GEOMORPHIC AND CLIMATIC INTERPRETATION OF SEDIMENT GEOCHEMISTRY IN MOOSE LAKE, BRITISH COLUMBIA, CANADA

Mineralogical and geochemical data are used to infer source area and production processes for lake and drainage basin sediments of the partially glacier covered uppermost Fraser River watershed of the Canadian Rocky Mountains. Moose Lake sediments examined from multiple cores record three major phases of deposition. Coarse sand and silt turbidite beds > 9120 ± 80 years in age are overlain conformably by massive to weakly laminated fine-grained sediments indicative of low rates of sediment input during most of the early to middle Holocene. At approximately 4100 yrs BP, sedimentation rates increased, coincident with the formation of varves which provide a chronology of more recent (Neoglacial) sediment inputs. The composition of Moose Lake sediments (6 major and 24 minor/trace elements) was determined at 10 cm intervals along our longest (12.5 m) core using Instrumental Neutron Activation Analysis. For comparison, the < 63 ìm fraction of 230 upstream river bed and floodplain samples was also analyzed. The results show that watershed source areas may be distinguished on the basis of elements associated with carbonate and aluminosilicate bedrock types, and that one major sub-basin (that of the Moose River tributary) contributes a disproportionate amount of the total fine-grained load due, in part, to its more extensive glacier cover. In lake sediments, carbonate content is substantially depleted due to solution, and redox sensitive elements are found to vary irregularly. However, systematic changes in the concentrations of more stable elements confirm that source variation is recorded and preserved in the lacustrine sequence, and can be interpreted in terms of climatic and geomorphic controls on basin sediment yield. Hence, middle Holocene sediments are progressively enriched in Al, K, and related trace elements derived from non-glacial areas of the watershed. Sediments of the last 4100 years contain an elevated but variable (Neo)glacial signal. Comparison of variations in composition, texture, and apparent accumulation rate (from varves) in the Moose Lake record allow discrimination of sediment production and sediment delivery mechanisms and is the subject of ongoing work.