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Abstract:
Empirical evidence suggested that the altitudinal dependence of hydrogen isotope ratios of leaf wax n-alkanes (dDwax) can
be used to estimate paleoaltitudinal changes. However, the application of dDwax-based paleoaltimetry remains difficult, as the
impacts of evaporative, transpirative and biosynthetic processes on hydrogen isotope fractionations in changing environments
and the influence of likely changing water vapor sources are not well explored. For this study, we sampled stream waters, soils
and plant leaves along two transects spanning large gradients of altitude, precipitation amount, vapor source, temperature
and vegetation type on the Tibetan Plateau (TP). dD values of stream water (as an approximation for dDp), soil water (dDsw)
and plant leaf water (dDlw) as well as leaf wax n-alkanes were measured in order to quantify isotopic fractionations in the
formation of leaf waxes.
Most interestingly, we found a strong negative correlation between the evapotranspirative enrichment of leaf water against
precipitation (elw-p), which combines the effects of soil evaporation and leaf transpiration, and the biosynthetic hydrogen isotope
fractionation (ewax-lw), which describes isotopic enrichment between leaf wax and leaf water. The relationship yields a
steady apparent isotopic enrichment factor (ewax-p) between leaf wax and precipitation, which is independent from climatic
parameters and has an average value of 107 ± 26‰ for grasses (monocotyledons) and 77 ± 22‰ for trees (dicotyledons).
Since the terrestrial n-alkanes, especially n-C27 and n-C29, in sediments are derived from trees and grasses, the likely change of
the vegetation type in the uplift of mountains can change the isotopic estimates by about ±30‰, which corresponds to an
altitudinal change of 1600 m. We, therefore, suggest that hydrogen isotope ratio of sedimentary n-C31 alkane, which is
mainly derived from grasses might be better proxies to reconstruct paleoaltitudes.
Our large dataset of dDwax from trees and grasses that aimed to mirror the variability of environmental factors over geological
time frames showed the lapse rates were significant, but much smaller than in previous studies. Most importantly our
result demonstrated that the lapse rate significantly differed for both transects (p = 0.0068), i.e. 0.87 ± 0.71‰/100 m
(R2 = 0.28, p = 0.2841, n = 6) and 2.28 ± 0.82‰/100 m (R2 = 0.34, p = 0.0135, n = 17) for Indian monsoon and Westerly dominated areas, respectively. This suggests that different moisture sources might strongly affected the observed lapse rates. In
consequences altitude reconstructions are strongly complicated in areas with likely changing air masses like the Tibetan
Plateau.
