The role of isostatic adjustment and gravitational effects on the dynamics of the Messinian salinity crisis

The Messinian Salinity Crisis (5.96 to 5.33 Ma) was one of the most dramatic periods of sea-level change in recent geologic history – a break in the longstanding connection between the Atlantic Ocean and Mediterranean Sea. Stratigraphic evidence throughout the Mediterranean basin records a series of sea-level oscillations during this period. We explore the contribution of isostatic adjustment and gravitational effects, in response to changing water loads in the Mediterranean, on sea-level change at the Gibraltar Strait using an ice-age sea-level model. During the filling of the Mediterranean basin, inflow is increased due to both the gravitational pull of the water in the basin and subsidence at the Gibraltar sill associated with the increased water load. These effects reverse as the basin empties. We show that sea-level changes at the sill depart dramatically from the average sea-level change in the global ocean, and that the relative contribution of deformational and gravitational effects to this departure varies with the period of the cyclicity. As a further illustration of the sea-level physics, we consider a model of the cyclical flooding and emptying of the Mediterranean driven by a competition between tectonic uplift and erosion at the Gibraltar Strait (with uplift leading to closure of the strait and erosion leading to opening of the strait). The positive feedback on both refilling and drying of the Mediterranean Sea associated with the sea-level physics significantly changes the uplift and erosion parameters that are required to generate the periodicity evident in the stratigraphy. We find that a tectonic uplift rate at the Gibraltar sill of 1.5 mm yr⁻¹, three times smaller than previously published, yields a periodic sea-level curve when the feedback is included. This rate is consistent with uplift rates estimated by geodynamic models appropriate for the Gibraltar arc (<2.65 mm yr⁻¹).