Sulfur cycle imbalance and environmental change during the Ediacaric Period

A different approach is proposed here to solve the problem of negative chemostratigraphic excursions during the Ediacárico, considering them in terms of a linked system of carbon-sulfide-oxygen, in which changes in the dynamics of oxidants would cause an excess of oxidation of organic carbon over the burial, which would result in a smaller deposit of DOM. The amount of oxidant required to achieve a negative carbon isotopic excursion through the oxidation of net organic carbon can reasonably result from the evaporitaa solution at the basin scale

Reconciling proxy records and models of Earth's oxygenation during the Neoproterozoic and Palaeozoic

A hypothesized rise in oxygen levels in the Neoproterozoic, dubbed the Neoproterozoic Oxygenation Event, has been repeatedly linked to the origin and rise of animal life. However, a new body of work has emerged over the past decade that questions this narrative. We explore available proxy records of atmospheric and marine oxygenation and, considering the unique systematics of each geochemical system, attempt to reconcile the data. We also present new results from a comprehensive COPSE biogeochemical model that combines several recent additions, to create a continuous model record from 850 to 250 Ma. We conclude that oxygen levels were intermediate across the Ediacaran and early Palaeozoic, and highly dynamic. Stable, modern-like conditions were not reached until the Late Palaeozoic. We therefore propose that the terms Neoproterozoic Oxygenation Window and Palaeozoic Oxygenation Event are more appropriate descriptors of the rise of oxygen in Earth's atmosphere and oceans

Sulfur cycle imbalance and environmental change during the Ediacaran Period = Desequilibrio del ciclo del azufre y cambio ambiental durante el Período Ediacárico

A different approach is proposed here to solve the problem of negative δ13C excursions during the Ediacaran, by viewing them in terms of a linked carbon-sulfur-oxygen system, whereby changes in oxidant dynamics caused an excess of organic carbon oxidation over burial, resulting in a smaller DOM reservoir. The amount of oxidant required to achieve a deep negative carbon isotope excursion through net organic carbon oxidation may reasonably result from basin-scale evaporite dissolution. / Se propone aquí un enfoque diferente para resolver el problema de las excursiones quimioestratigráficas negativas durante el Ediacárico, considerándolas en términos de un sistema vinculado de carbono-sulfuro-oxígeno, en el que los cambios en la dinámica de los oxidantes causarían un exceso de oxidación de carbono orgánico sobre el enterramiento, lo que resultaría en un depósito menor de DOM. La cantidad de oxidante requerida para lograr una excursión isotópica de carbono negativa a través de la oxidación de carbono orgánico neto puede resultar razonablemente de la disolución de evaporitaa a escala de cuenca

Weathering pathways and limitations in biogeochemical models: Application to Earth system evolution

Current biogeochemical box models for Phanerozoic climate are reviewed and reduced to a robust, modular system, allowing application to the Precambrian. It is shown that stabilisation of climate following a Neoproterozoic snowball Earth should take more than 10(7) years, due to long-term geological limitation of global weathering rates. The timescale matches the observed gaps between extreme glaciations at this time, suggesting that the late Neoproterozoic system was oscillating around a steady state temperature below the snowball threshold. In the model, the period of disequilibrium following snowball glaciations is characterised by elevated ocean nutrient and organic burial rates, providing fair correlation with available geochemical proxies. Extending the analysis to consider carbon removed from the ocean via seafloor carbonatization does not result in a signifi�cant reduction in stabilisation time. Model timeframe is extended over the last 2Ga. Predicted oxygen concentration is shown to depend on the balance between terrestrial and sea floor weathering, which alters the global nutrient delivery rate and therefore global productivity. Under reasonable assumptions, broad predictions for Proterozoic climate fall within, or close to the bounds imposed by geological proxies. A mechanism for atmospheric oxygenation over Earth history is proposed: the combination of declining mantle heat flux and increasing continental area, aided by colonising land biota, results in a steadily increasing ocean nutrient supply, driving increasing rates of organic carbon burial. Methods currently used for assessing Phanerozoic O2 assume only terrestrial weathering fluxes, and are found to give unreasonable results when applied to the Precambrian. Phanerozoic predictions from the model developed here show a significant�cant reduction in the large oxygen peak at 300Ma found in previous studies. This is due to consideration of terrestrial and sea floor weathering balance, and to the longer model timeframe - which allows prediction of crustal abundances in the Cambrian, rather than assuming present day conditions

