Comparison of δ2H values of leaf wax n-alkanes and n-alkanoic acids in subtropical angiosperms

Abstract

Hydrogen isotope compositions of leaf wax lipids are widely applied to reconstruct changes of hydroclimate in sedimentary archives. However, whether the δ2H signals from different classes of n-alkyl lipids yield consistent information is an important but poorly explored issue. In this study, we investigated seasonal variations in the molecular distributions and hydrogen isotope compositions of n-alkanes and n-alkanoic acids of three tree species (Cinnamomum camphora, Quercus chenii, Liquidambar formosana) growing in a humid subtropical monsoon climate. Seasonal differences exist in the concentrations and δ2H values of n-alkanes and n-alkanoic acids. These features may be linked to difference in the synthesis times of n-alkanes and n-alkanoic acids. The hydrogen isotope offset between n-alkanes and n-alkanoic acids (εalkane/acid) is variable across the seasons and among the three tree species. A survey of published data reveals that the εalkane/acid values are usually not significantly different among vegetation types, but the offset values can be significantly different in trees from tropical/subtropical and temperate regions. This difference may relate to different synthesis times of leaf waxes because of the climatic conditions in different climate zones. These findings encourage caution in interpreting differences in the δ2Hwax values of n-alkyl classes in paleoenvironmental reconstructions.