Carbon in Earth's interior: Storage, cycling, and life

Communications Earth & Environment, 2021

Evaluating carbon’s candidacy as a light element in the Earth’s core is critical to constrain the budget and planet-scale distribution of this life-essential element. Here we use first principles molecular dynamics simulations to estimate the density and compressional wave velocity of liquid iron-carbon alloys with ~4-9 wt.% carbon at 0-360 gigapascals and 4000-7000 kelvin. We find that for an iron-carbon binary system, ~1-4 wt.% carbon can explain seismological compressional wave velocities. However, this is incompatible with the ~5-7 wt.% carbon that we find is required to explain the core’s density deficit. When we consider a ternary system including iron, carbon and another light element combined with additional constraints from iron meteorites and the density discontinuity at the inner-core boundary, we find that a carbon content of the outer core of 0.3-2.0 wt.%, is able to satisfy both properties. This could make the outer core the largest reservoir of terrestrial carbon. A c...

Earth and Planetary Science Letters, 1993

In this paper we discuss the distribution, geochemical cycle, and evolution of CO 2 and N 2 in Earth's degassed mantle, and atmosphere plus continental crust. We estimate the present distribution of CO 2 and N 2 in Earth's degassed mantle based on amounts of He and Ar in the degassed mantle and observed outgassing ratios of CO2/He and N2/Ar at mid-ocean ridges. Estimated CO 2 in present degassed mantle is (1.8_ +9) x 1022 mol, representing (72 + 10)% of total degassable CO2, an amount far higher than fractions previously inferred for noble gases. This strongly suggests that most CO 2 has been recycled from Earth's surface into the degassed mantle through subduction, which is consistent with many recent discussions. For N2, the estimated amount in the present mantle is ~ 2.5 × 1019 mol, representing ~ 12% of total degassable N 2. Recycling of N 2 back to the mantle is also inferred, but on a much smaller scale. A simple model for the outgassing and recycling of CO 2 and N 2 in Earth is presented. Outgassing is assumed to be via melt-vapor partitioning of volatiles. Recycling back into the mantle via subduction is assumed to be proportional to the mass of the volatile component in the crust. This simple model is consistent with all currently available constraints. Difficulties arise from the dependence of the recycling constant on time. Hence, no single evolution history can be obtained for CO 2 based on the available data. Model results tentatively point to a higher CO 2 content on Earth's surface in the Archean and Proterozoic than at present. Important future constraints may come from records in sedimentary rocks, improved understanding of carbonate production, and better modeling of the recycling process.

Precambrian Research, 2005

Carbon and oxygen isotope measurements of 66 samples from the 60 m-thick variegated marble in the Upper Allochthon of the Norwegian Caledonides have a mean δ 13 C carb of −8.4 ± 0.9‰ (V-PDB), and a mean δ 18 O of 20.2 ± 2.2‰ (V-SMOW). The variegated marble is overlain by 150 m-thick pale grey marble characterised by mean δ 13 C carb of −6.5 ± 0.8‰ (n = 25) and underlain by dark grey marbles with a mean δ 13 C carb of +4.8 ± 1.1‰ (n = 61). This tripartite unit of an poorly constrained age-but between Neoproterozoic and Early Silurian-discontinuously developed over a distance of 500 km, is likely to represent one of the largest isotopically anomalous sedimentary carbonate formations yet reported. The marbles depleted in 13 C beyond the canonical mantle value of −6‰ show no obvious evidence of post-sedimentary repartitioning of carbon isotopes. Several other carbonate formations deposited within approximately 680-540 Ma time interval (Chenchinskaya, Nikolskaya and Torginskay in Siberia, Ingta in NW Canada, Shuram in Central Oman, Trezona and Wonoka in South Australia) are several hundred meters thick, developed over a distance of hundreds of kilometres, and all show a similar depletion in 13 C beyond the mantle value, for reasons that are not well understood. The existence of these carbonates represents a challenging problem for our current understanding of global carbon geodynamics. Changes in the ratio of reduced/oxidised carbon sequestered in sediments, a methane hydrate release or 'zero' biological productivity, if applied separately, cannot explain carbon isotope characteristics and formation of these carbonates. We tentatively propose that several factors associated with unusual geodynamic and palaeoclimatic scenarios developed between 600 and 540 Ma might have been involved in the extreme lowering of the isotopic composition of carbon entering the global Earth surface environment. This period was marked by the retreat of Neoproterozoic glaciers and break-up of the supercontinent Rodinia. The late-postglacial warming might have induced a massive release of methane hydrates extremely enriched in 12 C. The 'death' of Rodinia was marked by unusually rapid (approximately 20 cm/year) motion of newly formed continental plates suggesting vigorous mantle convection and an enhanced restructuring of the lowest compartments of the Earth. This could provide a flux of 12 C-rich material from the isotopically light asthenosphere-mantle source (δ 13 C = −25 to

2005

Carbon and oxygen isotope measurements of 66 samples from the 60 m-thick variegated marble in the Upper Allochthon of the Norwegian Caledonides have a mean δ 13 C carb of −8.4 ± 0.9‰ (V-PDB), and a mean δ 18 O of 20.2 ± 2.2‰ (V-SMOW). The variegated marble is overlain by 150 m-thick pale grey marble characterised by mean δ 13 C carb of −6.5 ± 0.8‰ (n = 25) and underlain by dark grey marbles with a mean δ 13 C carb of +4.8 ± 1.1‰ (n = 61). This tripartite unit of an poorly constrained age-but between Neoproterozoic and Early Silurian-discontinuously developed over a distance of 500 km, is likely to represent one of the largest isotopically anomalous sedimentary carbonate formations yet reported. The marbles depleted in 13 C beyond the canonical mantle value of −6‰ show no obvious evidence of post-sedimentary repartitioning of carbon isotopes.

