverse weathering buffer, destabilizing Earth’s climate system and lowering baseline $${p}_{{{\rm{CO}}}_{{\rm{2}}}}$$ . Elevated rates of reverse weathering within silica-rich oceans led to enhanced carbon retention within the ocean–atmosphere system, promoting a stable, equable ice-free climate throughout Earth’s early to middle ages.

The biogeochemical cycling of zinc (Zn) is intimately coupled with organic carbon in the ocean. Based on an extensive new sedimentary Zn isotope record across Earth's history, we provide evidence for a fundamental shift in the marine Zn cycle ~800 million years ago. We discuss a wide range of potential drivers for this transition and propose that, within available constraints, a restructuring of marine ecosystems is the most parsimonious explanation for this shift. Using a global isotope mass balance approach, we show that a change in the organic Zn/C ratio is required to account for observed Zn isotope trends through time. Given the higher affinity of eukaryotes for Zn relative to prokaryotes, we suggest that a shift toward a more eukaryote-rich ecosystem could have provided a means of more efficiently sequestering organic-derived Zn. Despite the much earlier appearance of eukaryotes in the microfossil record (~1700 to 1600 million years ago), our data suggest a delayed rise to ecological prominence during the Neoproterozoic, consistent with the currently accepted organic biomarker records.