Thursday, 13 July 2006

Holocene Soil and Landscape Dynamics Reconstructed by Sediment Analysis in the French Alps.

Brice Mourier1, Jérome Poulenard1, Aurélie Genries2, Christopher Carcaillet2, Pierre Faivre1, and David Williamson3. (1) University of Savoie (CARRTEL Laboratory), Domaine universitaire - Savoie Technolac, Le Bourget du Lac, 73376, France, (2) Centre de Bio-Archéologie et d’Ecologie, Institut de botanique - 163, rue A. Broussonet, Montpellier, 34090, France, (3) Centre Européen de Recherche et d'Enseignement des Géosciences de l'Environnement, Europôle Méditerranéen de l'Arbois, Aix-en-Provence, 13 545, France

The time required to initiate and complete pedogenetic processes remains largely to be researched. It is one key issue for paleo-environmentalists. The rate and intensities of pedogenesis vary as a result of substrate and environmental properties. Moreover, disturbances, e.g. fires, might strongly affect vegetation cover and as a consequence landscape evolution processes. A multi-proxies analysis based on charcoal and geo-chemical datasets was performed on lacustrine sediments to reconstruct fire history, erosion, vegetation changes and soil dynamics. Sediments were extracted from a pond situated in the subalpine belt at 2020 m asl in the Maurienne valley (Alps, Savoie, France). High-resolution analyses of the 4-m length core were carried out to detect erosion based on Loss-On-Ignition (LOI) and magnetic susceptibility. The chronology was secured by AMS 14C and 210Pb measurements. Soil development and dynamics were reconstructed using different geochemical measurements both on present-day soils as references and on selected sediment depth. Common soil extractions on Fe, Al and Si (pyrophosphate, citrate-bicarbonate-dithionite and ammonium oxalate), total analyses (major and trace-elements) and phosphorus geochemistry were chosen to be applied to both lake sediments and present-day soils. The base of the chronology is dated from 8995 cal yr BP. The sedimentation rate varies between 0.5 and 1.6 mm yr-1. LOI and magnetic susceptibility highlight a significant change of sedimentation at 6595 cal yr BP. Detritical materials resulting from soil erosion dominate the oldest sediments, whereas peat-origin organic materials characterize the youngest sediments. The duration of the transition from a lake system to a peated-lake system is estimated to ca. 350 years. This transition is associated with 5 fire events during 300 years. Evidences of soil genesis are recorded in the sediment as attested by elementary accumulation rates and concentrations. We divide the core into zones owing to proxies trends. High influxes of inorganic silicate minerals (Si, Al, K, Mg and Cr) are proxies of physical weathering of silicate minerals from nutrient-poor soils. Large amount of Fe, Mn, Sr or Ba that are independent of the other inorganic elements, trace soil evolution. High mineral phosphorus concentrations reflect the setting of the modern soils after the glacial epoch. The increasing proportion of occluded and organic phosphorus is indicative of stable landscape (vegetation, soil). Fire history shows a change in frequency synchronous with the change in sedimentation rate and pattern (i.e. at approx 6995 cal. yr BP). The chemical analyses of sediments, compared with those from modern soil, indicate a change in dominant soil genesis process with a drastic decrease of podzolisation rate. One of the triggering mechanisms responsible of this ecosystem evolution is the modification of the fire frequency. We suspect that vegetation, affected by changes in fire frequency resulting from anthropic land-use changes, is the ultimate cause of soil dynamics.

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