Late Messinian Mediterranean base level fall : insights from sulfur isotopic variations coupled with isostasy-based paleo-depth estimates

At the end of the Miocene, the restriction of the Atlantic-Mediterranean seaway led to the deposition of a basin-wide salt giant (of up to 2.5km in thickness) in the Mediterranean. Drawdown(s) of the water level of the Mediterranean during this Messinian Salinity Crisis (MSC) have been proposed to have occurred. However, their number, timing and amplitude are still largely debated, with estimates ranging from 200m to 2km. While an important sea level fall could have efficiently blocked the Mediterranean outflow to the Atlantic Ocean, a limited drawdown would have allowed huge export of salt from the Mediterranean to the Atlantic Ocean, and in turn to the global ocean.  These two scenarios would thus have had opposite effects on thermohaline circulation and consecutively on climate. It is therefore crucial to estimate the water level of the Mediterranean during the MSC to understand its climatic and environmental effects both on a regional and global scale.

Here we investigated the deposits drilled by ODP and DSDP expeditions that characterize the terminal phase of the MSC in deep basins of the Mediterranean.  We measured the sulfur isotopic composition (both in sulfide and sulfate bearing minerals) in these deposits to constrain the maximal depth under which they formed, building on the observation that sea level is an important factor controlling sedimentary sulfur isotopic composition. We then used these maximal depths of deposition in a paleo-bathymetric reconstruction to constrain the amplitude of the base level fall that characterized the end of the Messinian Salinity Crisis. The result of this approach suggests a minimal water level drop of 1.1km in the western Mediterranean and a 1.7km drawdown at least in the eastern Mediterranean. This imply that these two basins were disconnected at the end of the MSC, and supports the findings of studies using independent seismic markers.