Hydrogen and carbon isotope responses to salinity in greenhouse-cultivated mangroves

Abstract

Abstract Paired hydrogen and carbon isotope ratios (2H/1H and 13C/12C) of mangrove lipids can be used to quantitatively reconstruct past salinity and 2H/1H ratios of environmental water, and in some cases precipitation rate. This approach is based on the observation that net 2H- and 13C-fractionation increases and decreases, respectively, with the salinity of environmental water. In order to better understand the mechanisms underlying these empirical observations and ultimately improve estimates of paleoprecipitation from the paired H and C isotope approach, we analyzed the isotopic composition of fatty acids from five species of mangroves cultivated in salinity treatments of 5–30\u202fppt (g/kg) for 3.5\u202fyears in a greenhouse. Decreased net 13C-fractionation with salinity in three mangrove species was attributed to increased water use efficiency and thus a 13C-enriched internal CO2 pool. Net 2H-fractionation decreased with salinity in three mangrove species, opposite to previous observations of mangroves growing along salinity gradients in lakes and estuaries. The difference between uncultivated and greenhouse-cultivated mangroves may result from variability of 2H/1H of environmental water in natural environments. In addition, decreased net 2H-fractionation with salinity could be due to temporal variability in 2H/1H of leaf water and timing of lipid production, and the use of stored carbohydrates in seeds. Due to the sensitivity of the salinity and 13C-fractionation relationship for calculating both salinity and water isotopes, optimization of mangrove lipid H and C isotopes as a paleohydrologic tracer may be best achieved through laboratory-based calibrations of the relationship between 13C-fractionation and salinity.