Soil production and hillslope transport in mid-latitudes during the last glacial-interglacial cycle: A combined data and modelling approach in northern Ardennes

The relative efficiency of various hillslope processes through Quaternary glacial-interglacial cycles in the mid-latitudes is not well constrained. Based on a unique set of topographic and soil thickness data in the Ardennes (Belgium), we combine the new CLICHE model of climate-dependent hillslope evolution with an inversion algorithm in order to get deeper insight into the ways and timing of hillslope dynamics under one such climatic cycle. We simulate the evolution of a synthetic hill reproducing the terrain attribute distributions of the hillslopes of a ~2500 km2 real area under a simple two-stage 120-kyr-long climatic scenario with linear transitions. The inversion method samples a misfit function in the model parameter space, based on estimates of the fit of topographic derivative distributions in classes of soil thickness and of the relative frequencies of the predicted soil thickness classes. Though the inversion results show convergence patterns for several parameters, no unique solution emerges. We obtain five clusters of good fits, whose centroids are taken as acceptable model solutions. Based on the predicted time series of average denudation rate and soil thickness, plus snapshots of the soil distribution at characteristic times, we compare these solutions with independent data not involved in the misfit function and identify the most realistic one. Providing first-order estimates of several parameters that compare well with published data, it shows that denudation rates increase dramatically at both warm-cold and cold-warm transitions when the mean annual temperature passes through the [0, -5°C] range. It also underlines the overwhelming importance of gelifluction in transporting soil and shaping hillslopes.