Non-Equilibrium Uranium as an Indicator of Global Climate Variations—The World Ocean and Large Lakes

Abstract

In natural water, as a rule, there is a violation of radioactive equilibrium in the chain 238U … → 234U → 230Th →. Groundwater usually has a 234U/238U ratio in the range of 0.8–3.0 (by activity). However, in some regions, the 234U/238U ratio reaches >10 and up to 50. Ultrahigh excesses of 234U can be explained by climatic variations. During a cold period, minerals accumulate 234U as a normal component of the radioactive chain, and after the melting of permafrost, it is lost from the mineral lattice faster than 238U due to its higher geochemical mobility. This hypothesis was tested using data on the isotopic composition of uranium in the chemo- and bio-genic formations of the World Ocean and large lakes, which are reservoirs that accumulate continental runoff. The World Ocean has the most significant 234U enrichments in the polar and inland seas during periods of climatic warming in the Late Pleistocene and Holocene. In the bottom sediments of Lake Baikal, the 234U/238U ratio also increases during warm periods and significantly exceeds the 234U excess of the World Ocean. Furthermore, the 234U/238U ratio in the water of Lake Baikal and its tributaries increases from north to south following a decrease in the area of the continuous permafrost and has a seasonal variation with a maximum 234U/238U ratio in summer. The behavior of 234U in large water reservoirs is consistent with the hypothesis about the decisive influence of permafrost degradation on the anomalies in 234U/238U ratios in groundwater.