Mass balance study of Holocene sediment accumulation between the Ems and Weser estu-aries on the German North Sea coast
by
Christian Hoselmann
Hessisches Landesamt für Umwelt und Geologie, PO Box 3209, D-65022 Wiesbaden, Germany
Coauthors: Hansjörg Streif (Niedersächsisches Landesamt für Bodenforschung, PO Box 510151, 30631 Hannover, Germany)

A research programme, funded by the Deutsche Forschungsgemeinschaft (DFG), deals with a mass balance of the Holocene sediment accumulation in the coastal zone between the estuaries of the rivers Ems and Weser. This region is highly suitable for this kind of research because it is built up by various coastal sedimentary environments comprising barrier islands with sheltered tidal flats, bay flats (Dollart, Jadebusen), estuaries (Unterweser, Unterems), and extended coastal marshlands which may effect the accumulation processes.

Under the influence of 25 m of sea-level rise a wedge-shaped body of coastal deposits ac-cumulated in this zone in the course of the last 8,500 years. At the seaward margin, in the region of the barrier islands, these deposits are more than 35 m thick and consist of marine to beach sands which are locally overlain by up to 25 m thick dunes. Thinner Holocene se-quences consisting of clastic tidal-flat to brackish deposits with intercalated peat layers occur in the subsoil of the present day wadden sea area and the coastal marshlands and wedge out against the Pleistocene hinterland or merge into alluvial deposits of the river valleys. The peat layers become thicker towards the landward margin of the coastal lowland area, and in many places continuous peat sequences occur close to the Pleistocene hinterland. The re-search programme aims at a quantification of the total sediment accumulation. Additionally, cumulative mass balances have to be established concerning the clastic sedimentary com-ponents which were supplied from marine or riverine sources as well as of sedentary peat which was formed in coastal bogs.

Basic data of the mass balances are geological borehole records stored in the archive of the Geological State Survey of Niedersachsen (NLfB). Starting from this material two sets of maps 1:25,000 at scale are manually constructed:

- The one is a structure contour map depicting contour lines of the base of the Holocene related to the German zero datum NN with depth intervals of 1 m.

- The other is a sequence map representing the spatial extension of clastic deposits and of sedentary peat.

In the 4950 km2 wide study area, Holocene deposits occur in the interval between NN –42 m (fossil tidal gully systems) and NN +25 (dunes on the East Friesian barrier islands). The en-tire volume of the deposits amounts 41 billions m3. Cumulative balances demonstrate that about 90% of this volume is made up by clastic sediments whereas peat makes up about 10%.

A comparison of the present day sediment load of the rivers Ems, Weser, and Elbe gives some hints that about 90% of the clastic components of the coastal Holocene stem from marine sources. Reworking of Pleistocene and Holocene sediments and redeposition under marine and tidal conditions seem to be the predominating mechanisms of sedimentation. Only about 10% of the clastic material stems directly from riverine sources.

As the investigated area comprises a great variety of coastal environments, e.g., sheltered tidal flats, open tidal flats, bay flats, estuaries, and coastal marshlands, the results of the study can serve as a model for general mass-transport and accumulation of sediment con-nected with transgression onto coastal lowlands. Additionally the results can be used for predictive statements on possible effects of future sea-level rise.

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".

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-63.