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.
Highlights
The violation of the radioactive equilibrium of the 238 U chain (238 U → 234 Th . . . 234 U →→ . . . 206 Pb, hereafter referred to as the 234 U/238 U activity ratio) in natural water was discovered by P.I
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
Summary
The violation of the radioactive equilibrium of the 238 U chain In groundwater at middle and high latitudes of the Earth, there is a steady shift of the 234 U/238 U ratio to 5–20, and in some cases, it increases to 50 [6,7]. Such high enrichment in uranium-234 is explained by the release of recoil atoms (234 Th) from the mineral matrix into the water [8,9,10,11,12,13,14,15,16]. A more fundamental test of the hypothesis is possible on the basis of data on the isotopic composition of uranium in chemo- and biogenic formations in the World Ocean and large continental water bodies
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