Caspian — Black Sea Connection During MIS 5 (Late Pleistocene): Evidences from Drilling Data

The Caspian and Black Seas are adjacent inland bodies of water, each with its unique palaeogeographic history. The Black Sea has been connected to the World Ocean throughout its Quaternary history. In contrast, the Caspian Sea has been completely isolated since the beginning of the Middle Pleistocene. Since then, the Caspian Sea has occasionally discharged its excess waters through the Manych Depression into the Azov-Black Sea Basin. As a result of this isolation, unique species have developed in the Caspian Sea. The transgressive-regressive cycles of the Caspian Sea are associated with global and regional climate fluctuations since its sea level depends on its water balance. Due to the penetration of Caspian species into the Manych Depression and further into the Azov-Black Sea Basin, it is possible to determine episodes of its transgressive water discharge, assess the characteristics of the existing basins, and correlate these episodes not only with events within the Mediterranean—Black Sea—Caspian corridor but also with global events. The study of the connection between the Black and Caspian Sea basins dates back to the 18th century. Thereafter, numerous publications have addressed various aspects of the palaeogeography of the region. However, the events within these two basins during Marine Isotope Stage 5 (MIS 5) and their scales and characteristics remain debated. This research focused on studying the MIS 5 Epoch, which includes the Holocene-analogous Eemian interglacial period and the transition to the Early Weichselian glacial Epoch. To reconstruct the events in the Black Sea—Manych Depression—Caspian Sea during MIS 5, we conducted lithological, malacofaunistic, and geochronological analysis on six borehole sections in the Northern Caspian, four in the Manych Depression, and three in the northeastern sector of the Black Sea to identify events that occurred in basins, assess their environmental characteristics, conditions of sedimentation and time frames. We have discovered that the Karangatian transgression (analogous to the Eemian transgression in the Mediterranean Sea) ingressed into the Manych Depression 125–110 ka BP (MIS 5e–d) during its maximum phase and formed a gulf there with a water salinity of up to 18‰, featuring Mediterranean mollusc fauna. Later, the waters of the Hyrcanian transgression of the Caspian Sea exceeded the water divide of the Azov-Black Sea and Caspian Sea basins and began to spill over into the Manych Depression. This influx of water resulted in the desalination of the sea gulf that was present there, reducing its salinity to values of less than 14‰. This event occurred around 110–105 ka BP (MIS 5c). The Karangatian waters eventually left the Manych Depression, but Hyrcanian waters continued to flow into the Black Sea until ~ 100 ka BP. This is evidenced by characteristic Hyrcanian species, such as Didacna cristata and D. subcatillus, found in the upper part of Karangatian sediments in the Manych Depression and on the northeastern shelf of the Black Sea. The Hyrcanian water discharge marked the initial flow of Caspian water into the Black Sea during the late Pleistocene, occurring unilaterally without any exchange between these two basins. Reconstructing the transition from the interglacial to the glacial period during MIS 5 can provide valuable insights into the expected changes in the Black Sea—Manych Depression—Caspian Sea system as we move from the current Holocene interglacial into a new glacial period.