The Messinian Salinity Crisis
The event that changed our oceans
The Mediterranean Sea and its surrounds are a fascinating part of the world. Not only for their place in human civilization and its development but also for the continued use as a transport route, holiday destination, seafood resource and its economic contribution to those countries that share its shoreline. This 2.5 million square kilometer body of water is bounded by 21 countries and home to over 140 islands. With an average depth of 1500 meters, the volume of water contained in the Mediterranean is 3.75 million square kilometers. It is thought to be remnants of what was the Tethys Sea, a region made up between Gondwana and Laurasia between 250–65 million years ago.
The Mediterranean is classified as a concentration basin in that the level of evaporation is balanced via inflows from the Atlantic Ocean. However, the high rate of evaporation results in the waters of the Mediterranean being saltier than that of the ocean water that flows in. This results in the outflow of saltier water being heavier and cooler flowing below the warmer less salty Atlantic inflow. The Mediterranean volume is supplemented by large volumes of river runoffs of approximately 400–450 cubic kilometers/year. To highlight the extent of the evaporation on the waters of the Mediterranean, it is estimated that if the Straits of Gibraltar were to be closed off, the Mediterranean would dry up in around 1000 year. This is based on an evaporation rate of 188 cm/year and still allows for inflow from rain and rivers. This may seem like an astonishing feat but there is evidence that the Mediterranean did dry up quite recently, in geological terms.
The Messinian salinity crisis is estimated to have occurred around 5.96 million years ago when the Straits of Gibraltar closed and effectively shut off the inflow needed to keep the Mediterranean at a stable level. There is some conjecture as to the precise cause why the Straits closed but is agreed that a combination of geological, tectonic plate shifting, and climate both contributed. Even today, the Straits of Gibraltar are only 14.3 kilometers wide and vary from 300–900 meters in depth.
This closure resulted in a period of drying that lasted for 600,000 years. After an initial drying out period that lasted for over a thousand years, the Mediterranean was left as a dry basin that was up to 5 km deep in some places for a further 400,000 years until climate changes increased water flowing in to the basin from rivers.
Although there were theories around at the time following studies of gypsum deposits around the Mediterranean, it was confirmed during the 1970’s when a survey confirmed layers of salt below the Mediterranean that were up to 3 km’s thick. The fact that this layer was present throughout the region indicated that the sea had dried and that it was not only isolated to small pockets. There is no suggestion that it dried out completely and pockets of extremely saline water may have still existed during the period. However, the basin is expected to have been an incredibly hot and inhospitable place at times as has been evident in the findings of fossils that only exist in relatively hot water.
The effect of the Mediterranean drying had a global impact as well as a regional one. Fauna were able to migrate north from the African continent and populate areas within and surrounding the basin. There are fossils on Crete of Hippopotamuses than were left stranded on the island as water levels rose. As they evolved following the refilling of the Mediterranean, they underwent island dwarfism as they adapted to the ecosystem of the island.
The rivers that flowed in to the Mediterranean would have also cut deep beds as the water level dropped with the effect of water erosion over the period. The river Nile bed is estimated to have been 2400 meters deeper at the time and has left a buried canyon that exists under Cairo.
Globally, the water for the Mediterranean would have been redistributed to the worlds oceans and resulted in sea levels rising by 10 meters. The amount of salt that remains locked up below the Mediterranean meant that once the Straits of Gibraltar reopened, the salt level of the worlds oceans and seas was reduced forever.
The Messinian salinity crisis is a fascinating period in the earth’s geological history and one of the most spectacular over the last 20 million years. Its effect on the region and the globe were widespread and still exist. It is a body of water that we are all so familiar with and although it may be under growing pressure from human population and climate changes, it demonstrates the resilience of the planet through these major geological shifts.
