The water level in the ancient Caspian Sea was tens of meters higher than today due to paleoclimate changes

Scientists have shown that the extreme rise in the level of the Caspian Sea by tens of meters, which occurred 18-13 thousand years ago and was called the “Great Khvalynsk Transgression,” could have been caused, contrary to existing hypotheses, not by the melting of the glacier, but by natural changes in the paleoclimate. It turned out that due to the cold climate of that period, the vast areas from which the rivers flowing into the Caspian collected water were covered with permafrost. As a result, the masses of rain and melt water were almost not absorbed into the frozen soils and flowed into the sea, evaporation from the surface of which was small. All these factors led to an increase in the level of the Caspian Sea and an increase in the area of the sea by more than twice as compared to today. The data obtained will help clarify ideas about the scale of fluctuations in the level of the Caspian Sea under climate change.

The Caspian Sea is the world's largest body of water not directly connected to the ocean. Research indicates that the level of the Caspian Sea has varied greatly throughout the history of its existence: the difference between the minimum and maximum values reached more than 100 meters. The most famous and large-scale rise in sea level – the Khvalynsk transgression – occurred in the period 18-13 thousand years ago. During this time, the level of the Caspian Sea remained more than 50 meters higher than today. Scientists have put forward many hypotheses explaining the reasons for the Khvalynsk transgression.

Most of them boiled down to finding an answer to the question: what is the source of the huge influx of water necessary for such a high rise of the ancient sea in the cold and relatively dry climate of that time? Many scientists explained this phenomenon by an additional influx of water into the Caspian Sea, which arose, firstly, as a result of the melting of the giant Scandinavian glacier, and secondly, from glacial lakes with rivers that now do not flow into the Caspian Sea. Another confirmation of the enormous water content of the ancient rivers of the Caspian Sea basin was the traces of once-existing channels found on the Russian Plain, many times larger in size than modern rivers.

A group of paleogeographers and geomorphologists studied traces of ancient river beds on the Russian Plain, which carried their waters to the Caspian Sea. Having selected hundreds of soil samples and examined them using modern analytical equipment, scientists have shown that the age of these channels is 17.5–14 thousand years, that is, it coincides with the period of the Khvalynsk transgression. The calculated volume of water flowing into the Caspian Sea through these huge channels also turned out to be close to the estimates obtained from the modeling.

“The completed model calculations and the results of paleogeographic analysis confirmed the hypothesis that the rise in the level of the ancient Caspian Sea by tens of meters above the modern one could have occurred in a cold and relatively dry climate 18-13 thousand years ago, when the contribution of melted glacial waters was either already absent, since the glacier melted , or was insignificant. Independent estimates obtained by three groups of project implementers gave similar values: the influx of water into the Caspian Sea was up to one and a half times greater than today, which ensured the maintenance of such a high sea level with little evaporation from its surface. Such an influx in the absence of glacial runoff is explained by the spread of permafrost on the Russian Plain,” says the head of the project, supported by a grant from the Russian Science Foundation, Alexander Gelfan, Doctor of Physical and Mathematical Sciences, Corresponding Member of the Russian Academy of Sciences, chief researcher at the Institute of Water Problems of the Russian Academy of Sciences.

In the future, the authors plan to continue research on reconstructing large-scale rises and falls in the level of the Caspian Sea in other eras as a reflection of climate change.

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