Reconstructing the topography and water level of the Mediterranean Sea during the Messinian Salinity Crisis

Abstract

[eng] During the Messinian Salinity Crisis (MSC, 5.97 – 5.33 Ma), an environmental crisis unparalleled in recent geological history, thick evaporites were deposited in the Mediterranean Basin associated with major erosion of the continental margins. The MSC is thought to have led to a kilometre-scale water level drop by evaporation due to restriction of the Atlantic-Mediterranean marine connection, but the timing and amplitude of this drop have remained controversial. This is due to uncertainty in the post-MSC vertical motions and lack of clear correlations between the marginal and abyssal sedimentary records. In this thesis I aim at constraining the Messinian water level by way of providing a paleobathymetric reconstruction of the Mediterranean sub-basins and depth estimates for the emplacement of evaporite deposits and erosional markers. I constrain the magnitude of vertical motions induced by the accumulation of evaporite and other sediment units, isostatic and thermal subsidence, and tectonic deformation in three key regions, being: the Alboran Basin, the rest of the Western Mediterranean, and the Nile Delta. In the Alboran Basin (Chapter 3), erosional terraces were formed originally at a wide depth range. The shallowest terrace is reconstructed to 250-550 m depth, while the deepest terrace has a reconstructed depth range of 750-1500 m. This variation is interpreted as the result of fluctuating water levels during the drawdown phase and to a high-energy basin reflooding event. In the Western Mediterranean (Chapter 4), we use the MSC “Mobile Unit” halite and “Upper Unit” gypsum as markers for paleoshorelines, and we estimate them as having formed at depths of 1500 m and 1100 m respectively. In addition, halite is found in small silled basins originally as shallow as 500 m along the Balearic Promontory, suggesting that halite deposition during the evaporative drawdown spanned a wider depth range than suggested by its current preservation, and was subsequently removed by erosion during subaerial exposure during the drawdown and lowstand phase of the Messinian Salinity Crisis. Physics- based box modelling (Chapter 6) of water and salt fluxes to the Central Mallorca Depression on the Balearic Promontory allow to further evaluate this hypothesis, showing that the gypsum identified in this silled basin could only have formed by overall salinification of the Western Mediterranean at high water level, while the volume of halite suggests that its precipitation started only after the water level had dropped by at least 850 m. Also, the new, smaller, estimate of halite volume in the deep Western Mediterranean is in agreement with precipitation starting at or soon after the onset of drawdown, in contrast to that in the Eastern Mediterranean. In the Nile Delta (Chapter 5) topographical restoration shows the original depth of the geomorphological base level of the Nile River at ~600-m below present sea level, with a 400 m waterfall separating the downstream Messinian canyon from the older upper valley. This baselevel drop is 2-4 times smaller than derived from other criteria for the Eastern

basin, again suggesting fluctuations and diachronism of the MSC erosion episodes between the Western and Eastern Mediterranean. I show that the bathymetry of the Mediterranean basins was not radically different from the modern day in areas unaffected by fault tectonic deformation or plate subduction. Both the evaporites and the erosional features are found to have formed at a wide range of depths, not clearly linked to a single stable basinwide base level but rather affected by fluctuating water budgets in each subbasin. I propose that these variations in time and between subbasins were driven by variations in runoff from the continent and possibly by the capture of Paratethyan waters, during a stage of complete disconnection from the Atlantic.