GLACIAL MELTWATER VERSUS RIVER RUNOFF AS THE DRIVERS OF THE KHVALYNIAN TRANSGRESSION OF THE CASPIAN SEA (Invited Presentation)

The runoff of glacial melt waters has long been discussed as a possible cause of the Khvalynian transgression of the Caspian Sea at the end of the Late Pleistocene - the highest sea level rise for the entire Quaternary period. In this study, we tried to estimate the magnitude of this runoff based on the latest data from Quaternary geology of the region. It was established that there were no inter-basin overflows and the only source of glacial waters was the melting of the last Scandinavian ice sheet in the upper reaches of the River Volga in the period between 21 and 17-16 ka. For the deglaciation time, the calculations of the value of the glacial melt runoff were carried out using three models of the glacier. The upper estimate gives the value of 60-70 km³ per year, the lower estimate - 15 km³ per year, or about 5-25% of the current flow of the Volga into the Caspian, and the lower value seems to be more reliable. However, even the maximum assessment is not enough to initiate the Khvalynian sea level rise. Moreover, the main phase of the Khvalynian transgression began only 17 ka BP, i.e. the glacial melt water flow into the Caspian coincided mostly with the sea level lowstand.

Then what were the sources of water that caused the rise in sea level by more than 100 m? On the floodplains and terraces of the rivers of the Volga basin, big paleochannels, dated to the time of 18-13 ka, are ubiquitous. We used the parameters of paleochannels to estimate the river runoff for more than 120 small basins. Then, by interpolation, maps of the annual paleo-runoff layer were obtained for the entire Volga basin, and from these maps the value of the annual runoff was calculated - 420 km³, i.e. 1.7 times higher than the modern value. This volume is sufficient to maintain the Caspian at the levels of the maximum of the Khvalynian transgression (+ 48 m asl), provided that the value of effective evaporation (evaporation minus precipitation) over the sea was a quarter lower than the current one.

Thus, the main reason for the post-LGM phase of the Khvalynian transgression was the climatically conditioned rise of the river runoff. The role of the influx of glacial melt water could only be a slight decrease in the sea level drop during the LGM.

This research contributes to the RSF Project 19-17-00215.