Around 5.5 million years ago, tectonic changes interrupted the Mediterranean’s connection to the Atlantic. As a result, the Mediterranean became increasingly salty due to evaporation and at times almost completely dried out. A study has now examined how this so-called Messinian salt crisis affected the species native to the Mediterranean. According to the study, most species died out and were replaced by new ones after the Mediterranean basin was flooded again. However, according to the researchers, it took at least 1.7 million years for biodiversity to recover.
To this day, a kilometer-thick layer of salt beneath the Mediterranean bears witness to a catastrophe that occurred there between 5.97 and 5.33 million years ago: the Messinian Salt Crisis. During this time, the Mediterranean lost its connection to the Atlantic. The supply of fresh water from rainfall and rivers was not enough to compensate for the evaporation. Similar to the Dead Sea today, the Mediterranean became increasingly salty and at times dried out almost completely. When renewed tectonic changes reopened the connection to the Atlantic through the Strait of Gibraltar 5.33 million years ago, the dry salt basin was flooded again within a very short time and developed into the Mediterranean as we know it today.
Great extinction
A team led by Konstantina Agiadi from the University of Vienna has now investigated how these events affected biodiversity in the Mediterranean. The researchers analyzed more than 22,000 fossils dating from 12 to 3.6 million years ago that were found on land in the Mediterranean countries and in sediment cores from the sea floor. These included the fossil remains of tiny plankton, mollusks and corals, as well as skeletons of large marine animals, including sharks and dolphins.
“Our study provides the first statistical analysis of this major ecological crisis,” says Agiadi. In 2006, the team identified various species in the Mediterranean before the Messinian salt crisis, of which they classified almost 800 as endemic, i.e. species that only occurred in the Mediterranean. In the fossils from the period after the crisis, Agiadi and her team found only 86 of the endemic species. “At least 693 of the presumed endemic species, however, disappeared,” reports the team. “It can be assumed that some of the remaining species found refuge in the neighboring Atlantic during the height of the crisis. Fossil evidence of this, however, is rare.”
Slow recovery
After the Mediterranean was flooded again, species gradually returned. Some that had previously lived in the region and survived in the Atlantic returned to their original home. Many others, including great white sharks and ocean dolphins, were new arrivals. Overall, around two-thirds of the species that colonized the Mediterranean after the crisis were different from those that had lived there before.
According to Agiadi and her team, it took at least 1.7 million years for biodiversity to return to a level similar to that before the Messinian Salt Crisis. This revealed a changed pattern of biodiversity: while before the Mediterranean became salinized and dried up, the greatest biodiversity was found in the eastern part, after the crisis, biodiversity decreased from west to east – a pattern that is still evident today.
“The results raise a number of new, exciting questions,” says co-author Daniel García-Castellanos from Geosciences Barcelona in Spain. “How and where did some of the endemic species survive the salinization of the Mediterranean? And how did other, similar events change the respective ecosystems?” The researchers want to address these questions in future studies.
Source: Konstantina Agiadi (University of Vienna, Austria) et al., Science, doi: 10.1126/science.adp3703