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.</p

The n-alkanol paleoclimate records in two peat deposits: a comparative study of the northeastern margin of the Tibetan Plateau and Northeast China

A number of studies have revealed that the climate in the eastern margin of the Tibetan Plateau and Northeast China is sensitive to postglacial changes. Unfortunately, the link of the past climate evolution between the two regions is not well understood. In this study, two cores are analyzed to determine this link directly. The high-resolution n-alkanol distribution patterns from two typical peat sequences covering the past 16,000 cal years in the northeastern margin of the Tibetan Plateau and Northeast China, respectively, are closely examined by gas chromatograph-mass spectrometry analysis. In combination with other palaeoclimatic proxies, it is proposed that the n-alkanol average chain length and (C(22) + C(24))/(C(26) + C(28)) ratio could reflect past climate changes in the two peat sequences. The n-alkanol proxies reveal several climatic intervals in the period from the last deglaciation through the Holocene. A comparison of n-alkanol records between the northeastern margin of the Tibetan Plateau and Northeast China indicates that the start and end of the warm Holocene Optimum differed at the two locations. The spatially asynchronous pattern of climatic change is possibly a result of different responses to change in solar radiation. The evolution of the Holocene paleoclimate is more consistent with changes in Northern Hemisphere solar radiation in Northeast China than on the Tibetan Plateau. The Holocene Optimum began and terminated earlier in Northeast China than in the northeastern margin of the Tibetan Plateau. Thus, the two n-alkanol proxies provide valuable insights into the regional Holocene climate and local environmental conditions.</p

n-Alkan-2-one distributions in a northeastern China peat core spanning the last 16 kyr

Most research on long chain methyl ketones has focused on their origins and distributions. Their application in paleoclimate studies is less common than that of other n-alkyl lipids. The goal of this research was to explore this potential by studying n-alkan-2-ones from the Hani peat sequence in northeastern China. They were identified using gas chromatography-mass spectrometry (GC-MS) and showed a distribution ranging from C(19) to C(31) with a strong odd/even predominance. This type of distribution is considered to derive from Sphagum and microbial oxidation of n-alkanes. Comparison with climate sensitive indicators and macrofossil analysis shows that microbial oxidation of n-alkanes derived from higher plants was enhanced during the warm early Holocene period. This led us to develop three n-alkan-2-one proxies - C(27)/Sigma C(23-31) (C(27)/HMW-KET), carbon preference index (CPI(H)-KET) and average chain length (ACL((27-31))-KET) - as possible indicators of paleoclimate in the peat-forming environment. These proxies, in combination with C(27) n-alkane delta D values and peat cellulose delta(18)O records, might allow examination of paleo-ecosystem behavior during climatic evolution in northeastern China over the past 16,000 yr.</p