Tectonic versus climatic control on the evolution of a loess–paleosol sequence at Beremend, Hungary: an integrated approach based on paleoecological, clay mineralogical, and geochemical data
Introduction
Quaternary loess–paleosol records preserve the long history of global climatic changes and regional climatic fluctuations (e.g., Kukla et al., 1988, Hovan et al., 1989, Ding et al., 1993). Many investigations have been carried out on a number of loess sections worldwide, employing a diverse range of analytical tools, all aimed at deciphering and reconstructing the climatic changes through glacial and interglacial times (e.g., Gallet et al., 1996, Jahn et al., 2001, Yang et al., 2006, Jeong et al., 2008, Bokhorst et al., 2009, Buggle et al., 2009). In contrast to the relatively unweathered loess layers, paleosols exhibit darker colors, finer grain sizes and geochemical signatures indicative of weathering and pedogenesis. Many previous studies were focused on the development of useful climatic proxies, including traditional grain-size distribution for changes in wind intensity and source aridity (e.g., Sun et al., 2002, Nugteren et al., 2004, Prins et al., 2007, Machalett et al., 2008), magnetic susceptibility (MS) as a function of precipitation and temperature (e.g.,Maher et al., 1994, Maher and Thompson, 1995, Maher et al., 2002, Geiss et al., 2008, Buggle et al., 2009), and pedo-chemical elemental ratios and chemical indices as sensitive recorders of variations in weathering intensity (e.g., Gallet et al., 1996, Chen et al., 1999, Guo et al., 2000, Yang et al., 2006, Bokhorst et al., 2009, Sheldon and Tabor, 2009, Buggle et al., 2011).
Despite a long history of research, paleoenvironmental studies generally regarded varying degrees of chemical weathering solely as a function of climate. However, relative rates of physical erosion and chemical weathering are determined both by tectonism and climate (Nesbitt et al., 1997, Liu et al., 2005). Loess deposits are mostly associated with young orogenic belts characterized by intensive physical erosion during the Pleistocene. Similarly, silty material of loesses in the Pannonian Basin may have originated from the Alpine–Carpathian mountain belt, where erosion rates dramatically increased from 2.7 Ma onwards (Hinderer, 2001, Kuhlemann, 2007, Willett, 2010). Previous studies showed that chemical weathering and physical erosion may be coupled, because the physical breakdown of rock controls the supply of fresh minerals with high reactivity surfaces to soils, thereby tectonic uplift exert a strong influence on chemical weathering (Riebe et al., 2001, Riebe et al., 2004, West et al., 2005). If chemical weathering is also affected by tectonism and not just by climate, this additional factor should be taken into consideration in CIA (Chemical Index of Alteration; Nesbitt and Young, 1982) based paleoreconstructions on time scales of many hundred thousands or million years. In a recent study, significant autocorrelations of weathering indices in loess profiles were observed (Xiao et al., 2010), suggesting that existing understanding of loess deposition in terms of climate conditions requires re-examination. Furthermore, Kemp (2001) demonstrated that loess–paleosol sequences cannot be treated as quasi-continuous time series owing to hiatuses, syn- and post-depositional pedogenic modifications causing complications in soil–climate relationships.
In this study, macrofauna (molluscs), bulk rock and clay mineralogy, and major and trace element geochemistry have been investigated from Quaternary loess and paleosol samples at Beremend, SW Hungary (Fig. 1) with the following main objectives: (a) to describe and interpret a multi-proxy dataset in terms of paleoclimate and tectonism, and (b) to highlight the complexity of geologic systems implying that single values such as CIA may not adequately reflect paleoclimatic changes in certain cases.
Section snippets
Molluscs and paleoenvironmental reconstructions
Terrestrial molluscs are considered to be reliable indicators of paleoenvironments in loess–paleosol sequences (e.g., Ložek, 1964, Ložek, 2001, Rousseau and Wu, 1999), as most species of the Quaternary mollusc fauna exist today and the ecological demands of living species is thought to be well-known (Sparks, 1964, Sümegi and Krolopp, 2002). Composition of the snail assemblages reflects the microclimate and local vegetation cover thereby allowing precise reconstructions of past environments.
Geological setting and stratigraphy
The studied profile is located on the western slope of Szőlő Hill at Beremend (45°47′38″N, 18°26′15″E, ∼110 m a.s.l.) which consists of Cretaceous limestone (Marsi and Koloszár, 2004; Fig. 2). This block of the Mesozoic basement rises above the surrounding alluvial plain of the Drava River. In general, basement rocks of the study area are covered by Upper Pannonian (Upper Miocene–Pliocene, ca. 9 to 3 Ma) deposits such as gravel, sand, silt, clays interrupted by thin lignite and dolomite layers.
Sampling
A total of 28 loess and paleosol samples were collected from the Beremend section in southeastern Transdanubia. Profile was carefully cleaned before sampling and samples for malacological, mineralogical, and geochemical analysis were taken at every 40 cm (Fig. 1, Fig. 2). Sediment blocks of ca 6 kg spanning about 20 cm in the vertical sense were cut from the cleaned profile for malacological investigations. Samples were numbered from top to bottom, so the numbering increases downwards in the
Mollusc assemblages
Shells of 2228 individuals representing 28 terrestrial and 2 aquatic species were found in 28 samples of the Beremend section. Higher mollusc abundances can be observed in loess layers of YLS (mean: 187 individuals per sample) compared to those of OLS (51 individuals per sample on average) and fossil soils (mean: 13 individuals/sample). Two ecological groups dominated the mollusc assemblages, the warm requiring, drought resistant species and the elements with higher ecological tolerance (Fig. 3
Paleoecological history
In general, warm requiring (thermophilous), drought resistant species such as Chondrula tridens, Helicopsis striata, Granaria frumentum, Pupilla triplicata and species with high ecological tolerance (Pupilla muscorum and Vallonia costata) dominated the loess mollusc assemblages, implying the predominance of semi-arid open grassland environments during loess formation phases (Fig. 3). The entire absence of cryophilous, cold indicator species and the continuous presence of Chondrula and Helicopsis
Conclusions
A multi-proxy dataset (molluscs, bulk rock and clay mineralogy, major and trace element geochemistry) has been evaluated to gain insight into climatic and tectonic controls on the evolution of a loess–paleosol sequence at Beremend, Hungary. Two major unconformities were identified in the section suggesting significant impacts of tectonism on sedimentation. Mollusc assemblages indicate open, in some phases semi-open or more closed environments during loess formation and possible shell
Acknowledgments
The authors are grateful for the field assistance from Antal Újvári and Ferenc Varga. We wish to thank Szabolcs Czigány and László Merényi for their help with sample preparation and the GeoAnalytical Laboratory of Washington State University (Zsuzsanna Balogh-Brunstad, especially) for the sample analysis by XRF. Thoughtful reviews by two anonymous referees contributed to a significant improvement of the manuscript.
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