A recent study has advanced our understanding of how life rebounds following an ecological disaster in the Mediterranean Sea that occurred around 5.5 million years ago. An international team, spearheaded by Konstantina Agiadi from the University of Vienna, quantified the effects of the Mediterranean’s salinization on marine life: just 11 percent of the unique species managed to survive this event, and it took at least another 1.7 million years for biodiversity to rebound. This research has been published in the journal Science.
Throughout Earth’s history, movements in the lithosphere have periodically isolated regional seas from the global ocean, resulting in massive salt accumulations. Geologists have found extensive salt formations, or salt giants, in regions like Europe, Australia, Siberia, the Middle East, and beyond. These salt deposits have served as significant natural resources, having been mined from ancient times up to the present day, with locations such as the Hallstatt mine in Austria and the Khewra Salt Mine in Pakistan being notable examples.
Located beneath the Mediterranean Sea is the Mediterranean salt giant, a layer of salt that is over a kilometer thick and was first identified in the early 1970s. This formation occurred around 5.5 million years ago, resulting from the disconnection from the Atlantic Ocean during the Messinian Salinity Crisis. In a study published in Science, an international research team consisting of 29 scientists from 25 institutions across Europe, led by Konstantina Agiadi from the University of Vienna, has quantified the loss of marine biodiversity in the Mediterranean due to this crisis and examined the subsequent recovery.
Significant effects on marine biodiversity
After decades of meticulous examination of fossils dating from 12 to 3.6 million years, discovered in areas surrounding the Mediterranean and in deep-sea sediment cores, the research team found a striking shift: nearly 67% of marine species in the Mediterranean Sea following the crisis were different from those present before it. Only 86 out of 779 endemic species (those that exclusively inhabited the Mediterranean before the event) survived the dramatic changes in their environment caused by the separation from the Atlantic. This alteration in the configuration of sea routes led to rapid changes in salinity and temperature, as well as disturbed migration paths for marine organisms, larval flow, and plankton, severely affecting key ecosystem processes. As a result, a considerable number of Mediterranean species from that time, including tropical reef-building corals, went extinct. After reconnecting with the Atlantic and the introduction of new species like the Great White shark and oceanic dolphins, the Mediterranean’s marine biodiversity took on a new distribution pattern, with species richness decreasing from west to east, a trend that remains today.
Longer recovery times than anticipated
Since peripheral seas such as the Mediterranean function as crucial biodiversity hotspots, it was anticipated that the formation of salt giants through geological history would greatly influence marine life, although this had not been quantified until now. “Our study presents the first statistical analysis of this significant ecological disaster,” remarks Konstantina Agiadi from the Department of Geology. Additionally, it establishes for the first time the timeline for recovery following a marine environmental disaster, which turned out to be much longer than expected: “Biodiversity, in terms of species count, did not recover for over 1.7 million years,” explains the geoscientist. The study’s methodology also offers a framework linking plate tectonics, the formation and demise of oceans, salt, and marine life that could be applied to other global regions.
“These findings raise numerous intriguing questions,” says Daniel GarcÃa-Castellanos from Geosciences Barcelona (CSIC), the senior author of this study: “How and where did the 11% of species endure the salinization of the Mediterranean? In what ways did earlier, more extensive salt deposits affect ecosystems and the Earth’s system?” These questions await further investigation, including within the upcoming Cost Action Network “SaltAges,” where researchers will explore the social, biological, and climatic impacts of historical salt periods starting in October.
