The sedimentary history of loess : sources, deposition, and reworking of aeolian sediments as indicators for palaeoenvironmental changes in the Danube Basin

Pötter, Stephan; Lehmkuhl, Frank (Thesis advisor); Markovic, Slobodan B. (Thesis advisor)

Aachen : RWTH Aachen University (2021)
Dissertation / PhD Thesis

Dissertation, Rheinisch-Westfälische Technische Hochschule Aachen, 2021

Abstract

Loess is a windblown dust deposit, which covers approximately 10% of the Earth’s surface. Especially the Eurasian mid-latitudes are mantled by a belt of loess deposits spanning from southern England towards the steppes of Central Asia. Despite its terminal atmospheric deposition, the formation of loess is a complex chain including several geomorphological and sedimentological processes. Aeolian processes cannot solely produce dust particles in sufficient amounts. During the Pleistocene, grinding of Alpine glaciers and continental ice sheets – as well as physical weathering phenomena such as frost shattering in periglacially influenced high-latitudes and -altitudes were among the essential formation processes of silt production. Rivers are another important factor, since they erode and transport unconsolidated sediments and increase dust formation due to abrasion during riverine transport. After the deposition of fluvial and alluvial de-posits, fine particles, mostly in the silt fraction, can be deflated and transported by wind. Topographic barriers or biotic components, such as vegetation or biocrusts, trap the particles. The aeolian dust undergoes post-depositional alterations, so-called loessification processes, which help to preserve the deposit. Despite these processes, loess is prone to erosion, especially to reworking by water. In Europe, the distribution of loess and other aeolian sediments is strongly related to (palaeo-)geographic conditions and the (palaeo-)climate influencing the process chain of loess formation. The catchment of the Danube River, especially in its middle and lower course, is one of the most important loess regions in Europe since loess covers vast areas of the basin and the records reach back to the Early Pleistocene. Due to thicknesses, which in some instances exceed 50 m, the loess deposits of the Danube Basin are a crucial archive for palaeoclimatic and palaeoenvironmental changes in Central and Southeast Europe. The interplay of the Atlantic, Mediterranean, and continental climate regimes influences this region. The present dissertation casts light on the sedimentary history of loess deposits of the Danube Basin to highlight its applicability as an archive of palaeoenvironmental changes. The sedimentary history includes the process chain of loess formation and possible post-depositional alteration such as reworking by water. Especially dust deflation, transport, and deposition via sedimentary pathways, the vegetation patterns trapping aeolian mineral dust and related ecosystem qualities, and the post-depositional reworking of aeolian sediments due to landscape instability are investigated to understand palaeoclimatic and palaeoenvironmental changes. This dissertation examines the physical and chemical properties of seven loess-palaeosol sequences and one archaeological excavation from various geomorphological settings to unravel regional environmental patterns for the Middle and Lower Danube Basin. To reconstruct the deflation and aeolian transport of dust, sedimentary pathways for Pleniglacial loess deposits from the Lower Danube Basin are reconstructed based on geochemical compositions of five loess-palaeosol sequences, distributed along a cross-section from the plateau of the Dobrogea in the south towards the northern Wallachian Plain. The comparison with loess depos-its of reference regions revealed that the alluvial deposits of the Danube and its tributaries draining the Carpathians and Balkan Mountains are the primary source for the loess deposits of the southern Lower Danube Basin indicating westerly palaeowind directions. Northwards, a consider-able amount of dust derived from the rivers draining the flysch zones of the Eastern Carpathians, such as Siret, Prut, or Dniester, pointing to stronger north-westerly winds. Based on stable carbon and nitrogen isotope analyses of two loess-palaeosol sequences from the Middle Danube Basin, Irig and Semlac, the vegetation of the basin during the Middle to Late Pleistocene was dominated by C3 plants, most likely grasses, which acted as an efficient dust trap. The isotopic composition of the two sequences also reveals a palaeoclimatic gradient in the basin, as the South of the basin seems to have been experiencing drier conditions compared to the north. The geochemical evidence highlights the importance of aridity as a palaeoecological factor that influenced vegetation patterns and ecosystem qualities during interglacial and interstadial periods. The Upper Palaeolithic findspot of Temereşti Dealu Vinii was investigated to reconstruct site-formation processes of a reworked archaeological open-air site. Temereşti shows no evident stratification, and lithic artefacts from the Aurignacian and (Epi-)Gravettian cultural units permeate the sequence with (typological) age inversions and no horizontal concentrations. The primarily aeolian sediments were (sub-)continuously translocated by fluvial processes. Based on geomorphological evidence and the lack of wear on the found artefacts, the range of transport is constrained to the width of the river terrace on which the site is located. Reworking took place during known phases of landscape instability during the Holocene in the area. This dissertation illuminates the steps of the sedimentary history of loess as palaeoenvironmental indicators for the Pleistocene in the Danube Basin. It highlights the importance of multi-proxy approaches by the application of three different courses on loess deposits from different milieus. These approaches are combined to unravel regional palaeoenvironmental patterns of Southeast Europe in the context of the dispersal of anatomically modern humans into the subcontinent during the Pleniglacial.

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