ABSTRACT: Holocene paleoclimatic transitions inferred from diatom assemblages in sediments of Kusawa Lake, Yukon Territory, Canada.

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Abstract

The southwest Yukon Territory, Canada, has long been recognized as an important region for recovering sensitive records of Holocene paleoclimatic change, owing to pronounced moisture and elevation gradients, sharp vegetation transitions, and the abundance of glaciated alpine watersheds. More information is required, however, to refine the magnitudes and timing of major Holocene climatic transitions, and associated impacts on ecosystems. Lake sediments are important repositories of paleoclimatic information although watershed- specific processes need to be identified prior to the extraction of the regional climate signal. Smaller lakes and ponds have been the focus of many studies. We used analyses of both physical properties of the sediments and biological proxies in cores from Kusawa Lake (60°19'55°N; 136°4'48°W; 671 m a.s.l) to record the sensitivity of this large (142 km2), deep (max. 135 m) freshwater ecosystem to Holocene climatic and environmental transitions. An depth-age model was constructed for a single core in the deepest part of the lake using five AMS radiocarbon dates and a tephra layer. A high-resolution sequence of paleolimnological change was produced through the analysis of diatoms (microscopic algae) in the core, and measurement of biogenic silica (BSi). Fluctuations in diatom primary production as documented both by BSi and valve concentrations suggest that the Holocene Thermal Maximum (HTM) began ~9000 cal yrs BP, terminating ~7000 cal yrs BP. Coincident changes in relative abundance of key planktonic taxa, increases in species diversity, and comparisons of those dates with other records from the region, help to corroborate the existence of these regional climatic changes at that time. Stratigraphy and grain size data show frequent sand beds and laminae alternating with silt-clay laminated sections between 9700 cal yrs BP and ~6500 cal yrs BP, indicating a period of energetic turbidity currents, alternating with quiet water deposition. The late Holocene shows less frequent sand turbidite inputs but higher rates of primarily silt-clay deposition as distinct couplets reflecting the cooler and wetter Neoglacial climates.

Bibliography of Canadian Geomorphology