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BIOMARKERS AND CARBON ISOTOPIC ANOMALY FROM THE MIOCENE LACUSTRINE OIL SHALES IN HOH XIL BASIN OF NORTHERN TIBETAN PLATEAU

YI Hai-shen 1, LIN Jin-hui 1, WANG Cheng-shan 1, PENG Ping-an 2 (1.Chengdu University of Technology, China; 2. Guangzhou Institute of Geochemistry, Chinese Academy of Science)  
The oil shale and lacustrine sediment samples from the Miocene Wudaoliang Formation sections of the Hoh Xil basin in northern Tibetan plateau are studied in order to rescontruct paleolake environment and to deduce paleoclimatic change information. Twelve samples are examined by total organic carbon analysis and Rock-Eval pyrolysis. The solvent extracts and kerogens from oil shales and marls are separated and examined by organic petrographic, GC-MS, and GC-IRMS methods. The relative abundances of biomarkers and carbon isotopic composition of individual hydrocarbons are used to explain organic source and paleoecosystem. The individual carbon isotopic compositions of short-chain n-alkanes suggest the input from aquatic organisms, and the long-chain n-alkanes characterized by maximum at n-C 29 and an odd-over-even predominance in the range C 23-C 31 are derived from terrestrial plants. Pristane and phytene commonly reflect the input from algae and cyanobacteria in the shallow photic zone. Hopanoid biomarker is produced by organisms inhabiting the lower part of the water column of paleolake. Hopanes and moretanes are likely from methanotrophic bacteria community and chemoautrophic bacteria living at the lacustrine oxycline interface or above anoxic bottom water. The authors oberserved that the oil shale and marl in the lower part of stratigraphic section show higher values in gammacerane and β-carotane, its depositional environment is interpreted as a stratified paleolake system. β-carotane could be related to halophilic archaebacteria or phototrsynthetic bacteria in deeper photic zone. Gammacerane is derived from primary producers and green sulfer bacteria living at or below chemocline and is an indicator for water column stratification. The upper oil shale samples are clearly rich in 13C compared to the lower oil shale samples, δ 13C in the kerogen is -17.76‰, the average value of δ 13C of the individual n-alkanes is -21.97‰, and δ 13C of the n-C 23 maximiumg peak is about -15.01‰. The novel heavier 13C enrichment is proposed to correspond to colonial green algae (Bostryococcus) bloom or indicate C 4-type vegetation expansion. This also implies that the Tibetan plateau probably uplifted to a critical height and lacustrine ecosystem were adjusted and updated in the late Miocene.
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