Ingrid Blanchard, Eleanor Jennings, Ian A. Franchi, Xuchao Zhao, Sylvain Petitgirard, Nobuyoshi Miyajima, Seth Andrew Jacobson, Dave Rubie

Carbon is an essential element for the existence and evolution of life on Earth, constitutes up to 50% of dry biomass, and is likely a requirement for all life in the universe. Its high abundance in Earth’s crust and mantle (the Bulk Silicate Earth, BSE) is surprising because carbon is strongly siderophile (metal-loving) and should have segregated almost completely into Earth’s core during accretion. Estimates of the concentration of carbon in the mantle lie mostly in the range of 80–120 ppm, which is much higher than expected based on simple models of core–mantle differentiation. Here we show through experiments at 49–71 GPa and 3600–4000 K that carbon is significantly less siderophile at such conditions than at the low pressures (≤ 3 GPa) of previous studies. We derive a new parameterization of the pressure–temperature dependence of the metal–silicate partitioning of carbon and apply this in a state-of-the-art model of planet formation and differentiation that is based on astrophysical N-body accretion simulations. Results show that BSE carbon concentrations increase strongly starting at a very early stage of Earth’s accretion and, depending on the concentration of carbon in accreting bodies, can easily reach or exceed estimated BSE values. In contrast, simple models of “continuous core formation” require all BSE carbon to be accreted after core formation ended, but this is not consistent with astrophysical models of accretion.