Dr. Fűköh Levente szerk.: Malakológiai Tájékoztató 27. (Eger, 2009.)

SÁNDOR, R.-SÜMEGI,P.: New preliminary data for evolution of the Holocene Hungarian Mollusc fauna

Three major lithostratigraphic units could have been identified on the undisturbed, continuous core of the 280 cm deep borehole. The first unit located between the depths of 280 and 200 cm is made up of alternating layers of fine laminated unweathering sandy sediment. These assumptions are justified by findings of radiocarbon measurements. According to radiocarbon data the development of these silty intercalations must have taken place between 13.000-9.000 BP at the end of the Pleistocene and at the transition zone of lateglacial/postglacial age. The next sedimentary unit is located between the depths of 200 and 160 cm. This horizon is composed of dark brown muds with a low carbonate and high organic content. This sequence must have been deposited in a relatively shallow, lacustrine system with eutrophic conditions. This lacustrine phase characterized by the deposition of organic rich mud must have existed between 9000 and 7.000 BP in the area. From the depth of 160 cm there is a sudden decrease in the carbonate content accompanied by a prominent increase in the organic content. The distribution of organic matter is not homogenous and by no means dispersed but resembles a downward flame like structure made up of the remnants of reed (Phragmites), and Mg, Na rich peat sediment accumulated between 160 and 100 cm. According to the radiocarbon dating the sedimentation of peat started from the beginning of the Atlantic phase. From the depth of 100 cm there was a gradually change in the peat composition. The reed remains decline gradually and the dominance of the sedge remains started increasing. The radiocarbon data suggest that this change in the peat composition developed from 3000-3500 BP years ago. Malacological results According to the Mollusc fauna four malacological zones can be divided on this sequence from the Late Glacial until Late Holocene. The pollen, macrobotanical and geochemical data suggest that the malacological changes followed the paleohydrological and vegetation changes on this filling river bed. The first malacological zone is located between the depths of 280-200 cm. According to radiocarbon data this horizon must have developed between 13.000 and 9300 BP. The ratio of species with a preference for moving water habitats is above 60 % in this horizon (Valvata piscinalis, Lithoglyphus naticoides, Lymnaea stagnalis, Planorbis cf. carinatus, Unio cf. crassus, Pisidium amnicum). This was the so called Valvata piscinalis-Lithoglyphus naticoides paleoassociation include cold resistant (widespread during the Pleistocene in the Carpathian Basin) (eg.: Valvata pulchella, Bithynia leachi) and thermophilous (widespread during the Holocene in the Carpathian Basin) (e.g.: Lithoglyphus naticoides, Bithynia tentaculata) elements as well. The composition of this fauna as well as the lithology of the embedding sediments is fully identical with the Lithoglyphus naticoides — Valvata piscinalis biozone of Fűköh (1991, 1992, 1997). Macrobotanical remains suggest that a living water environment with Equisetetum fluitantis and Nymphaeetum albo-luteae paleocommunities developed in the ancient Danube channel during this transition zone of lateglacial/postglacial periods. The paleochannel could have been a spill-stream of River Danube during this period.