Leaf water deuterium enrichment shapes leaf wax n-alkane δD values of angiosperm plants II: Observational evidence and global implications

Leaf wax n-alkanes are long-chain hydrocarbons that can persist in sedimentary records over geological timescales. Since their hydrogen isotopic composition (expressed as a delta D value) can be correlated to the delta D values of precipitation, leaf wax n-alkane delta D values have been advocated as new and powerful proxies for paleohydrological research. The exact type of hydrological information that is recorded in the delta D values of leaf wax n-alkanes remains, however, unclear. In a companion paper we provide experimental evidence showing that the delta D values of leaf wax n-alkanes of angiosperm plants grown under controlled environmental conditions not only reflect delta D values of precipitation - as has often been assumed - but that evaporative deuterium (D)- enrichment of leaf water has an additional critical effect on their delta D values. Here we present a detailed observational study that illustrates that evaporative D-enrichment of leaf water also affects the delta D values of leaf wax n-alkanes in plants from natural ecosystems along a 1500 km climate gradient in Northern Australia. Based on global simulations of leaf water D-enrichment we show that the effects of evaporative D-enrichment of leaf water on leaf wax n-alkane delta D values is relevant in all biomes but that it is particularly important in arid environments. Given the combined influence of precipitation delta D values and leaf water D-enrichment we argue that leaf wax n-alkane delta D values contain an integrated signal that can provide general hydrological information, e.g. on the aridity of a catchment area. We also suggest that more specific hydrological information and even plant physiological information can be obtained from leaf wax n-alkanes if additional indicators are available to constrain the plant- and precipitation-derived influences on their delta D values. As such, our findings have important implications for the interpretation of leaf wax n-alkane delta D values from paleohydrological records. In addition, our investigations open the door to employ delta D values of leaf wax n-alkanes as new ecohydrological proxies in contemporary plant and ecosystem sciences. (C) 2012 Elsevier Ltd. All rights reserved.