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Author(s) of the publication: Ivar MURDMAA, Natalia KELLER

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1912 saw the publication of a new science journal Priroda (Nature). Its founding father and first editor were: Prof. Vladimir Wagner - a prominent zoologist and the founder of comparative psychology in Russia, and Prof. Lev Pisarzhevsky, an outstanding chemist who later became an Acting Member of the Academy of Sciences. Right from the start the new journal attracted the prime the Russian science. Its subsequent history is inseparably associated with prominent personalities like the mineralogist Acad. Alexander Fersman, biologist and geneticist, Corresponding Member of the St. Petersburg Academy N. Koltsov, physicist and optician Acad. Otto Schmidt. And this list could be continued. Leading names of Russian science who wished to cooperate with the journal included Vladimir Vernadsky, Ivan Pavlov, Vladimir Bekhterev, Ivan Mechnikov, Nikolai Umov, Pyotr Lazarev, Vladimir Obruchev and Pyotr Semyonov-Tian-Shansky. Volumes of Priroda piled up over the past century could make up an anthology of Russian and Soviet popular-science classics. Today we present two articles published in one of the issues for 2004.

Pages. 95


by Ivar MURDMAA, Dr. Sc. (Geol. & Mineral.); Natalia KELLER, Cand. Sc. (Biol), RAS Institute of Oceanology named after P. Shirshov

The undersea threshold (ridge) of the Strait of Gibraltar located at the depth of 100 - 300 m, stretches over 18 km as an arc bent towards the Atlantic. It is located westwards of the narrowest passage separating the south-western extremity of the Pyrenean Peninsula and the north-western coast of Africa. There its depth exceeds 800 m.

When there appeared the ambitious idea of building a railway tunnel under the Gibraltar (the reason for our expedition being sent there), designers chose a longer route under the threshold, because otherwise it would be necessary to build the tunnel at the depth of one kilometer.

We looked at the site through a narrow porthole of our research submarine ARGUS. And all we saw were barren rocks stripped of loose sediments. We saw a kind of parallel walls several meters thick and up to 10 m high, covered withbenthic fauna. The

Pages. 96


flat hard bottom between them "glistens", being washed clean with currents. The walls are vertically located strata of sandstone, and the softer clayey deposits (argillites) between them being all eroded by water.

In the geological history of the Mediterranean there was a time when the sea almost dried out. That was at the very end of the Miocene (6.3 - 5.3 mn years ago) and was caused by the closure of a strait which was then located to the South from the present-day Gibraltar. The sea turned into a giant lake which was rapidly drying out in the conditions of a hot and dry climate. Water salinity was rising until it reached the point of saturation. A layer of salt up to 2,500 m thick accumulated on the bottom of the basin. It underlay like a foundation, the mass of younger and friable sediments piled up over the last several millions years.

Frankly speaking, it is hard to imagine in the place of the Mediterranean a cavity, or pit, of more than 2 km in depth separated from the Atlantic with a narrow isthmus. A geological structure of this kind could not have remained there for long. The breakthrough happened some 5.3 mn years ago, and it took so much water to fill this hole in the ground that the level of the World Ocean dropped appreciably.

The first task given by researches to members of the expedition was to try and investigate the flat plato in the middle of the Gibraltar threshold at the depth of some 200 m. Our British predecessors who conducted detailed surveys of the relief and soil along the route of the proposed tunnel "picked up" by pipes several bits of corals. This suggested the idea that located there was a submerged ancient coral reef - a massif of porous limestones. If that was so, the tunnel project would have been more difficult and much more expensive to accomplish. And it was up to us to either confirm or deny these suppositions.

Solving the problem was the responsibility of the head of our expedition, marine geologist Prof. Kazimeras Shimkus. When we were going down at the southern edge of the platoeau, he was observing nearly vertical strata of sandstone stretching under branching thickets of corals. And it

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was clear that there was no ancient coral reef there, and that the very young colonies of corals were proliferating upon a strong foundation of Cretaceous sandstones, crushed into folds.

The maximum velocity of the surface and also benthic currents in the middle of Gibraltar reaches 80 - 100 с m/s., and exceeds 200 cm/s in some places. Members of the ARGUS crew experienced on more than one occasion the violent "temper" of these currents and treacherous internal waves developing on the border of the lower Mediterranean and upper Atlantic waters. It was therefore quite natural to see an abundance of dead corals exactly in the middle part of the ridge, or threshold. They accumulate like bars and "snow-drifts" on lower rocky grounds and as for living organisms, they dwell on rock ledges and on top of the plateau.

Radiocarbon dating of rolled, or rounded, samples of the two most common local samples of Caryophyllia indicated an age of 1,700 +/- 70 and 2,040 +/- 90 years. That suggests that at the start of our chronology, when corals were developing on the brink of the middle part of the Gibraltar, the living conditions for them were more favorable than now.

The dieing out of deep-sea corals in the Mediterranean was caused by the harmful effect of high water temperature and by upsets of the vertical circulation of masses of water caused by climate warming. It has been established that the temperature of bentic strata of the Mediterranean was constantly rising after the maximum of the last glaciation (18 thous. years ago) and now reached +13°C. This must have been the cause of the extinction of cold-water coral species.

One can assume that during glaciations and at a low level of the World Ocean, the inflow of Mediterranean water into the Atlantic was weaker and this water itself was cooler and richer in oxygen than during the warm periods. This promoted the development of "cold-loving" fauna of deep-sea corals both at sea and on the Gibraltar threshold.

Our datings of Caryophillia corals prove that they existed at relatively cold periods which preceded the "warm" Middle Ages.

And as for reactivating the plans of building an undersea railway tunnel between Africa and Europe, according to a recent report in the media, Spain and Morocco are resuming talks on the project after an interval of almost a decade. An it could be that our modest contribution to the studies of the geological history of the Gibraltar threshold, made in 1994, will help to implement the 21st-century project.


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Ivar MURDMAA, Natalia KELLER, GLIMPSE AT THE MEDITERRANEAN // London: Libmonster (LIBMONSTER.COM). Updated: 27.09.2018. URL: https://libmonster.com/m/articles/view/GLIMPSE-AT-THE-MEDITERRANEAN (date of access: 06.12.2021).

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