Pastoralism increased vulnerability of a subalpine catchment to flood hazard through changing soil properties

Soil erosion is strongly linked to both precipitation patterns and land-use. We examine the effects of erosion and its drivers (i.e. human or/and climate) on soil evolution from the study of lacustrine archives in the northern French Alps. Multi-proxy analyses of the Lake Gers sediment sequence combined with the study of soils and rocks of its catchment allowed to reconstruct its evolution over the past 4.6 kyr. This included 14C dating, short-lived radionuclides, geochemistry, loss on ignition, grain-size analyses, as well as plant and mammal DNA analyses. A total of 127 instantaneous deposits were identified among the continuous sedimentation in the lake and 93 were interpreted as flood deposits. Erosion was quantified for the whole catchment considering both the continuous sedimentation and the flood deposit thicknesses. The catchment is mainly formed by andesitic sandstones over which Andosols and Podzols can develop. However, low weathered materials from colluviation constitute the main input to the lake. Four main phases of changes in soil weathering were recorded by increases in K2O/TiO2 ratios, associated with an increase in both erosion and flood-frequency. Two phases were associated with climate cooling in the Western Alps, from 2.65 to 2.5 and from 1.45 to 1.3 cal kyr BP. The two others (1.9–1.75 and 0.9–0.4 cal kyr BP) were triggered with deforestation, with cow grazing during the Roman period, and with grazing of cows, sheep and goats during the Medieval period, amplified by the onset of the Little Ice Age. The increase in erosion and flood frequency after 950 cal yr BP indicates substantial damages to soils of the catchment including increase of their erodibility over the last millennia.