Inverse relationships between salinity and hydrogen isotope fractionation of n-alkanes in the Aegiceras corniculatum leaves and surface sediments from Zhanjiang mangrove estuary of China

Abstract

Hydrogen isotope ratios of mangrove lipids have recently gained increasing interest because of their potential for reconstructing past water salinity and paleohydrology conditions. Although it has been suggested that the deuterium fractionation in mangrove leaf lipids appears to be primarily affected by water salinity, it remains an important research topic if this inverse relationship between salinity and hydrogen fractionation in leaf wax can be universally applied. In this regard, we have investigated the hydrogen isotopic composition of n-alkanes in leaves of Aegiceras corniculatum and surface sediments and the environmental water (surface water; αn-alkane-Water), collected along a 30 ppt salinity gradient in the Zhanjiang mangrove estuary of China, aiming to evaluate the impact of water salinity on the hydrogen isotopic fractionation of leaf wax derived n-alkanes. δ2H values of A. corniculatum leaf wax n-alkanes were between −183‰ and − 127‰ for C27, C29, and C31n-alkanes, and their αn-alkane-Water values range from 0.823 to 0.887. Sedimentary δ2H values range from −171‰ to −132‰ for C27, C29, and C31n-alkanes, and their αn-alkane-Water values were between 0.832 and 0.887. Inverse relationships between αn-alkane-Water and salinity were observed in mangrove A. corniculatum and surface sediment. This relationship for mangrove A. corniculatum may result from the selective utilization of freshwater during the rain event, changes in biosynthetic fractionation, and the relatively high humidity and/or larger changes in water vapour isotopic composition. For surface sediment, the inverse relationship could be mainly attributed to the mangrove-derived organic matter input, as the principal component analysis (PCA) results demonstrated a strong connection between the studied mangrove tree and the surface sediment. The consistent negative correlations between αn-alkane-Water and salinity for mangrove leaf waxes and surface sediments provide encouraging evidence for applying the hydrogen isotopic composition of mangrove leaf wax for paleohydrological reconstruction.