Quantification and regulation of organic and mineral sedimentation in a late-Holocene floodplain as a result of climatic and human impacts (Taligny marsh,Parisian Basin,France)

Quantification in grams per metres squared per year of the sediment accumulation in a flood plain (‘marsh’) located in the southwestern Parisian basin showed that there is no close relationship between the accumulation of organic matter (OM) and mineral matter (MM) during the late Holocene, and provided an accurate view of the distinct yield and storage conditions of both sediment components. Endogenic OM accumulation in peaty sediments is not related to the climate but to felling of the alder forest and its substitution by Cyperaceae and paludal taxa in the marsh (Iron Age and Middle Ages). MM accumulation expresses mainly the sediment yield on the slopes, determined by landuse. During an initial phase (from the late Neolithic to the early Middle Ages), land-use change from crop cultivation to pastureland, possibly related to climate deterioration, led to a decrease in the sediment yield. During a second phase, since the early Middle Ages, the considerable development of crop cultivation over pasturing, even during periods of climate deterioration (such as the ‘Little Ice Age’), led to a sharp increase in sediment yield. However, although sediment yield was high, the hydrodynamics in the fen did not favour particle retention. Thus, since the Neolithic, yield and storage of OM and MM sediment have been marked by human activities, initially with high climatic stress, but since the Middle Ages without significant climatic stress.