Approche multidisciplinaire d'une séquence lacustre holocène dans les Alpes du Sud au Lac Petit (Mercantour, alt. 2200 m, France): Histoire d'un géosystème dégradé

Abstract

High resolution multiproxy analyses, including sedimentological, geochemical and palynological investigations, were carried out on a 144-cm-long sediment profile from Lake Petit (2200 m a.s.l., Vallon des Millefonts, Mercantour, in the French Southem Alps). This allowed us to reconstruct complex interactions between climate, environment and human during the Late Holocene. We identify three phases: (1) a first phase (4 750 to 4 350 cal. BP) characterized by a dominant lake sedimentation and developed soils on slopes, (2) a second period (4 350 to 1 550 cal. BP) marked by an abrupt switch in sediment source and an input of organic allochtho-nous matter linked with soil erosion and a more open landscape, (3) a final phase (since 1550 cal. BP) of intense destabilization of slopes. Even if climate could have triggered such drastic changes, discreet but continuous human impact until 1550 cal. BP might be the main factor of degradation of slopes and ecosystems in the valley. The record of abundant terrigenous input also suggests that high-intensity rainfall events might have occurred, while agro-pastoralism and local mining activities continuously weakened slopes.