The Mediterranean dried up
Around six million years ago the Mediterranean basin largely evaporated, leaving salt kilometres thick. When the Atlantic broke back in, the sea returned in a flood.
The Messinian salinity crisis was identified from salt and gypsum cored out of the deep Mediterranean floor, layers that form when seawater evaporates, and from canyons cut by rivers that had to fall much farther to reach a vanished shoreline. Kenneth Hsü and colleagues put the case in 1973: the Strait of Gibraltar had closed, the rivers could not keep up with the Sun, and the basin drew down. Parts of it went dry. Parts became brine lakes. The drawdown may have happened more than once.
The refilling is the Zanclean flood. A 2009 model of the incision at Gibraltar has the Atlantic cutting back through the sill and pouring in at a rate that could refill the basin in months to a couple of years. That duration is a calculation from the shape of the channel, not a tide gauge. The salt, the erosion, and the fact of a deep dry basin are the part that is drilled. The speed of the last flood is the part that is modelled.
Caveats
- ‘Dried up’ does not mean a single empty bathtub for half a million years. The crisis was long, and the level rose and fell.
- Months-to-years is one published model of the final refill. It is not a consensus stopwatch, and it is not the whole crisis.
Sources
- 1Hsü K. J., Ryan W. B. F., Cita M. B.. Late Miocene Desiccation of the Mediterranean. Nature 242, 1973.
- 2Garcia-Castellanos D. et al.. Catastrophic flood of the Mediterranean after the Messinian salinity crisis. Nature 462, 2009.