A Lake Sediment Study of Particulate Flux in the Humber Catchment Using Magnetic Techniques

At present there is a poor understanding of changes in rates of erosion over long time periods and in the link between erosion and sediment delivery in Britain. Using a catchment study approach sediment accumulation rates in cores from the three lakes Semer Water, Gormire and Hornsea Mere have been used to reconstruct changes in sediment yield over time periods of up to 10 000 years, and estimate the mean annual flux of sediment to the Humber estuary. Each of the sites lies in a catchment of differing land use, relief and geology but taken together they are representative of the Humber catchment. Sediment cores obtained from Gormire and Hornsea Mere have been correlated successfully using variations in mineral magnetic signature. The magnetic minerals are dominated by bacterial magnetosomes at Hornsea Mere and by a combination of bacterial magnetosomes and topsoil at Gormire. Sediment accumulation rates have been converted into sediment yields using a "'Cs and 210Pb chronology, sediment accumulation area, sediment density, organic carbon and carbonate data. At Gormire yields increased from just 7 t km 2 a1 in the early Holocene to 78 t km 2 a 1 for the period 1949-1994. Sediment yields at Hornsea Mere increased dramatically from 12 t krn -2 a 1 in the period 1730-1963 to 52 t km 2 a 1 between 1963 and 1979, but declined slightly to 41 t km 2 a 1 in the most recent period 1979-1994. Sediment accumulation in three cores from Semer Water has been used to determine a mean sediment yield of 6.3 t km 2 a 1 since 1950. Unusually thick sediment sequences were identified upstream of the existing lake Semer Water in the Raydale valley. Resistivity profiles and gouge cores were used to map the extent of these deposits and 14C and pollen analysis used to establish their chronology. The combined sediment mass of Semer Water and Raydale deposits has been calculated at 11 million tonnes. This translates into a mean Holocene sediment yield of 28 t km a 1. The topography of five representative gullies was used to calculate the potential volume of sediment produced from gully erosion in the catchment. This technique indicates that the entire mass of sediment deposited in Raydale during the Holocene may have been produced from gully and channel erosion. Sixteen catchment and land use characteristics have been determined for 30 British lakes and reservoirs previously studied for sediment flux. Using stepwise regression a model has been constructed relating sediment flux catchment area, catchment soil erosion susceptibility and a slope length factor. The regression equation has an R2 value of 79%. Combining a sediment delivery ratio with sediment flux estimates from (i) Semer Water and Gormire, and (ii) published lake and reservoir sediment studies from sites in the Humber catchment, sediment flux to the Humber estuary is estimated at between 40000 and 110 000 t a'.