An international research team has a hypothesis regarding the mystery of why the Mediterranean Sea dried up around 5.6 million years ago. The event, known as the Messinian Salinity Crisis (MSC), saw the Mediterranean become a 1.5 kilometer deep basin for around 270,000 years, and left a kilometers-deep layer of salt due to seawater evaporation.

The cause of the MSC has been the subject of speculation and debate, but now an international team of researchers
 say they have solved it. After the MSC was discovered in the 1970s, the early hypothesis was that movements of the African, Arabian and Eurasian tectonic plates had led to the Mediterranean becoming landlocked. Later on, other scientists suggested that it was a global fall in sea levels due to growing ice sheets that cut the sea off from the Atlantic Ocean.

To try and find answers, University of Otago
geologist Dr. Christian Ohneiser and colleagues examined 60 sedimentary drill cores from around the edge of Antarctica and then ran simulations to try and hone in on what happened. "We found that the Antarctic ice sheet had an uneven effect on the global sea level because its growth resulted in a complex interplay between gravitational and rotational effects and the deformations to the Earth's crust caused by ice advance and retreat." 

The simulations showed that as the Mediterranean Sea evaporated, the Earth's crust around the Strait of Gibraltar began rising up because the overlying load from the water was removed. 

"This kept the Mediterranean isolated from the Atlantic Ocean until the crust began to relax and sink. At the same time, Antarctica began to melt, raising sea levels again."

By around 5.33 million years ago, the rising sea level was just enough to wash over the thin land bridge at Gibraltar, resulting in a catastrophic flood that refilled the sea, he says.

Other researchers have previously shown that this deluge, known as the Zanclean flooding event, took only a few years to fully replenish the Mediterranean.

Dr Ohneiser says that one of the key implications of the study is that changes in global sea-level are uneven when ice sheets expand or retreat. "Future melting of the large Southern or Northern hemisphere ice masses will result in an uneven rise in sea-level around the world, and this should be factored into future climate change scenarios," he says. 

 Published in Nature Communications.