HOLOCENE CLIMATE VARIABILITY OF THE SOUTHERN ALBERTA ROCKY MOUNTAINS RECONSTRUCTED USING OXYGEN ISOTOPE ANALYSIS OF CLOSED-BASIN LAKE SEDIMENT

Analyses of oxygen isotope ratios in authigenic lacustrine carbonate sediment from South Hogarth Lake (Alberta, Canada) provide a ~10,000 year record of variations in precipitation-evaporation balance in the southern Canadian Rocky Mountains. South Hogarth Lake is a small, alkaline, closed-basin system that is at the highest elevation of a series of three connected lakes. The calcareous sediment is authigenic calcite (CaCO₃), as determined through SEM and XRD analyses. The age model for the core is based on a series of radiocarbon and tephra dates and indicates that sediment has been continuously deposited in the basin since deglaciation. We sampled the lake sediment cores at 1-10mm intervals and measured the isotopic composition of fine-grained, authigenic CaCO₃ in the samples. In closed-basin lakes, negative δ¹⁸O values in both water and sediment generally indicate wetter conditions, and positive δ¹⁸O values indicate drier conditions. Water isotope measurements show that South Hogarth Lake loses substantial amounts of water through evaporation (i.e. lake water samples are isotopically enriched relative to local meteoric water), and because of this the lake responds strongly to both short and long term shifts in the balance between precipitation and evaporation. Previous work on similar lakes in the Pacific Northwest suggests that mid-Holocene climate was drier than present overall, but wetter conditions persisted during the cold season. Several climate model simulations from the Paleoclimate Modeling Intercomparison Project Phase 3 (PMIP3) support the idea that the mid-Holocene in the Pacific Northwest was characterized by enhanced hydroclimatic seasonality, although this result is not consistently expressed by all models. Oxygen isotope measurements from South Hogarth Lake provide insight on climate change in an underrepresented region of western North America and thus provide a new spatial perspective for testing hypotheses on ocean-atmosphere control of precipitation-evaporation balance variability during the middle through late Holocene.