Holocene sediment budget for the Thue and the Mue catchments (NW France): European comparison and methodological issues
Résumé
To contribute to the understanding of Holocene sediment dynamics in Western Europe, a Holocene sediment budget is proposed for the Thue and Mue catchments (52 and 97 km 2 respectively) located in NW France. In this budget, soil erosion as well as colluvial sediment storage is estimated from soil profiles analysis for a representative catchment and extrapolated to the entire catchments. Alluvial sediment storage, including a chronology of the alluvial fills, has been quantified by augerings cores distributed on several transects located along the floodplain of the two rivers. The proposed sediment budget highlights significant sediment storage in the catchment: 12.3 Mt (32.5 %) is stored as colluvium and 15.5 Mt as alluvium (40.8 %), i.e. more than 70 % of eroded sediments on the slopes. The remaining eroded sediments (10.2 Mt) are considered to have been exported out of the catchments. The timing of slope erosion and colluvial storages indicates the initiation of soil erosion from the Neolithic period and an acceleration over the last 3 millennia. However, a long gap is observed between forest clearance, soil erosion and the acceleration of alluvial filling underlying a progressive improvement of the connectivity induced by a progressive transformation of the running water circulation within the catchment. The results thus highlight the complexity of Holocene sediment dynamics across spatial and temporal scales and emphasize the necessity to integrate the question of the efficiency of the sediment cascade particularly induced by the landscape structure and drainage density.
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