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Interpreting the Carbon Isotope Record of Mass Extinctions

Mass extinctions are global-scale environmental crises marked by the loss of numerous species from all habitats. They often coincide with rapid changes in the stable carbon isotope ratios (13C/12C) recorded in sedimentary carbonate and organic matter, ratios which can indicate substantial inputs to the surface carbon reservoirs and/or changes in the cycling of carbon. Models to explain these changes have provided much fuel for debate on the causes and consequences of mass extinctions. For example, the escape of methane from gas hydrate deposits or the emission of huge volumes of gaseous carbon from large-scale volcanic systems, known as large igneous provinces, may have been responsible for decreases of 13C/12C in sedimentary deposits. In this article, we discuss the challenges in distinguishing between these, and other, alternatives.

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Earth’s Outgassing and Climatic Transitions: The Slow Burn Towards Environmental “Catastrophes”?

On multimillion-year timescales, outgassing from the Earth’s interior provides the principal source of CO2 to the ocean–atmosphere system, which plays a fundamental role in shaping the Earth’s baseline climate. Fluctuations in global outgassing have been linked to icehouse–greenhouse transitions, although uncertainties surround paleo-outgassing fluxes. Here, we discuss how volcanic outgassing and the carbon cycle have evolved in concert with changes in plate tectonics and biotic evolution. We describe hypotheses of driving mechanisms for the Paleozoic icehouse–greenhouse climates and explore how climatic transitions may have influenced past biotic crises and, in particular, how variable outgassing rates established the backdrop for carbon cycle perturbations to instigate prominent mass extinction events.

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Deep Carbon and the Life Cycle of Large Igneous Provinces

Carbon is central to the formation and environmental impact of large igneous provinces (LIPs). These vast magmatic events occur over geologically short timescales and include voluminous flood basalts, along with silicic and low-volume alkaline magmas. Surface outgassing of CO2 from flood basalts may average up to 3,000 Mt per year during LIP emplacement and is subsidized by fractionating magmas deep in the crust. The large quantities of carbon mobilized in LIPs may be sourced from the convecting mantle, lithospheric mantle and crust. The relative significance of each potential carbon source is poorly known and probably varies between LIPs. Because LIPs draw on mantle reservoirs typically untapped during plate boundary magmatism, they are integral to Earth’s long-term carbon cycle.

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The Influence of Large Bolide Impacts on Earth’s Carbon Cycle

Human society’s rapid release of vast quantities of CO2 into the atmosphere is a significant planetary experiment. An obvious natural process capable of similar emissions over geologically short time spans are very large bolide impacts. When striking a carbon-rich target, bolides significantly, and potentially catastrophically, disrupt the global biogeochemical carbon cycle. Independent factors, such as sulfur-rich targets, redox state of the oceans or encountering ecosystems already close to a tipping point, dictated the magnitude of further consequences and determined which large bolide strikes shaped Earth’s evolution. On the early Earth, where carbon-rich sedimentary targets were rare, impacts may not have been purely destructive. Instead, enclosed subaqueous impact structures may have contributed to initiating Earth’s unique carbon cycle.

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On the Origin(s) and Evolution of Earth’s Carbon

The isotopic “flavor” of Earth’s major volatiles, including carbon, can be compared to the known reservoirs of volatiles in the solar system and so determine the source of Earth’s carbon. This requires knowing Earth’s bulk carbon isotope value, which is not straightforward to determine. During Earth’s differentiation, carbon was partitioned into the core, mantle, crust, and atmosphere. Therefore, although carbon is omnipresent within the Earth system, scientists have yet to determine its distribution and relative abundances. This article addresses what we know of the processes involved in the formation of Earth’s carbon reservoirs, and, by deduction, what we know about the possible origins of Earth’s carbon.

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Earth Catastrophes and Their Impact on the Carbon Cycle

Carbon is one of the most important elements on Earth. It is the basis of life, it is stored and mobilized throughout the Earth from core to crust and it is the basis of the energy sources that are vital to human civilization. This issue will focus on the origins of carbon on Earth, the roles played by large-scale catastrophic carbon perturbations in mass extinctions, the movement and distribution of carbon in large igneous provinces, and the role carbon plays in icehouse–greenhouse climate transitions in deep time. Present-day carbon fluxes on Earth are changing rapidly, and it is of utmost importance that scientists understand Earth’s carbon cycle to secure a sustainable future.

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Palaeoweathering: How Do Weathering Rates Vary with Climate?

A feedback between Earth surface weathering and climate is thought to be fundamental in maintaining Earth’s habitability over long timescales, but investigating this control in the modern world is difficult. The geologic record of cycles between glacial and interglacial conditions of the last 2.6 million years allows us to study weathering feedback in action. A suite of mineral, element and isotope proxies have been applied to address how weathering rates have varied over glacial cycles. Despite evidence for substantial local changes, the emerging answer at a global scale seems to be, “not very much”.

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Combating Climate Change Through Enhanced Weathering of Agricultural Soils

Rising levels of atmospheric carbon dioxide (CO2) are driving increases in global temperatures. Enhanced weathering of silicate rocks is a CO2 removal technology that could help mitigate anthropogenic climate change. Enhanced weathering adds powdered silicate rock to agricultural lands, accelerating natural chemical weathering, and is expected to rapidly draw down atmospheric CO2. However, differences between enhanced and natural weathering result in significant uncertainties about its potential efficacy. This article summarizes the research into enhanced weathering and the uncertainties of enhanced weathering due to the key differences with natural weathering, as well as future research directions.

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Breaking it Down: Mechanical Processes in the Weathering Engine

Since land plants emerged from swampy coastlines over 400 million years ago, they have played a fundamental role in shaping the Earth system. Roots and associated fungi increase rock weathering rates, providing access to nutrients, while altering atmospheric CO2. As soils weather, the dissolution of primary minerals forces plants to rely on recycling and atmospheric deposition of rock-derived nutrients. Thus, for many terrestrial ecosystems, weathering ultimately constrains primary production (carbon uptake) and decomposition (carbon loss). These constraints are most acute in agricultural systems, which rely on mined fertilizer rather than the recycling of organic material to maintain production. Humans now mine similar amounts of some elements as weather out of rocks globally. This increase in supply has myriad environmental consequences.

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How Plants Enhance Weathering and How Weathering is Important to Plants

Since land plants emerged from swampy coastlines over 400 million years ago, they have played a fundamental role in shaping the Earth system. Roots and associated fungi increase rock weathering rates, providing access to nutrients, while altering atmospheric CO2. As soils weather, the dissolution of primary minerals forces plants to rely on recycling and atmospheric deposition of rock-derived nutrients. Thus, for many terrestrial ecosystems, weathering ultimately constrains primary production (carbon uptake) and decomposition (carbon loss). These constraints are most acute in agricultural systems, which rely on mined fertilizer rather than the recycling of organic material to maintain production. Humans now mine similar amounts of some elements as weather out of rocks globally. This increase in supply has myriad environmental consequences.

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