Permafrost in the Caspian Basin as a Possible Trigger of the Late Khvalynian Transgression: Testing Hypothesis Using a Hydrological Model

A quantitative estimate is given to the hypothesis, explaining the paleogeographic data on the extremely high water abundance in rivers in the Caspian Basin in the period when the Late Khvalynian Transgression was forming, by postglacial permafrost in this basin, which could contribute to a decrease of soil infiltration and evapotranspiration, and an increase of river flow. The estimate was based on numerical experiments with a physical-mathematical ECOMAG model of Volga runoff formation. Two series of experiments were carried out to simulate deep soil freezing all over the territory of the present-day Volga basin. In these experiments, the process of seasonal thawing of frozen soils was described taking into account their thawing during the warm (summer–autumn) season and the formation of annual thawed layer (1); in the cases where there was no seasonal thawing, lower values of soil heat conductance were specified (2). If we assume that the climate conditions in the territory of the present-day Volga basin during the formation of the Late Khvalynian transgression were similar to those we see now, the increase in the water abundance in rivers compared with its present-day level can be partly attributed to the occurrence of postglacial permafrost in this basin. At the same time, the increase in the normal annual runoff due to seasonal thawing of permafrost is about 20% (the runoff coefficient increases from 0.36 to 0.44) and does not agree with the huge inflow (>450 km³/year) estimated by paleogeographic data. Under the current climate conditions, an inflow 85% greater than the estimated value (an increase in the runoff coefficient up to 0.67) can be formed in the absence of seasonal thawing of the frozen catchment, i.e., at air temperature lower than its current value.