Hydrogen isotope response to changing salinity and rainfall in Australian mangroves

Hydrogen isotope ratios (H-2/H-1, delta H-2) of leaf waxes covary with those in precipitation and are therefore a useful paleohydrologic proxy. Mangroves are an exception to this relationship because their delta H-2 values are also influenced by salinity. The mechanisms underlying this response were investigated by measuring leaf lipid delta H-2 and leaf and xylem water delta H-2 and delta O-18 values from three mangrove species over 9.5 months in a subtropical Australian estuary. Net H-2/H-1 fractionation between surface water and leaf lipids decreased by 0.5-1.0% ppt-1 for n-alkanes and 0.4-0.8% ppt-1 for isoprenoids. Xylem water was H-2 depleted relative to surface water, reflecting H-2 discrimination of 4-10% during water uptake at all salinities and opportunistic uptake of freshwater at high salinity. However, leaf water H-2 enrichment relative to estuary water was insensitive to salinity and identical for all species. Therefore, variations in leaf and xylem water delta H-2 values cannot explain the salinity-dependent H-2 depletion in leaf lipids, nor the 30 parts per thousand range in leaf lipid delta H-2 values among species. Biochemical changes in direct response to salt stress, such as increased compatible solute production or preferential use of stored carbohydrates, and/or the timing of lipid production and subsequent turnover rates, are more likely causes.