Résumé
At the scale of northwestern Europe, the valley bottoms of the River Somme basin make remarkable wetlands characterized by a significant fluviogenic peat accumulation (4 to 6 m on average). These fluviogenic peat (fen) sequences provide outstanding archives to describe the environmental evolution of northwestern France over the last 14,000 years. They are also fragile environments forming huge carbon sink. In this context, a research initiative has been initiated to highlight the respective roles of climatic and anthropogenic forcing factors (drainage, slope soil erosion) on peat formation/degradation processes and the modification of the related fluvial environments. The study is based on two stratigraphic transects of the valley (about 600 m each) based on more than one hundred manual boreholes and mechanical corings. The reconstruction of both sedimentation dynamics and palaeoenvironments evolution is based on a multiproxy approach combining sedimentology, geochemistry, palynology, and plant macro-remains identification, supplemented by forty-two 14C dates. This work reveals that the first peat deposits were restricted to channel filling at the beginning of Lateglacial (14.6 - 14.0 ka cal. BP). This first peaty event was then interrupted during the Younger Dryas by the deposition of highly calcareous overbank silts (CaCO3 > 40%) in the whole alluvial plain. Typical peat formation with high TOC values (> 45%) then restarted at the beginning of the Preboreal period around 12 ka cal. BP and rapidly extended to the full valley bottom (0.07 cm/year). During the Subboreal, the reactivation of the river flow is then indicated by the development of a deep meandering channel progressively filled in with laminated silty-organic deposits and a slower peat accumulation rate in the alluvial plain (0.03 cm/year). These modifications are contemporaneous with the generalized opening of the landscape (palynology), associated with the acceleration of anthropic erosion processes on the slopes during the Sub-Atlantic (2.9 ka cal. BP). Since the end of the Middle Age, organic silts fed by the erosion of loess soils have rapidly buried the peaty system, which is today essentially inactive and fossilized.