Late Neogene red clay in the Carpathian basin and its paleoclimatological implications

The red clays in Hungary (Tengelic Red Clay Formation: TRCF; Kerecsend Red Clay Formation: KRCF) is overlain by loess paleosol sequences. The thickness of the red clay (in general) ranges from 4 to 90 m. The red clay sediments in the Carpathian basin are known from both exposures and boreholes. The age of these formations is ~3.5–1.0 Ma. Elemental oxide analyses of red clays were determined by x-ray florescence (XRF), and x-ray powder diffraction (XRD) was used for mineral identification. The degree of chemical weathering in soils by hydrolysis increases with available precipitation and temperature. Both water and warmth serve to accelerate the depletion of alkali and alkaline earth elements (Ca, Mg, Na, K) at the expense of refractory elements such as aluminum (Al). These relationships within soils have been used to derive transfer functions for estimating paleoclimatic variables from paleosols of comparable parent material composition and degree of development. This study extends these techniques to paleosols formed during the Late Pliocene–Early Pleistocene in the Carpathian basin. The chemical index of alteration without potassium (CIA = 100×mAl₂O₃/(mAl₂O₃+mCaO+Na₂O) in mol) increases with mean annual precipitation in modern soils. Paleotemperature of paleosols can be derived from alkali content (C = (mK₂O+mNa₂O)/mAl₂O₃, in mol) which decreases in modern soils with mean annual temperature. The equation for mean annual temperature is: MAT (°C) = 46.94C + 3.99 (R² = 0.72, standard error ±182 mm); for mean annual precipitation is: MAP (mm) = 14.265(CIAK)-37.632 (R² = 0.96, standard error ±0.6°C). The older type (Beremend Member, age ~3.5–2.0 Ma) of the TRCF is red kaolinitic clay containing typically disordered kaolinite, mixedlayer smectite/kaolinite, smectite and little gibbsite. It was formed in the local subaerial weathering crust in warm, humid, subtropical or monsoon climate (MAT: 13–15°C; MAP: 1200–1400 mm). The younger member (age ~2.0–1.0 Ma) of the TRCF contains red (or “reddish”) clay beds. It contains relatively fresh material (illite, chlorite), the weathering products are predominantly smectite and goethite formed under warm and drier climate in environmental conditions of savannah and steppe or forest steppe (MAT: 10–13°C; MAP: 1100–1300 mm). The basal red clay layers of the Paks Loess Fm. and KRCF (age ~1.0–0.5 Ma) contain similar material as the underlying red clays belonging to the younger member of the TRCF. The slightly but significantly lesser degree of weathering (more illite and chlorite, less smectite) indicates cooling of the climate (MAT: 8–10°C; MAP: 900–1000 mm). It appears from the sedimentological data that the main part of the red clay is of wind-blown origins, other is weathering crust of the underlying material. The Neogene red clay accumulated under persistent weak winds and a rather steady warm–arid climate. This material later was modified by post-depositional weathering under warm–humid climate.
   Acknowledgements: This contribution was made possible through financial support for J. K. by the Hungarian Academy of Sciences, Bolyai János Research Grant.