André-Valentin Munaut

Response of the Selle River to climatic modifications during the Lateglacial and Early Holocene (Somme Basin-Northern France)

Abstract Research on Lateglacial sequences from the Selle valley leads to an overview of its evolution in relation to climatic variations between the end of the Weichselian Upper Pleniglacial and the beginning of the Holocene. The first major modification of the fluvial morphology is dated at the Upper Pleniglacial/Lateglacial transition (13,000 14 C-yr BP). At that time, the response to climatic improvement and environmental modifications is marked by downcutting and evolution from a braided river to a transitional river pattern (Bolling infilling in the newly created channels). After a short cold phase recorded in a thin calcareous bed at the top of the Bolling peat attributed to the Older Dryas (Dr. II), the Allerod is characterized by the deposition of organic overbank silts within a large single channel meandering system. In lower slope environments, this period is also marked by slow rates of colluvial accumulation and by the development of upbuilding soils (Allerod soil). On the other hand, the end of the Lateglacial, is characterised by the infilling of the whole valley by fine calcareous overbank silts during the Younger Dryas cold phase (overflow of a large single channel and lateral input of chalk mud). A second major downcutting phase occurs at the beginning of the Holocene at around 10,000 BP, in parallel with another rapid climatic improvement and the renewed spread of vegetation. From a general point of view, the evolution of fluvial environments in the Selle valley is comparable with many other river valleys in NW Europe, showing that fluvial systems react very quickly to climatic variations of short duration (1000 to 100 years). Finally, in the Upper Selle River, incision events occur clearly before the main modifications of the vegetal cover. They are most likely linked to a rapid shift in the balance between water discharge and sediment supply, caused by climate modifications (shift to more temperate and oceanic conditions), and the resulting environmental changes: cessation in aeolian sedimentation, strong reduction of slope processes, permafrost disappearance and soil development.