An Old Salt's Tale

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Science  25 Apr 2003:
Vol. 300, Issue 5619, pp. 553
DOI: 10.1126/science.300.5619.553a

About 6 million years ago, the Mediterranean Sea began to dry up—an event marked in the rock record by substantial evaporite deposits, some sitting directly atop deep-water sediments such as black shales. This “Messinian salinity crisis,” during which the Mediterranean became an evaporitic lake for a million years, apparently stemmed from closure of the narrow Atlantic-Mediterranean seaway. This closure is usually attributed to climate-driven sea level fall at the end of the Miocene, though recent studies have emphasized a possible role for tectonics as well.

Roveri et al. limn that tectonic role for the Mio-Pliocene Vena del Gesso basin, the record of which is exposed in rocks in the Italian Apennines. Integrating field and subsurface data, they show that evaporite precipitation there was tightly controlled by pre-Messinian and Messinian tectonic events that isolated the basin. They also establish that some shallow-water Messinian evaporites may have been transported by gravity sliding into adjacent areas and onto deeper-water deposits, near the close of the salinity crisis. The supposedly abrupt transition from deep-water shales to shallow-water evaporites that marked the beginning of the Messinian event may thus be partly a tectonic artifact, and the sea level drop that precipitated the salinity crisis may have been far smaller than generally thought.—SW

Geol. Soc. Am. Bull. 115, 387 (2003).

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