ABSTRACT: Late glacial and Holocene environmental change inferred from sedimentary archives of Kusawa Lake, Boundary Range Mountains, Yukon Territory, Canada.

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Abstract

Modern Kusawa Lake (60°19'55”N, 136°4'48”W, 142 km2), drains a 4290 km2 catchment, 4.7 % of which is glacier covered. The drainage basin straddles the Boundary Ranges between the Yukon Territory and British Columbia. Glacial Lake Champagne, which formed during the Late Pleistocene when westward flowing glaciers in the Takhini valley coalesced with eastward flowing glaciers from the St. Elias complex, occupied all of Kusawa Lake and the surrounding valleys. As a trunk glacier occupied the southern portion of Kusawa Lake, sediment accumulation was derived in response to the complete drainage of Glacial Lake Champagne ~10.5 ka BP. Given the post-glacial sedimentary environment, Kusawa Lake and the upper Takhini River basin provide the opportunity to assess the utility of large lake records in reconstructing Holocene environmental change. Nine sediment cores in the northern region, and two cores in the central and southern regions of Kusawa Lake were extracted using a Rossfelder vibra corer. Analysis of sediment properties including percent organic matter by loss-on-ignition and grain size distribution help distinguish watershed-specific processes from the local climate signal. Sediments in the deep, southern part of the lake show at least 15 major and 25 minor episodes of high-energy turbidity deposits (sand). An age-depth model was constructed using 5 C14 dates and a tephra layer. The 2.8 m long core indicates an average accumulation rate of 0.27 to 0.48 mm/yr, thus the upper 3 m documents the entire Holocene sediment input. These rates correspond with estimates made from a core taken in the central region of the lake. The early Holocene shows frequent sand laminae deposits alternating with silt-clay. The late Holocene shows infrequent sand turbidite inputs but higher rates of silt-clay deposition as distinct couplets reflect the cooler, wetter Neoglacial climate. A sequence of paleolimnological change was produced through the analysis of diatoms in the core. Fluctuations in diatom primary production suggest that the Holocene Thermal Maximum (HTM) began ~ 9 ka BP, terminating ~ 7 ka BP. Coincident changes in key planktonic taxa abundance, species diversity, and comparisons of dates with other records from the region, helped corroborate the HTM boundary. Sediment properties of cores in northern Kusawa Lake near the Primrose River delta show alternating sequences of finely laminated silts (0.2 ± 0.078 cm) and clays (1.17 ± 0.8 cm), which are interrupted by sand beds (5.2 ± 4.1 cm). These indicate periods of energetic turbidity currents, alternating with quiet water deposition. Cores proximal to the Kusawa Campground Alluvial fan show a different style of sedimentation ranging from thick silt, to sand beds (2 - 7 cm) with grading and evidence of erosional contacts, to organic beds (< 15 cm). From laminae couplet thickness in cores, the average rate of deposition adjacent to the alluvial fan is ~ 5mm/yr. The frequency of sand deposits appears to have increased since 1850 AD (Chow, 2007).

Bibliography of Canadian Geomorphology