Imprints of Chemical Weathering in a Middle Pleistocene Loess-Paleosol Sequence at Beremend (South-Hungary)

Újvári, G.¹, Varga, A.², Raucsik, B.³, Varga, G.⁴, Kovács, J.¹⁴

¹Geodetic and Geophysical Research Institute of the Hungarian Academy of Sciences, Csatkai, E. u. 6-8, H-9400 Sopron, Hungary

²Department of Petrology and Geochemistry, Eötvös Loránd University, Pázmány Péter, sétány 1/c., H-1117 Budapest, Hungary

³Department of Earth and Environmental Sciences, University of Pannonia, Egyetem utca 10, H-8200 Veszprém, Hungary

⁴Department of Geology, University of Pécs, Ifjúság u. 6, H-7624 Pécs, Hungary

Chemical weathering is primarily controlled by climate through the amount of precipitation and the temperature, and indirectly through the kinds of vegetation that can cover the land. As a result of Quaternary climate fluctuations alternating cold, dry and warm, humid environmental conditions appeared in the Carpathian Basin leading to the formation of loesspaleosol sequences which hereby preserved the history and rate of chemical weathering. In our study we made an attempt to track the effects and imprints of chemical weathering in a loess record using grain size, mineralogical and geochemical data.

The loess section at Beremend represents the Old Loess Series (OLS) and partially the Young Loess Series (YLS) exposing three reddish brown forest soils (Be- S1, Be-S2, Be-S3) and a brown forest or forest steppe soil (Be-S4). Correlation of the lower part of the section is based on an age constraining biostratigraphical datum, the appearance of species Neostyriaca corynodes (Held, 1836) which lived between about 600 and 120 ka in the Carpathian Basin, and it could not be found in older (i.e. Early Pleistocene) or younger (i.e. Late Pleistocene) deposits up to now. This species appeared in a highly weathered loess layer at the bottom of Beremend section (10.80 m) with a mild climate indicator fauna and in which did not appear any cryophilous species. According to previous investigations, Neostyriaca corynodes occurs in “loess fauna” which indicates cold climate only in loess deposits formed during the Riss glaciation, as long as its accompanying fauna indicates milder climate in older loess horizons formed during the Günz-Mindel glaciations. Assuming an older Middle Pleistocene age (Günz) for the basal loess layer the lower two paleosols (Be-S1, Be-S2) can be correlated with the PD1 (Paksi Double 1) and Phe (sandy humous soil of Paks) soils (MIS 13-15 and 17). It is worth mentioning that there is a significant unconformity on the top of the lowermost paleosol (Be-S1) showing itself as truncation of the soil. Another unconformity can be found on the top of the third fossil soil (Be-S3) which is supposed to be correlated with the MB (Mendei Base) paleosol (MIS 11). The uppermost fossil soil which is a less developed, brown forest or forest steppe soil might be correlated with the BD (Basaharci Double) or MF2 (Mendei Upper 2) soils (MIS 5-7).

Having a basically unimodal grain size distribution (Me: 18-21 micron; Mo: 26-29 micron; average grain size: 33-42 micron) loess deposits in the Beremend profile are distinctly, but generally less affected by chemical weathering than paleosols being characterized by bimodal distributions (Me: 15-16 micron; Mo: 23- 24 micron; average grain size: 26-30 micron) with a local secondary maximum around 2 micron. The reasons for the accretion of the fine fraction are unequivocally pedogenetic processes which significantly altered the parent material. The average bulk mineral composition of sediments estimated from XRD data shows that quartz and smectite are the dominant minerals, and the loess samples contain high amounts of calcite and illitic material as well. Chlorite, kaolinite, albite, K-feldspar, dolomite, and goethite are the typical minor components with amorphous material. In the clay fraction (<2 μm) of the sediments, varying amounts of smectite, illitic material, chlorite, and kaolinite are present. The paleosol samples can be characterized by a smectite-dominance relatively to the loess samples which contain higher amounts of illite±muscovite. Changes in kaolinite/illite ratio deduced from estimated bulk composition show significant differences between the paleosols and loesses suggesting fluctuations in the intensity of the coeval continental hydrolisis.

The absolute concentrations of the major elements were affected by carbonate dilution effect as indicated by the strong negative correlation between CaO and SiO2 (and Al2O3) in the Beremend loess-paleosol section. Loss on ignition (LOI) showed a strong positive correlation with total CaO, suggesting that it was predominantly associated with carbonate minerals. In the loess samples, the concentrations of the large ion lithophile elements (LILE) were variable, and K2O showed a strong positive correlation with Rb and Ba (r=0.97 and r=0.97, respectively), suggesting that their abundances were exclusively governed by the various amounts of K-bearing minerals (e.g., K-feldspar and illitic material). The high field strength elements (HFSE: Th, Zr, Nb, Y) showed positive correlations with TiO2, an expected trend due to their incompatible behavior. Concentrations of the transition metals (TM: V, Cr, Ni, Cu, Zn) and Ga were also variable, and they were positively correlated with Al2O3 (and TiO2, FeO). Compositional variations among the paleosol samples, except for CaO content, were low. The LILE concentrations showed relatively narrow ranges. K2O showed a moderate positive correlation with Rb (r=0.78), and CaO was positively correlated with Sr (r=0.85). The HFSE (except for Y) showed moderate to strong positive correlations with TiO2. Regarding the TM, Cr and Ni were positively correlated with Al2O3 (and TiO2, FeO). Chemical Index of Alteration (CIA) values for the OLS loess and paleosol samples range from 71 to 77 and from 74 to 78, respectively. CIA values for the YLS loess and paleosol samples range from 67 to 70 and from 70 to 73, respectively. Based on these results, a climatic trend could be suggested from warmer and relatively wet climatic conditions with moderate to intense chemical weathering during the deposition of the OLS to cooler and dryer conditions with weak to moderate weathering during the deposition of the YLS.

Corresponding author: Újvári, G. | FALI MAIL !!!