Earth's heat flux revised and linked to chemistry

Abstract

Recent estimates of the heat flux from the oceanic crust rest on the validity and accuracy of the half-space cooling (HSC) model. The known discrepancies between calculated and measured heat fluxes are not due to hydrothermal circulation, as commonly assumed, because the magmatic source provides too little energy, and the Rayleigh number is too small to foster vigorous convection on an oceanic scale. The half-space cooling model errs in assuming constant thermal conductivity ( k), but more importantly, provides infinite flux along the ridge centers over all time for any form for k. This unrealistic global singularity strongly impacts the derived mean flux. We develop three independent methods to ascertain Earth's mean oceanic heat flux directly from compiled heat flow data. The results are congruent, insensitive to uncertainties in the dataset, corroborate previous spherical harmonic analyses, provide the same average flux as from the continents, and constrain the global power as 31±1 TW. Geological observations, inferred mantle overturn rates, estimated mantle cooling rates, and recent geodynamic models independently suggest that neither delayed secular cooling nor primordal heat are currently significant sources, necessitating that current heat production predominately originates in radioactive decay and is quasi-steady-state. Models of Earth's bulk composition based on CI chondritic meteorites provide an unrealistically low radioactive power of ∼20 TW, whereas enstatite chondrites are sufficiently radioactive to supply the observed heat flux, contain enough iron metal to account for Earth's huge core, and have the same oxygen isotopic ratios as the bulk Earth. We devise a method to obtain K/U/Th ratios for the Earth and other planetary bodies from their power, including secular delays, and use this to constrain Earth's cooling rate.