Decomposition of ocean waters by potassium-40 radiation 3800 Ma ago as a source of oxygen and oxidizing species

Abstract

Abstract The presence of potassium salts and radioactive decay of its isotope 40 K (half-life 1250 Ma) have provided an ideal in situ source of energy for a steady decomposition of primitive ocean waters. Computer simulation based on a free radical model of water radiolysis is used to obtain an insight into the process induced 3800 Ma ago by the β rays (1.31 MeV) and γ rays (1.46 MeV) of radioactive 40 K. The model system is an ocean water with sodium, magnesium, calcium, and potassium as principal cationic constituents. Its chemical composition and subsequently the radiation chemistry, are sodium chloride dominated and the radiolytic generation of O 2 and H 2 O 2 is not affected by a potential presence of inorganic (Fe 2+ , Mn 2+ ) and simple organic oxidizible substances. The energy deposited by radioactive decay is calculated to be 2.95×10 −4 J kg −1 yr −1 . Over a geologic time scale significant amounts of water decomposition products have been generated: 9.5×10 17 mol O 2 M ocean −1 (100 Ma) −1 , 3.2×10 16 mol H 2 O 2 M ocean −1 (100 Ma) −1 and 1.9×10 16 mol H 2 M ocean −1 (100 Ma) −1 , where mol is the molecular weight (in gram) and M ocean = 1.4×10 24 g. Annual generation rates were low (10 −11 to 10 −12 mol dm −3 yr −1 ) and could have played a role in shaping the environment where oxygen-tolerant life-forms evolved. This work shows that the early ocean had an intrinsic oxidizing capacity due to the radiolysis of water. The amounts of water decomposed and the oxidizing radiolytic species formed depended only on the concentration of 40 K, which represented 0.1% of K in nature, 3800 Ma ago. At variance with other sources of O 2 (photodissociation and photosynthesis), the radiolytic generation could have occurred throughout the water volume including the deep ocean.