Annual review of earth and planetary sciences

[1] The alteration of upper oceanic crust entails growth of hydrous minerals and loss of macroporosity, with associated large-scale fluxes of H2O, CO2, Cl−, and K2O between seawater and crust. This age-dependent alteration can be quantified by combining a conceptual alteration model with observed age-dependent changes in crustal geophysical properties at DSDP/ODP sites, permitting estimation of crustal concentrations of H2O, CO2, Cl−, and K2O, given crustal age. Surprisingly, low-temperature alteration causes no net change in total water; pore water loss is nearly identical to bound water gain. Net change in total crustal K2O is also smaller than expected; the obvious low-temperature enrichment is partly offset by earlier high-temperature depletion, and most crustal K2O is primary rather than secondary. I calculate crustal concentrations of H2O, CO2, Cl−, and K2O for 41 modern subduction zones, thereby determining their modern mass fluxes both for individual subduction zones and globally. This data set is complemented by published flux determinations for subducting sediments at 26 of these subduction zones. Global mass fluxes among oceans, oceanic crust, continental crust, and mantle are calculated for H2O, Cl−, and K2O. Except for the present major imbalance between sedimentation and sediment subduction, most fluxes appear to be at or near steady state. I estimate that half to two thirds of subducted crustal water is later refluxed at the prism toe; most of the remaining water escapes at subarc depths, triggering partial melting. The flux of subducted volatiles, however, does not appear to correlate with either rate of arc magma generation or magnitude of interplate earthquakes.

Subduction Fluxes of Water, Carbon Dioxide, Chlorine, and Potassium

Mountain building results in high erosion rates and the interaction of rocks with the atmosphere, water and life. Carbon transfers that result from increased erosion could control the evolution of Earth’s long-term climate. For decades, attention has focused on the hypothesized role of mountain building in drawing down atmospheric carbon dioxide (CO2) via silicate weathering. However, it is now recognized that mountain building and erosion affect the carbon cycle in other important ways. For example, erosion mobilizes organic carbon (OC) from terrestrial vegetation, transferring it to rivers and sediments, and thereby acting to draw down atmospheric CO2 in tandem with silicate weathering. Meanwhile, exhumation of sedimentary rocks can release CO2 through the oxidation of rock OC and sulfide minerals. In this Review, we examine the mechanisms of carbon exchange between rocks and the atmosphere, and discuss the balance of CO2 sources and sinks. It is demonstrated that OC burial and oxidative weathering, not widely considered in most models, control the net CO2 budget associated with erosion. Lithology strongly influences the impact of mountain building on the global carbon cycle, with an orogeny dominated by sedimentary rocks, and thus abundant rock OC and sulfides, tending towards being a CO2 source. By increasing erosion, mountain building can steer the evolution of atmospheric carbon dioxide (CO2) and global climate. This Review expands from the canonical focus on silicate weathering to consider the net carbon budget of erosion, including both CO2 sinks (silicate weathering, organic-carbon burial) and CO2 sources (oxidative weathering).

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Mountains, erosion and the carbon cycle

One popular view of Venus' climate history describes a world that has spent much of its life with surface liquid water, plate tectonics, and a stable temperate climate. Part of the basis for this optimistic scenario is the high deuterium to hydrogen ratio from the Pioneer Venus mission that was interpreted to imply Venus had a shallow ocean's worth of water throughout much of its history. Another view is that Venus had a long lived (approximately 100 million year) primordial magma ocean with a CO2 and steam atmosphere. Venus' long lived steam atmosphere would sufficient time to dissociate most of the water vapor, allow significant hydrogen escape and oxidize the magma ocean. A third scenario is that Venus had surface water and habitable conditions early in its history for a short period of time (< 1 Gyr), but that a moist/runaway greenhouse took effect because of a gradually warming sun, leaving the planet desiccated ever since. Using a general circulation model we demonstrate the viability of the first scenario using the few observational constraints available. We further speculate that Large Igneous Provinces and the global resurfacing 100s of millions of years ago played key roles in ending the clement period in its history and presenting the Venus we see today. The results have implications for what astronomers term "the habitable zone," and if Venus-like exoplanets exist with clement conditions akin to modern Earth we propose to place them in what we term the "optimistic Venus zone."

