f Paleoclimate investigation based on magnetic and electric properties of sediments in the basin of Lake Ferto (Neusiedlersee)

Lake Ferto (Neusiedlersee) (309 km2) is located on the Austro-Hungarian border south (20 km) of the Danube. It is a shallow (average water depth 50 - 60 cm) water body with large water level fluctuations over the historical period. More than one-third of its surface is now overgrown with uliginal plants. The age of the lake is estimated to be nearly 20 kyr and its origin is related to the simultaneous formation of Hanság and Ferto basins. Large depositions of gravel and sands by Danube and Rába rivers at the end of Pleistocene formed a barrier separating the two basins. During high water periods the two basins were frequently connected. The lake is supplied by local rainfall (78%) and the Vulka and Rákos rivers (20%); groundwater feed represents 2% (Boroviczényi et al 1992). In its natural state lake Ferto has no outlet, its water level is regulated by the discharge through the Hanság channel. Lake Ferto is a part of the Ferto-Hansag (Neusiedler See - Seewinkel) National Park. After some former geophysical experiments (e.g. Fritsch, 1961, Ádám et al 1964, then Kohlbeck et al 1993, 1994) the present study of the area of lake Ferto (Neusiedlersee) is being carried out since 1997 with an aim to understand the past of its basin and environment during the post-glacial period, its actual state and possibly future evolution. Physical properties of sediments measured from the surface (resistivity by geoelectric end electromagnetic methods) and directly from cores are investigated. Preliminary study was realised in 1997 in 6 sites using short (up to 3.5 m) hand-drilled cores and vertical electric soundings. It has shown by comparison of high precision electrical sounding (20 data/decade, millimetre range geometry of electrode positioning for AB distances shorter than 6.4 m; a combination of potential electrodes at the surface and at a depth of 40 cm) with direct measurements of physical parameters (resistivity, water content and low field magnetic susceptibility) on the cores that although very thin sedimentary layers and local inhomogenities cannot be identified by surface measurements, the geoelectric inversion methods allowing smooth layer transitions and stochastic Bayesian method provide a realistic model of the subsurface structure while classical few-layer inversion techniques fail (Kohlbeck et al 1999). Variation of some environmental parameters in sediments were investigated using changes of magnetic minerals which reflect variation in salinity and organic material contents in waters and sediments of the lake and therefore give information about the evolution of hydrogeological system of the lake basin (Jelinowska et al 1999).