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Lake Balaton is the largest shallow-water lake in Central Europe. The main objective of the research on its Quaternary lacustrine sediments was to reconstruct the evolution of Lake Balaton from its formation until today. One of the key parameters in answering this question is to retrace lake-level changes. There is a lot of evidence, such as sedimentological, mineralogical, geochemical, archaeological, etc. to reveal these changes. Each type is of different reliability depending upon the sensitivity of the lake to level change and to preservation of the traces of those changes as a result of geologic, climatologic or human factors. The average thickness of Quaternary sediments accumulated in Lake Balaton is 5 m. From the lake bottom toward the surface lacustrine sediments are constituted by clastic deposits, peat and calcareous mud with upward-increasing carbonate content (calcite, dolomite, Mg-calcite, protodolomite and aragonite). Towards the end of Pleistocene, approximately 15,000-17,000 years BP, several shallow ponds with clean and cold water formed in the site of the present Lake Balaton. Inundation followed progressively from west to east. A warming climate and increasing precipitation brought about the rise of the water level. Moreover, abrasion progressively destroyed the dams separating the ponds and a uniform lake was formed. Later, as a function of changing climate the water level of the lake varied approximately between 103.0 and 108.0 m aASl. (above Adriatic Sea level). The average rate of total lacustrine sediment accumulation is 0.38 mm/year.

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The Sárrét marshland is situated along the northeastern foothills of the Bakony Mountains, along a NE-SW-trending neotectonic zone. Investigation of the marshland and the reconstruction of its evolution are especially interesting to compare to the present and the expected future stage of Lakes Balaton and Velence. Based on the sedimentological characteristics of the core sequence and the geochemical, physical, and mineralogical composition of the layers, three sedimentological cycles could be separated. The three cycles represent the most important evolutionary phases of lake formation: flourishing and progressive aging. First, a nutrient-poor, open-water lake existed, where clastic sediments (sand and silty clay) were deposited. During the second phase, phytoplankton-produced autogenic lime mud was deposited in the progressively more eutrophic water. Peat accumulation during the third stage indicates the marsh phase of the lake.The results suggest that the studied sequence developed from Late Glacial to Middle Holocene. According to radiocarbon dating the accumulation rate in Sárrét (Sümegi, this volume) corresponds to the sedimentation rates in the Tapolca Basin and Lake Balaton.

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Lake Balaton is a large and shallow lake that is of great economic and cultural importance in landlocked Hungary. Even though the lake has been studied extensively in the last century from a large number of scientific aspects, the mineralogy of its sediments has not been fully explored. The mud at the bottom of the lake consists mostly of silt-sized grains of carbonate minerals with compositions between those of calcite (CaCO3) and dolomite CaMg (CO3)2. In order to understand the processes of carbonate precipitation and the influence of water budget fluctuations on the mineralogical character of the sediment, we used X-ray powder diffraction to analyze the changes of cell parameters of carbonate minerals in the upper half meter of the sediment. The major carbonate phase is Mg-calcite that shows a distinct reduction in cell parameters from west to east, reflecting an increase of its Mg-content, in parallel with a gradient of dissolved Mg/Ca ratio in the water. Intriguingly, dolomite, the other widespread carbonate phase in the sediment, also shows a change in cell parameters from west to east, with the deviations from values of stoichiometric dolomite being largest in the Eastern Basin of the lake. The similar pattern of cell parameter changes of Mg-calcite and dolomite suggests that ordered dolomite with slightly anomalous, Ca-rich composition also forms in the lake, probably by direct precipitation from the water. In contrast, protodolomite forms within the sediment through diagenetic processes. Based on our X-ray powder diffraction measurements, we propose a model of carbonate mineral formation and transformation in Lake Balaton. Since the Mg/Ca ratio of the water appears to be the major factor in controlling the compositions of carbonate minerals, and this ratio in turn is governed by the amount of water supply, the properties of the precipitating carbonate minerals are affected by the actual level of the lake water.

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Central European Geology
Attila Kovács
Ágnes Rotár Szalkai
Zsolt Kercsmár
, and
Tibor Cserny

A coupled groundwater flow and heat transport model was developed for a trans-boundary geothermal reservoir located in the Alpokalja area. The study area lies in the western part of the Pannonian Basin, at the border between Hungary and Austria. The study area contains several famous geothermal water utilizations on both sides of the border, which has an impact on natural groundwater conditions. The aim of the modeling study was to evaluate the natural-state and production-state groundwater conditions, and to make predictions on cross-boundary interferences. A three-dimensional finite element-type coupled geothermal model was constructed to provide a coherent quantitative representation of geothermal flow systems. The model described the hydraulic behavior of the flow system, the interaction between different reservoirs, and geothermal conditions.

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