Leaf-Wax Lipid Hydrogen Isotope Ratios as a Paleohydrologic Proxy: Environmental and Taxonomic Controls
First, to interpret hydrogen isotope ratios of leaf waxes quantitatively in terms of relative humidity requires constraining the source water (precipitation or soil water) isotope ratio. Several potential methods for doing this will be discussed including isotopic measurements of short-chain n-alkanes, tooth enamel, and pedogenic clays. Second, in addition to climatic factors, plant type also affects δD signatures of leaf wax lipids. For example, enrichment factors (εlipid-water) between leaf-wax n-alkanes and precipitation differ between C3 and C4 grasses (monocots) and between monocots and dicots. Observed variations are on the order of 20 to 40 per mil and, at least within the grasses, can be accounted for by inter-veinal distances. Differences between monocots and dicots are still being explored, but are likely related to leaf anatomy and plant water budgets. Interestingly, no significant difference is emerging in lipids from coniferous gymnosperms and dicotyledonous angiosperms. Because δD signatures can vary among plant types, plant community change could affect isotopic records in the geologic past irrespective of climate. Therefore, any interpretation of leaf-wax δD records in terms of hydrologic condition must take into account the potential effects of changes in the relative abundance of C3 vs. C4 grasses and grasses vs. non-grasses.