How much did the climate force the Messinian salinity crisis? Quantified climatic conditions from pollen records in the Mediterranean region.

International audienceThe latest Miocene (5.96 to 5.33 Ma) is characterised by an outstanding event: the desiccation of the Mediterranean Sea (Messinian salinity crisis). It has been suggested that this was caused by a tectonic event, with no climatic change playing a role in desiccation. Quantifying the climate of the region during this period will help support or refute this hypothesis. An effective method for reconstructing the climate from Neogene pollen data is the “Climatic Amplitude Method” based on the modern climatic requirements of plants to interpret fossil data. It has been conceived especially for periods devoid of modern vegetation analogue. Twenty Messinian to Lower Zanclean pollen sequences are now available in the peri-Mediterranean region. Most of them do not cover the whole Messinian interval, particularly those along the Mediterranean shorelines where sedimentation was interrupted during the sea's desiccation. In contrast, sedimentation was almost continuous in such areas as the Atlantic side of Morocco, along the Adriatic coast (including the Po Valley), and to a lesser extent the Black Sea. The Mediterranean sites nonetheless provide a reliable if not a discontinuous record of vegetation variability in time and space. A first examination of the pollen diagrams reveals a high regional variability controlled by local conditions, and throughout the interval a southward increase in herb pollen frequency in contrast to the tree pollen frequency. This indicates that open and probably dry environments existed in the southern Mediterranean region prior to, during and after the salinity crisis. Trees developed in areas close to mountains such as in the Po Valley, in Cerdanya and in the Black Sea region. Most variations observed in the pollen diagrams are constrained by fluctuations of Pinus pollen amounts, indicating eustatic variations. Climatic quantification from pollen data does not show obvious climatic changes due to the desiccation of the Mediterranean Sea, especially in the dry and warm southwestern Mediterranean area (Sicily, southern Spain and North Africa). At Maccarone, along the Adriatic Sea, a decrease in temperatures of the coldest month and, less importantly, a decrease in mean annual temperatures, corresponding to a drastic vegetation change, are reconstructed. These temperature variations are assumed to be controlled by regional environmental changes (massive arrival of waters in this basin) rather than to reflect cooling, because some authors link the second phase of evaporite deposition to a period of global warming. Some migrations of plants probably occurred as a response to Mediterranean desiccation. But the climatic contrast which has probably existed at that time between the central Mediterranean and the peripheral areas might be amplified. Climatic reconstruction from pollen data in the western Mediterranean area shows that climate is not the direct cause of the Mediterranean desiccation, as the Mediterranean region had experienced continuously high evaporation long before the crisis. Therefore the main factor leading to this event seems to be the successive closures of the Betic and Rifian corridors, isolating the Mediterranean Sea from the Atlantic Ocean