HOLOCENE CLIMATE CHANGE AT SPINDLY PINE LAKE, SOUTHWESTERN YUKON, INFERRED FROM THE HYDROGEN ISOTOPE COMPOSITION OF PLANT WAXES

The isotopic composition of precipitation is a tracer for hydrological change, and is sensitive to temperature, seasonality and amount of precipitation, and moisture trajectory among other processes. The hydrogen isotopic composition of fossil plant waxes (δD_wax) in sediments is a proxy for precipitation δD and, thus, paleoclimate, but is offset from δD_precip due to a large net fractionation from several biotic and abiotic factors. We collected a ~5 m sediment core from Spindly Pine Lake in southwest Yukon Territory (60.30°N, 134.99°W) that spans the full Holocene with a basal date of 12.1 ka cal. BP, and measured the δD_wax of n-C₂₈ alkanoic acids at ~400-yr resolution. The δD_wax record has a mean of -228‰ (1σ = 8‰, n = 22) and a 26‰ range. The record is characterised by a negative trend from ca. 12 to 2 ka cal. BP, and a recovery to intermediate values after 2 ka cal BP. Higher frequency (centennial) trends are also present, and result in local minima at ca. 8.6 and 4.2 ka cal. BP, the latter being coincident with a prominent δD excursion in the Mt. Logan ice core record. While our δD_wax record exhibits some variability shared by other Holocene temperature proxies in this region, other trends are inconsistent. Our record is not likely to be driven solely by mean annual temperature since the δD_precip-temperature sensitivity for this region is ~3.1‰·°C^-1, which implies a shift of ~8°C given the δD_wax range of our record, and is larger than previous estimates for the Holocene. Other factors that may explain part of the signal include ecology (which can influence net fractionation), moisture source and precipitation seasonality. Pollen data show a clear shift from spruce- to pine-dominated forest at 3.8 ka, but δD_wax appears to be insensitive to this transition suggesting that ecology is not a primary driver of the δD_wax record. Aleutian Low strength is often cited as a major driver of precipitation isotopes and precipitation amount/seasonality in this region. Precipitation δD in southwest Yukon Territory is characterized by strong seasonal variation, with an annual range of 70‰ and summer values being ~30‰ more enriched than the annual mean. Therefore, it is conceivable, and perhaps probable that North Pacific atmospheric circulation accounts for a significant fraction of variance in the Spindly Pine δD_wax record.