Geologists: 10 Million Years Ago, Mlynská Dolina was Underwater

Bratislava, 20 December 2021: During the Tertiary Period Mlynská dolina in Bratislava was covered by a large lake. It was inhabited by bivalve molluscs, Ostracods and other animals typical for the brackish waters of sea coasts, with an inflow of fresh water from streams. Those are some of the findings of a paper written by geologists from Comenius University Bratislava and the Slovak Academy of Sciences. The paper was published in the open-access journal Rivista Italiana di Paleontologia e Stratigrafia. The team also included scientists from France and Austria.

21. 12. 2021 11.25 hod.
By: CU External Relations Office

The inland sea which the scientists named Lake Pannon had brackish water similar to the present-day Caspian Sea. It reached its maximum extent more than 10.6 million years ago, stretching from Vienna to Belgrade and Transylvania in Romania. In places it reached depths of one kilometre. Lake Pannon was later filled by Danube sediments and receded south towards Serbia, where it disappeared 4 million years ago.

"The presence of Neogene Period deposits had been known in Mlynská dolina, but the exact age or nature of the environment of these sediments has not been previously determined," says the lead author of the study, Michal Šujan from Comenius University. The staff of the Department of Geology and Palaeontology of Comenius University carried out excavation work on the slope of the university campus in Mlynská dolina.

The scientists manually excavated an outcrop and observed an alternation of layers of mud deposited below the surface of the Pannonian Lake, layers of sand deposited by waves, and layers of fine gravel particles deposited by streams flowing from the Malé Karpaty Mountains, which formed a system of islands. Wave action near the bottom of the lake created dunes which resemble sand dunes formed normally in deserts by winds.

"Between the strata we could clearly make out bioturbations - traces left by the animals inhabiting the Paleo-lake bottom in search of food and digging tunnels. We also found fossils of nanoplankton and Ostracods," continues Michal Šujan. "This fossil fauna indicates that the coastal areas were dynamic and fresh water was flowing in from streams. For animals, such conditions are not ideal but mildly stressful, resulting in a reduced variety of fossils," he continues.

The authors dated the layers using a relatively new dating method based on the concentration of the cosmogenic nuclide Beryllium, which is being applied and improved at the Department of Geology and Palaeontology of the Faculty of Natural Sciences of Comenius University. Using this method the scientists concluded that the layers were deposited on the bottom of the ancient sea between 10.9 and 10.6 million years ago. "This type of sediment is challenging to research, because the fossils we encounter often include older species whose shells were recycled by erosion, like those deposited in the Paratethys sea which formed the Sandberg sands," concludes Michal Šujan from Comenius University.

The knowledge that the slopes of the Malé Karpaty Mountains were once shores of Lake Pannon up to the level of today's university campus tells us that the waters which used to cover the present-day Záhorská nížina and Podunajská nížina basins were once connected.

"I am pleased that our geologists and palaeontologists chose to explore the area of Mlynská dolina with which we are all familiar. The results are extremely interesting from a historic stand-point, but at the same time they allow us to better understand the uplift of Malé Karpaty Mountains. This, in turn, leads to a better knowledge of the fault movements and the potential risks for Bratislava," said the Rector of Comenius University Marek Števček. "Understanding the changes that have taken place over tens of millions of years in this area will, among other things, help us to better understand the current climate changes," he added.

The research received support of the Slovak Research and Development Agency through grants APVV-16-0121 and APVV-20-0120.