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Lake sediments as records of palaeoenvironmental change : Kwoiek Creek, Coast Mountains, British Columbia Souch, Catherine Jane


It has been suggested that the dominant controls on alpine sediment transfers during the Holocene Epoch relate to climate change, specifically paraglacial sedimentation and Neoglacial activity. Alpine lakes with appropriate geometry and hydraulic conditions trap a high proportion of sediments inflowing from their surrounding drainage basins. Thus alpine lake sediments have the potential to yield a comprehensive, integrated signal of drainage-basin geomorphic activity through time, which may be interpreted as a proxy record of Neoglacial activity. This study is concerned with the interpretation of alpine lake sediments in glacierized drainage basins as records of Neoglacial activity. It adopts an explicitly geomorphological approach that integrates an understanding of the drainage basin sedimentary system, specifically sediment sources and transfers, with the interpretation of lake sediment deposits and extends existing models of alpine sedimentary response down-valley, away from the immediate proglacial environment. A down-valley sequence of four valley bottom lakes, Kha, Klept, Kokwaskey and Kwoiek, within the Kwoiek Creek watershed, southeastern Coast Mountains of British Columbia, were studied. Sub-bottom sounding and multiple cores from each lake allowed identification of lake-wide changes in sediment input through time; in addition terrain mapping and characterisation of sediment sources provided a framework within which to identify the sources of the lake sediments and their fluctuations through time. Preliminary characterization of the sediments broadly separated organic and clastic components. Detailed laboratory analyses revealed organic matter content to be a good inverse indicator of sedimentation rates. Grain size analyses revealed three distinct textural populations. Graphical partitioning of the cumulative grain size distributions identified each fraction for further analysis. The provenance of the coarsest and intermediate fraction was determined through SEM surface texture analysis of a statistically representative number of grains. The coarsest fraction was derived from localized colluvial sources. The intermediate fraction was derived from glacial sources and strongly filtered downsystem. The finest fraction was characterised as glacial in origin because of consistent trends in its variability at the drainage basin scale through time. Fluctuations in the total influx of the intermediate and finest fractions are interpreted as a proxy record of Neoglacial activity in the watershed. Analysis of persistence in the sedimentation data indicates history of the order 100 yrs, which is interpreted as an index of the relaxation time of sedimentary stores. Basal dates on the sediments provide the earliest dates for deglaciation in the southern Coast Mountains, suggesting that extensive areas of southwestern British Columbia were ice free prior to 11 500 B.P. Three phases of Neoglacial activity centred 6000 to 5000 B.P., 3500 to 2900 B.P. and post 750 B.P are suggested by increased sedimentation rates for glacially-derived material. When compared with reconstructions from a pollen study conducted within the watershed and regional chronologies reported in the literature, there is remarkable consistency. The major advantage of the lake sediment approach as developed in this study is the continuity and apparent sensitivity of the derived proxy records. These records permit a consideration of both the magnitude and frequency of palaeoenvironmental change, specifically Neoglacial activity, at one site. Such a record has not been found elsewhere in British Columbia, where discontinuous terrestrial records have been used.

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