Publications by authors named "Benjamin J W Mills"

18 Publications

  • Page 1 of 1

The rise of angiosperms strengthened fire feedbacks and improved the regulation of atmospheric oxygen.

Nat Commun 2021 01 21;12(1):503. Epub 2021 Jan 21.

Global System Institute, University of Exeter, Exeter, EX4 4QE, UK.

The source of oxygen to Earth's atmosphere is organic carbon burial, whilst the main sink is oxidative weathering of fossil carbon. However, this sink is to insensitive to counteract oxygen rising above its current level of about 21%. Biogeochemical models suggest that wildfires provide an additional regulatory feedback mechanism. However, none have considered how the evolution of different plant groups through time have interacted with this feedback. The Cretaceous Period saw not only super-ambient levels of atmospheric oxygen but also the evolution of the angiosperms, that then rose to dominate Earth's ecosystems. Here we show, using the COPSE biogeochemical model, that angiosperm-driven alteration of fire feedbacks likely lowered atmospheric oxygen levels from ~30% to 25% by the end of the Cretaceous. This likely set the stage for the emergence of closed-canopy angiosperm tropical rainforests that we suggest would not have been possible without angiosperm enhancement of fire feedbacks.
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http://dx.doi.org/10.1038/s41467-020-20772-2DOI Listing
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC7820256PMC
January 2021

Past climates inform our future.

Science 2020 11;370(6517)

Department of Oceanography, Texas A&M University, College Station, TX, USA.

As the world warms, there is a profound need to improve projections of climate change. Although the latest Earth system models offer an unprecedented number of features, fundamental uncertainties continue to cloud our view of the future. Past climates provide the only opportunity to observe how the Earth system responds to high carbon dioxide, underlining a fundamental role for paleoclimatology in constraining future climate change. Here, we review the relevancy of paleoclimate information for climate prediction and discuss the prospects for emerging methodologies to further insights gained from past climates. Advances in proxy methods and interpretations pave the way for the use of past climates for model evaluation-a practice that we argue should be widely adopted.
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http://dx.doi.org/10.1126/science.aay3701DOI Listing
November 2020

An enormous sulfur isotope excursion indicates marine anoxia during the end-Triassic mass extinction.

Sci Adv 2020 Sep 9;6(37). Epub 2020 Sep 9.

School of Earth and Environment, University of Leeds, Leeds, UK.

The role of ocean anoxia as a cause of the end-Triassic marine mass extinction is widely debated. Here, we present carbonate-associated sulfate δS data from sections spanning the Late Triassic-Early Jurassic transition, which document synchronous large positive excursions on a global scale occurring in ~50 thousand years. Biogeochemical modeling demonstrates that this S isotope perturbation is best explained by a fivefold increase in global pyrite burial, consistent with large-scale development of marine anoxia on the Panthalassa margin and northwest European shelf. This pyrite burial event coincides with the loss of Triassic taxa seen in the studied sections. Modeling results also indicate that the pre-event ocean sulfate concentration was low (<1 millimolar), a common feature of many Phanerozoic deoxygenation events. We propose that sulfate scarcity preconditions oceans for the development of anoxia during rapid warming events by increasing the benthic methane flux and the resulting bottom-water oxygen demand.
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http://dx.doi.org/10.1126/sciadv.abb6704DOI Listing
September 2020

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

Interface Focus 2020 Aug 12;10(4):20190137. Epub 2020 Jun 12.

School of Earth and Environment, University of Leeds, Leeds LS2 9JT, UK.

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.
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http://dx.doi.org/10.1098/rsfs.2019.0137DOI Listing
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC7333907PMC
August 2020

Permo-Triassic boundary carbon and mercury cycling linked to terrestrial ecosystem collapse.

Nat Commun 2020 06 11;11(1):2962. Epub 2020 Jun 11.

School of Earth and Environments, University of Leeds, Leeds, LS2 9JT, UK.

