HYDROCLIMATE CHANGE IN SOUTHWESTERN BRITISH COLUMBIA, CANADA REVEALED BY A 7000 YEAR LAKE SEDIMENT OXYGEN ISOTOPE RECORD

Holocene climate variability in the Pacific Northwest has not been well characterized due to the scarcity of continuous, high-resolution records and inconsistencies 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 isotope 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°, 807 m), 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 three meters 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, 10 ¹⁴C measurements, and one tephra layer and analyzed 374 samples of authigenic carbonate (the <63µm fraction) collected at 2 mm intervals. The Turquoise Lake δ¹⁸O record exhibits highly positive values at ~7000 years BP, indicating low lake levels and dry conditions at this time, consistent with a similar δ¹⁸O record from Castor Lake, WA. δ¹⁸O values over the remainder of the Holocene exhibit marked multi-century scale variability, indicating changes in hydroclimate that likely far exceed those observed historically. The Turquoise Lake δ¹⁸O 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 should improve our overall understanding of past hydroclimate changes in North America.