FROM THE FIELD
Italy's Ring of Fire
A Dry Mediterranean
The Crotone Basin accumulated sediments for nine million years before the forearc uplifted above sea level. Each layer of sand, clay, and conglomerate in the basin contains information about the environment at the time that layer was deposited.
About six million years ago, halite and gypsum were deposited in the Crotone Basin. Geologists refer to both rocks as evaporites. All bodies of water on the Earth’s surface contain dissolved ions, most commonly sodium (Na+), chloride (Cl-), magnesium (Mg2+), calcium (Ca2+), and sulfides (SO42-). When water starts to evaporate, the dissolved ions bond together and precipitate out of the solution, forming evaporites (halite = NaCl, salt; gypsum = CaMg2SO4). Most commonly we find evaporites in deserts environments that sometimes receive influxes of water, like the Great Salt Lake in Utah. Since halite and gypsum are found in the Crotone Basin, we think that water must have evaporated from the basin about six million years ago.
As it turns out, evaporite deposits are found across the Mediterranean Sea during the same time period. Drill cores have turned up three kilometers of evaporites in some areas. To crystallize this much salt over such a wide area, geologists think that the entire Mediterranean Sea must have evaporated–an event called the Mediterranean Salinity Crisis (or Messinian Salinity Crisis) which lasted from 5.96 million years ago to 5.33 million years ago.
The Mediterranean Sea is located in the desert latitudes, where evaporation exceeds precipitation. The water level remains constant because water from the Atlantic Ocean enters the basin through the Straits of Gibralter.
But this wasn’t always the case. During the Messinian, a global sea level drop and local tectonics caused the land at the Straits to rise, cutting off the Mediterranean from the ocean. Since evaporation was so high, the water level dropped, concentrating the dissolved ions, and crystallizing evaporites; just like the Dead Sea in Israel, which crystallizes halite on its seafloor. Halfway through the Salinity Crisis, the four kilometers of water that filled the Mediterranean disappeared. A vast, desert basin is all that remained.
Nano and I are studying Messinian river deposits. Before and after the Salinity Crisis, rivers carried sediments from the mountains west of the basin. During the Messinian, however, something different happened.
The rivers seem to have flowed from east to west, exactly opposite from today. They may also have carried chert, a rock made of silica and formed only within deep ocean basins. Chert is not found in the mountains to the west, but is found offshore below current sea level. This suggests there may have been mountains east of the Crotone Basin during the Salinity Crisis.
So, how did the mountains form and where did they go? The water in the Mediterranean Sea pushes down on and depresses the crust, much as glaciers do on land. If water is removed (as it was during the Salinity Crisis), the crust rebounds. Therefore, uplift and local tectonics may have formed mountains of deep-sea rock east of Calabria. When the the Mediterranean Sea came flooding in, the mountains would have been obliterated.