Records suggest that the Permo-Triassic mass extinction (PTME) involved one of the most severe terrestrial ecosystem collapses of the Phanerozoic. However, it has proved difficult to constrain the extent of the primary productivity loss on land, hindering our understanding of the effects on global biogeochemistry. We build a new biogeochemical model that couples the global Hg and C cycles to evaluate the distinct terrestrial contribution to atmosphere-ocean biogeochemistry separated from coeval volcanic fluxes. We show that the large short-lived Hg spike, and nadirs in δHg and δC values at the marine PTME are best explained by a sudden, massive pulse of terrestrial biomass oxidation, while volcanism remains an adequate explanation for the longer-term geochemical changes. Our modelling shows that a massive collapse of terrestrial ecosystems linked to volcanism-driven environmental change triggered significant biogeochemical changes, and cascaded organic matter, nutrients, Hg and other organically-bound species into the marine system.
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http://dx.doi.org/10.1038/s41467-020-16725-4DOI Listing
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC7289894PMC
June 2020

Deep CO in the end-Triassic Central Atlantic Magmatic Province.

Nat Commun 2020 04 7;11(1):1670. Epub 2020 Apr 7.

Lithosphere Fluid Research Lab, Institute of Geography and Earth Sciences, Eötvös Loránd University, Budapest, H-1117, Hungary.

Large Igneous Province eruptions coincide with many major Phanerozoic mass extinctions, suggesting a cause-effect relationship where volcanic degassing triggers global climatic changes. In order to fully understand this relationship, it is necessary to constrain the quantity and type of degassed magmatic volatiles, and to determine the depth of their source and the timing of eruption. Here we present direct evidence of abundant CO in basaltic rocks from the end-Triassic Central Atlantic Magmatic Province (CAMP), through investigation of gas exsolution bubbles preserved by melt inclusions. Our results indicate abundance of CO and a mantle and/or lower-middle crustal origin for at least part of the degassed carbon. The presence of deep carbon is a key control on the emplacement mode of CAMP magmas, favouring rapid eruption pulses (a few centuries each). Our estimates suggest that the amount of CO that each CAMP magmatic pulse injected into the end-Triassic atmosphere is comparable to the amount of anthropogenic emissions projected for the 21 century. Such large volumes of volcanic CO likely contributed to end-Triassic global warming and ocean acidification.
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http://dx.doi.org/10.1038/s41467-020-15325-6DOI Listing
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC7138847PMC
April 2020

Stepwise Earth oxygenation is an inherent property of global biogeochemical cycling.

Science 2019 12 11;366(6471):1333-1337. Epub 2019 Dec 11.

School of Earth and Environment, University of Leeds, Leeds LS2 9JT, UK.

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.
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http://dx.doi.org/10.1126/science.aax6459DOI Listing
December 2019

A tectonically driven Ediacaran oxygenation event.

Nat Commun 2019 06 19;10(1):2690. Epub 2019 Jun 19.

Global Systems Institute, University of Exeter, Exeter, EX4 4QE, UK.

The diversification of complex animal life during the Cambrian Period (541-485.4 Ma) is thought to have been contingent on an oxygenation event sometime during ~850 to 541 Ma in the Neoproterozoic Era. Whilst abundant geochemical evidence indicates repeated intervals of ocean oxygenation during this time, the timing and magnitude of any changes in atmospheric pO remain uncertain. Recent work indicates a large increase in the tectonic CO degassing rate between the Neoproterozoic and Paleozoic Eras. We use a biogeochemical model to show that this increase in the total carbon and sulphur throughput of the Earth system increased the rate of organic carbon and pyrite sulphur burial and hence atmospheric pO. Modelled atmospheric pO increases by ~50% during the Ediacaran Period (635-541 Ma), reaching ~0.25 of the present atmospheric level (PAL), broadly consistent with the estimated pO > 0.1-0.25 PAL requirement of large, mobile and predatory animals during the Cambrian explosion.
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http://dx.doi.org/10.1038/s41467-019-10286-xDOI Listing
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC6584537PMC
June 2019

Possible links between extreme oxygen perturbations and the Cambrian radiation of animals.

Nat Geosci 2019 Jun;12(6):468-474

London Geochemistry and Isotope Centre (LOGIC), Institute of Earth and Planetary Sciences, University College London and Birkbeck, University of London, London, WC1E 6BT, UK.

