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