A new study is paving the way to understanding the recovery of biological life after an ecological crisis in the Mediterranean Sea about 5.5 million years ago.

An international team, led by Konstantina Agiadi from the University of Vienna, has been able to quantify how marine life was impacted by the salinisation of the Mediterranean. Only 11% of endemic species survived the crisis, and biodiversity did not recover for at least another 1.7 million years.

Movement of Earth's mantle, throughout its history, have repeatedly led to the isolation of regional seas from the world ocean, along with massive accumulations of salt. Giants salt deposits of thousands of cubic kilometres have been found by geologists in Europe, Australia, Siberia, the Middle East, and elsewhere.

These salt accumulations present valuable natural resources and have been exploited from antiquity until the present day in mines around the world, such as the Hallstatt mine in Austria or the Khewra salt mine in Pakistan.

The Mediterranean salt giant is a one-kilometre-thick layer of salt beneath the Mediterranean Sea, which was first discovered in the early 1970s. It formed about 5.5 million years ago because of the disconnection of the Mediterranean from the Atlantic during the Messinian Salinity Crisis.

In a study published in the journal Science, an international team of researchers – comprising 29 scientists from 25 institutes across Europe - was able to quantify the loss of biodiversity in the Mediterranean Sea due to the Messinian Crisis and biotic recovery afterwards.

Marine impact

After several decades of painstaking research on fossils dated from 12 to 3.6 million years ago, found on land in the peri-Mediterranean countries and in deep-sea sediment cores, the team found that almost 67% of the marine species in the Mediterranean Sea after the crisis were different than those prior. Only 86 of 779 endemic species living exclusively in the Mediterranean before the crisis survived the enormous change in living conditions after the separation from the Atlantic.

The change in the configuration of the gateways, which led to the formation of the salt giant itself, resulted in abrupt salinity and temperature fluctuations, but also changed the migration pathways of marine organisms, the flow of larvae and plankton and disrupted central processes of the ecosystem. Due to these changes, a large proportion of Mediterranean inhabitants of that time, such as tropical reef-building corals, died out.

After natural reconnection to the Atlantic and invasion of new species like the great white shark and oceanic dolphins, Mediterranean marine biodiversity presented a novel pattern, with the number of species decreasing from west to east, as it does today.

Because peripheral seas like the Mediterranean are important biodiversity hotspots, it was very likely that the formation of salt giants throughout geologic history had a great impact, but that had not been quantified up to now.

"Our study now provides the first statistical analysis of such a major ecological crisis," explains Agiadi, from the department of geology.

Furthermore, it also quantifies for the first time the timescales of recovery after a marine environmental crisis, which is actually much longer than expected, she says.

"The biodiversity in terms of number of species only recovered after more than 1.7 million years."

The methods used in the study also provide a model connecting plate tectonics, the birth and death of oceans, salt and marine life, that could be applied to other regions of the world.

"The results open a bunch of new exciting questions," says Daniel García-Castellanos from Geosciences Barcelona at the Consejo Superior de Investigaciones Científicas (CSIC), who is senior author of the study. “How and where did 11% of the species survive the salinisation of the Mediterranean? How did previous, larger salt formations change the ecosystems and the Earth System?”

These questions will be explored in a new 'SaltAges' project, starting in October 2024, where researchers are invited to explore the social, biological and climatic impacts of salt ages. The research is a result of the Late Miocene Mediterranean Marine Ecosystem (ReMarE) Crisis project, which is funded by the Austrian Science Fund and was published in Science journal.