Geochemical Analysis Reveals Climatically Driven Changes in Sediment Sources, Lillooet River Basin, British Columbia, Canada

Lillooet Lake contains a high-quality record of clastic varves spanning the Holocene Epoch. We recovered multiple, 10-12 m long sediment cores comprising thick (> 1 cm) silt-clay varves that span the past 800 years. Dominant sediment sources to Lillooet Lake include landslides in hydrothermally altered, Plio-Pleistocene volcanic rocks of the Mount Meager Volcanic Complex (MMVC), and glaciers draining granitic and metamorphic terrain. In an effort to understand changes in sediment sources to the lake through time, we used ICP-MS and laser ablation ICP-MS methods to measure trace and rare earth elements in the clay-size fraction of sediment samples collected from streams in the Lillooet Lake catchment and from the lake sediment cores. High concentration of rare earth elements and medium-to-high concentrations of Be, Zr, and Cs characterize sediments derived from the MMVC complex. In August 2010, a 48 x 10⁶m³ landslide sourced from the MMVC entered Lillooet River valley and provided large amounts of sediment to Lillooet Lake. Samples from the landslide deposit are likewise enriched in elements common to the MMVC. Varves dating to AD 1623-1900, have low concentrations of Be, Zr, and Cs and measured rare earth elements, suggesting that glacially derived sediments from non-MMVC sources were the dominant sediment source to the lake over that period. In contrast, concentrations of Be, Zr, Cs and the rare earth elements are relatively high in varves deposited since about 1900, which was a period of frequent large landsides in the MMVC. These elements steadily increase throughout the 20^th century, suggesting an increase in MMVC-derived sediments reaching the lake. The observed geochemical trend may be explained by: (1) an increase in extreme precipitation events, which caused an unusually large number of landslides in the MMVC; (2) an increase of MMVC-derived sediments due to rapid glacier retreat; (3) a decrease in glacigenic sediments originating from non-volcanic areas in the watershed; or (4) a combination of these factors.