A 10,000-YEAR LAKE-SEDIMENT BASED RECONSTRUCTION OF PRECIPITATION ISOTOPE VALUES IN THE CANADIAN ROCKY MOUNTAINS

Variations in precipitation timing, amount, and phase affect water availability in the Rocky Mountains and the nearby Great Plains, with implications for agriculture and industry. Previous research demonstrates that the oxygen isotope composition of lake sediment can provide long time-scale records of changes in hydroclimate, specifically precipitation source and/or precipitation-evaporation balance.

We collected sediment cores from Shark Lake located in Peter Lougheed Provincial Park, Alberta, Canada in order to produce a Holocene length record of hydroclimate change. Shark Lake is an open-basin lake with water isotope values consistent with local meteoric water. Authigenic carbonate minerals form in the water column and record the isotopic composition of water at the time of mineral formation. We used IRMS to measure the sediment oxygen (δ¹⁸O) and carbon (δ¹³C) isotopic composition. Sediment texture, mineral, and elemental composition were determined using loss-on-ignition (LOI), X-ray fluorescence (XRF), and X-ray diffraction (XRD). An age model produced using 17 AMS ¹⁴C ages of terrestrial macrofossils, one tephra, and ¹³⁷Cs dating of surface sediment indicates continuous sediment deposition over the last ~10,000 years.

The Shark Lake δ¹⁸O record exhibits a transition from lower to higher average values from the middle to the late-Holocene at around 4500 yr BP. This is consistent with previous findings of changes in Pacific North American (PNA) pattern-like atmospheric circulation during the middle Holocene, when a gradual shift from a more negative to a more positive mean state phase of the PNA likely 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. Consistent with this hypothesis, the Shark Lake δ¹⁸O times series has a positive, significant correlation with other similar records from the Pacific Northwest, i.e. 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).