A 7000 YEAR OXYGEN ISOTOPE RECORD OF CLIMATE CHANGE IN SOUTHWESTERN BRITISH COLUMBIA, CANADA

Holocene climate variability in the Pacific Northwest is the subject of ongoing debate due to the scarcity of continuous, high-resolution records and differences in proxy sensitivity. Analyses of the oxygen isotopic composition of lake sediment carbonates contribute to this research by providing information on past changes in water δ¹⁸O values (δ¹⁸O), which respond to a variety of climate variables including changes in the balance between precipitation and evaporation, lake level, and/or the isotopic composition of precipitation. Here we present a decadally resolved carbonate oxygen isotope record spanning the last ~7000 yr BP from Turquoise Lake (N50.83°, W121.69°, 807m), a small, alkaline, semi-closed basin system located in southwestern British Columbia. Turquoise Lake surface water isotope values fall along the local evaporation line but are not substantially enriched relative to local meteoric water, indicating that water losses from the lake occur principally through overflow and groundwater outseepage. We measured the δ¹⁸O values of carbonates in 3m of sediment recovered from the depocenter of Turquoise Lake, in order to make inferences on climate change during the Holocene.

We dated the Turquoise Lake record using ²¹⁰Pb, ¹³⁷Cs, ten ¹⁴C measurements, and one tephra layer and analyzed 293 samples of authigenic carbonate (the <63µm fraction) collected at 2mm intervals. The Turquoise Lake δ¹⁸O record exhibits an increasing trend from -17‰ to -16‰ over the last 4000 years. During this time period, five 400-600 year intervals occur in which δ¹⁸O values gradually increase then abruptly decrease thereafter. Greater variability in δ¹⁸O occurs from 4000 yr BP to 1500 yr BP (between -17.5 and -15‰), while after 1500 yr BP the record is less variable (-17.3 to -16‰). More negative δ¹⁸O values occur from 1100 yr BP to 500 yr BP, implying wetter conditions during the Medieval Climate Anomaly, followed by a steady increase until 400 yr BP, which indicates a drier climate early in the Little Ice Age. The results provide insight on the influence of Pacific ocean-atmosphere dynamics including the El Niño Southern Oscillation, Pacific Decadal Oscillation, and the Pacific North American Pattern on climate in the Pacific Northwest, and thus will improve our overall understanding of past hydroclimate changes in North America.