PLACING RECENTLY OBSERVED DRYING TRENDS IN THE PEACE-ATHABASCA DELTA, CANADA, INTO AN HISTORICAL CONTEXT USING PALEOLIMNOLOGY: STABLE ISOTOPE RESULTS

The Peace-Athabasca Delta (PAD) consists of hundreds of lakes whose water and nutrient balances require periodic replenishment by ice-jam flooding on the Peace and Athabasca Rivers. The PAD provides extensive wildlife habitat and is recognized internationally for its ecological significance. Over the past 30 years, concern over drying of the PAD has been heightened by construction of the Bennett Dam on the Peace River in 1968 and the absence of a major ice-jam flood event for 20+ years from the mid-1970s to the mid-1990s. However, climate and hydrological records are too short to evaluate the significance of these recent observations. In order to develop a framework of natural variability, we are using a multi-proxy approach to reconstruct hydrological and ecological conditions over the past 1000 years from several lake sediment cores.

Stable isotope studies in the PAD have focussed on assessing the modern hydrological conditions of lakes and reconstructing paleohydrology from lake sediment cores. Results show that seasonal changes in water balance are reflected by lake water oxygen and hydrogen isotope composition, with isotopically-depleted summer flood signatures clearly distinct from isotopically-enriched signatures related to evaporative draw-down. The broad range of isotopic values related to flood versus non-flood status provide a useful template for reconstructing past hydrological conditions from the oxygen isotope composition of aquatic cellulose in sediment cores, which can be used as a tracer of lake water oxygen isotope composition. At Jemis Lake, a perched basin in the PAD, cellulose-inferred lake water oxygen isotope enrichment during the 19th century is similar to that observed after 1950, suggesting this most recent dry interval is within the range of natural variability of the past 200 years. Conversely, more depleted values in the cellulose-inferred lake water oxygen isotope record during the early part of the 20th century suggests relatively wet conditions. Overall, the multi-decadal dry-wet-dry cycle evident in the sediment record from Jemis Lake is consistent with a 175-year observational record of Peace River ice-jam flood frequency, indicating that climate-driven shifts in hydrology are a natural feature of the PAD, independent of anthropogenic influence.