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Rock Metamorphism and the Global Carbon Cycle.

Record Type:	Electronic resources : Monograph/item
Title/Author:	Rock Metamorphism and the Global Carbon Cycle./
Author:	Stewart, Emily M.
Published:	Ann Arbor : ProQuest Dissertations & Theses, : 2020,
Description:	173 p.
Notes:	Source: Dissertations Abstracts International, Volume: 83-03, Section: B.
Contained By:	Dissertations Abstracts International83-03B.
Subject:	Petrology. -
Online resource:	https://pqdd.sinica.edu.tw/twdaoapp/servlet/advanced?query=28149995
ISBN:	9798538107469

Rock Metamorphism and the Global Carbon Cycle.
Stewart, Emily M.

Rock Metamorphism and the Global Carbon Cycle. - Ann Arbor : ProQuest Dissertations & Theses, 2020 - 173 p.

Source: Dissertations Abstracts International, Volume: 83-03, Section: B.

Thesis (Ph.D.)--Yale University, 2020.

This item must not be sold to any third party vendors.

Global carbon cycling has long been recognized as a fundamental control on Earth's evolution, driving changes in both the deep Earth and our surface environment. Yet our knowledge of the carbon cycle in the past and present is ultimately limited by our ability to constrain the magnitude of major carbon fluxes. The addition of CO2 to the atmosphere via a diffuse metamorphic flux was historically a major uncertainty in global geochemical models. In fact, it was often assumed to be negligible when compared to volcanic outgassing. The work in this dissertation demonstrates that this assumption is categorically incorrect. New field-based studies in combination with the development of novel modeling techniques explore the importance of metamorphism to global carbon cycling across time and space.Following Chapter 1's introduction to the problem, Chapter 2 focuses on Carbonation/decarbonation reactions as the primary mechanisms for transferring carbon between the solid Earth and the ocean-atmosphere system. Here, we summarize their significance as part of the Deep Carbon Observatory's "Earth in Five Reactions" project. In the forward direction, carbonation reactions 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, prograde metamorphic decarbonation reactions 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.In Chapter 3 we undertake thermodynamic pseudosection modeling of metacarbonate rocks in the Wepawaug Schist, Connecticut, USA, and examine the implications for CO2 outgassing from collisional orogenic belts. Two broad types of pseudosections are calculated: (1) a fully closed-system model with no fluid infiltration and (2) a fluid-buffered model including an H2O-CO2 fluid of a fixed composition. This fluid-buffered model is used to approximate a system open to infiltration by a water-bearing fluid. In all cases the fully closed-system model fails to reproduce the observed major mineral zones, mineral compositions, reaction temperatures, and fluid compositions. The fluid-infiltrated models, on the other hand, successfully reproduce these observations when a water-rich fluid is included. Fluid-infiltrated models predict significant progressive CO2 loss, peaking at ∼50% decarbonation at amphibolite facies. The closed-system models dramatically underestimate the degree of decarbonation, predicting only ∼15% CO2 loss at peak conditions, and, remarkably, <1% CO2 loss below ∼600 ⁰C. We propagate the results of fluid-infiltrated pseudosections to determine an areal CO2 flux for the Wepawaug Schist. This yields ∼1012 mol CO2 km−2 Myr−1, consistent with multiple independent estimates of the metamorphic CO2 flux, and comparable in magnitude to fluxes from mid-ocean ridges and volcanic arcs. Extrapolating to the area of the Acadian orogenic belt, we suggest that metamorphic CO2 degassing is a plausible driver of global warming, sea level rise, and, perhaps, extinction in the mid-to late-Devonian.In another tectonic setting, the fate of subducted CO2 remains the subject of widespread disagreement, with different models predicting either wholesale (up to 99%) decarbonation of the subducting slab or extremely limited carbon loss and, consequently, massive deep subduction of CO¬2. To resolve this debate, Chapter 4 presents a regional study of metamorphic carbon loss in subducted rocks of the Cycladic Blueschist Unit, Greece. We find pervasive water rich conditions (mole fraction CO2 < 0.01) and carbon and oxygen isotope signatures consistent with open-system processes. A new technique is used to calculate carbon loss in high-grade metamorphic rocks, revealing a wide range in the degree of decarbonation that varies systematically with lithology (e.g., pure limestones and carbonated metabasalts released ~0 and ~80% of their CO2, respectively). Globally we predict ~40% to ~65% CO2 in subducting crust is released via metamorphic decarbonation reactions at forearc depths (<70 km), with most carbon loss occurring in a thermally-controlled pulse around 525 ⁰C. These results preclude the possibility of pervasive deep subduction of most CO2 and suggest that the mantle may have become more depleted in carbon over geologic time.

