Important role of seawater radiolysis of the World Ocean in the chemical evolution of the early Earth

Abstract

It is generally recognized that the evolution of early Earth was affected by an external energy source: radiation from the early Sun. We also propose and substantiate the hypothesis of important role of natural radioactivity, as an internal energy source, in the evolution of early Earth. Radioactive isotopes have been present on Earth since its formation. The decay of long-lived 232Th, 238U, 238U, and 40K was an important source of energy and, together with accretion of interplanetary matter and the action of gravitational forces, it promoted melting and differentiation of matter and tectonic activity of early Earth. At the same time, radioactive isotopes initiated radiation-chemical transformations of the Earth's matter and contributed to the chemical evolution of the planet. The initial content of radioactive isotopes in the World Ocean of the early Earth and the kinetics of their decay, the values of the absorbed dose and dose rate, as well as the efficiency of seawater radiolysis as a function of time are calculated. It is shown that the decay of long-lived isotopes initiated radiation chemical reactions of compounds dissolved in the ocean and gave rise to oxygen. The ocean served as a “reservoir” that collected components of the early atmosphere and products of their transformations and simultaneously as a “converter” in which further chemical reactions of these compounds took place. The chemical transformations of dissolved inorganic compounds boosted interrelated processes important for evolution: formation of organic matter, including prebiotic molecules, purification of the ocean, and oxygenation of hydro- and atmosphere of Earth. As a result, ocean became the “cradle” for the emergence of life. Radical mechanisms for the formation of simple amino acids, sugars, and nitrogen bases, i.e., the key structures of all living matter, were proposed. The amounts of organic matter and oxygen accumulated with time were calculated; the results confirmed the role of natural radioactivity in the evolution of Earth's matter and the emergence of life. We expect that this hypothesis would become the starting point for considering the important role of natural radioactivity and radiation-chemical processes in the general picture of Earth's evolution and emergence of life.