Quantitative evaluation of human and climate forcing on erosion in the alpine Critical Zone over the last 2000 years

Abstract

Soil erosion is one of the main environmental threats affecting the Critical Zone (CZ) and thus ecosystem services and human societies. Through time, physical erosion is linked to both climate variations and the landscape evolution under long-term human pressures. In mountainous areas where erosion is highest a combination of large spatial and temporal approaches allows to assess the effect of these forcing factors on erosion rates. Here, we apply a retrospective approach based on lake sediments to reconstruct the long-term evolution of erosion in alpine landscapes. Lake Iseo in the Italian Alps acts as a natural sink for all the erosion products from a large catchment (1777 km2). This catchment is representative of the southern Italian Alps, where Holocene human activity and climate fluctuations are well known. The approach combines a source-to-sink method, using isotopic geochemistry (εNd, 87Sr/86Sr) associated to a multiproxy study of a lacustrine sediment section covering the last 2000 years. The applied method 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, the high-altitude part of the Iseo catchment, where glacier advances and retreats drive the erosion, presents isotopic signature different from those of the sedimentary rocks located in the lower part of the catchment, where both human activities and precipitations impacted erosion through time. A chronicle of glacial erosion over the last 2000 years was produced. Once the climatic trend was highlighted, the signal of erosion of sedimentary rocks was investigated to understand the influence of humans. From the Roman Period to the Industrial Age several periods of deforestation and increased human pressure were documented. The past sediment yield inferred for sedimentary rocks exhibits the highest values (>80 t km−2. yr−1) at periods of intense human practices. Hence, since the late Roman Period, human activities seem to be the dominant forcing factor of the physical erosion in mountainous environment of northern Italy. This study presents the first reconstruction through time of sediment yield derived from lake sediments associated with sediment sources identification and quantitative evaluation of the CZ erosion drivers.