The Salt of the Earth - Ocean Worlds: The Story of Seas on Earth and Other Planets - page 79

replenish the sea—only to dry out in turn, leading to yet another
layer of salt building up on the surface. This has been explained in
terms of competition between the tectonics that built the barrier at
the Straits of Gibraltar, and the forces of erosion that wore it down. 52
And so, over the next 700,000 years, there were successive brief
influxes of Atlantic water. Each influx soon dried out, and so the salt
built up and up.
In total, an estimated 1 million cubic kilometres of salt lie buried
beneath the present-day Mediterranean Sea floor.53 53 That is more than
50 Mediterranean Sea's worth—and was about 5 per cent of the salt
in the global ocean. Thus, within the space of less than a million
years, the ocean salt content dropped by that 5 per cent forever—or
at least for as long as the salt stays locked up in those strata beneath
the sea floor.
What would have been the effect of this salt transfer? Clearly, within
the area of the Mediterranean virtually all of the marine biology would
have been killed off as the area became a toxic desert. But outside it?
The salinity reduction would have had wider effects, in raising the
freezing point of water and making it easier to form sea ice, for instance.
Did this have any effect on global climate? No one yet knows.
The rebirth of the Mediterranean was even more sudden than its
death. Above the thick salt deposits there is a sudden change to normal
marine sediments, this happened simultaneously across the basin 5.3
million years ago. The Gibraltar barrier, then, must have finally broken
completely to allow the Atlantic water to rush back in, in what geolo-
gists term the Zanclean flood. Estimates of the refilling, based upon deep
scours still preserved around the Straits, suggest that it took only some
two years to accomplish. 54 If so, it must have been one of the most spec-
tacular floods in Earth's history. Perhaps the thundering waters were
witnessed, in mute terror, by the ancestors of today's Gibraltar apes.
Let's hope they had a safe perch, high on the island's peak.