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

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 drain- ages, 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 （R2 = 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 εn-c25- 31/p （-95 ‰） is evidently higher than the value observed in European wet regions （-128 ‰）; besides, εn-C31 （about -116 ‰） is constant along the study transect （SD = 9）, which indicates that n-C31 is a useful proxy for the environment.