Causes and consequences of the Messinian salinity crisis

Salt giants are massive salt deposits (hundreds of metres thick) that form during the evaporation of semi-enclosed seas. The drivers of salt giant formation and their feedbacks on global and regional environmental change remain debated. In this Review, we summarize the boundary conditions, causes and consequences of the Mediterranean Messinian salinity crisis (MSC; 5.97-5.33 million years ago). Salt giant formation is more complex than the simple evaporation of an enclosed sea. Instead, the tectonic setting of an evaporative basin largely determines the timing and mode of salt formation, with superimposed impacts of orbital-scale climate and sea-level fluctuations. These drivers triggered precipitation of carbonates, gypsum, halite and bittern salts, with well-defined orbital cyclicities in carbonate and gypsum phases. Removal of Ca2+ during salt giant deposition decouples the oceanic Ca2+ and HCO3- sinks, causing reduced CaCO3 burial and, consequently, increased ocean pH, lower atmospheric partial pressure of CO2, and global cooling. Salt giants, which reflect a net evaporite-ion extraction of similar to 7-10% from oceans and persist over million-year timescales, could therefore be an important climate driver but are currently underconsidered in long-term carbon cycle models. Future research should use advanced hydrogeochemical models of water-ocean exchange to further explore interactions between salt giants and environmental change.