Human and climate impacts on the alpine Critical Zone over the past 10,000 y.

Agropastoral activities have impacted the habitable part of our planet-the "Critical Zone"-for thousands of years, triggering a major increase in soil erosion in mountain environments. Understanding and quantifying the impact of these activities on soil is central to the well-being of our societies. Here, we investigate the isotope ratios of the trace element lithium in detrital sediments of Lake Bourget, European Alps, and provide a reconstruction of the impact of human activities on the evolution of alpine soil during the Holocene. We demonstrate that during the Early Holocene, soil formation was altered by the development of pastoralism followed by tillage. This led to three major erosive surges (3.8 to 3.0, 2.8 to 1.6, and 1.6 ky cal BP to modern times), thinning soils down to a state close to that of their early development 10,000 y ago. The detailed study of the lithium detrital signal reveals the appearance of an altitudinal decoupling in the response of the Critical Zone in the Alps following the development of the agropastoral activities during the Iron Age. The onset of agropastoral activities disrupted the balance between soil formation and erosion, leading to erosion rates 3 to 10 times faster than soil production since the end of the Ice Age.