Venusian Habitable Climate Scenarios: Modeling Venus through time and applications to slowly rotating Venus-Like Exoplanets

14 The existence of stabilizing feedbacks on Earth is generally thought to be necessary for the persistence of 15 liquid water and life. Earth’s atmospheric composition appears to have adjusted to the gradual increase in 16 solar luminosity over time, resulting in persistently habitable surface temperatures. With limited exceptions, 17 the Earth system recovered rapidly from climatic perturbations. Carbon dioxide (CO2) regulation via negative 18 feedbacks within the coupled global carbon-silica cycles are classically viewed as the main processes giving 19 rise to climate stability on Earth. Here we review the long-term global carbon cycle budget and how the 20 processes modulating Earth’s climate system have evolved over time. Specifically, we focus on the relative 21 roles that shifts in carbon sources and sinks have played in driving long-term changes in atmospheric pCO2. 22 We make a case that marine processes are an important component of the canonical silicate weathering 23 feedback, and have played a much more important role in pCO2 regulation than traditionally imagined. The 24 weathering of marine sediments and off-axis basalt alteration are major carbon sinks. However, this sink 25 was potentially dampened during Earth’s early history when oceans had higher levels of dissolved silicon 26 (Si), iron (Fe) and magnesium (Mg), and instead likely fostered more extensive reverse weathering—which in 27 turn fostered higher ocean-atmosphere CO2. 28 29

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Evolution of the Global Carbon Cycle and Climate Regulation on Earth

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 CO2. The fluid history of subducted rocks lies at the heart of this debate: rocks that experience significant infiltration by a water-bearing fluid may release orders of magnitude more CO2 than rocks that are metamorphosed in a closed chemical system. Numerical models make a wide range of predictions regarding water mobility, and further progress has been limited by a lack of direct observations. Here we present a comprehensive field-based study of decarbonation efficiency in a subducting slab (Cyclades, Greece), and show that ~40% to ~65% of the CO2 in subducting crust is released via metamorphic decarbonation reactions at forearc depths. This result precludes extensive deep subduction of most CO2 and suggests that the mantle has become more depleted in carbon over geologic time. The fate of subducted CO2 remains debated, with estimates mainly from numerical predictions varying from wholesale decarbonation of the shallow subducting slab to massive deep subduction of CO2. Here, the authors present field-based data and show that ~40% to ~65% of the CO2 in subducting crust is released via metamorphic decarbonation reactions at forearc depths.

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Pervasive subduction zone devolatilization recycles CO2 into the forearc.

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). $\\text{CaSi}{{\\text{O}}_{\\text{3}\\,\\,\\left( \\text{wollastonite} \\right)}}+\\text{C}{{\\text{O}}_{\\text{2}\\,\\,\\left( \\text{gas} \\right)}}\\leftrightarrow \\text{CaC}{{\\text{O}}_{\\text{3}\\,\\,\\left( \\text{calcite} \\right)}}+\\text{Si}{{\\text{O}}_{\\text{2}\\,\\,\\left( \\text{quartz} \\right)}}.$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.

Carbonation and decarbonation reactions: Implications for planetary habitability

Infiltration-driven metamorphism, New England, USA: Regional CO2 fluxes and implications for Devonian climate and extinctions

We report the results of Sm–Nd garnet geochronology on a mafic granulite from the metamorphic sole of the Ballantrae Ophiolite, Scotland. The resultant age of 477.6 ± 1.9 Ma (2σ) is interpreted to represent the obduction of the Ballantrae Ophiolite and the onset of the Grampian phase of the Caledonian Orogeny in Scotland. Thermodynamic modelling indicates peak conditions of c . 825°C and at least 10 kbar for the unit. In conjunction with a biostratigraphic constraint on metamorphic garnet deposition in sediments at 465 ± 2.5 Ma, this new age more precisely constrains the duration of Grampian metamorphism in Scotland to 12.6 ± 3.1 myr (2σ). In combination with this new age, previous geochronological work on sillimanite-zone garnet indicates a time-averaged initial heating rate of 106 ± 78°C myr −1 (2σ), a rapid rate that may be inconsistent with orogenic self-heating via crustal overthickening alone. The calculated heating rate for the associated garnet zone is also rapid and more precisely constrained at 47 ± 14°C myr −1 (2σ). These results together with the short overall duration of the event support the idea that the Grampian phase of the Caledonian Orogeny in Scotland included both rapid heating and correspondingly fast exhumation.

E.M. Stewart

Papers

Evolution of the Global Carbon Cycle and Climate Regulation on Earth

Pervasive subduction zone devolatilization recycles CO2 into the forearc.

Carbonation and decarbonation reactions: Implications for planetary habitability

Infiltration-driven metamorphism, New England, USA: Regional CO2 fluxes and implications for Devonian climate and extinctions

Initiation and duration of Grampian orogenesis constrained by refined Sm–Nd garnet geochronology of the Ballantrae ophiolite, Scotland