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

• Published in: Water
• Main Authors: Igor Tokarev, Evgeny Yakovlev
• Format: Article in Journal/Newspaper
• Language: English
• Published: MDPI AG 2021
• Subjects: non-equilibrium uranium; 234 U/ 238 U ratio; climate variations; permafrost; World Ocean; Lake Baikal; Hydraulic engineering; TC1-978; Water supply for domestic and industrial purposes; TD201-500
• Online Access: https://doi.org/10.3390/w13243514; https://doaj.org/article/dda64165690e4d17ae1bed4fed07c1ff

In natural water, as a rule, there is a violation of radioactive equilibrium in the chain 238 U … → 234 U → 230 Th →. Groundwater usually has a 234 U/ 238 U ratio in the range of 0.8–3.0 (by activity). However, in some regions, the 234 U/ 238 U ratio reaches >10 and up to 50. Ultrahigh excesses of 234 U can be explained by climatic variations. During a cold period, minerals accumulate 234 U as a normal component of the radioactive chain, and after the melting of permafrost, it is lost from the mineral lattice faster than 238 U 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 234 U 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 234 U/ 238 U ratio also increases during warm periods and significantly exceeds the 234 U excess of the World Ocean. Furthermore, the 234 U/ 238 U 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 234 U/ 238 U ratio in summer. The behavior of 234 U in large water reservoirs is consistent with the hypothesis about the decisive influence of permafrost degradation on the anomalies in 234 U/ 238 U ratios in groundwater.