ISBN: 9798538107469Subjects--Topical Terms:

535210
Petrology.
Subjects--Index Terms:

Carbon dioxide

Rock Metamorphism and the Global Carbon Cycle.
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300 $a 173 p.
500 $a Source: Dissertations Abstracts International, Volume: 83-03, Section: B.
500 $a Advisor: Ague, Jay J.
502 $a Thesis (Ph.D.)--Yale University, 2020.
506 $a This item must not be sold to any third party vendors.
520 $a Global carbon cycling has long been recognized as a fundamental control on Earth's evolution, driving changes in both the deep Earth and our surface environment. Yet our knowledge of the carbon cycle in the past and present is ultimately limited by our ability to constrain the magnitude of major carbon fluxes. The addition of CO2 to the atmosphere via a diffuse metamorphic flux was historically a major uncertainty in global geochemical models. In fact, it was often assumed to be negligible when compared to volcanic outgassing. The work in this dissertation demonstrates that this assumption is categorically incorrect. New field-based studies in combination with the development of novel modeling techniques explore the importance of metamorphism to global carbon cycling across time and space.Following Chapter 1's introduction to the problem, Chapter 2 focuses on Carbonation/decarbonation reactions as the primary mechanisms for transferring carbon between the solid Earth and the ocean-atmosphere system. Here, we summarize their significance as part of the Deep Carbon Observatory's "Earth in Five Reactions" project. In the forward direction, carbonation reactions 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, prograde metamorphic decarbonation reactions 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.In Chapter 3 we undertake thermodynamic pseudosection modeling of metacarbonate rocks in the Wepawaug Schist, Connecticut, USA, and examine the implications for CO2 outgassing from collisional orogenic belts. Two broad types of pseudosections are calculated: (1) a fully closed-system model with no fluid infiltration and (2) a fluid-buffered model including an H2O-CO2 fluid of a fixed composition. This fluid-buffered model is used to approximate a system open to infiltration by a water-bearing fluid. In all cases the fully closed-system model fails to reproduce the observed major mineral zones, mineral compositions, reaction temperatures, and fluid compositions. The fluid-infiltrated models, on the other hand, successfully reproduce these observations when a water-rich fluid is included. Fluid-infiltrated models predict significant progressive CO2 loss, peaking at ∼50% decarbonation at amphibolite facies. The closed-system models dramatically underestimate the degree of decarbonation, predicting only ∼15% CO2 loss at peak conditions, and, remarkably, <1% CO2 loss below ∼600 ⁰C. We propagate the results of fluid-infiltrated pseudosections to determine an areal CO2 flux for the Wepawaug Schist. This yields ∼1012 mol CO2 km−2 Myr−1, consistent with multiple independent estimates of the metamorphic CO2 flux, and comparable in magnitude to fluxes from mid-ocean ridges and volcanic arcs. Extrapolating to the area of the Acadian orogenic belt, we suggest that metamorphic CO2 degassing is a plausible driver of global warming, sea level rise, and, perhaps, extinction in the mid-to late-Devonian.In another tectonic setting, the fate of subducted CO2 remains the subject of widespread disagreement, with different models predicting either wholesale (up to 99%) decarbonation of the subducting slab or extremely limited carbon loss and, consequently, massive deep subduction of CO¬2. To resolve this debate, Chapter 4 presents a regional study of metamorphic carbon loss in subducted rocks of the Cycladic Blueschist Unit, Greece. We find pervasive water rich conditions (mole fraction CO2 < 0.01) and carbon and oxygen isotope signatures consistent with open-system processes. A new technique is used to calculate carbon loss in high-grade metamorphic rocks, revealing a wide range in the degree of decarbonation that varies systematically with lithology (e.g., pure limestones and carbonated metabasalts released ~0 and ~80% of their CO2, respectively). Globally we predict ~40% to ~65% CO2 in subducting crust is released via metamorphic decarbonation reactions at forearc depths (<70 km), with most carbon loss occurring in a thermally-controlled pulse around 525 ⁰C. These results preclude the possibility of pervasive deep subduction of most CO2 and suggest that the mantle may have become more depleted in carbon over geologic time.
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650 4 $a Geochemistry. $3 539092
650 4 $a Minerals. $3 542841
650 4 $a Acids. $3 922327
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650 4 $a Estimates. $3 3561047
650 4 $a Equilibrium. $3 668417
650 4 $a Earth. $3 643955
650 4 $a Sediments. $3 3566210
650 4 $a Mineralogy. $3 516743
650 4 $a Lithosphere. $3 2055717
650 4 $a Geology. $3 516570
650 4 $a Monte Carlo simulation. $3 3564916
650 4 $a Solid solutions. $3 3566211
650 4 $a Quartz. $3 1569706
650 4 $a Open systems. $3 3566212
650 4 $a Rocks. $3 535223
650 4 $a Atmosphere. $3 542821
650 4 $a Temperature. $3 711968
650 4 $a Carbon dioxide. $3 587886
650 4 $a Isotopes. $3 664463
650 4 $a Geophysics. $3 535228
650 4 $a Carbon cycle. $3 3566213
650 4 $a Alkalinity. $3 3566214
653 $a Carbon dioxide
653 $a Global carbon cycle
653 $a Metamorphic decarbonation
653 $a Metamorphic petrology
653 $a Orogenesis
653 $a Subduction
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