A 10,000-year lake-sediment based reconstruction of precipitation isotope values in the Canadian Rocky Mountains and implications for past changes in North American hydroclimate

Abstract

Hydroclimate change in the Canadian Rocky Mountains is an important area of research, as demand for water resources in the Great Plains has been continuously increasing in recent years. The population of Alberta has one of the fastest growth rates in Canada, and cities like Calgary and Edmonton are dependent on surface and groundwater resources that originate from precipitation in the mountains. Recent increases in petroleum exploration have also amplified demands for water, along with the growing water requirements of industrial agriculture. The application of sound water resource management policies is essential, and historical records span only ~200 years, a time frame too short to capture the full range of climate variability. The development of paleoclimate proxy records from the Rocky Mountains is therefore necessary to attain a thorough perspective on potential changes in climate. Such data can, for example, inform water resource managers of possible shifts in precipitation seasonality and drought/pluvial events on timescales of decades to millennia. To this end, we present a 10,000-year oxygen isotope record as a proxy for precipitation seasonality linked to the Pacific North American pattern (PNA) which adds to a growing body of research in a region of high spatial complexity of hydroclimate. Shark Lake in Alberta, CA (50.8412°N, 115.3990°W; 1857 m above sea level) is a hydrologically open basin lake in the Canadian Rocky Mountains with one large outlet and numerous small inlets and groundwater springs. Winter precipitation is more depleted in the heavier 18O isotope relative to summer precipitation due to equilibrium fractionation from rain-out and phase changes. δ18O and δD values of Shark Lake water samples (collected during the summer) range from -18.9 to -20.0 ‰ and -141 to -150 ‰, respectively. The annual weighted average precipitation isotope ratios are -16.6 and -126 ‰ for oxygen and hydrogen, respectively. This indicates that Shark Lake principally receives water inputs from runoff or shallow groundwater that originated as precipitation during the cold season. We collected 1 m and 1.5 m long sediment cores using a modified Livingstone corer and used loss-on-ignition (LOI), x-ray diffraction (XRD), x-ray fluorescence (XRF) analyses to destermine sediment texture and composition. The sediment was dated using 14C from terrestrial plant fossils, as well as 210Pb and 137Cs for the surface sediment. We analyzed the isotopic composition (δ18O) of authigenic carbonate sediment from Shark Lake using mass spectrometry and applied these results as a proxy for precipitation seasonality. Lake water oxygen isotope ratios are captured by authigenic carbonates, which form during the spring and summer in response to pH changes associated with primary production. These carbonate minerals (calcite) precipitate in isotopic equilibrium and are deposited on the lakebed where they are preserved. The Shark Lake δ18O record demonstrates a transition from lower to higher average δ18O values from the middle to the late-Holocene at around 4500 yr BP. This is consistent with previous findings of changes in PNA-like atmospheric patterns during the middle Holocene, when a gradual shift from a more negative to a more positive mean state phase of the PNA occurred. This produced enhanced zonal atmospheric circulation in the Pacific Basin that led to a reduction in winter precipitation in northwestern North America and drier conditions in the southwestern part of the continent in the late Holocene relative to the middle Holocene. The Shark Lake δ18O record has a positive, significant correlation with other similar records from the Pacific Northwest, specifically those from Lime Lake (WA) and the OCNM (OR) speleothem, and has a negative, significant correlation with records from the southern Rocky Mountains and eastern North America, namely those from Bison Lake (CO), Cheeseman Lake (NL), Grinnell Lake (NJ), and the Buckeye Creek Cave speleothems (WV). Decadally resolved records are useful for tracking changes in PNA state and its interaction with other related climate oscillations such as the El Niño Southern Oscillation (ENSO) and the associated Pacific Decadal Oscillation (PDO). Open-basin lake records can also be combined with hydrologically closed-basin lake records to reconstruct pluvial and drought periods over the Holocene. The Shark Lake record increases the spatial resolution of mid- to late Holocene hydroclimate climate data in the Rocky Mountains and provides a baseline for natural variability in precipitation seasonality in a hydrologically important region.