Predicting the Hydrogen Isotope Ratios of Leaf Waxes Across Landscapes

Leaf wax n-alkanes ²H/¹H ratios are widely used as a proxy of paleoprecipitation in climate reconstruction. While the broad nature of the relationships between n-alkane δ²H values and climate are appreciated on geologic scales, the quantitative details of what this proxy is reflecting remain ambiguous on plant and ecosystem levels. Areas of uncertainty on these smaller scales of importance to geologic interpretations are both the biosynthetic fractionation and the leaf-growth interval that is recorded by the isotope signal. To clarify these details, we designed a series of experiments in which modern plants were grown under controlled and monitored conditions.

To determine the biosynthetic fractionation, we analyzed n-alkanes from plant grown hydroponically on isotopically distinct waters and under contrasting and controlled humidities. We observed δ²H values of n-alkane were linearly related to growth water δ²H values, but with slope differences associated with humidity. These findings suggested leaf water were central controls on δ²H values of n-alkane and support a relatively constant biosynthetic fractionation factor between leaf water and n-alkanes. To determine the interval that the leaf wax isotope signal reflects, we studied a species naturally growing on water with a constant δ²H value. Here we found the δ²H values of n-alkanes recorded only a two-week period during leaf flush and did not vary thereafter. These data indicated the δ²H values of n-alkanes record conditions early in the season, rather than integrating over the entire growing season.

Using these data, we are beginning to develop geospatial predictions of the δ²H values of n-alkane across landscapes for given climate conditions, plant phenologies, and ecosystems. These emerging modeling tools may be used to assess modern ecosystem dynamics, to estimate weathering of leaf waxes to geologic repositories, and to define and test paleoclimate reconstructions from the δ²H values of n-alkanes.