Climatic significance of n-alkanes and their compound-specific δD values from lake surface sediments on the Southwestern Tibetan Plateau

Abstract

The content and hydrogen isotope ratios of n-alkanes extracted from recent lake surface sediments sampled from nine lakes in three different climate zones on the Southwestern Tibetan Plateau were analyzed. Values were compared with n-alkanes in plants from lake drainages, and δD values of meteoric water, lake water, and mean annual precipitation. The results showed that n-C23 was predominantly derived from aquatic plants, and n-C27–n-C33 from terrestrial higher plants. The average carbon chain length of n-C27–n-C33 (ACL27–33) was positively correlated with the mean annual precipitation. δD values of the long-chain n-alkanes n-C29 and n-C31 of terrestrial origin (varying between −214 ‰ and −169 ‰, and −226 ‰ and −185 ‰, respectively) were inversely correlated with mean annual precipitation; but in accordance with the average annual variations in δD (OIPC), δD values of n-C31 were strongly related to the δD values of growing season meteoric water (R 2 = 0.74). The large difference between δD values of n-C23 of aquatic origin and n-C31 (an average of about 27 ‰) demonstrates the enrichment of the lake water δD compared to precipitation, caused by strong evaporation in the semiarid–arid areas of the southwestern Tibetan Plateau. Average value of \( \varepsilon_{n\text{-C}_{25-31}/\text{p}} \) (−95 ‰) is evidently higher than the value observed in European wet regions (−128 ‰); besides, \( \varepsilon_{n\text{-C}_{31}} \) (about −116 ‰) is constant along the study transect (SD = 9), which indicates that n-C31 is a useful proxy for the environment.