50th anniversary review of the Mediterranean desiccation hypothesis

Abstract

The first deep-sea drilling expedition in the Mediterranean chanced upon unanticipated discoveries when recovering deeply buried sediments of Late Miocene age that had once accumulated in ultra-shallow water and had later experienced subaerial exposure. Among these deposits were potassium and magnesium chlorides, desiccation cracks, fluvial gravels and shale bearing bottom-dwelling diatoms requiring sunlight, all indicative that the Mediterranean had evaporated one or more times to near dryness during what became called the Messinian Salinity Crisis. The initial presentation of these findings in 1973 was met with hesitancy. Had the present 2 to 4 km deep Mediterranean Basins been much shallower? How does one explain sediment sandwiched between beds of anhydrite and gypsum that hosted microfossils belonging to both ocean seawater and species that lived in fresh-to-brackish water lagoons? Did all of the evaporites originate on salt pans rimmed by alluvial aprons, or was there a deep-water period during which most of the 1 to 2 km thick layer of salt layer had arrived? How do the Late Miocene evaporites and mudstones outcropping in mobile belts along numerous Mediterranean margins fit with the scenario of a near empty Mediterranean? These inquiries are addressed in the style of a historical narrative reviewing 50 years of investigations by researchers turning their attention to the peripheral deposits, including those from the Paratethys. The stable isotopes of oxygen, carbon, sulfur, deuterium and strontium become crucial evidence in support of a substantial Mediterranean desiccation that harmonizes what appears to be unresolved conflicts among prior and even contemporary interpretations.