Modeling the magnitude and timing of evaporative drawdown during the Messinian salinity crisis

Abstract

The Mediterranean and Red Sea salt layers ofMessinian age reach up to 3 km in thickness in the subsurface of themodern abyssal plains and are rare to absent on the contemporary margin rims. Deposition of the full evaporite suite of carbonates, sulfates, halite, potash, dolomitic marl and debris from pervasive margin erosion took place during a regional Late Miocene salinity crisis once the restriction of two-way water flow through the portal from the Atlantic reached a threshold that triggered a rise in salinity everywhere sufficient for precipitation to commence on a regional scale. For the first 0.35my of the crisis the surface of the Mediterranean remained more or less at the level of the global ocean except possibly during brief episodes when brine reflux ceased as the consequence of sea-level fall in the exterior Atlantic. Evaporative drawdown on a large scale is an intra-salinity-crisis phenomenon and coincides with the precipitation of the bulk of the halite that makes up the giant flowing salt layer observed in reflection profiles. We present an integrated quantitative model that reproduces the closure of the Atlantic spillway, evaporative drawdown and eventual opening of the Gibraltar Strait for the Pliocene refilling. The model incorporates time-varying global eustacy, solar insolation, mixing of brine across sills between the Mediterranean and Red Sea depressions and water flow through subsurface aquifers. The model accounts for the precipitation of more than one million km3 of halite alone in just a few precession cycles. The closure results from the weight of the growing brine body and salt deposit that depresses the basin center and elevates its rims. Ground water leakage under high pressure from the Atlantic into the nearly empty Mediterranean corrodes and opens fissures in the Gibraltar barrier leading to eventual break through at this particular location followed by a run-away flood.