Around 5.5 million years ago, the Mediterranean Sea almost entirely dried up, leaving behind deposits of salt of up to 3 kilometres thick known as salt giants. What drove this event is debated, but a study of the super-salty Dead Sea offers some new clues.
女生小视频s agree that the so-called Messinian salinity crisis was caused by a diminished flow of water from the Atlantic into the Mediterranean, which meant that evaporated water wasn鈥檛 replaced. But scientists argue about important aspects, such as whether the flow was completely cut off.
Up to now, those arguments have ignored important physical insights about the behaviour of saline water, says at the GFZ Helmholtz Centre for Geosciences in Potsdam, Germany. In particular, he says, existing models of the event do not reflect how salinity hinders the evaporation of water. As a sea gets saltier, it should stop shrinking altogether, unless the air over it is abnormally dry or considerably cooler than the water.
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To get to the bottom of this, Guillerm and his colleagues looked at a modern analogue. The Dead Sea was, for a long time, in a stable state. Dense, salty water was overlaid by a layer that was less dense, because a sustained influx of fresh water from the river Jordan kept it at a lower salinity, compensating for water that evaporated from it.
This changed as towns and agriculture used the river more, reducing the flow into the lake. Since 1979, the density difference between the two layers has broken down every winter as the now saltier top layer cools and becomes even denser. That kicked off other major changes: since then, the level of the Dead Sea has gone down 40 metres, it has warmed by 3掳C and it has deposited some 3- to 4-billion tonnes of salt on the bottom.
The warming happens because, in summer, the reduced contrast between the two layers makes it easier for heat to flow down to the bottom water. When that water reaches the surface in winter, it is warmer than the air over it. That helps evaporation more than the salinity hinders it, allowing water loss to continue.
Applying the same idea to a water body the size of the Mediterranean, Guillerm calculated that cutting it off from the ocean would result in 20掳C of warming and kilometre-scale drawdown in just a few thousand years.
鈥淚t is an interesting idea, and probably very relevant for understanding the water level in the Dead Sea,鈥� says at Utrecht University in the Netherlands, who wasn鈥檛 involved with the study. For the Mediterranean, however, he thinks existing models of the Messinian salinity crisis are fine, because they already show extreme decline of sea levels in a climate similar to today鈥檚.
But, says Guillerm, if those models took account of the detrimental effect of salinity on evaporation, they would struggle to lose so much water. Only when the annual overturning is also included would they show such extensive drying of the Mediterranean.
Guillerm thinks his work could also clear up the question of whether the connection to the Atlantic was completely cut off. Only then would it have behaved like the Dead Sea, losing water and, crucially, warming up. 鈥淭he next step is therefore to reconstruct the evolution of water temperature in the Mediterranean Sea during the Messinian salinity crisis,鈥� he says.
Geophysical Research Letters
