Geol. Soc. Am. Bull. http://doi.org/j7p (2012)

About 6 million years ago, the water exchange between the Atlantic Ocean and the Mediterranean Sea closed down, leaving the Mediterranean to desiccate. A reconstruction of climate at that time suggests that falling sea level may have been the final trigger.

Gonzalo Jiménez-Moreno of the Universidad de Granada, Spain, and colleagues collected a core of marine sediments from the Atlantic side of the connection with the Mediterranean. They analysed pollen grains and marine fossils, and found that sea level was relatively high in the Atlantic ocean in the 350,000 years leading up to the isolation of the Mediterranean. However, immediately preceding the desiccation, the climate cooled and sea level dropped rapidly. The magnitude of the sea level change could have been as little as a few tens of metres. However, earlier tectonic activity in the region of what is today the Strait of Gibraltar had left only a shallow gateway between the Mediterranean and Atlantic.

Once sea level fell, the main source of water inflow to the Mediterranean was cut off and the ocean began to evaporate. The authors suggest that the negative water balance may have been compounded by regional aridity, which would also have limited continental runoff into the basin.