HYDROGEN ISOTOPES IN LEAF WAX N-ALKANES AND RIVER WATERS ALONG TWO ELEVATION TRANSECTS IN MONTANA, USA

Hydrogen isotope values (δD_lf) of n-alkanes derived from the leaf wax of terrestrial plants have been utilized as a viable paleoclimate and paleoelevation proxy in some parts of the globe for it records the local paleo-precipitation hydrogen isotopic signature (δD_w). Understanding the controlling factors of δD_w and the relationship between δD_lf and δD_w in modern environments is fundamental to the application of this proxy but has not been well examined in North America. In this study, we collected 14 sets of samples of river water, topsoil, and river sediment, along a transect in northwestern Montana to the west of the continental divide and a transect between northwestern Wyoming and eastern Montana to the east of the continental divide, for δD_w and δD_lf analyses. Each sample set was collected at the same elevation to examine the leaf wax-water δD offset (ε_w/wax). River sediments were studied for leaf wax likely from the catchment above the sampling site, while river water is a good approximation of the mean catchment precipitation. River water sampling was expanded to examine the controlling factors of δD_w and further understanding of the offset. The δD_lf values of soil and river sediment at each site are similar, both show no correlations with elevation along the two transects. The river water δD_w values in the western transect do not show correlation with elevation, but those in the eastern transect show a low δ¹⁸O_w (δD_w) lapse rate of 1.0 ‰/km (r²=0.8) (3.7‰ /km, r²=0.4), a combined result of drainage effect, evaporation in eastern Montana, and eastward decrease (increase) of Pacific (recycled continental) moisture. The ε_w/wax values vary between -113‰ and -71‰, within the global range (-140‰ to -50‰). The similarity between soil and river sediment δD_lf values suggests that the leaf wax preserved in the river sediments were most likely primarily recycled from the adjacent soils, but the recycling of leaf wax from river sediments into local soils cannot be fully ruled out. The absence of a δD_lf lapse rate in the eastern transect may be mainly a result of vegetation change. Western Montana has more trees, but grassland dominates eastern Montana, and trees produce significantly higher δD_lf (~50‰) values than grasses using the same water. Although vegetation is a major factor influencing ε_w/wax variation, a spatial trend expected from the vegetation change is not observed from our ε_w/wax.