The role of oxygen as a driver for early animal evolution is widely debated. During the Cambrian explosion, episodic radiations of major animal phyla occurred coincident with repeated carbon isotope fluctuations. However, the driver of these isotope fluctuations and potential links to environmental oxygenation are unclear. Here, we report high-resolution carbon and sulphur isotope data for marine carbonates from the southeastern Siberian Platform that document the canonical explosive phase of the Cambrian radiation from ~524 to ~514 Myr ago. These analyses demonstrate a strong positive covariation between carbonate δC and carbonate-associated sulphate δS through five isotope cycles. Biogeochemical modelling suggests that this isotopic coupling reflects periodic oscillations in atmospheric O and the extent of shallow ocean oxygenation. Episodic maxima in the biodiversity of animal phyla directly coincided with these extreme oxygen perturbations. Conversely, the subsequent Botoman-Toyonian animal extinction events (~514 to ~512 Myr ago) coincided with decoupled isotope records that suggest a shrinking marine sulphate reservoir and expanded shallow marine anoxia. We suggest that fluctuations in oxygen availability in the shallow marine realm exerted a primary control on the timing and tempo of biodiversity radiations at a crucial phase in the early history of animal life.
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http://dx.doi.org/10.1038/s41561-019-0357-zDOI Listing
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC6548555PMC
June 2019

Stepwise oxygenation of the Paleozoic atmosphere.

Nat Commun 2018 10 4;9(1):4081. Epub 2018 Oct 4.

School of Earth and Environment, University of Leeds, Leeds, LS2 9JT, UK.

Oxygen is essential for animal life, and while geochemical proxies have been instrumental in determining the broad evolutionary history of oxygen on Earth, much of our insight into Phanerozoic oxygen comes from biogeochemical modelling. The GEOCARBSULF model utilizes carbon and sulphur isotope records to produce the most detailed history of Phanerozoic atmospheric O currently available. However, its predictions for the Paleozoic disagree with geochemical proxies, and with non-isotope modelling. Here we show that GEOCARBSULF oversimplifies the geochemistry of sulphur isotope fractionation, returning unrealistic values for the O sourced from pyrite burial when oxygen is low. We rebuild the model from first principles, utilizing an improved numerical scheme, the latest carbon isotope data, and we replace the sulphur cycle equations in line with forwards modelling approaches. Our new model, GEOCARBSULFOR, produces a revised, highly-detailed prediction for Phanerozoic O that is consistent with available proxy data, and independently supports a Paleozoic Oxygenation Event, which likely contributed to the observed radiation of complex, diverse fauna at this time.
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http://dx.doi.org/10.1038/s41467-018-06383-yDOI Listing
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC6172248PMC
October 2018

Early Palaeozoic ocean anoxia and global warming driven by the evolution of shallow burrowing.

Nat Commun 2018 07 2;9(1):2554. Epub 2018 Jul 2.

School of Earth and Environment, University of Leeds, Leeds, LS2 9JT, UK.

The evolution of burrowing animals forms a defining event in the history of the Earth. It has been hypothesised that the expansion of seafloor burrowing during the Palaeozoic altered the biogeochemistry of the oceans and atmosphere. However, whilst potential impacts of bioturbation on the individual phosphorus, oxygen and sulphur cycles have been considered, combined effects have not been investigated, leading to major uncertainty over the timing and magnitude of the Earth system response to the evolution of bioturbation. Here we integrate the evolution of bioturbation into the COPSE model of global biogeochemical cycling, and compare quantitative model predictions to multiple geochemical proxies. Our results suggest that the advent of shallow burrowing in the early Cambrian contributed to a global low-oxygen state, which prevailed for ~100 million years. This impact of bioturbation on global biogeochemistry likely affected animal evolution through expanded ocean anoxia, high atmospheric CO levels and global warming.
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http://dx.doi.org/10.1038/s41467-018-04973-4DOI Listing
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC6028391PMC
July 2018

Long-Term Planetary Habitability and the Carbonate-Silicate Cycle.

