Altitude effect of leaf wax n-alkane δD in surface soils along the Longmen Shan, eastern Tibetan Plateau

Abstract:

.Stable isotope data from the Tibetan plateau may largely reflect the evolution of air masses sourced from the Pacific Ocean and crossing the eastern plateau margin rather than the Himalayas (Blisniuk and Stern, 2005). We present 30 hydrogen isotope values of leaf wax-derived n-alkanes (δD_wax, the abundance-weighted average δD values of C₂₉ and C₃₁ n-alkanes) in surface soils and 23 δD of soil water (δD_sw) along the Longmen Shan, eastern Tibetan Plateau (altitude range 780-4200m* above sea level, representing Southeast Asian Monsoon from the Pacific Ocean air mass). Altitude is the predominant factor in determining soil δD_wax values, which varied from −160‰ to −223‰ with a lapse rate of −1.8‰/100m (R²= 0.81; n=30). We observed the same correlation for δD_sw values with altitude (R²=0.79). These δD_wax values were more enriched than the eastern slope of the Gongga Shan transect (−203‰ to −268‰ from 1230 to 4275 m; Bai et al., 2011). Additionally, our ε_wax/sw values of soil (the isotopic fractionation between the δD_wax remain approximately constant at –100‰ (ranging between −99 and –101‰), which demonstrates that the isotopic altitude effects of soil water (precipitation) control the n-alkane δD_wax altitudinal gradients for all of the sample localities in the Southern Himalaya (SH) and the southern TP (Bai et al., 2015). Additionally, there is a difference in ε_wax-rw with the increasing elevation, and which are smaller at low elevation (with ε_wax/rw of −107‰, average d=4.4) than those at high elevation (with larger ε_wax/rw of −137‰, average d=–0.3). Hence, it is likely that river water may not represent the actual source water on the eastern edge of the Tibetan Plateau, especially above ~3 km, and it is cautious when establish the modern isotope-elevation relationship using river water for any future paleoenvironmental and paleoelevation reconstructions.