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Abstract: 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 18.4Ma; 6.25 to 5.5Ma; and two major humid stages at ~18.4Ma and ~5.5Ma. A warm-humid climate was identified for the ~10Ma to ~9Ma period, which turned to an arid-cold climate from ~8Ma to ~7.5Ma, by assessing changes in the (n-C17–n-C21)/(n-C27–n-C31) ratios. Warm-humid conditions gradually increased from ~7.5Ma to ~5.5Ma, as has been confirmed by multiple geochemical climate indicators. A distinctive climate change toward arid-cold conditions at ~8Ma 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 ~8Ma. [Copyright &y& Elsevier]

Abstract: This study reports the first observation of predominant even C-numbered n-alkanes from sediments in the continuous lacustrine-sedimentary section (Maogou) from the Late Miocene to the Early Pliocene (13–4.4Ma) in the Linxia Basin, NE Tibetan Plateau. The n-alkanes showed a bimodal distribution that is characterised by a centre at n-C16–n-C20 with maximum values at n-C18 and n-C27–n-C31 as well as at n-C29. The first mode shows a strong even C-number predominance (OEP16–20 0.34–0.66). In contrast, the second mode has a strong odd C-number predominance (OEP27–31 1.20–2.45). Certain types of special autochthonous bacteria are a possible source for this distribution of even C-numbered n-alkanes in lacustrine sediments. These bacteria may have a high production rate in weak oxic–anoxic and arid depositional environments, in which a variety of geochemical parameters have recorded palaeoclimate change. [ABSTRACT FROM AUTHOR]