A sedimentological insight into the deglacial reorganization of the Mediterranean thermohaline circulation

Abstract

During the last deglaciation and early Holocene significant oceanographic changes have been observed in both western and eastern Mediterranean basins. Post glacial sea level rise strongly reduced the northwestern overturning cell from 14.5 to 9 ka BP, which favored the formation of an organic rich layer in the Alboran Sea. Increased river runoff in the east-Med enhanced water column stratification leading to the formation of Sapropel 1 between 10.8-6.1 ka BP. Although asynchronous and triggered by independent mechanisms, these basin-scale oceanographic changes had a profound impact on the water exchange between both basins and the Mediterranean thermohaline circulation, and consequently, in the amount and properties of Mediterranean waters exported to the north Atlantic region. Our grain-size and geochemical records show coetaneous decrease/increase from cores at deep/ intermediate levels of the west-Med confirming a major reduction of deep water currents in the deepest part of the basin related to the post glacial sea level rise, while increased intensity of currents at intermediate levels. These results likely suggest a significant change in the water masses properties during the deglaciation that would favored formation of intermediate water. On the other hand, the sedimentological records from an Ionian Sea sediment core show enhanced deep water currents during the first phase of the deglaciation, and a very marked reduction in the 10.8-7.5 ka time-interval, coinciding with the Sapropel 1 formation. Our results show that intermediate levels circulation was reinforced in both eastern and western Mediterranean basins during the first phase of the deglaciation likely suggesting a major export of intermediate water masses from the eastern to the western basin. Finally, the strong stratification leading formation of the Sapropel 1 produced a major reduction of deep water ventilation in the eastern basin that could also affect westward export of intermediate waters.