Abstract 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.
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Deep Earth carbon reactions through time and space
Abstract Reactions involving carbon in the deep Earth have limited manifestations on Earth's surface, yet they have played a critical role in the evolution of our planet. The metal-silicate partitioning reaction promoted carbon capture during Earth's accretion and may have sequestered substantial carbon in Earth's core. The freezing reaction involving iron-carbon liquid could have contributed to the growth of Earth's inner core and the geodynamo. The redox melting/freezing reaction largely controls the movement of carbon in the modern mantle, and reactions between carbonates and silicates in the deep mantle also promote carbon mobility. The 10-year activity of the Deep Carbon Observatory has made important contributions to our knowledge of how these reactions are involved in the cycling of carbon throughout our planet, both past and present, and has helped to identify gaps in our understanding that motivate and give direction to future studies.
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- Award ID(s):
- 1751664
- NSF-PAR ID:
- 10136606
- Date Published:
- Journal Name:
- American Mineralogist
- Volume:
- 105
- Issue:
- 1
- ISSN:
- 0003-004X
- Page Range / eLocation ID:
- 22 to 27
- Format(s):
- Medium: X
- Sponsoring Org:
- National Science Foundation
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Abstract As the reaction product of subducted water and the iron core, FeO2 with more oxygen than hematite (Fe2O3) has been recently recognized as an important component in the D” layer just above the Earth's core-mantle boundary. Here, we report a new oxygen-excess phase (Mg, Fe)2O3+δ (0 < δ < 1, denoted as “OE-phase”). It forms at pressures greater than 40gigapascals when (Mg, Fe)-bearing hydrous materials are heated over 1,500 kelvin. The OE-phase is fully recoverable to ambient conditions for ex-situ investigation using transmission electron microscopy, which indicates that the OE-phase contains ferric iron (Fe3+) as in Fe2O3 but holds excess oxygen through interactions between oxygen atoms. The new OE-phase provides strong evidence that H2O has extraordinary oxidation power at high pressure. Unlike the formation of pyrite-type FeO2Hx which usually requires saturated water, the OE-phase can be formed with under-saturated water at mid-mantle conditions, and is expected to be more ubiquitous at depths greater than 1,000 km in Earth's mantle. The emergence of oxygen-excess reservoirs out of primordial and subducted (Mg, Fe)-bearing hydrous materials may revise our view on the deep-mantle redox chemistry.more » « less
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- Free Publicly Accessible Full Text
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Accepted Manuscript1.0
- Journal Article:
- https://doi.org/10.2138/am-2020-6888CCBY
