DISTRIBUTION OF N-ALKANES IN THE LUGUHU LAKE SEDIMENTS AND THEIR PALEOVEGETATION SIGNIFICANCE

Using GC/MS analysis, n-alkanes have been identified in the sediments of the Luguhu Lake, southeastern Tibetan Plateau.The n-alkanes for all the samples are distributed in a bimodal pattern, indicating a mixed source. They are classified into two types. The Type Ⅰ found in seven samples is characterized by significant abundance of C₂₇,C₂₉ and C₃₁, indicating a substantial input of greater and higher plant. The other 9 samples, which fall in Type Ⅱ, are characterized by lower microorganisms input. The significant abundance of the medium-chain(C23 and C25) n-alkanes and the value of P_aq and the ratio of (C₁₅+C₁₇+C₁₉)/(C₂₇+C₂₉+C₃₁) indicate an input of greater aquatic plant. The grater endogenous input maybe one of the reasons for higher TOC in the Luguhu Lake. The parameters of (C₂₇+C₂₉)/(C₃₁+C₃₃) and ACL, which represents relatively high woody plants compared to grassy plants, is 1.73~4.28 and 26.19~28.1, respectively, indicating that wood plants have dominated the Luguhu Lake area since 30 kaBP. It is consistent with the present vegetation distribution pattern in the Luguhu Lake area. The vertical distribution of (C₂₇+C₂₉)/(C₃₁+C₃₃) and ACL does not suggest a obvious change trend. It means the change in relative contribution of woody plants and grassy plants recoded by n-alkanes was rather small. It is worthy to note that the paleoclimate of the Luguhu Lake area was cold and dry about 12 kaBP, which could be the response to the Younger Dryas Event.