Sensitivity of floodplain geoecology to human impact: a palynological approach

Floodplain deposition rates have increased markedly under influence of human impact throughout the Late Holocene in many Western and Central European catchments. These variations in sedimentation rates have changed the geomorphology and ecology of many floodplains. In this study we discuss the human impact and its influence on floodplain geoecology during the Middle and Late Holocene for the Dijle catchment (760 km²), located in the Belgian loess belt. Based on sedimentological and palynological data from 6 study sites, the geoecology of the floodplain and the regional vegetation was reconstructed. Age-depth models for each of the studied sequences were obtained through 60 radiocarbon dates. Human impact in the catchment was (semi-)quantified based on statistical analysis of the pollen data (cluster analysis and non-metric multidimensional scaling). Our data shows that until ca. 2500 cal BP, human impact was nearly absent or localized with no discernible influence on the floodplain geoecology. The river environment was in a stable phase and consisted of a marshy environment were organic material could accumulate, which is interpreted as the natural state of the floodplain. From ca. 2500 cal BP onwards, human impact gradually increased. However, only when human impact in the catchment crossed a threshold, the floodplain geoecology changed with clearing of the Alder carr forest, the dominance of minerogenic overbank sedimentation and the emergence of a single channel meandering river. Spatial and temporal variability in the coupling between increasing human impact and changes in floodplain geoecology can be attributed to differences in timing and intensity of agricultural activities at catchment scale, to differences in the hillslope-floodplain connectivity, or to the location within the catchment. Overall, this study shed new lights on the indirect effect of anthropogenic forces on floodplain geoecology. It also shows that the contemporary morphology of the Dijle River floodplain contrasts widely with that of the Middle Holocene, which was dominated by peat formation in marshes and gyttja deposition in floodplain lakes. These changes in floodplain geoecology are the result of human disturbances in the catchment.status: publishe