Organic Geochcmical Characterization Of Early-mid Holocenc Limnological Sediments From Eastern Qinghai-Tibet Plateau:Palenenviromental Implications

Biomarkers preserved geologic sediments can provide rich useful information about source, preservation, diagenetic process and historical changes in organic matter under paleoenvironmental changes. Biomarkers have become increasingly utilized for paleoenvironmental reconstructions. Zoige Basin located in the eastern margin of the Qinghai-Tibet Plateau is a topographic depression region in the uplift of Qinghai-Tibet Plateau during the Quaternary and in the influence of Asian Summer Monsoon (ASM) to become the largest highland marsh deposit in China. This provides excellent sediments for paleoenvironmental reconstruction. Hongyuan biomarkers study confirm that biomarkers can complement traditional geochemical proxies, and provide more specific environmental information regarding organic sources, vegetation, climate changes and so on. However, biomarkers studies on Zoige Marsh are in early stage. It is necessary to need more lipid researches on the sediments from Zoige Marsh that enhance deep understanding of the ecology and climate changes there.For these reasons, we investigated the organic matter (OM) composition characterization of a early-mid Holocene limnological sedimentary section/sequence from Langmusi bog in Zoige Marsh (eastern Qinghai-Tibet Plateau) using lithology, total organic carbon (TOC) content, Rock-Eval pyrolysis, and organic carbon isotope and biomarker assemblage. Based on the profile distributions of these proxies and previous studies, we discuss these proxies especially lipid compounds response to papleoenvirionment information such as organic sources and past climate changes, and reconstruct the early-mid evolution history of vegetation and climate in the Langmusi basin. The main results can be summarized as follows:1. Through comparison of the Langmusi profile δ13C record and the Hongyuan peat Car ex muliensis cellulose δ13C record, it is found that the peat Car ex m. δ13C record in the upper part of Langmusi profile have similar trend as the Hongyuan peat record, and bulk δ13C record in the nether is almost no related with the Hongyuan record. These features suggest that the two study sites of Langmusi and Hongyuan are controlled by the same climate system. Consequently, the peat Carex muliensis δ13C in the paper is a useful proxy of papleoclimate that can well record the past climate changes, while the bulk δ13C are affected by plant kinds and are difficult to reflect past climatic variations.2. A series of lipids, including n-alkanes (C16～C35),n-alkens (C19～C28), pristine (Pr) and phytane (Ph), n-alkan-2-ones (C19～C35), n-fatty acid methyl esters (C14～C30), are detected in the Langmusi profile by gas chromatography-mass spectrometry (GC-MS).3. HI have an average value of around250mg HC/g TOC. Lipid extraction yields are dominated by asphaltenes and non-hydrocarbon fractions, and have a relatively small amount of saturated and aromatic hydrocarbon fractions. The identified lipid compounds are characteristic of dominant component of long-chain (>C22) and strong odd/even (or even/odd) predominance. These features suggest that the major sources of the organic matter of the Langmusi profile origin from higher plant.4. The n-alkan-2-one results of the Langmusi section and other studies show that the peat sediments from Zoige Marsh are characterized of n-alkan-2-one with strong maximum at C23. However, n-alkan-2-ones in the lacustrine samples from the nether profile are mainly composed of C27-31odd carbon homologues. Consequently, the n-alkan-2-ones parameters (Cmax-KET、CPI-KET�'�ACL-KET) are useful papleoenvironmental proxies to sensitively indicate the sedimentary evolution of Langmusi basin.5. A homologous series of n-fatty acid methyl esters (n-FAMEs) are identified in the Langmusi samples reflect that the Langmusi basin deposits have specially biochemical process. Although substantial formation process of n-Es remains unknown, its parameter C16/C24is a useful proxy for environmental changes.6. The decreasing trend of CPI of n-alkanes,n-alkan-2-one and n-fatty acid methyl esters between124cm and34cm are attributed to relatively strong microbial activity and degradation.7. The sedimentary revolution of Langmusi basin is reconstructed according to lithology and organic geochemistry proxies in the paper. Langmusi basin mainly experienced two marked different hydrological conditions during the early-mid Holocene:1) from346cm (ca.11.37cal ka BP) to203cm depth (ca.7.33cal ka BP) lacustrine conditions accompanied by relatively high water levels and proportion of aquatic macrophytes and land plants organic contributions;2) from203cm upwards full swampy peat conditions with relatively low levels and relatively high herbaceous plant organic contributions. Moreover, this natural evolution has been superimposed by transitory significant hydrological fluctuations suggesting the instability of Asian Summer Monsoon (ASM) in the early Holocene. The profile does not contain sediment since the last ca.4500cal yr BP, probably due to the declining water table leaded by universal weakening Asian Summer Monsoon system in the eastern Qinghai-Tibet Plateau and thus a disruption in the sediment accumulation.