Influence of salinity on hydrogen isotope fractionation of n-alkanes in mangrove leaves and surface sediments: A comparison across various geomorphological settings

Abstract

Increasing attention has recently been paid to hydrogen isotope ratios of mangrove lipids due to their potential for reconstructing past salinity and hydrology. This proxy is based on several observations that net 2H/1H fractionation, εn-alkane/water, between mangrove leaf waxes and surface water, increases with surface water salinity. However, the extent to which hydrogeomorphic conditions, on a regional scale, affect the hydrogen isotope fractionations of n-alkanes in mangrove trees and sediments is not yet understood. This study aimed to assess the robustness of negative εn-alkane/water – salinity for six mangrove species and adjacent surface sediments from the Zhanjiang mangrove across different hydrogeomorphic settings. The results indicated that a significant inverse relationship exists between εn-C31/water and εn-C33/water and salinity for all samples from the three fringe and basin/interior mangroves, with εn-C31/water decreasing by −2.57 ± 0.87 ‰ ppt−1 and εn-C33/water decreasing by −3.64 ± 1.14 ‰ ppt−1. These trends, as well as interspecific differences among mangrove species, could be attributed to biochemical changes in response to salt stress, such as using different proportions of amino acids and soluble carbohydrates as compatible solutes. In contrast, the relationship between sedimentary εn-alkane/water and salinity differed significantly across hydrogeomorphic conditions and land-use changes, possibly due to variations in hydrology and input of allochthonous organic matter. A comparison of fitted parameters of linear regressions among riverine, fringe, and interior mangroves suggests that the slop and intercept describing the linear relationship between sedimentary εC27–33/water of basin/interior mangroves and salinity are generally consistent with that of the corresponding mangrove trees. This suggests that the basin/interior mangroves and their sedimentary deposits may be better candidates for paleoenvironmental reconstruction using hydrogen isotope ratios of long-chain n-alkanes.