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"Loess is not just the accumulation of dust....."

Björn Buggle1, Ulrich Hambach2, Martin Kehl3, Michael Zech4, Natalia Gerasimenko5, Slobodan Markovic6, Bruno Glaser7, Ludwig Zöller2
1 zur Zeit :Univ.Bayreuth, Lehrstuhl Geomorphologie /im September ETH Zürich,Department Erdwissenschaften
2 Universität Bayreuth Lehrstuhl Geomorphologie
3 Universität Köln, Geographisches Institut
4 Universität Bayreuth, Lehrstuhl Geomorphologie
5 Tarasa Shevchenko Universität Kiev, Department Erdwissenschaften und Geologie
6 LAPER, University of Novi Sad
7 Martin-Luther Universität Halle-Wittenberg, Lehrstuhl Bodenbiogeochemie

Key Note 2.1 in Löss und terrestrische Archive

17.09.2012, 16:00-16:30, H8

 

“Loess is not just the accumulation of dust...

or

... Reconstruction of the Mid- and Late Pleistocene climate and landscape history in SE-Central Europe using a paleopedological and geochemical multiproxy approach in loess-paleosol studies

 

Björn Buggle1,2, Ulrich Hambach1, Martin Kehl3, Michael Zech1, Natalia Gerasimenko4, Slobodan Markovic5, Bruno Glaser6 and  Ludwig Zöller1

 

1 Chair of Geomorphology, University of Bayreuth, Bayreuth

2Chair of Biogeoscience, ETH Zürich,

3 Institute of Geography, University of Cologne

4 Earth Science and Geomorphology Department, Tarasa Shevchenko National University of Kyiv, Ukraine.

5 Chair of Physical Geography, Faculty of Sciences, University of Novi Sad, Serbia

6 Chair of Soilbiogeochemistry, Martin-Luther University Halle-Wittenberg

 

Introduction

“Loess is not just the accumulation of dust” (Pécsi, 1990). This phrasal originally refers to the diagenetic processes associated with the transformation of dust into loess. Slightly modified to “loess is not just accumulated dust”, this phrasal emphasizes fairly well the relevance of loess for geosciences, especially as archive for past environmental conditions.

 The work presented here, summarizes the essential results of the author`s PhD project on the reconstruction of the Quaternary climate and landscape history of SE-Central European lowlands  from loess paleosol sequences. The investigations focus on key sections (Mircea Voda, Batajnica, Stari Slankamen) in the lower Danube Basin and the middle Danube Basin (= Carpathian Basin, Pannonian Basin). These sections are several decameters thick and comprise several glacial interglacial cycles (An impression of the Mircea Voda section is given in figure 1). Hence they represent valuable and unique archives for the long-term Quaternary climate and landscape history.

 

Methods

The interpretation of any paleoenvironmental proxy data, requires basic knowledge on the nature of the studied archives as well as a reliable chronostratigraphy. Hence, investigations on the origin of the loess, genesis of the archives and their sedimentary homogeneity as well as the development of a chronostratigraphy for the sites represent the basic backbone of the project.  For reconstructing the Mid- and Late Pleistocene climate and landscape history from these archives a paleopedological – geochemical multiproxy approach was applied involving 1) grain size analyses, 2) micromorphological observations, 3) geochemically based weathering indices, 4) diffuse reflectance spectroscopy for the determination of the iron oxide assemblage 5) rock magnetic parameters as well as 6) n-alkanes as biomarkers for the tree vs. grass abundance. The focus of the present study is on the interglacial climate conditions.

 

Findings – Provenance of the loess, sedimentary homogeneity

According to its geochemical fingerprint loess in the Danube Basin formed from dust blown out from alluvial sediments of the Danube and its tributaries.  Furthermore, the geochemical composition of the loess in the Pannonian and lower Danube Basin is very similar but shows distinct differences to the composition of Ukrainian loess samples deriving from glaciofluvial sediments of the Fennoscandinavian ice sheet. Regarding the silicate assemblage, the sedimentary homogeneity of the studied profiles could be confirmed – a prerequisite for the application of geochemical weathering indices.

