Earth's Carbon Deficit Caused By Early Loss Through Irreversible Sublimation

By Keith Cowing
April 6, 2021
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Earth's Carbon Deficit Caused By Early Loss Through Irreversible Sublimation
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 + 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 (Table S4, Table 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 1-7% carbon-rich source material that survived sublimation (Table S6). The bulk silicate Earth (BSE, small dark blue box) with 140±40 ppm carbon by weight (1.7±0.5x10-3 relative to Mg and CI) plots well below the upper bounds, possibly due to sequestration of carbon by the core.

Carbon is an essential element for life but its behavior during Earth’s accretion is not well understood.

Carbonaceous grains in meteoritic and cometary materials suggest that irreversible sublimation, and not condensation, governs carbon acquisition by terrestrial worlds. Through astronomical observations and modeling we show that the sublimation front of carbon carriers in the solar nebula, or the soot line, moved inward quickly so that carbon-rich ingredients would be available for accretion at 1 au after the first million years.

On the other hand, geological constraints firmly establish a severe carbon deficit in Earth, requiring the destruction of inherited carbonaceous organics in the majority of its building blocks. The carbon-poor nature of the Earth thus implies carbon loss in its precursor material through sublimation within the first million years.

Jie Li, Edwin A. Bergin, Geoffrey A. Blake, Fred J. Ciesla, Marc M. Hirschmann

Comments: 21 pages including main article and supplementary materials
Subjects: Earth and Planetary Astrophysics (astro-ph.EP); Geophysics (physics.geo-ph)
Journal reference: Science Advances published 02 Apr 2021: Vol. 7, no. 14, DOI: 10.1126/sciadv.abd3632
Cite as: arXiv:2104.02702 [astro-ph.EP] (or arXiv:2104.02702v1 [astro-ph.EP] for this version)
Submission history
From: Edwin A. Bergin
[v1] Tue, 6 Apr 2021 17:51:39 UTC (1,636 KB)

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