n-Alkanes δ13C and salinity correlation in mangrove Aegiceras corniculatum leaves and surface sediments from Zhanjiang estuaries, China

Carbon isotope ratios (δ¹³C) in plant lipid biomarkers yield insights into historical climates by reflecting plant responses to environmental stressors. Previous research has established a correlation between increased salinity and elevated δ¹³C in Avicennia marina leaf wax n-alkanes, yet it remains unclear if these isotopic signatures are universally applicable across different species and regions or consistently preserved in sedimentary records. We investigated the influence of a 30 ppt salinity gradient on Aegiceras corniculatum in Zhanjiang, China, examining δ¹³C values in leaf wax n-alkanes and adjacent sediments. Leaf wax δ¹³C values ranged from −36.7 ‰ to −31.4 ‰, while sediment n-alkanes ranged from −35.6 ‰ to −28.2 ‰. Sediments consistently exhibited higher δ¹³C values than leaf wax, and their total lipid and n-alkane δ¹³C exhibited a positive correlation with salinity, increasing by 0.06 ± 0.02 ‰ and 0.1 ± 0.04 ‰ per ppt respectively. This positive correlation may be attributed to factors such as marine material input, seasonal variations in lipid synthesis, and plant community succession. In contrast, no significant correlation with salinity was observed in A. corniculatum's bulk leaf tissues or total lipids. This lack of correlation can be attributed to the unique physiological conditions and varying salt exclusion mechanisms among mangrove species, which influence water use efficiency. Accordingly, while sediment δ¹³C values can indicate past vegetation, the variability necessitates caution in using mangrove sediment δ¹³C for paleoenvironmental reconstruction.