Messinian Salinity Crisis


The Messinian Salinity Crisis is a rather important geologic event which caused massive changes for Europe. In short, this event was the drying up of the Mediterranean Sea, with a far deeper level of the sea of up to 300 m below sea level (the world sea level this time) and much of the area which is now the Mediterranean Sea being land. This event happened around 5.96 to 5.33 Ma, during the Messinian, which is an age of the Miocene epoch. It took about 600,000 years and started with the closing of the Strait of Gibraltar by geological movements and ended with its reopening.

The Mediterranean has rivers which flow into the sea, and seawater evaporates from its surface due to energy provided by the sun. At the current longitude, the climate is dominated by a dry season during summer and a wet season during winter. The rainfall of this longitude is not sufficient to replace the water which evaporates, and after all the sea will evaporate completely after some time. This obviously does not happen, because there is another influence, the Atlantic Ocean provides additional water through the opening at Gibraltar. Seawater flows into the Mediterranean at the bottom of the strait, while Mediterranean Sea water flows into the Atlantic at the surface. But the stream into the Mediterranean is much bigger and replaces the difference completely. This is a simple closed loop: if there is not enough water, the level of the Mediterranean lowers and more water flows in because its steeper, if there is more water from rivers the sea level rises, the inflow becomes shallower and so there is less inflow. It's actually quite simple.

But the so-called crisis was the blockade of this connection. The water of the mediterranean decreased continually, became saltier and saltier, the sea floor became land which was fertile, but later when the salt precipitated it was covered by salt, similar to the salt lakes of the Rocky Mountains. But there were also massive changes to the hydrology of the surrounding countries, despite the fact that the rivers had to flow much further to reach the sea. The drainage was not only farther away, it was also much deeper. As a result, the groundwater level went down in many areas, especially in karstified areas with massive underground drainage. In other areas, the rivers cut deep into the rocks. And finally cave systems developed far below sea level, draining to the depression of the Mediterranean.

When the connection opened again, there was an event called the Zanclean Flood. The Atlantic reclaimed the basin, the sea level went up rather fast, all the areas below sea level were flooded, and the drainage had to adapt once more. The rivers, which flowed through gorges below zero, became estuaries and slowly filled with gravel and other sediments. The cave systems below the sea were also flooded, but as there was no outflow any more, or better as the seawater at the outflow had the same pressure as the sweet water from the land, there was no movement any more. The water had to drain much higher up in the rock, where new caves started to form.

This geological event is of enormous influence to all karst areas around the Mediterranean Sea. The temporary drainage to a lower level caused the formation of caves which cannot be explained with the current drainage. The most spectacular such caves can be found at the Ardeche river. The cavers had big problems to explain the formation of cave system which are far too low for the current receiving waters. This is the reason why the Messinian salinity crisis is so important for cave development in the entire Mediterranean region.

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