Messinian salinity crisis: A novel unifying shallow gypsum/deep dolomite formation mechanism

The Messinian Salinity Crisis is a dramatic event that took place between 5.96 and 5.33 Ma, and resulted not only in the desiccation and reflooding of the Mediterranean but also in the deposition of 0.3–3 km thick evaporites at its seafloor. There has been considerable controversy on the modes of their formation. The first-stage gypsum deposits are considered to have only formed at silled marginal basins, whereas anoxic marls and dolostones are found at deeper settings. We agree with these observations but reject the coincidental presence of similar sills as being too fortuitous to explain gypsum formation in silled basins alone. Alternatively, we present a unifying mechanism in which gypsum precipitation takes place basin-wide at all shallow-water depths but its preservation is limited to silled marginal basins. Taking a realistic early Messinian scenario, including a moderate primary production rate, the sulphate-consumption rate in the deep water appears to exceed the sulphate supply rate as deduced from reported shallow-water gypsum deposition rates. The consequent lower dissolved sulphate content limits gypsum preservation but permits dolomite formation to take place in the deep water. The often observed concurring anoxic environment and absence of gypsum formation is thus related to diminished dissolved sulphate content via organic matter degradation and not to oxygen-free conditions as such.