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Holocene sediment yield and geomorphic sensitivity in alpine landscapes, Cathedral Lakes, British Columbia Evans, Martin


Holocene patterns of sediment yield reconstructed from sediments in four lakes are used to assess the geomorphic sensitivity of four alpine-subalpine basins in Cathedral Provincial Park, British Columbia. A three stage process was used to assess basin sensitivity. Stage one involved appraisal of the potential generalisability over the landscape of the results to be derived from the lake basins. The four lakes, Quiniscoe , Glacier, Pyramid and Lake of the Woods lie at or just below treeline. Glacier, Pyramid and Quiniscoe lakes lie in cirque basins. Slope frequency analyses show that whilst slopes in these basins are representative of those in the cirques of Cathedral park they differ significantly from the wider landscape. Stage two of the process involved identification of Holocene variability in the sediment yield record. Cores from each of the lakes were correlated using magnetic susceptibility measurements and lithostratigraphy to define chronostratigraphic units. The mass of allochthonous mineral sediment deposited in each of these units was estimated using measured sediment density, carbonate content, loss on ignition, and biogenic silica. Sediment yield to the lake for each unit was calculated based on dating of zone boundaries, estimates of trap efficiency, and the mineral mass estimate. In order to identify significant variations in the record, a new method was developed for estimating error associated sediment yield estimates derived from lake sediments. Sediment yield estimates were compared with the published Holocene climate record to make an initial assessment of sensitivity. Stage three involved discriminating between true sensitivity to climate change in the record and coincidental correlation. The approach was to identify process linkages between the observed changes in sediment yield and the climate change record. Two methods of inferring process were used. Firstly process change was inferred directly from the sedimentology of the lake sediments. Secondly magnetic and mineralogical characterisations of lake and catchment sediments were used to identify changing sediment sources. The results show considerable variability in the patterns of Holocene sediment yield amongst the study basins. In particular, Glacier and Quiniscoe Lakes show a marked increase in sediment yield under cooler conditions ca. 3390 BP, which is not apparent at Pyramid and Lake of the Woods. The increase is driven by increased surface erosion in the two higher basins under cooler conditions, and with retreating treeline. The results suggest that the sensitivity of alpine and subalpine basins is both temporally and spatially contingent. The links between climate change, process change and variation in sediment yield are not simple, rather they is mediated by a series of 'resistances'. The variability in sediment yield was compared with the variable nature of the catchments and four important controls were identified: Sediment production, vegetation, relief, and the extent of fluvial development in the catchment. Maximum sensitivity was observed in steep ecotonal sites with a well developed fluvial system.

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