Uranium-234/uranium-238 ratios in sea water

Abstract

The importance of radioactive disequilibrium within the natural uranium-238 series in various natural systems, since the initial contributions of Hans Pettersson, has steadily gained importance especially in geochronological and geochemical studies. Recently, Russian workers have shown that the parent member, U-238, and the daughter, U-234, in groundwaters do not exist in radioactive equilibrium concentrations. It has been postulated that while U-238 exists in crustal rocks primarily in the +4 state, as a result of the radioactive decays producing the daughter U-234, electrons are lost from the nucleus such that a part of the U-234 ends up in the +6 state. The latter form is preferentially brought into solution by weathering waters due to the formation of the strong complexes formed between uranyl and carbonate ions. Thus, the activity ratio A(U-234)/A(U-238) often exceeds one in surface waters.Experimentally determined values of the activity ratio in surface and deep sea waters from the Atlantic and Pacific Oceans and the Mediterranean Sea are uniform with a value of 1·14 ± 0·014. The error arises from the counting statistics in the radioactive assay of the alpha particles emitted by these two isotopes of uranium. This constancy in the ratio is consistent with the long residence time of uranium and relative lack of reactivity in sea waters.