Introducing a water activity coefficient to account for abundance and isotopic compositions of gypsum and halite in the Mediterranean's salt giant deposit

Abstract

A simple box model is developed to access the astronomical-modulated exchange of ocean seawater with the Mediterranean during the Late Miocene salinity crisis. The model differs from previous ones by using a unidirectional portal as the method to get salinity to increase to saturation for gypsum and halite. Flow through the entry portal switches in direction as climate oscillates from arid to wet during each precession cycle. The calculations incorporate a water activity coefficient that reduces the rate of evaporation as salinity increases. As a result, the accumulation rate of gypsum and halite declines with time. The sodium, sulfate, potassium and magnesium ions observed from fluid inclusions in gypsum and halite are replicated in the model. The reduction in input from the exterior ocean and eventual closure of the gateway conforms with the sedimentological and geochemical behavior of the gypsum deposits on margins and halite in salt mines and reflection profiles. By showing that rain and rivers supply eight times more water to the Mediterranean brine than seawater during the hypersaline stage of the Late Miocene salinity crisis, the calculations resolve the geochemical riddle of “low salinity gypsum”. The sulfur isotopes in gypsum derive from ocean seawater and the oxygen and hydrogen isotopes in the gypsum hydration water from atmospheric precipitation.