The Mediterranean hydrologic budget from a Late Miocene global climate simulation

During the Late Miocene the Mediterranean experienced a period of extreme salinity fluctuations known as the Messinian Salinity Crisis (MSC). The causes of these high amplitude changes in salinity are not fully understood but are thought to be the result of restriction of flow between the Mediterranean and Atlantic, eustatic sea level change and climate. Results from a new Atmospheric General Circulation Model (AGCM) simulation of Late Miocene climate for the Mediterranean and adjacent regions are presented here. The model, HadAM3, was forced by a Late Miocene global palaeogeography, higher CO₂ concentrations and prescribed sea surface temperatures. The results show that fluvial freshwater fluxes to the Mediterranean in the Late Miocene were around 3 times greater than for the present day. Most of this water was derived from North African rivers, which fed the Eastern Mediterranean. This increase in runoff arises from a northward shift in the intertropical convergence zone caused by a reduced latitudinal gradient in global sea surface temperatures. The northwards drainage of the Late Miocene Chad Basin also contributes. Numerical models designed to explore Late Miocene salt precipitation regimes in the Mediterranean, which typically make use of river discharge fluxes within a few tens of percent of present-day values, may therefore be grossly underestimating these fluxes.Although the AGCM simulated Late Miocene river discharge is high, the model predicts a smaller net hydrologic budget (river discharge plus precipitation minus evaporation) than for present day. We discuss a possible mechanism by which this change in the hydrologic budget, coupled with a reduced connection between the Mediterranean and the global ocean, could cause the salinity fluctuations of the MSC.     

Nannoplankton biostratigraphic calibration of the evaporitic events in the Neogene Fortuna Basin (SE Spain)

The Fortuna Basin is an example of a marginal Mediterranean basin with evaporitic sedimentation during the Late Tortonian and Messinian. This basin shows an early restriction event before the main Messinian Salinity Crisis (MSC) that allows the Tortonian Salinity Crisis (TSC) to be proposed as a tectonic uplift event isolating the eastern Betic basins. Four evaporitic events are present in the central part of the Fortuna Basin, from bottom to top: Los Baños Marls Formation (composed by Fenazar Conglomerate Bed, Lower Gypsum Member [Mb] and Sanel Mb), Tale Gypsum Formation (Fm), Chicamo Diatomites and Gypsum Cycles Fm, and Rambla Salada Gypsum Fm. The present work documents the first biostratigraphic dating based on calcareous nannoplankton of these events. The lowest occurrence (LO) of Amaurolithus primus is registered at the upper part of the Sanel Mb, below the Tale Gypsum Fm. The LOs of Amaurolithus delicatus and Reticulofenestra rotaria, which mark the base of the Messinian, occur in the lower part of the Chicamo Cycles Fm, above the Tale Gypsum Fm, the Triquetrorhabdulus rugosus-Nicklithus amplificus integrate form and the LO of Nicklithus cf. amplificus in the upper part of the Chicamo Cycles Fm. Taking into account these results, a new calibration of the available magnetostratigraphic data is presented: the Chicamo Cycles Fm were formed during the reverse chron C3Ar and the Tortonian-Messinian boundary should be found within the Tale Gypsum Fm or near the top of the Sanel Mb. The onset of the TSC, the first restriction phase of the Fortuna Basin, is represented by the Fenazar Conglomerate Bed, bottom of the Los Baños Fm, and not by the Tale Gypsum Fm, as previously considered.     

Messinian vegetation maps of the Mediterranean region using models and interpolated pollen data

This study proposes to compare the outputs from the CARAIB vegetation model forced by results from the LMD General Circulation Model with interpolated pollen data (Kriging method) from the Mediterranean region during the Messinian. The vegetation maps that have been obtained represent distinct phases of the salinity crisis: before the crisis and during the marginal evaporitic phase (interpolated map), and during the complete desiccation phase (simulated map). However, they are comparable in terms of vegetation density and agree on a strong contrast between the Northern (forest vegetation) and Southern (open vegetation) Mediterranean regions. Main differences concern the type of forests in the northern Mediterranean region, which are explained by discrepancies between precipitation amount predicted by the model and that calculated by a transfer function using pollen records. The interpolation method has been successfully tested in France using interpolated current pollen records by comparison with the present-day potential vegetation map. The resulting Messinian map is useful to validate or improve model simulation which does not take into account the depth of the Mediterranean Basin when it dried up. The Southern Mediterranean landscapes were open, with a steppe-like vegetation to the West and a savannah-like vegetation to the East. Forests prevailed to the North, organized in a mosaic system mainly controlled by relief. Such a contrast provides some explanation of the large number of deep fluvial canyons cut on the Northern margin at opposed to the South during the Mediterranean desiccation.