Branched alkanes with quaternary carbon atoms in Chinese soils: Potential environmental implications

Abstract

Lipids extracted from 26 modern soils collected from different climate zones in China and Mongolia are analyzed using gas chromatographymass spectrometry. These extracts contain pseudohomologous families of 5,5-diethylalkanes (5,5-DEAs), 6,6-diethylalkanes, 5-butyl-5-ethylalkanes and 6-butyl-6-ethylalkanes, which are first detected in modern soils. All these pseudohomologous series display exclusively odd or exclusively even carbon numbers, clearly indicating a biosynthetic origin, and their carbon number distributions generally covary. The distributions of 5,5-DEAs exhibit strong regional variations, correlated with the spatial distributions of climate and vegetation. The lower molecular weight 5,5-DEAs, maximizing at C21 or C23, dominate in southeast and southwest China samples, while the higher molecular weight 5,5-DEAs with a mode at C29 dominate in cold, arid northwest China and Mongolia. The results demonstrate that the distribution of 5,5-DEAs can serve as a new independent proxy, indicating variations in soil ecosystems and climates, just like the well-known proxy of n-C15–21/n-C22–33 ratios.