Astrobiology 2018 05;18(5):469-480

6 School of Earth and Environmental Sciences, University of St. Andrews , St. Andrews, UK.

The potential habitability of an exoplanet is traditionally assessed by determining whether its orbit falls within the circumstellar "habitable zone" of its star, defined as the distance at which water could be liquid on the surface of a planet (Kopparapu et al., 2013 ). Traditionally, these limits are determined by radiative-convective climate models, which are used to predict surface temperatures at user-specified levels of greenhouse gases. This approach ignores the vital question of the (bio)geochemical plausibility of the proposed chemical abundances. Carbon dioxide is the most important greenhouse gas in Earth's atmosphere in terms of regulating planetary temperature, with the long-term concentration controlled by the balance between volcanic outgassing and the sequestration of CO via chemical weathering and sedimentation, as modulated by ocean chemistry, circulation, and biological (microbial) productivity. We developed a model that incorporates key aspects of Earth's short- and long-term biogeochemical carbon cycle to explore the potential changes in the CO greenhouse due to variance in planet size and stellar insolation. We find that proposed changes in global topography, tectonics, and the hydrological cycle on larger planets result in proportionally greater surface temperatures for a given incident flux. For planets between 0.5 and 2 R, the effect of these changes results in average global surface temperature deviations of up to 20 K, which suggests that these relationships must be considered in future studies of planetary habitability. Key Words: Planets-Atmospheres-Carbon dioxide-Biogeochemistry. Astrobiology 18, 469-480.
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http://dx.doi.org/10.1089/ast.2017.1693DOI Listing
May 2018

Nutrient acquisition by symbiotic fungi governs Palaeozoic climate transition.

Philos Trans R Soc Lond B Biol Sci 2018 Feb;373(1739)

Centre for Plant Sciences, School of Biology, University of Leeds, Leeds LS2 9JT, UK

Fossil evidence from the Rhynie chert indicates that early land plants, which evolved in a high-CO 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 CO 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 CO 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 CO 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-CO experiments, is key to resolving the question of how the first terrestrial ecosystems altered our planet.This article is part of a discussion meeting issue 'The Rhynie cherts: our earliest terrestrial ecosystem revisited'.
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http://dx.doi.org/10.1098/rstb.2016.0503DOI Listing
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC5745338PMC
February 2018

Elevated CO degassing rates prevented the return of Snowball Earth during the Phanerozoic.

Nat Commun 2017 10 24;8(1):1110. Epub 2017 Oct 24.

Earth System Science, College of Life and Environmental Sciences, University of Exeter, Exeter, EX4 4QE, UK.

The Cryogenian period (~720-635 Ma) is marked by extensive Snowball Earth glaciations. These have previously been linked to CO draw-down, but the severe cold climates of the Cryogenian have never been replicated during the Phanerozoic despite similar, and sometimes more dramatic changes to carbon sinks. Here we quantify the total CO input rate, both by measuring the global length of subduction zones in plate tectonic reconstructions, and by sea-level inversion. Our results indicate that degassing rates were anomalously low during the Late Neoproterozoic, roughly doubled by the Early Phanerozoic, and remained comparatively high until the Cenozoic. Our carbon cycle modelling identifies the Cryogenian as a unique period during which low surface temperature was more easily achieved, and shows that the shift towards greater CO input rates after the Cryogenian helped prevent severe glaciation during the Phanerozoic. Such a shift appears essential for the development of complex animal life.
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http://dx.doi.org/10.1038/s41467-017-01456-wDOI Listing
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC5736558PMC
October 2017

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

Philos Trans A Math Phys Eng Sci 2017 Sep;375(2102)

School of Earth and Environment, University of Leeds, Leeds LS2 9JT, UK.

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 10-10 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.This article is part of the themed issue 'Ocean ventilation and deoxygenation in a warming world'.
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http://dx.doi.org/10.1098/rsta.2016.0318DOI Listing
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC5559414PMC
September 2017

Tectonic controls on the long-term carbon isotope mass balance.

Proc Natl Acad Sci U S A 2017 04 10;114(17):4318-4323. Epub 2017 Apr 10.