Changing tectonic controls on the long-term carbon cycle from Mesozoic to present

Tectonic drivers of degassing and weathering processes are key long-term controls on atmospheric CO2. However, there is considerable debate over the changing relative importance of different carbon sources and sinks. Existing geochemical models have tended to rely on indirect methods to derive tectonic drivers, such as inversion of the seawater 87Sr/86Sr curve to estimate uplift or continental basalt area. Here we use improving geologic data to update the representation of tectonic drivers in the COPSE biogeochemical model. The resulting model distinguishes CO2 sinks from terrestrial granite weathering, total basalt weathering, and seafloor alteration. It also distinguishes CO2 sources from subduction zone metamorphism and from igneous intrusions. We reconstruct terrestrial basaltic area from data on the extent of large igneous provinces and use their volume to estimate their contribution to degassing. We adopt a recently published reconstruction of subduction-related degassing, and relate seafloor weathering to ocean crust creation rate. Revised degassing alone tends to produce unrealistically high CO2, but this is counteracted by the inclusion of seafloor alteration and global basalt weathering, producing a good overall fit to Mesozoic-Cenozoic proxy CO2 estimates and a good fit to 87Sr/86Sr data. The model predicts that seafloor alteration and terrestrial weathering made similar contributions to CO2 removal through the Triassic and Jurassic, after which terrestrial weathering increased and seafloor weathering declined. We predict that basalts made a greater contribution to silicate weathering than granites through the Mesozoic, before the contribution of basalt weathering declined over the Cenozoic due to decreasing global basaltic area

Stepwise Earth oxygenation is an inherent property of global biogeochemical cycling

Oxygenation of Earth’s atmosphere and oceans occurred across three major steps during the Paleoproterozoic, Neoproterozoic, and Paleozoic eras, with each increase having profound consequences for the biosphere. Biological or tectonic revolutions have been proposed to explain each of these stepwise increases in oxygen, but the principal driver of each event remains unclear. Here we show, using a theoretical model, that the observed oxygenation steps are a simple consequence of internal feedbacks in the long-term biogeochemical cycles of carbon, oxygen, and phosphorus, and that there is no requirement for a specific stepwise external forcing to explain the course of Earth surface oxygenation. We conclude that Earth’s oxygenation events are entirely consistent with gradual oxygenation of the planetary surface after the evolution of oxygenic photosynthesis

Ocean de-oxygenation, the global phosphorus cycle, and the possibility of human-caused large-scale ocean anoxia

The major biogeochemical cycles that keep the present-day Earth habitable are linked by a network of feedbacks, which has led to a broadly stable chemical composition of the oceans and atmosphere over hundreds of millions of years. This includes the processes that control both the atmospheric and oceanic concentrations of oxygen. However, one notable exception to the generally well-behaved dynamics of this system is the propensity for episodes of ocean anoxia to occur and to persist for 105–106 years, these ocean anoxic events (OAEs) being particularly associated with warm ‘greenhouse’ climates. A powerful mechanism responsible for past OAEs was an increase in phosphorus supply to the oceans, leading to higher ocean productivity and oxygen demand in subsurface water. This can be amplified by positive feedbacks on the nutrient content of the ocean, with low oxygen promoting further release of phosphorus from ocean sediments, leading to a potentially self-sustaining condition of deoxygenation. We use a simple model for phosphorus in the ocean to explore this feedback, and to evaluate the potential for humans to bring on global-scale anoxia by enhancing P supply to the oceans. While this is not an immediate global change concern, it is a future possibility on millennial and longer time scales, when considering both phosphate rock mining and increased chemical weathering due to climate change. Ocean deoxygenation, once begun, may be self-sustaining and eventually could result in long-lasting and unpleasant consequences for the Earth's biosphere

