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Suspended sediment characterization and tracing using a magnetic fingerprinting technique: Bassenthwaite Lake, Cumbria, UK

Centre for Environmental Magnetism and Palaeomagnetism, Lancaster Environment Centre, Department of Geography, University of Lancaster, Lancaster LA1 4YQ, UK

Barbara A. Maher

Centre for Environmental Magnetism and Palaeomagnetism, Lancaster Environment Centre, Department of Geography, University of Lancaster, Lancaster LA1 4YQ, UK, b.maher{at}lancs.ac.uk

Robust identification of catchment suspended sediment sources is a prerequisite both for understanding sediment delivery processes and targeting of effective mitigation measures. Fine sediment delivery can pose universal management problems, especially with regard to nutrient run-off and lake siltation. Here, 19 suspended sediment samplers were located within the three main tributary inflows to Lake Bassenthwaite, a key but vulnerable site of special scientific interest, with water quality problems linked to accelerated delivery of fine sediment. Magnetic properties of contemporary suspended sediments, collected on a monthly basis, were measured on a particle size-specific basis and compared with the lake sediment core-tops. Ferrimagnetic grain size and magnetic `hardness' vary significantly between the suspended sediments collected from the different tributaries, with the 8—31 µm and 31—63 µm clastic grain fractions displaying greatest magnetic contrasts. Postdepositional formation of bacterial magnetosomes is evident in the 2—8 µm and < 2 µm fractions of the lake sediments. Thus, for comparison with the potential source suspended sediments, we use only the detrital, clastic fractions, 8—31 µm and 31—63 µm. The lake sediment magnetic properties show little spatial variation, indicating through-lake transport of sediment (no evidence was found of postdepositional diagenetic sulphide formation). Magnetic comparisons between the potential sources and the lake surface sediments indicate that Newlands Beck, providing only ~ 10% of the lake's hydraulic load, is the main contributor of sediment to the deep basin of the lake. Sediments from the River Derwent subcatchment, contributing ~ 80% of the hydraulic load, are possibly stored either on the floodplain or in shallower areas of the lake.

Key Words: Environmental magnetism • sediment sourcing • suspended sediment • mineral magntics • magnetic susceptibility • magnetic fingerprinting • lake sediments • English Lake District • Bassenthwaite Lake.

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