5 millennia of Mediterranean mountain disturbance: soil erosion and vegetation dynamics recorded in Lake Petit (2200 m, South-Western Alps)

Description

Ancient human presence in the mountain landscapes of the Alps has been documented by archaeological and palaeoecological studies which show a change from gathering to pastoralism over the last 4 ka. Above 2000 m, landscapes have been exploited at least since the Bronze Age. Human activities rendered soils vulnerable to colluviation, increasing erodibility and erosivity. Most studies on long-term soil erosion have focused on gullying and hillslope erosion especially at lower altitudes. In mountain areas, well-dated deposits are scarce. Lake sediments are therefore a valuable landscape archive because they record a broad range of proxies responding to changes in slope stability and vegetation cover. We present high-resolution multiproxies interpretations of a cored profile from the Alpine Mediterranean Lake Petit covering the last 5 ka. Sedimentological, geochemical and botanical data from this core document weathering and erosion in relation to vegetation dynamics, climatic fluctuations and human activities. Lake Petit has recorded since 4770 cal. BP continuity of anthropogenic taxa identified by nitrate enrichment of Alpine grassland. Even though discrete, grazing activities were already extant at this time but did not lead to significant soil erosion. The first increase in anthropogenic taxa occurred at 3000 cal. BP, concomitant with an increase in Ericaceae and intensification of erosion. Soils, without tree protection and probably altered by domestic livestock, were frequently eroded. Four important detrital pulsations occurred synchronously with depletion in tree pollen, while pollen from anthropogenic-related taxa increased. Reported archaeological sites and mining activities also show the presence of Humans in the Lake Petit catchment at 1800 cal. BP and 500 cal. BP, respectively. Widespread deforestation occurred, probably to maintain Alpine pastures and to supply fuel for smelting, contributing to the continuity of anthropogenic landscape perturbation.

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