Abstract
The isotopic (234U/238U, 2H, 18O) and chemical composition of groundwater on the right bank of the Volga River along the middle reach (European Russia) was studied down to a depth of 400 m. These data allow diagnosis of the presence of a three-component mixture. The first component is modern/young fresh recharge water of the Holocene age. It has the isotopic composition of water δ18O → −12.9 ‰ and δ2H → −90 ‰, close to modern precipitations, and the equilibrium isotopic composition of uranium 234U/238U → 1 (by activity). The second component is slightly salted water of the late or postglacial period with δ18O → −17.0 ‰ and δ2H → −119 ‰, and a small excess of uranium-234 234U/238U ≈ 4. The third component is meltwater formed as result of permafrost thawing. It is brackish water with δ18O ≈ −15.0 ‰ and δ2H ≈ −110 ‰, and a maximum excess of uranium-234 234U/238U ≈ 15.7. The salinity of this water is associated with an increase of the SO42−, Ca2+ and Na+ content, and this may be due to the presence of gypsum in water-bearing sediments, because the solubility of sulfates increases at near-zero temperature. We explain the huge excess of uranium-234 by its accumulation in the mineral lattice during the glacial age and quick leaching after thawing of permafrost.
Highlights
During the Weichselian glaciation, continental glaciation occurred repeatedly and its characteristics have been studied quite well for the East European Plain [1–26]
In the Valdai cryochron, most of the East European Plain was an area of continuous permafrost
The proxy markers of the existence of permafrost were found in the isotope and chemical compositions of groundwater for the Pre-Volga region
Summary
Publisher’s Note: MDPI stays neutral with regard to jurisdictional claims in published maps and institutional affiliations. During the Weichselian glaciation (about 110–12 ka ago, the below mentioned Valdai cryochron according to the Russian notation), continental glaciation occurred repeatedly and its characteristics have been studied quite well for the East European Plain [1–26]. Geomorphological, textural, pedological, faunistic, micropaleontological, pollen, and geochemical signs indicate that a dry, cold and windy climate of the Arctic steppes with sparse cold-tolerant vegetation was prevalent in the periglacial area [27–35]. In the Valdai cryochron, most of the East European Plain was an area of continuous permafrost. In its central part during the MIS-2 the permafrost was 200–250 m thick, and the ground temperature was about
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