Origin of Earth's hydrogen and carbon constrained by their core-mantle partitioning and bulk Earth abundance

Hydrogen and carbon concentrations in the Earth's core are yet known. Here we determined their metal/silicate partition coefficients (D) simultaneously under typical conditions of core formation and found that D_H and D_C diminish in the presence of carbon and hydrogen, respectively, because of strong interactions between hydrogen and carbon in liquid metal, being markedly different from those separately examined in earlier experiments. With these partitioning data, we investigated the core and bulk Earth abundances of hydrogen and carbon based on core formation scenarios that are compatible with the bulk silicate Earth composition and the mass fraction and density of the core. The modelling results indicate that the Earth building blocks do not match enstatite chondrites in water abundance but require contributions by carbonaceous chondrites. The multi-stage core formation models combined with an Earth accretion scenario accounting for isotopic composition show 0.18-0.49 wt% H and 0.19-1.37 wt% C in the core, leading to 0.53-1.40 wt% H₂O (present as H in the core) and 0.07-0.44 wt% C in the bulk Earth. Our modelling also demonstrates that up to 53% and 72% of Earth's water (hydrogen) and carbon, respectively, could have been derived from non-carbonaceous chondritic materials.