Altitudinal Gradients of Stable Isotopes in Lee-Slope Precipitation in the Canadian Rocky Mountains.

Fresh snow samples were collected following seven snow accumulation events along an altitudinal transect of the Robertson Valley. This glacierized valley is on the eastern slopes of the Canadian Rockies at the Continental Divide and receives precipitation from both westerly (Pacific) air masses and from easterly (upslope) systems. Snow samples were collected over two winter seasons and were analyzed for δ¹⁸O, revealing altitudinal gradients that ranged from -0.3%/100 m to +1.8‰/100 m. Five of seven snow events had positive (inverse) isotopic gradients with altitude: ¹⁸O enrichment at higher altitudes. Surface and upper-air meteorological data were analyzed to classify the type of weather systems bringing precipitation to the area for each accumulation event. Three storm classifications were developed: westerly, upslope, and mixed/ northwesterly systems. Positive δ¹⁸O-elevation gradients were found under strong westerly and northwesterly flow, when the Robertson Valley acts as a leeward slope, while more conventional negative gradients correspond with upslope flow, when easterly winds make the Robertson Valley a windward snow deposition environment. We interpret the inverse isotopic gradients as evidence of ongoing Rayleigh distillation as westerly systems cross the Continental Divide. Position on the Rayleigh distillation curve had a strong influence on the magnitude of δ¹⁸O-elevation gradients. [ABSTRACT FROM AUTHOR]