Calculating solar equivalence ratios of the four major heat-producing radiogenic isotopes in the Earth's crust and mantle

Abstract

As part of the ongoing work in defining a consistent and unified geobiosphere emergy baseline (GEB) this paper considers the radiogenic component of the available energy from geothermal sources (one third of the global tripartite: solar radiation, dissipation of tidal momentum, and geothermal exergy). Recent literature suggests that Earth's geothermal energy results from two very different sources, decay of radioisotopes and primordial heat (heat left from Earth's accretion). In previous baseline computations, the radiogenic component of geothermal exergy was added to primordial heat, given various names like “deep earth heat”, and a single transformity was computed for the combined sources. With the acknowledgment that the geothermal component of the GEB had two different sources, it became apparent that a single transformity may no longer be appropriate, thus a method of computing separate transformities was necessary. In a novel approach, this paper uses gravity as the primary input to both solar radiation and heavy radionuclides and computes gravitational transformities for both. Then solar equivalence ratios (SERs) are computed between solar radiation and the four major crustal radionuclides (238U, 235U, 232Th, 40K). The SERs are combined with published radiogenic geothermal exergy data to calculate the solar equivalent exergy of the radiogenic component of the geothermal flux. This equivalence method can be used to derive a theoretically and methodologically consistent calculation for the other inputs to the global emergy baseline (i.e. tides and primordial geothermal heat flux) that can be similarly quantified in terms of gravitational exergy required to produce them.