Radioliza wody – niedoceniony czynnik geologiczny

Abstract

 Radiolityczny rozkład wody jest głównym źródłem wolnego tlenu w skorupie ziemskiej. Produkty radiolitycznie zainicjowanych reakcji chemicznych odgrywają zasadniczą rolę w stopniu utleniania składników chemicznych w skałach i substancji rozpuszczonych w wodach. Są one aktywne w uwodornianiu substancji organicznej i tworzeniu się prekursorów ropy naftowej. Przyczyniają się też do „spalania się” rozproszonej w skałach substancji humusowej. W skałach można też dostrzec specyficzne zdefektowanie liniowe wywołane promieniowaniem jonizującym.

RADIOLYSIS OF WATER – AN UNDERESTIMATED GEOLOGICAL FACTOR

In the nature, ionizing radiation is mainly due to isotopes of uranium, thorium and, on smaller scale, potassium ⁴⁰K. It produces thermal energy, being at the same time highly effective source of energy introduced into chemical systems. The latter aspect was hitherto almost completely neglected in geological studies. However, it should be noted that chemical processes initiated by radiolytic decomposition of water have been of primary importance in evolution of chemical composition of the matter forming at present the most external zones of the Earth (atmosphere, hydrosphere and litosphere). Products of radiolytic decomposition of water represent either very strong oxidants (radicals OH and H₂O₂) or strong reducers (atomic hydrogen, hydrated electron, molecular hydrogen). Only a part of them form again water particles in natural environments of waters and rocks. Omnipresence of oxidant acceptors in these environments results in marked shift of chemical equillibrium. Radiolysis has been responsible for origin of products of oxidation and reduction, in amounts significant for water ballance in hydrosphere. Rocks display numerous traces of direct effects of physical and chemical action of ionizing radiation. Preliminary quantitative estimations of that phenomenon, supported by results of experimental studies and model calculations, indicate significant loss of water (equal even 50% in some geological environments). Freed oxigen has been mainly used in oxidation of inorganic compounds and, at the same time, for formation of carbon dioxide, trivalent iron compounds, sulfates, and other compounds representing high level of oxidation. In turn, the overhelming part of hydrogen escaped into interplanetary space. The ballance of oxygen produced in the above mentioned way unequivocally shows predominating role of water radiolysis in its production throughout the whole history of the Earth.

Quantitative share of processes of photosynthesis in production of oxigen was overestimated. The processes appear responsible for production of merely 12% of oxigen used in oxidation of inorganic

components. Products of radiolysis contribute to “burning out” organic matter buried in rocks as well

as reintroduction of its carbon in subsequent cycle in the form of CO₂, This phenomenon is traceable in Precambrian rocks and several younger series (limestones, dolomites, saliferous formation rocks).

Gaseous hydrogen freed by processes of radiolytic decomposition has been playing highly significant role in hydrogenisation of buried organic matter and formation of precursors of oil. Radiolysis products also bear active influence on mineral matter present in rocks. They take part in alteration of clay minerals, facilitate remobilization of silica, contribute to changes in concentration of salts in pore waters and stimulate sealing of rocks. The resulting gaseous products – H₂, CO₂ and N₂ – markedly contribute to increase of layer pressure.

The share of radiolysis processes in production of oxigen has been the greatest at early stages of the Earth history, i.e. in times of activity of short-living isotopes and high activity of uranium and potassium in producing the ionizing radiation. The survival of life on the Earth from the beginning of dispersal of phytoplankton in surface waters (at least since 3.5 milliard years) has been only possible thanks to establishment of a dynamic ballance in supply of carbon dioxide, increasing limited resources of the latter in biosphere. In result of prolonged action of that mechanism, the overhelming part of carbonates and organic matter became concentrated in Phanerozoic rocks. In turn, the changes in concentration of carbon dioxide in atmosphere appear highly limited The level of content of carbon dioxide in atmosphere has been controlled by precipitation of carbonates and sedimentation of organically-bound carbon, compensating supply of carbon dioxide from deep-seated layers in connection with both metamorphic processes (leading to decomposition of carbonates and “burning out” organic carbon) and those of degasification of the Mantle.