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 chain238U … →234U →230Th →. Groundwater usually has a234U/238U ratio in the range of 0.8–3.0 (by activity). However, in some regions, the234U/238U ratio reaches >10 and up to 50. Ultrahigh excesses of234U can be explained by climatic variations. During a cold period, minerals accumulate234U as a normal component of the radioactive chain, and after the melting of permafrost, it is lost from the mineral lattice faster than238U 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 significant234U 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, the234U/238U ratio also increases during warm periods and significantly exceeds the234U excess of the World Ocean. Furthermore, the234U/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 maximum234U/238U ratio in summer. The behavior of234U in large water reservoirs is consistent with the hypothesis about the decisive influence of permafrost degradation on the anomalies in234U/238U ratios in groundwater.