For the first four billion years of Earth's history, climate was marked by apparent stability and warmth despite the Sun having lower luminosity. Proposed mechanisms for maintaining an elevated partial pressure of carbon dioxide in the atmosphere ([Formula: see text]) centre on a reduction in the weatherability of Earth's crust and therefore in the efficiency of carbon dioxide removal from the atmosphere. However, the effectiveness of these mechanisms remains debated. 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 formation-enhanced the retention of carbon within the ocean-atmosphere system, leading to an elevated [Formula: see text] 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 conditions 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 [Formula: see text]-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 organisms and a resulting decline in silica-rich conditions dampened the reverse weathering buffer, destabilizing Earth's climate system and lowering baseline [Formula: see text].

The existence of stabilizing feedbacks within Earth's climate system is generally thought to be necessary for the persistence of liquid water and life. Over the course of Earth's history, Earth's atmospheric composition appears to have adjusted to the gradual increase in solar luminosity, resulting in persistently habitable surface temperatures. With limited exceptions, the Earth system has been observed to recover rapidly from pulsed climatic perturbations. Carbon dioxide (CO2) regulation via negative feedbacks within the coupled global carbon‐silica cycles are classically viewed as the main processes giving rise to climate stability on Earth. Here we review the long‐term global carbon cycle budget, and how the processes modulating Earth's climate system have evolved over time. Specifically, we focus on the relative roles that shifts in carbon sources and sinks have played in driving long‐term changes in atmospheric pCO2. We make the case that marine processes are an important component of the canonical silicate weathering feedback, and have played a much more important role in pCO2 regulation than traditionally imagined. Notably, geochemical evidence indicate that the weathering of marine sediments and off‐axis basalt alteration act as major carbon sinks. However, this sink was potentially dampened during Earth's early history when oceans had higher levels of dissolved silicon (Si), iron (Fe), and magnesium (Mg), and instead likely fostered more extensive carbon recycling within the ocean‐atmosphere system via reverse weathering—that in turn acted to elevate ocean‐atmosphere CO2 levels.

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.

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.

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.

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The evolution of the global carbon and silicon cycles are thought to have contributed to the long-term stability of the Earth's climate [1][2][3] . Many questions remain, however,

Iron speciation and trace metal proxies are commonly applied together in efforts to identify 21 anoxic settings marked by the presence of free sulfide (euxinia) or dissolved iron (ferruginous) in the 22 water column. Here, we use a literature compilation from modern localities to provide a new empirical 23 evaluation of coupled Fe speciation and Mo concentrations as a proxy for pore water sulfide accumulation 24 at non-euxinic localities. We also present new Fe speciation, Mo concentration, and S isotope data from 25 the Friends of Anoxic Mud (FOAM) site in Long Island Sound, which is marked by pore water sulfide 26 accumulation of up to 3 mM beneath oxygen-containing bottom waters. For the operationally defined Fe 27 speciation scheme, 'highly reactive' Fe (Fe HR) is the sum of pyritized Fe (Fe py) and Fe dominantly present 28 in oxide phases that is available to react with pore water sulfide to form pyrite. Observations from FOAM 29 and elsewhere confirm that Fe py /Fe HR from non-euxinic sites is a generally reliable indicator of pore fluid 30 redox, particularly the presence of pore water sulfide. Molybdenum (Mo) concentration data for anoxic 31 continental margin sediments underlying oxic waters but with sulfidic pore fluids typically show 32 authigenic Mo enrichments (2-25 ppm) that are elevated relative to the upper crust (1-2 ppm). However, 33 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 35 positives and false negatives. False positives are most frequently found in sediments from low oxygen 36 water columns (for example, Peru Margin), where Mo concentration ranges can also overlap with values 37 commonly found in modern euxinic settings. FOAM represents an example of a false negative, where, 38 despite elevated pore water sulfide concentrations and evidence for active Fe and Mn redox cycling in 39 FOAM sediments, sedimentary Mo concentrations show a homogenous vertical profile across 50 cm 40 depth at 1-2 ppm. A diagenetic model for Mo provides evidence that muted authigenic enrichments are 41 derived from elevated sedimentation rates. Consideration of a range of additional parameters, most 42 prominently pore water Mo concentration, can replicate the ranges of most sedimentary Mo 43 concentrations observed in modern non-euxinic settings. Together, the modern Mo and Fe data 44 compilations and diagenetic model provide a framework for identifying paleo-pore water sulfide 45 accumulation in ancient settings and linked processes regulating seawater Mo and sulfate concentrations 46 and delivery to sediments. Among other utilities, identifying ancient accumulation of sulfide in pore 47 waters, particularly beneath oxic bottom waters, constrains the likelihood that those settings could have 48 hosted organisms and ecosystems with thiotrophy at their foundations. 49 INTRODUCTION 50 Iron speciation and molybdenum concentrations have been well calibrated in mod...