Radiolysis of ocean and crustal water initiated by the decay of radioactive natural isotopes (40K, 235U, 238U and 232Th): Implications for the chemical evolution of the early Earth

Abstract

Radioactive isotopes have been an integral part of the Earth's matter since its formation 4.6 billion years ago (4.6 Ga). Long-lived and potent energy sources such as ²³²Th, ²³⁸U, ²³⁵U, and ⁴⁰K have played an important role in the Earth formation. They determined, to a large extent, the chemical evolution of the early Earth and probably influenced the appearance and development of life. This study compares the contributions of various isotopes to energy release and to the effect on the radiation chemical reactions in the Earth near-surface layer, including the Global Ocean and the Earth's crust. The considerable difference in the isotope weights and distributions has a crucial influence on the mechanisms and efficiency of the occurring reactions. In particular, only dissolved ⁴⁰K is present in the ocean, while the crust contains all heavy isotopes, together with ⁴⁰K. The energy released in the Earth's crust considerably exceeds that released in the Global Ocean. Indeed, at 4.0 Ga, 3.2 × 10²⁶ J was released in the ocean and 1.2 × 10³⁰ J was released in the crust. Currently, due to the 12-fold decrease in the ⁴⁰K weight, the decay of ²³⁸U and ²³²Th started to predominate in the radiation transformations in the Earth's crust. Comparison of the energies and weights demonstrates that the specific activity of the crustal matter expressed as energy to weight ratio is approximately 100 times higher than that for sea water. It was shown that the radiation chemical transformations in the ocean and in the crust are mainly caused by water radiolysis. In the crust, the radiolysis occurs upon the direct action of radiation on water (content of ∼3 wt %). Heavy isotopes are characterized by a high linear energy transfer by the α-particles that are emitted upon their decay. As a result, molecular products, Н₂ and Н₂О₂, are mainly formed in the crust. Conversely, in the case of light ⁴⁰K isotope, the linear energy transfer from the emitted γ-rays and β-particles is moderate. Therefore, ion and radical products, e_aq⁻, ^•H, and ^•OH, and mainly produced in the ocean. This circumstance dictates the difference between the radiation-induced chemical transformations that occur in the ocean and in the crust. The ocean served as the collector for components of the early Earth's atmosphere and the products of their reactions that entered the ocean and simultaneously was the converter that hosted further radiation chemical transformations involving e_aq⁻, ^•H, and ^•OH. As a result, organic matter including prebiotic molecules and oxygen was generated in the Global Ocean. Hydrogen generation predominated in the crust. The calculated global rate of hydrogen formation was ∼6.6 × 10¹² mol year⁻¹ at 4.5 Ga, while in the last billion years, this rate has been ∼1.5 × 10¹² mol year⁻¹. The radiation conversion of the inorganic matter initiated by the decay of naturally occurring radioactive isotopes was among the important factors of chemical evolution of the Early earth. The radiolytic Н₂ was and, what is important, still is a permanent energy source for the deep subsurface microbial communities.