Rainshadow effect on hydrogen isotopes in leaf wax n-Alkanes across the Cascade Mountains of Washington, USA

International audienceHydrogen isotope ratios are sensitive tracers of the water cycle with the potential to constrain the timing of mountain building episodes in the geologic past. This study presents hydrogen isotope ratios (2H/1H or δ2H) of n-alkanes derived from the leaf wax of terrestrial plants, collected from trees and soils along an east–west transect across the Cascade mountain range of Washington State, USA. Along this transect, the δ2H values of n-alkanes (δ2Halkane) in surface soils and gymnosperm leaves are well-correlated with the mean annual δ2H of precipitation (δ2Hprecip) derived from instrument-model reanalysis products (R2 = 0.346 to 0.558, p < 0.001 for soil alkanes, R2 = 0.667 to 0.844, p < 0.001 for gymnosperm alkanes) and with measured δ2H values of nearby surface waters (R2 = 0.451, p < 0.001). All data from this study show increasing deuterium (2H) depletion with distance from the coast in response to Rayleigh distillation induced by the Cascade Mountains. Assessed within the context of a global dataset of 235 published soil δ2Halkane values vs δ2Hprecip, the Cascades results are statistically indistinguishable. However, the gymnosperm leaf δ2Halkane vs δ2Hprecip data from the Cascades differ from a global set of 87 such sites, with a steeper relationship. This is attributed to regional differences between the Cascades and other locations that include higher latitude, higher altitude, and the presence of a strong, narrow climatic gradient. The strong imprint of the Cascade rainshadow on δ2Halkane validates its use as a proxy for the timing of uplift of this mountain range when measured in suitable sedimentary rocks