Causes and consequences of the Messinian salinity crisis

Salt giants are massive salt deposits (at least hundreds of m thick) that form during the evaporation of semi-enclosed seas. However, 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 Ma), and highlight the impacts of salt extraction and ion return on the Earth system. 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 even bittern salts in the Mediterranean, with well-defined orbital cyclicities in carbonate and gypsum phases. Removal of Ca2+ during salt deposition decouples the oceanic Ca2+ and HCO3- sinks, causing a CaCO3 burial decrease and, consequently, increased ocean pH, lower atmospheric pCO2, and global cooling. Salt giants, which reflect a ~7 to 10 % net evaporite-ion extraction from oceans that persists over million-year timescales, could therefore be an important climatic driver, but are currently not considered in long-term carbon cycle models. Future research should target more advanced hydrogeochemical models of water exchange with the open ocean to provide critical context for understanding interactions between salt giants and environmental change.