Termination I: Fluvial response to rapid climate change in small catchments in Central German basins
by
Wolfgang Andres
Institute of Physical Geography, University of Frankfurt a.M., Germany
Coauthors: Johanna A.A. Bos (Laboratory of Palaeobotany and Palynology, Utrecht University, Netherlands), Peter Houben, Arie J. Kalis (Institut für Vor- und Frühgeschichte, University of Frankfurt a.M:, Germany), Sabine Nolte, Holger Rittweger, Jürgen Wunderlich

An interdisciplinary approach allows to distinguish spatially divergent process responses of different sub-catchments to the Younger Dryas (YD) cooling. The investigations were carried out in valley reaches of the Ohm- and Wetter rivers as those small-sized catchments reflect local to regional environmental conditions. The studied Sites are located in basins, the Amöneburger Becken and the Wetterau, respectively. The upland sub-catchments mainly consist of Tertiary basalts (Vogelsberg), while the lowland areas are largely covered with loess.

Open sections as well as closely spaced corings were studied. Sample material was submitted to extensive lithological and pedological analyses, including grain size analysis, carbon, calcium carbonate, heavy minerals, metal contents (Al, Cr, Cu, Fe, Mn, Ni), stable isotopes (C, N, S), magnetic volume susceptibility, analysis of plant and animal remains, and pollen analysis. The chronological framework was based on numerous ¹⁴C-data (AMS, CRA), pollen-stratigraphy, and tephrochronological marker horizons.

The results indicate that the Alleroed flood-plain was covered by overbank fines overlying Pleniglacial sands and gravels. Rivers were characterised by a straight to meandering fluvial style. At some localities a prominent clayey A-horizon developed. Birch-pine woodlands were covering valley slopes, whilst the floodplain was grown with willow and birch. About 12, 900 cal BP widespread deposition of Laacher See tephra fall-out (LST) draped underlying sediments and soils. Reworked LST is up to 60 cm thick in the Ohm valley.

In Lower Younger Dryas reworked coarse-grained deposits, channel-fill crossbeds, and lower erosional surfaces of YD age cut into underlying deposits. This points to the formation of multi-channel braidplains at the beginning of the YD. ¹⁴C data inversions and reworked Tertiary palynomorphs indicate the erosion and re-deposition of older sediments. Despite the overall cooling open pine forests persisted on lowland valley slopes. On floodplains dwarf shrub and heliophilous herbs were present. The vegetation cover suggests a moderate climatic deterioration in climatically protected lowland areas. On the other hand fluvial sediments indicate a meltwater driven regime corresponding to the YD cooling. This shows that discharge even in the lower catchment areas was controlled by processes in the more elevated sub-catchments.

Overbank fines of the Upper Younger Dryas reveal a return to straight to meandering river courses. The initial deposition of fines in abandoned channels was passing into organic muds due to shallowing. The fluvial change may be attributed to a shift to rainwater driven regime. The palynological record suggests open pine forests and varying groundwater tables on floodplains.

This poster belongs to a poster cluster of the DFG priority programme "Changes of the Geo-Biosphere during the last 15.000 years. Continental sediments as evidence for changing environmental conditions".

Reference:

Andres, W., J.A.A. Bos, P. Houben, A.J. Kalis, S. Nolte, H. Rittweger, & J. Wunderlich (2001): Environmental change and fluvial activity during the Younger Dryas in Central Germany.- Quaternary International, 79/1: 89-100; Amsterdam.

http://www.uni-frankfurt.de/fb11/ipg/spp/Postergallery/Postergallery.htm

Date received: March 29, 2001

Copyright © 2001 by the author(s). The author(s) of this document and the organizers of the conference have granted their consent to include this abstract in Atlas Conferences Inc. Document # cagc-49.