A Multiple Proxy Lake Sediment Record Of Middle Through Late Holocene Climate Change In Southwestern British Columbia, Canada

Abstract

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 debate 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 ~7500 years BP from Turquoise Lake (N50.83°, W121.69°, 807m), a small, alkaline, semi-closed basin system located in southwestern British Columbia. Turquoise Lake waters are dominated by cold season meltwater and 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, twelve 14C measurements, and one tephra layer and analyzed 375 samples of authigenic carbonate (<63µm fraction), collected at 2-10mm intervals. Multiple turbidite sequences possibly related to the Mazama ash deposition complicate the δ18O signal, but high-resolution X-ray fluorescence and magnetic susceptibility are used to identify and remove distinct intervals of detrital influence from the δ18O record. Detailed characterization of the sediment by scanning electron miscroscopy and powder x-ray analysis allowed for the distinction between useful authigenic sediment and clastic allochthonous input. With clastic turbidites removed, the Turquoise Lake δ18O record exhibits a decreasing trend of average δ18O values from -15 to -17‰ over the last 7500 years implying a dry middle Holocene and a transition to a wetter late Holocene. The past 4000 years are marked by overall steady conditions punctuated by decadal positive spikes in δ18O reflective of long-term drought events. More negative δ18O values occur from 1100 yr BP to 500 yr BP, implying wetter conditions during the Medieval Climate Anomaly (MCA). A steady increase in δ18O values from 400 yr BP indicates a drier climate early in the Little Ice Age (LIA), followed a steep decline at 200 yr BP, indicating a wetter late LIA. The results provide insight on middle to late Holocene climate and add to the growing network of lacustrine δ18O records and thus will improve our overall understanding of past hydrologic changes in North America.