 

Findings – Chronostratigraphy

 

 The chronostratigraphy of the studied sections is based on the correlation of characteristic fingerprints of the magnetic susceptibility to proxy curves for the global ice volume. This is further supplemented by pedo-, and tephrostratigraphic marker allowing correlations to previously established chronostratigraphies from profiles in the region as well as from Chinese stratotype sections.

The results show that the studied loess paleosoll  sequences comprise at least the last 700 ka of climate history i.e. the last 17 Marine Isotope Stages.

 

Findings – paleoenvironmental evolution

 

Following methodological investigations on various geochemical weathering proxies, the molar ratio of Al2O3 / (Al2O3 + Na2O) x 100 was introduced as Chemical Proxy of Alteration (CPA), representing the most suitable indicator for silicate weathering intensity in loess. This CPA shows a decrease of silicate weathering intensity of interglacial paleosols from early Mid-Pleistocene to the present. Also micromorphological parameters (c/f related distribution pattern, type of b-fabric), as well as grain size proxies for pedogenesis (< 5 µm fraction) indicate a decrease of pedogenesis intensity for the interglacial paleosols during the Mid-Pleistocene. On the other hand, the U-ratio indicates an increase of wind strength.

Hence, this multiproxy dataset gives evidence for a decrease of rainfall in the middle and lower Danube Basin during the Mid-Pleistocene. In contrast, rock magnetic parameters (especially the ARM/IRM) and the hematite to goethite ratio indicate more oxidative soil environment in the early Mid-Pleistocene interglacials. This precludes that higher rainfall in the older interglacials appeared in summer time or as today in early summer.

Hence, our multiproxy dataset points towards high summer temperatures and a pronounced estival dry period combined with mild and wet winter condition in the interglacials of the early and middle Mid-Pleistocene. According to the paleopedologic findings soil development in these interglacials peaked in (chromic) Luvisols and Cambisols under this Mediterranean type of climate. The progressive decrease of rainfall over the Mid-Pleistocene can be linked to a shift in seasonality towards a steppe like climate of the interglacials, reflecting increasing continentality. Also the n-alkane results suggest that summer dryness was a persistent feature of interglacial climate in the  middle and lower Danube Basin, limiting the expansion of trees. Present climatic differences between the Serbian (~ 680 mm mean annual precipitation, MAP) and Romanian site (~ 400 mm MAP) are sustained during most glacial and interglacial stages, according to the multiproxy dataset.  The increasing continentality during the last 700 ka is likely related to a rise in paleoelevation of the Alps and Carpathian mountains in the order of up to 50-70 m per 100 ka. Hence, rain shadow effects and changes in the atmospheric circulation induced by mountain uplift explain the expansion of the Eurasian steppe belt as far west as to SE-Central Europe during the Mid-Pleistocene.

To conclude, the present study highlights the potential of a paleopedologic – geochemical multiproxy approach to derive information on the natural baseline of environmental change in the past, even with respect to seasonality of climate variables.

 

Figure 1. The loess paleosol sequence Mircea Voda (Romania) . Major Interglacial pedocomplexes are denoted with "S" and numbered according to the Chinese nomenclature. The chronostratigraphic placement of the pedocomplex is as follows: S1 = MIS 5, S2 = Mis 7, S3 = MIS 9, S4 = MIS 11, S5 = MIS 13-15, S6 = MIS 17
Figure 1. The loess paleosol sequence Mircea Voda (Romania) . Major Interglacial pedocomplexes are denoted with "S" and numbered according to the Chinese nomenclature. The chronostratigraphic placement of the pedocomplex is as follows: S1 = MIS 5, S2 = Mis 7, S3 = MIS 9, S4 = MIS 11, S5 = MIS 13-15, S6 = MIS 17



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Letzte Änderung 26.07.2012