Leaf-Wax Lipid Hydrogen Isotope Ratios as a Paleohydrologic Proxy: Environmental and Taxonomic Controls

Compound-specific hydrogen isotope ratios of leaf-wax lipids hold great promise as a paleohydrologic proxy. Although precipitation sets the starting hydrogen isotope ratio of water in an ecosystem, this water can be enriched in D as a result of evaporation from soils and from leaves (transpiration). Arid conditions yield greater evaporation with leaf-wax compounds recording the degree of evaporative isotopic enrichment. Thus, the deuterium signature recorded in sedimentary plant lipids reflect precipitation patterns and evaporative conditions. Despite our understanding of fundamental controls on plant signatures, specific challenges remain in the calibration and application of this proxy.

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 (ε_lipid-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.