Earth’s carbon deficit caused by early loss through irreversible sublimation

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Science Advances  02 Apr 2021:
Vol. 7, no. 14, eabd3632
DOI: 10.1126/sciadv.abd3632
  • Fig. 1 The sublimation sequence of carbon in the solar nebula.

    An element’s relative abundance to Mg and CI is the ratio of its relative abundance to Mg to that in CI, calculated as (C/Mg in Earth or solar)/(C/Mg in CI) in wt %. On the high-temperature side, the volatility trend (blue-shaded band) describes the relative abundances of lithophile elements (rock loving, blue circles) in the bulk silicate Earth (mantle and crust) as a function of their half-mass condensation temperatures (1). Siderophile elements (iron loving, red circles) plot below the volatility trend, presumably due to preferential incorporation into the core. On the low-temperature side, the sublimation sequence of carbon (gray thick line) traces the falling relative abundance of condensed carbon in the solar nebula (excluding H and He) as the disk warms up (tables S4 and S5). The abundance (relative to Mg and CI) of condensed carbon starts at 9.48 (long dashed line) and is reduced to 4.74 after carbon-carrying ices transform into gases. It falls precipitously by more than one order of magnitude when the nebular temperature reaches ~500 K. The maximum amount of carbon in the bulk Earth (large dark blue box), represented by a generous upper bound of 1.7 ± 0.2 wt % carbon (0.30 ± 0.03 relative to Mg and CI) and a probable bound of 0.4 ± 0.2 wt % carbon (0.07 ± 0.04 relative to Mg and CI), corresponds to a maximum fraction of 1 to 7% carbon-rich source material that survived sublimation (table S6). The bulk silicate Earth (BSE; small dark blue box) with 140 ± 40 ppm (parts per million) carbon by weight (1.7 ± 0.5 × 10−3 relative to Mg and CI) plots well below the upper bounds, possibly due to sequestration of carbon by the core.

  • Fig. 2 Schematic illustration of the soot line in a protoplanetary disk.

    The soot line (red parabola) delineates the phase boundary between solid and gaseous carbon carriers. In the accretion-dominated disk phase, it is located far from the proto-Sun and divides carbon-poor dust and pebbles (green dots) from carbon-rich ones (dark blue dots). Within 1 Ma, as a result of the transition to a radiation-dominated, or passive, disk phase, the soot line migrates inside Earth’s current orbit. Note that the Si-rich and C-rich solids do not represent distinct reservoirs because carbonaceous material is likely associated with silicates. They are provided for ease in illustration.

  • Fig. 3 Generous upper bounds on carbon in Earth’s core, from density constraints.

    The density of Fe─C alloy at the inner core boundary (ICB) or the core-mantle boundary (CMB) as a function of carbon content is estimated from that of iron and Fe─C alloys at 330 GPa and 5500 ± 1500 K [red squares (13)] and that at 136 GPa and 4500 ± 1500 K [red-filled circles (1415)], respectively. The thick trends are linear fits through experimental data on solid Fe─C alloys and compounds (top) and liquid Fe─C alloys (bottom). The horizontal black lines are the inferred densities at the ICB and CMB based on the Preliminary Reference Earth Model (PREM) (16). The horizontal arrows denote the ranges of carbon concentrations in iron-carbon alloys to match the core densities, considering uncertainties in core temperatures and mineral physics measurements. The maximum estimated carbon in the liquid outer and the solid inner core are 5.0 ± 0.6 and 4.5 ± 0.5 wt %, assuming that carbon is the sole light element in the core to account for the observed densities (16).

Supplementary Materials

  • Supplementary Materials

    Earth’s carbon deficit caused by early loss through irreversible sublimation

    J. Li, E. A. Bergin, G. A. Blake, F. J. Ciesla, M. M. Hirschmann

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