Proxy value of n-alkan-2-ones in the Hongyuan peat sequence to reconstruct Holocene climate changes on the eastern margin of the Tibetan Plateau

n-Alkan-2-ones in the Hongyuan peat core from the eastern margin of the Tibetan Plateau were investigated to assess the paleoclimate proxy value of variations in their compositions over the last 13,500 cal years. A homologous series of these compounds that ranged from C(19) to C(31) was identified throughout the peat sequence, maximizing at C(23) and showing a strong odd-over-even carbon chain length predominance. To help evaluate the possible paleoclimatic value of the n-alkan-2-ones in the Hongyuan peat core, we compared our molecular results with the pollen records from the same sequence. Results show that past changes in monsoon precipitation amounts as revealed by the ratio of Abies/Cyperaceae (firs/sedges) are also recorded in the n-alkan-2-one (C(23) + C(25))/(C(27) + C(29) + C(31))-KET ratio and CPI-KET (Carbon Preference Index) value. Stratigraphic variations of n-alkane and n-alkanoic acid ratios suggest that changes in the moisture-sensitive microbial alpha-oxidation of odd-chain n-alkanes and decarboxylation of even-chain length n-alkanoic acids is recorded in the abundance of n-alkan-2-ones. Changes in the monsoon rain intensity on millennial scales are also evident in the n-alkan-2-one proxies, indicating that their variations in the peat sequence are correlative with changes in microbial activity controlled by the Asian monsoon. (C) 2011 Published by Elsevier B.V.