Author: Federico Andreeto – Utrecht University (Neetherlands)
Can a sea disappear? No one would answer; that a sea could disappear would seem science fiction. On the other hand, if we could travel back tens or hundreds of millions of years, moving towards the most remote ages, we would be amazed to find a planet so different from the current one. Seas and entire oceans formed and disappeared during the geological eras mainly due to the slow, inexorable, and imperceptible to the human senses, movement of continental masses driven by the powerful internal forces of the planet generated by the convective motions of the earth’s mantle. Going forward for millennia and millions of years, these geological processes led to the shrinking and occlusion of seas or ocean basins and culminated with the emergence of new lands and the raising of mountain ranges, such as the Apennines, the Alps, the Urals, the Himalayas, the Rocky Mountains and the Andes. Appearance and disappearance of seas can also occur in geologically short times. During the Pleistocene glaciations, for example, the conditions of cold and arid climate on a global scale led to the formation of glaciers and ice caps at the Poles and on the highest reliefs of the continents with the subtraction of large amount of water from the atmosphere and hydrosphere. The widespread formation of ice caused lowering of tens and hundreds of meters of marine waters, generating the emergence of entire territories until then submerged.
The last example of the disappearance of an ocean due to tectonic movements on a global scale concerns the Alpine Tethys, an ocean branch placed in an East-West direction that in the Permo-Triassic (250 Ma ca.) separated the two main continental masses (Laurasia in the north and Gondwana in the south) that constituted the supercontinent Pangea. The removal of the two parts of the Pangea continued until the Jurassic, when the movements of the tectonic plates were reversed and a contraction of the Tethys Ocean began. The collision of the Adriatic plate with the European continent closed the Tethys in the central region of the Mediterranean, giving rise to the Alps. The Cenozoic Alpine orogeny caused the partition of Tethys into the Mediterranean and Paratethys. While during the Late Neogene the Mediterranean attained its final configuration, eastwards, the Paratethys, underwent complex changing in landsea distribution. Isolated from the World Ocean since the Serravallian, it disintegrated progressively into a series of smaller internal seas separated by shallow sills and characterized by restricted connectivity and poorly oxygenated environments.
These (sub) basin were grouped in three geographic clusters: Western, Central and Eastern Paratethys, surrounded by fresh to brackish water system. Recurrent fragmentation of Paratethys and repeated isolation of individual sub-basins led to the flourishing of an endemic brackish to fresh water molluscan faunas association. These oligohaline inhabitants of the vestigial seas of the Paratethys led to consider that the different areas inhabited by these faunas were no longer seas, but constituted a “sea-lake” (in Russian), which was translated “Lac-Mer” and “Lago Mare” in the French and Italian scientific literature, respectively.
But what does the Paratethys have to do with the catastrophic events that have marked the Mediterranean Sea during the Messinian stage and that are the object of study by the Salt Giant project?
During the latest Miocene (5.97–5.33 Myr ago), the Mediterranean became a very restricted basin as a result of the progressive deterioration of connections with the Atlantic in the Betic and Gibraltar area and developed hypersaline waters from where evaporites ultimately precipitated during the Messinian Salinity Crisis (MSC).
During the last decade, evidence is accumulating that evaporites from the first two MSC steps (5.97-5.55 Ma) require regular sea-level and at least one continuously open Atlantic connection to provide the salts for repeated events of gypsum and halite deposition. Furthermore, gypsum formation, always linked with an increase in water salinity, was linked to a potential freshwater origin, possibly provided by the Eastern Paratethyan basins, which connection with the Mediterranean Sea has been proved to be present at least throughout the first phase of the MSC (5.97-5.6 Ma). But the greatest influence of the Paratethyan low-salinity waters in the Mediterranean circulation occurred possibly in the last 90 kyr of the salinity crisis, just before returning to normal marine conditions. All over the Mediterranean region, sandwiched between the evaporitic facies and the marine Zanclean sediments, local evaporites and terrigenous sediments containing fresh to brackish water fauna and flora occurred. These low salinity assemblages which increase in abundance and diversity with time and spread progressively westward, resemble those found in the brackish-water Paratethyan lake system. This suggests prolonged and increasing connectivity between the Mediterranean and Paratethys, although the location of the Mediterranean’s Paratethyan gateway at this time is just as enigmatic as its Atlantic counterpart. Creating a marine connection between this sea (i.e. the Paratethys) draining large parts of central Europe and Central Asia and a highly restricted Mediterranean Sea, would have affected the hydrological budget and potentially disturbed the circulation pattern in the Mediterranean Sea. Therefore, it is important to know the extent of the Paratethys Sea through time to assess at which stage of the MSC rivers currently draining into the Caspian Sea (e.g. Volga, Ural and Amu Darya) contributed to the Mediterranean hydrological budget.
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