Distribution of n-alkanes in Miocene loess in Qinan, western Chinese Loess Plateau, and its palaeoenvironmental implications

Abstract

Neogene eolian successions are one of the most important terrestrial palaeoenvironmental archives in East Asia. However, they have received far less attention than Quaternary loess deposits, especially in the case of lipid biomarker analysis. In order to obtain a better insight into the early-middle Miocene palaeoenvironment, we conducted a study of n-alkane biomarkers from sediments of the QA-I section (Qinan) in the western Chinese Loess Plateau, and compared the results with those of previous n-alkane analyses of eolian and aquatic sediments of varying age. Our principal results are as follows: (1) All QA-I samples contain n-alkanes ranging from C14 to C35, among which the relative content of short-chain n-alkanes (C14–C20) from microorganisms is significantly greater than that of long-chain n-alkanes (C26–C35) from the waxes of terrestrial higher plants; the main peak is at C16–C18. All samples have a relatively lower abundance of medium-chain n-alkanes (C21–C25) than that of long- and short-chain n-alkanes, similar to strongly weathered palaeosols in Quaternary loess and Late Miocene-Pliocene Hipparion Red-Earth; however, this distribution is significantly different from that in weakly-weathered loess of Quaternary loess and Late Miocene-Pliocene Hipparion Red-Earth, as well as from aquatic sediments. (2) Despite some odd-over-even carbon predominance of long-chain n-alkanes in the QA-I samples, the carbon preference index (CPI) values are significantly lower than those of most of the weakly-weathered sediments. Our results show that strong weathering and microbial processes have significantly altered the n-alkanes in the Miocene eolian deposits in Qinan, and led to a significant oxidation and degradation of long-chain n-alkanes and the predominance of short-chain n-alkanes from bacteria. Therefore, the contribution of microorganism to total organic carbon (TOC) and its resulting in carbon isotopic composition should be carefully assessed in future studies.