Human activities : main drivers of the erosion in the northern French Alps over the last 3800 years

Abstract

Land is the principal support for human livelihoods as it supplies food, freshwater and multiple other ecosystem services. In the current context of global anthropization and climate change, soil erosion is becoming a threat for human societies and one of the question that most deserve the attention of the entire world & scientific community. Given the large uncertainties underlying the drivers of land erosion, it is crucial to assess the impact of human activities and climate fluctuations over erosion, especially in mountain areas, where erosion is the highest. Only studies combining large spatial and temporal approaches allow to assess the effect of the different forcing factors on soil erosion rates. Here, we apply a retrospective approach based on lake sediments to reconstruct the long-term evolution of erosion in alpine landscapes. Lake Bourget, located in the northern French, acts as a natural sink for a fraction of the erosion products from its large catchment. We combined a multiproxy study of lacustrine sediment sections covering the Holocene with a source-to-sink method, using isotopic geochemistry (εNd). The applied methodology allows us to disentangle the role of climate and land use as erosion forcing factors through their differential impact on the various rock types present in the catchment. Indeed, high-altitude areas of the study site, where the erosion is dominated by precipitation and glacier advances, present isotopic signature different from those of the sedimentary rocks located in the rest of the catchment, where both human activities and precipitations impact erosion through time. To understand the effect of human activities, erosion signals from high-altitude and the rest of the catchment were compared to local and regional indexes of human activities, obtained from pollen and environmental DNA studies conducted on lake sediment sequences. For the last 3.8 kyr, climate fluctuations alone cannot explain measured erosion trends. Between the Late Bronze Age and the modern times, human activities are at the origin of a two-fold increase of the erosion rates in the Alps. Human activities, by modifying the soil erodibility through land-use (agriculture, grazing, ore extraction and deforestation) is the dominant forcing factor of the physical erosion in mountainous environment of the European Alps at least for the last 3800 years.