Increased Deuterium Discrimination in Micronesian Mangroves Growing at High Salinity: Insights from Leaf and Xylem Water Isotopes

Hydrogen isotope ratios of plant leaf waxes are increasingly used as a proxy for past hydrologic variability. However, a number of environmental variables influence the net fractionation between leaf waxes and the environmental water from which their hydrogen is ultimately derived. Salinity effects are of particular importance in coastal tropical and subtropical settings, where deuterium discrimination increases by 1.5‰ per salinity unit in the leaf wax n-alkanes of the grey mangrove, Avicennia marina. It is not possible to tell whether sedimentary n-alkanes are from mangrove plants, or from terrestrial plants that are not exposed to salt water. The salinity component of hydrogen isotope fractionation therefore complicates leaf wax hydrogen isotopes in most tropical coastal marine and lacustrine settings. However, a strong relationship between salinity and a more specific mangrove lipid biomarker could provide the basis for a paleosalinity and water isotope proxy in low-latitude coastal environments.

Here we present results from a calibration study of Rhizophora spp. (red mangroves) growing on the Micronesian islands of Pohnpei and Palau, using taraxerol, a biomarker that is largely specific to this genus in these settings. We observed an increase in net deuterium discrimination between surface water and taraxerol of 1.2‰ per salinity unit. We investigated potential mechanisms for this increase at high salinity by measuring the hydrogen isotopic composition of leaf and xylem water from Rhizophora spp. Contrary to most terrestrial plants, xylem water in these trees is depleted relative to surface water, with greater relative depletion at higher salinities. This could be the result of increased deuterium discrimination during water uptake, as a greater percentage of salt is excluded by roots at higher salinity. Alternatively, it could indicate that some of the water in the xylem is from relatively depleted freshwater (rain and or dew) that enters the plant through the leaves, and that this source becomes increasingly important at higher salinities. We also observed decreased leaf water enrichment relative to xylem water at higher salinity, which is consistent with foliar uptake of fresh water for mangroves growing in saline conditions.