Hydraulic theory of sea straits applied to the onset of the Messinian Salinity Crisis

Abstract

Theory for the dynamics of flow in sea straits holds promise to provide, in addition to geological evidence, insight into the configuration of the connection between the Mediterranean Sea and the Atlantic Ocean at the onset of the Messinian Salinity Crisis. This paper, for the first time, systematically explores the application of hydraulic control theory to the question of how, about 6 Ma ago, Mediterranean salinity could have risen to values associated with gypsum saturation. The theory is based on the notion that it is the greatest constriction of the flow between basin and ocean that acts to limit the exchange. The response of basin salinity to strait depth, strait width, and relative thickness of the outflow layer proves to be highly nonlinear. For strait width on the order of kilometres, an asymptotic rise in basin salinity occurs when the strait depth is on the order of a few tens of metres. Completely blocked outflow takes place when the depth is reduced to metres. The nonlinear nature of the system implies that even a slow gradual reduction in the sill depth leads to an event-like rise in basin salinity. For values of basin salinity approaching gypsum saturation the response of the basin to changes in the strait depth is significantly delayed.