Effect of precipitation regime on δD values of soil n-alkanes from elevation gradients – Implications for the study of paleo-elevation

Abstract

We measured dD values of long chain n-alkanes isolated from 30 surface soil samples along two elevation transects on the Tibetan Plateau differing in precipitation regime and water source. The East Asian Monsoon precipitation dominates the wetter regime on the eastern slope (from 1230 to 4300 m) of Gongga Shan on the eastern Tibetan Plateau. Precipitation from the Polar Westerlies dominates the drier region on the slope from 1900 to 5000 m in the West Kunlun Shan on the northwestern Tibetan Plateau. The decrease in dD value with elevation in the wetter region greatly exceeded that in drier region by, � 1.9 ± 0.1‰/100 m and � 1.4 ± 1.0‰/100 m respectively. The apparent fractionation between leaf wax and precipitation ewax–p values in the wetter region (ca. � 164‰) were more negative than those in drier region (ca. � 125‰ above 3200 m). We also measured dD values in leaves of six common living trees (values from � 287‰ to � 193‰) from Gongga Shan, ranging from about 2900–4200 m. The abundance-weighted average values of the nalkanes (dDwax) show a strong reverse correlation with sample source elevation (R 2 0.78 for soils from 2 0.85 for soils from West Kunlun Shan above 3200 m), suggesting that n-alkane dDwax faithfully records the precipitation dD and that the isotopic altitude effect of precipitation controls dDwax altitudinal gradients in the mountains. The data show a fairly strong monotonic dependency of n-alkane dD values on elevation for the eastern Plateau, but a complex relationship between n-alkane dD values and elevation for the northwestern Plateau. The dDwax values at sites below 3200 m from the Kunlun Shan area exhibit an unexpected positive correlation with elevation. The study confirms the potential for using sediment dDwax values to reconstruct paleo-elevation in wetter regions, but suggests caution in applying the approach to dry regions. Our results also show it is essential to consider the intricacy of the pattern of atmospheric circulation and water sources and their influence on the lapse rate of dD values with elevation.