School of Earth and Environment, University of Leeds, Leeds LS2 9JT, United Kingdom

The long-term, steady-state marine carbon isotope record reflects changes to the proportional burial rate of organic carbon relative to total carbon on a global scale. For this reason, times of high δC are conventionally interpreted to be oxygenation events caused by excess organic burial. Here we show that the carbon isotope mass balance is also significantly affected by tectonic uplift and erosion via changes to the inorganic carbon cycle that are independent of changes to the isotopic composition of carbon input. This view is supported by inverse covariance between δC and a range of uplift proxies, including seawater Sr/Sr, which demonstrates how erosional forcing of carbonate weathering outweighs that of organic burial on geological timescales. A model of the long-term carbon cycle shows that increases in δC need not be associated with increased organic burial and that alternative tectonic drivers (erosion, outgassing) provide testable and plausible explanations for sustained deviations from the long-term δC mean. Our approach emphasizes the commonly overlooked difference between how net and gross carbon fluxes affect the long-term carbon isotope mass balance, and may lead to reassessment of the role that the δC record plays in reconstructing the oxygenation of earth's surface environment.
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http://dx.doi.org/10.1073/pnas.1614506114DOI Listing
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC5410774PMC
April 2017

Atmospheric oxygen regulation at low Proterozoic levels by incomplete oxidative weathering of sedimentary organic carbon.

Nat Commun 2017 02 2;8:14379. Epub 2017 Feb 2.

Earth System Science Group, Department of Geography, College of Life and Environmental Sciences, University of Exeter, Laver Building (Level 7), North Parks Road, Exeter EX4 4QE, UK.

It is unclear why atmospheric oxygen remained trapped at low levels for more than 1.5 billion years following the Paleoproterozoic Great Oxidation Event. Here, we use models for erosion, weathering and biogeochemical cycling to show that this can be explained by the tectonic recycling of previously accumulated sedimentary organic carbon, combined with the oxygen sensitivity of oxidative weathering. Our results indicate a strong negative feedback regime when atmospheric oxygen concentration is of order pO∼0.1 PAL (present atmospheric level), but that stability is lost at pO<0.01 PAL. Within these limits, the carbonate carbon isotope (δC) record becomes insensitive to changes in organic carbon burial rate, due to counterbalancing changes in the weathering of isotopically light organic carbon. This can explain the lack of secular trend in the Precambrian δC record, and reopens the possibility that increased biological productivity and resultant organic carbon burial drove the Great Oxidation Event.
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http://dx.doi.org/10.1038/ncomms14379DOI Listing
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC5296660PMC
February 2017

Earliest land plants created modern levels of atmospheric oxygen.

Proc Natl Acad Sci U S A 2016 08 15;113(35):9704-9. Epub 2016 Aug 15.

Department of Environmental Science and Analytical Chemistry, Stockholm University, SE-114 18 Stockholm, Sweden.

The progressive oxygenation of the Earth's atmosphere was pivotal to the evolution of life, but the puzzle of when and how atmospheric oxygen (O2) first approached modern levels (∼21%) remains unresolved. Redox proxy data indicate the deep oceans were oxygenated during 435-392 Ma, and the appearance of fossil charcoal indicates O2 >15-17% by 420-400 Ma. However, existing models have failed to predict oxygenation at this time. Here we show that the earliest plants, which colonized the land surface from ∼470 Ma onward, were responsible for this mid-Paleozoic oxygenation event, through greatly increasing global organic carbon burial-the net long-term source of O2 We use a trait-based ecophysiological model to predict that cryptogamic vegetation cover could have achieved ∼30% of today's global terrestrial net primary productivity by ∼445 Ma. Data from modern bryophytes suggests this plentiful early plant material had a much higher molar C:P ratio (∼2,000) than marine biomass (∼100), such that a given weathering flux of phosphorus could support more organic carbon burial. Furthermore, recent experiments suggest that early plants selectively increased the flux of phosphorus (relative to alkalinity) weathered from rocks. Combining these effects in a model of long-term biogeochemical cycling, we reproduce a sustained +2‰ increase in the carbonate carbon isotope (δ(13)C) record by ∼445 Ma, and predict a corresponding rise in O2 to present levels by 420-400 Ma, consistent with geochemical data. This oxygen rise represents a permanent shift in regulatory regime to one where fire-mediated negative feedbacks stabilize high O2 levels.
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http://dx.doi.org/10.1073/pnas.1604787113DOI Listing
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC5024600PMC
August 2016