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In this study, n-alkanes, isoprenoids and n-alkyl-ketones were detected in lacustrine sediments in the Maogou section of the Linxia Basin, NE Tibetan Plateau, NW China. The distribution characteristics of these compounds correspond to arid climate change in inland Asia from the upper Oligocene to the Pliocene. The characteristic bimodal distribution of n-alkanes, which was observed in the investigated samples, is centered on n-C17–n-C20 and has maximum values at n-C18 in all samples; n-C27–n-C31 has maximum values at n-C29 in some of the samples. The front mode shows a weak even carbon number predominance of short-chain n-alkanes (CPI17–21 0.55–0.89); in contrast, the back mode has a strong odd carbon number predominance of long-chain n-alkanes (CPI25–31 1.41–2.42). Changes in the nC27/nC31 ratio (woody plants/grassy plants) along the entire section corresponded to three climate stages: an arid to humid climate stage from ~ 22.5 to 18.4 Ma; 6.25 to 5.5 Ma; and two major humid stages at ~ 18.4 Ma and ~ 5.5 Ma. A warm-humid climate was identified for the ~ 10 Ma to ~ 9 Ma period, which turned to an arid-cold climate from ~ 8 Ma to ~ 7.5 Ma, by assessing changes in the (n-C17–n-C21)/(n-C27–n-C31) ratios. Warm-humid conditions gradually increased from ~ 7.5 Ma to ~ 5.5 Ma, as has been confirmed by multiple geochemical climate indicators. A distinctive climate change toward arid-cold conditions at ~ 8 Ma corresponded to a striking increase in the high-carbon-numbered n-alkanes and a rapid decrease in the (n-C17–n-C21)/(n-C27–n-C31) and n-C27/n-C31 ratios of n-alkanes, which suggests a large input of higher plants. Our observation from the biomarker analysis is in agreement with previous reports that used palynofloras to determine that climate change in the investigated region responded to the uplift of the Tibetan Plateau at ~ 8 Ma. |
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