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The standard model of Earth’s core evolution has the bulk composition set at formation, with slow cooling beneath a solid mantle providing power for geomagnetic field generation. However, controversy surrounding the incorporation of oxygen, a critical light element, and the rapid cooling rates needed to maintain the early dynamo have called this model into question. The predicted cooling rates imply early core temperatures that far exceed estimates of the lower mantle solidus, suggesting that early core evolution was governed by interaction with a molten lower mantle. Here we develop ab initio techniques to compute the chemical potentials of arbitrary solutes in solution and use them to calculate oxygen partitioning between liquid Fe-O metal and silicate melts at the pressure-temperature (P-T) conditions expected for the early core-mantle system. Our distribution coefficients are compatible with those obtained by extrapolating experimental data at lower P-T values and reveal that oxygen strongly partitions into metal at core conditions via an exothermic reaction. Our results suggest that the bulk of Earth’s core was undersaturated in oxygen compared to the FeO content of the magma ocean during the latter stages of its formation, implying the early creation of a stably stratified oxygen-enriched layer below the core-mantle boundary (CMB). FeO partitioning is accompanied by heat release due to the exothermic reaction. If the reaction occurred at the CMB, this heat sink could have significantly reduced the heat flow driving the core convection and magnetic field generation.

The Earth's magnetic field is powered by energy supplied by the slow cooling and freezing of the liquid iron core. Efforts to determine the thermal and chemical history of the core have been hindered by poor knowledge of the properties of liquid iron alloys at the extreme pressures and temperatures that exist in the core. This obstacle is now being overcome by high-pressure experiments and advanced mineral physics computations. Using these approaches, updated transport properties for Fe-Si-O mixtures have been determined at core conditions, including electrical and thermal conductivities that are higher than previous estimates by a factor of two to three. Models of core evolution with these high conductivities suggest that the core is cooling much faster than previously thought. This implies that the solid inner core formed relatively recently (around half a billion years ago), and that early core temperatures were high enough to cause partial melting of the lowermost mantle. Estimates of core-mantle boundary heat flow suggest that the uppermost core is thermally stratified at the present day.

Recent theory and experiment suggest the thermal and electrical conductivities of the Earth’s core are 2–4 times higher than previously thought. This has important consequences for the core’s thermal history and behaviour of the geodynamo. The conductivities increase with depth, with a discontinuous jump at the inner core boundary caused by the change in composition and phase change to a solid. Properties of putative core alloys are now sufficiently well known to make it worth exploring the effects of their variation with depth within the core. The magnetic decay times are increased to 58 kyr for the whole core, considerably longer than the advection time (the time it takes fluid to traverse the outer core), and 9 kyr for the solid inner core. Heat conducted down the adiabat through the core–mantle boundary is in excess of 15 TW, which is one third of the Earth’s total heat loss and 2–3 times higher than most estimates. The core can be stirred by chemical convection against a stable thermal gradient, but at a cost that reduces the effective power available for generating magnetic field. We estimate the minimum heat flux required to sustain thermal dissipation alone to be 5–8 TW, but this is almost certainly a gross underestimate because it leaves nothing for convective or dynamo processes. Conduction gradients for cooling rates corresponding to these minimum heat fluxes are subadiabatic in the top 740 km of the core, which is also unlikely because geomagnetic secular variation requires upwelling somewhere near the core surface. Lateral variations in heat flux at the core–mantle boundary could easily be large enough to exceed the adiabatic value in some places, leading to mixing throughout the upper core. This not only reduces the total heat flux required to produce a well-mixed core, but also explains how mantle anomalies can exert a strong influence on core convection and the form of the geomagnetic field at the core surface. We propose a model of core convection that is vigorous in the lower part and very weak in the upper part.