2015

Studies in mineral ecology exploit mineralogical databases to document diversity-distribution relationships of minerals-relationships that are integral to characterizing "Earth-like" planets. As carbon is the most crucial element to life on Earth, as well as one of the defining constituents of a planet's near-surface mineralogy, we focus here on the diversity and distribution of carbonbearing minerals. We applied a Large Number of Rare Events (LNRE) model to the 403 known minerals of carbon, using 82,922 mineral species/locality data tabulated in mindat.org (as of 1 January 2015). We find that all carbon-bearing minerals, as well as subsets containing C with O, H, Ca, or Na, conform to LNRE distributions. Our model predicts that at least 548 C minerals exist on Earth today, indicating that at least 145 carbon-bearing mineral species have yet to be discovered. Furthermore, by analyzing subsets This is a preprint, the final version is subject to change, of the American Mineralogist (MSA) Cite as Authors (Year) Title. American Mineralogist, in press. (DOI will not work until issue is live.

Proceedings of the National Academy of Sciences, 2013

The Review of High Pressure Science and Technology, 2017

Carbon, the fourth most abundant element in the solar system, is believed to be an important light element constituent in the Earth's core. The high carbon content of carbonaceous chondrites (3.2 wt.) compared to bulk earth estimates, the presence of graphite/diamond and metal carbides in Iron meteorites, the high solubility of carbon into iron melts in the Fe-C system, all suggests the plausible presence of carbon in the Earth's core. However, the distribution of carbon isotopes in the core and deep mantle remains elusive. Newly reported experimental data and theoretical estimates on equilibrium carbon isotope fractionation between graphite/diamond and carbide phases suggests that iron carbide melt will preferentially gather 12 C than 13 C. These results are consistent with the carbon isotope distribution between graphite and cohenite (Fe 3 C) observed in iron meteorites. The temperature dependent fractionation of carbon isotopes between carbide phases and elemental carbon can be an effective mechanism that might have created a 12 C-enriched core. If the Earth's core is a large reservoir of 12 C-enriched carbon, then it can result in large perturbations in surface carbon cycle caused by the flux of isotopically lighter carbon from the core-mantle boundary. [carbon isotopes, earth's core, iron carbide, isotope fractionation]

Faraday Discussions, 2014

Given the central role of carbon in the chemistry of life, it is a fundamental question as to how carbon is supplied to the Earth, in what form and when. We provide an accounting of carbon found in solar system bodies, in particular a comparison between the organic content of meteorites and that in identified organics in the dense interstellar medium (ISM). Based on this accounting identified organics created by the chemistry of star formation could contain at most ∼15% of the organic carbon content in primitive meteorites and significantly less for cometary organics, which represent the putative contributors to starting materials for the Earth. In the ISM ∼ 30% of the elemental carbon is found in CO, either in the gas or ices, with a typical abundance of ∼ 10 −4 (relative to H 2). Recent observations of the TW Hya disk find that the gas phase abundance of CO is reduced by an order of magnitude compared to this value. We explore a solution where the volatile CO is destroyed via a gas phase processes, providing an additional source of carbon for organic material to be incorporated into planetesimals and cometesimals. This chemical processing mechanism requires warm grains (> 20 K), partially ionized gas, and sufficiently small (a grain < 10 µm) grains, i.e. a larger total grain surface area, such that freeze-out is efficient. Under these conditions static (non-turbulent) chemical models predict that a large fraction of the carbon nominally sequestered in CO can be the source of carbon for a wide variety of organics that are present as ice coatings on the surfaces of warm pre-planetesimal dust grains.

Energy Procedia, 2013

Mineral carbonation is a process whereby CO 2 reacts with ultramafic rocks to form carbonate minerals such as calcite (CaCO 3 ) and magnesite (MgCO 3 ). This process can be induced artificially at high pressures and temperatures and therefore has potential to be adapted as a carbon capture and storage (CCS) technology. Large-scale surface and subsurface carbonate deposits of probable Quaternary age are associated with major faulting across the Oman-UAE ophiolite. Here, fractured rock forms a natural fluid pathway and increases the surface area available for carbonation. Modern springs along these faults typically discharge hyperalkaline (pH ~11), Ca(OH) 2 -rich waters that precipitate carbonates on reaction with atmospheric CO 2 . Carbonates formed by absorption of atmospheric CO 2 into Ca(OH) 2 13 13

a b s t r a c t Editor: R.W. Carlson Keywords: carbon isotopes iron carbide core deep carbon cycle