Use of the geochemical record preserved in floodplain deposits to reconstruct recent changes in river basin sediment sources

Abstract

The geochemical record preserved in overbank floodplain deposits is used in combination with a composite fingerprinting technique and the chronology provided by 137Cs dateable horizons, to reconstruct recent changes in sediment sources in the Exe and Severn basins, UK. Composite fingerprints for discriminating potential source materials are identified from a suite of properties including heavy metals, trace metals and cation bases. These fingerprints are then compared with those of historical floodplain deposits using a numerical sediment mixing model. Relative contributions from distinct spatial sources characterised in terms of the Devonian, Carboniferous and Permian geological sub-areas of the Exe and the Ordovician-Silurian, Devonian-Carboniferous-Permian and Triassic-Jurassic sub-areas of the Severn are shown to fluctuate over the recent past in response to intrinsic (e.g. the occurrence of extreme hydrologic events) and extrinsic (e.g. anthropogenic disturbance associated with land use change) controls. The potential for fingerprint property transformation in association with floodplain storage is assessed and the scope for further research using fingerprinting to reconstruct longer-term sediment sources is discussed.