Radiation-induced oxidation of iron in the ocean of the early Earth

Abstract

Radiation-chemical mechanism of bivalent iron oxidation in the World Ocean is suggested and substantiated. The decay of primordial radioactive isotopes 40K, 235U, 238U, and 232Th, present on the Earth since the moment of its formation (4.5 Ga), initiated the water radiolysis with the generation of ionic and radical species (eaq–, •H, •OH) and formation of molecular products (H2O2 and H2). The mechanism of the radiation-chemical oxidation of Fe2+ in the World Ocean depending on pH and the presence of oxygen is considered. The Fe(III) hydrolysis is responsible for the iron transfer from the freely dissolved state in the ocean into the solid phase. Low solubility of the hydroxide favors the formation of insoluble oxides, magnetite and hematite. The calculations reveal the decisive role of 40K in the radiation-induced oxidation. The process occurred throughout the volume of the World Ocean with the efficiency corresponding to the solubility of potassium salts in it. Quantitative estimation shows that, in the period from 4.3 to 2.5 Ga, i.e., before the occurrence of the Great Oxidation Event, about ~1021 g of iron could be oxidized by the radiation-chemical mechanism.