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 (14–15)], 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).