Nutrient acquisition by symbiotic fungi governs Palaeozoic climate transition

Fossil evidence from the Rhynie chert indicates that early land plants, which evolved in a high-CO2 atmosphere during the Palaeozoic Era, hosted diverse fungal symbionts. It is hypothesized that the rise of early non-vascular land plants, and the later evolution of roots and vasculature, drove the long-term shift towards a high-oxygen, low CO2 climate that eventually permitted the evolution of mammals and, ultimately, humans. However, very little is known about the productivity of the early terrestrial biosphere, which depended on the acquisition of the limiting nutrient phosphorus via fungal symbiosis. Recent laboratory experiments have shown that plant–fungal symbiotic function is specific to fungal identity, with carbon-for-phosphorus exchange being either enhanced or suppressed under superambient CO2. By incorporating these experimental findings into a biogeochemical model, we show that the differences in these symbiotic nutrient acquisition strategies could greatly alter the plant-driven changes to climate, allowing drawdown of CO2 to glacial levels, and altering the nature of the rise of oxygen. We conclude that an accurate depiction of plant–fungal symbiotic systems, informed by high-CO2 experiments, is key to resolving the question of how the first terrestrial ecosystems altered our planet

Exploring multiple steady states in Earth's long-term carbon cycle

The long-term carbon cycle regulates Earth's climate and atmospheric CO2 levels over multimillion-year timescales, but it is not clear that this system has a single steady state for a given input rate of CO2. In this paper we explore the possibility for multiple steady states in the long-term climate system. Using a simple carbon cycle box model, we show that the location of precipitation bands around the tropics and high mid-latitudes, coupled with the response of the terrestrial biosphere to local surface temperature, can result in system bi-stability. Here, maximum CO2 drawdown can occur when either the tropics or high mid-latitudes are at the photosynthetic optimum temperature of around 25°C, and a period of instability can exist between these states. We suggest that this dynamic has influenced climate variations over Phanerozoic time, and that higher steady state surface temperatures may be easier to reach than is commonly demonstrated in simple ‘GEOCARB style’ carbon cycle models

COPSE reloaded: an improved model of biogeochemical cycling over Phanerozoic time

The ‘COPSE’ (Carbon, Oxygen, Phosphorus, Sulphur and Evolution) biogeochemical model predicts the coupled histories and controls on atmospheric O2, CO2 and ocean composition over Phanerozoic time. The forwards modelling approach utilized in COPSE makes it a useful tool for testing mechanistic hypotheses against geochemical data and it has been extended and altered a number of times since being published in 2004. Here we undertake a wholesale revision of the model, incorporating: (1) elaboration and updating of the external forcing factors; (2) improved representation of existing processes, including plant effects on weathering and ocean anoxia; (3) inclusion of additional processes and tracers, including seafloor weathering, volcanic rock weathering and 87Sr/86Sr; (4) updating of the present-day baseline fluxes; and (5) a more efficient and robust numerical scheme. A key aim is to explore how sensitive predictions of atmospheric CO2, O2 and ocean composition are to model updates and ongoing uncertainties. The revised model reasonably captures the long-term trends in Phanerozoic geochemical proxies for atmospheric pCO2, pO2, ocean [SO4], carbonate δ13C, sulphate δ34S and carbonate 87Sr/86Sr. It predicts a two-phase drawdown of atmospheric CO2 with the rise of land plants and associated cooling phases in the Late Ordovician and Devonian-early Carboniferous, followed by broad peaks of atmospheric CO2 and temperature in the Triassic and mid-Cretaceous – although some of the structure in the CO2 proxy record is missed. The model robustly predicts a mid-Paleozoic oxygenation event due to the earliest land plants, with O2 rising from ~ 5% to > 17% of the atmosphere and oxygenating the ocean. Thereafter, atmospheric O2 is effectively regulated with remaining fluctuations being a Carboniferous–Permian O2 peak ~ 26% linked to burial of terrestrial organic matter in coal swamps, a Triassic–Jurassic O2 minimum ~ 21% linked to low uplift, a Cretaceous O2 peak ~ 26% linked to high degassing and weathering fluxes, and a Cenozoic O2 decline