Zanclean flood refills the Mediterranean

About 5.33 million years ago, at the start of the Pliocene, the Atlantic Ocean broke through a barrier near the Strait of Gibraltar and poured into the Mediterranean basin. The Mediterranean had been cut off and had largely dried out during the Messinian salinity crisis, leaving a deep, partly salt-covered basin up to a couple of kilometres below sea level in places. The refilling is called the Zanclean flood, after the Zanclean stage of the Pliocene.¹²

The idea that the Mediterranean had dried out was proposed by Kenneth Hsu after the Glomar Challenger drilled salt layers in 1970. In 2009 Daniel Garcia-Castellanos and colleagues reported seismic images of a long channel carved across the Gibraltar sill and the Alboran sea floor and argued that the water entered in a catastrophic flood. They estimated a peak discharge of around 100 million cubic metres per second, hundreds of times the flow of the Amazon, with the basin refilling in months to a couple of years and water rising by metres a day at the peak.¹²

Not everyone agrees on how sudden it was. Some studies of sediments on the Mediterranean margin and in the Atlantic point to a slower or two-stage refilling. What is agreed is that the Mediterranean returned to the sea at the start of the Pliocene, restoring the connection between Atlantic and Mediterranean life. Its story is also the basis of recent studies on how rivers carve channels under large floods.³⁴