Hydroclimatic Controls of the Hydrogen Isotope Composition of n-alkanes in Melaleuca quinquenervia Leaves: Implications for Understanding Past Environmental Change

The hydrogen isotope composition of plant leaf wax n-alkanes (δ2Hwax) have increasingly been used to reconstruct past hydroclimates from across the globe, over a range of timescales. One of the principal controls on δ2Hwax is the δ2H composition of precipitation (δ2Hppt), but δ2Hwax is depleted compared to δ2Hppt because of a fractionation factor (ɛapp) that encompasses both biosynthetic fractionation and environmentally controlled transpiration enrichment of leaf water. A number of studies using lake surface sediments and living plants have helped to constrain ɛapp, which has provided mechanistic understanding of isotope controls of δ2Hwax, but it is clear there are species-specific and environmental differences that might confound the sediment record. Therefore reconstructing past climates using the δ2Hwax of a single species throughout a sedimentary record means we have the potential to isolate the main driver of isotope change: δ2Hppt and give confidence to the palaeoclimate record. Fortunately, the leaves of the evergreen tree Melaleuca quinquenervia have been found preserved in sediment cores recovered from perched lakes on North Stradbroke and Fraser Islands, Queensland, Australia. Here, we examine the potential of using M. quinquenervia δ2Hwax as a tracer of past precipitation by analyzing the δ2H composition of its leaves. We then examine the relationship between climate and δ2Hwax on a quadrennial basis for an 11-year time series (1992-2003) of leaves collected in litter trays from a wetland in Queensland, to explore whether variability in local δ2Hppt is recorded in δ2Hwax.