Ocean levels were then 120 meters lower. A giant river, the Channel, flowed over a plain between France and England. In Central Europe, the Black Sea was not connected to the Mediterranean Sea and formed a large lake some 100 meters below today's level, exposing 100,000 km2 of land. The Danube and Dnieper rivers shared a common mouth in the Black Sea. While the Danube drained meltwater and sediment released by Alpine glaciers, the Dnieper collected sediment from the Fennoscandian cap.
This particular situation has made the sediments deposited at the bottom of the Black Sea a prime archive for reconstructing and understanding the profound environmental reorganizations that accompanied the end of the last Ice Age in Central Europe. Just a few meters below the bottom of the Black Sea lies a series of four layers of red sediment, in stark contrast to the bluish-gray that commonly characterizes the sedimentary column. For some ten years, these red layers had been suspected to represent the sedimentary imprint in the Black Sea of the melting European ice. However, this had yet to be proven, the cap at the origin of their deposition had to be determined and the dynamics of their genesis had to be understood.
Various geochemical indicators measured in the sediments of a core collected as part of the Assemblage project by IPEV's French oceanographic research vessel Marion Dufresne in 2004, enabled us to reconstruct the activity of the rivers feeding the Black Sea and reveal the origin of the sediments they carried over the last 25,000 years.
Two organic geochemical indicators sensitive to soil erosion by rivers showed that each of the red layers was deposited in response to a strong and sudden increase in river activity. A comparison of the neodymium isotopic signature of the sediments forming the red layers with that of many present-day soils in Central Europe showed that only the soils currently present at the headwaters of the Dnieper River (present-day Belarus) corresponded. Taken together, these results prove that the formation of the red layers in the Black Sea, between 17,000 and 15,500 years ago, was due to major inflows of meltwater from the disintegration of the Fennoscandian ice cap.
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Article published in La Lettre du Collège de France n° 37, December 2013
