Holocene climate variability in the lower Rhône plains (Provence, Mediterranean France)
by
Hélène Bruneton
CEREGE, B.P. 80, 13545 Aix-en-Provence, cedex 04, France
Coauthors: Claude Vella, Gilles Arnaud-Fassetta, Mireille Provansal

The goal of the poster is to assess the relationships between the hydro-morphological changes in the lower Rhône area and the local Mediterranean climate variability during the Holocene. Indeed, such changes as floodplain and channel mobility, variations in flooding frequency and fluctuations of marsh levels may also depend (1) on the water and sediment fluvial discharge, coming from the non-Mediterranean upstream parts of the Rhone catchment, (2) on the downstream lateral and vertical shoreline mobility and (3) on a possible geomorphological response-time of the fluvial system.

The field data issue from 4 categories of sites. (1) A group of 2 cores in the Baux marsh, 15 km east from the Rhône channel in Arles, supplied authigenic sediments dated from Younger Dryas to present. The site was separated from the floodplain by a geological threshold until the Little Ice Age, so that it constitutes a valuable source for local climate reconstruction. (2) Cores and geoarcheological excavations along the present and past channels from Arles to the sea allow to study the frequency and energy of the Rhône floods, together with the channels vertical and lateral mobility, from c. 8 kyrs cal. BP to present. (3) A 5 m long core in Castelet marsh, 4 km east from the Rhône channel, showed the palustrine sediments of the distal floodplain, giving an idea of changes in the mean fluvial aquifer level from c. 7 kyrs cal. BP. (4) Several cores from the Fos gulf area, in the eastern part of the delta, allow to reconstitute the marine shoreline mobility.

Every core is dated by radiocabon, archaeology and/or palynology. Grain-size analyses characterize the energy of the marine or fluvial deposits. The chalk, peat and fauna (ostracode) studies indicate variations in the environmental conditions, especially for the marine and wetland sites : changes in salinity (of marine or continental origin), presence of a seasonal drying. Stratigraphical reconstitutions show the changes in the deltaic floodplain geomorphology.

A Holocene marsh-level curve is produced for the Baux marsh, interpreted as changes in the local climate moisture.

In the Rhône floodplain area, there is a good similarity between the geomorphological episodes that reacted to extreme meteorological events, the lateral shoreline mobility that depended partly on the amount of the fluvial sediment load, and the marsh-levels that fluctuated with the mean hydrological abundance. A good synchronism is observed between hydro-morphological events along the Rhône catchment, so that there appears no sensible intertia of the fluvial system. The long-term continued rise of the aquifer levels in the floodplain appears to be linked with the sea-level rise, but the shorter, millennial to centennial fluctuations correspond with upstream-initiated hydrological variations.

The comparison between the local and the Rhône curves reveals periods of divergences, the local marsh being dry and the Rhône carrying a high water discharge. The most remarkable event takes place between 6.5 and 5.5 kyrs cal. BP. It shows that important environmental changes in the lower Rhône happened without any local climate influence, especially during the Atlantic chronozone. Important amounts of freshwater flowed through the fluvial axis in the Mediterranean sea from temperate, non-Mediterranean Europe.

Date received: April 3, 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-95.