An Empirical Evaluation of Modern Tree Leaf Wax Isotopic Signals

The advent of compound-specific hydrogen and carbon isotopic analysis has extended the range of paleoclimatic information entrained within leaf wax lipids. The primary objective of this research was to evaluate empirically the environmental controls on δD and δ^13C values of specific terrestrial higher plant normal alkanes, since these compounds are refractory and readily preserved in the geologic record. Our approach was to extensively sample leaves and stems of seven species of living trees (Acer rubrum, Quercus alba, Quercus rubra, Juniperus virginiana, Platanus occidentali, Pinus taeda, Pinus strobus) that span and co-occur along the Eastern United States climatic gradients (pedimont plateau, Maine to Florida). One hundred and thirty-three trees from forty sites were collected along with topsoils and surface waters from each locality in order to assess leaf wax input to soils and establish a δD baseline for the transect. Red maple (A. rubrum) and Eastern redcedar (J. virginiana) normal alkanes δ^13C have been analyzed and range from -38.0 to -30.2‰ and -34.7 to -29.1‰, respectively, and demonstrate no relationship with latitude, elevation, aspect, canopy cover, tree height, or leaf height. In addition, the gymnosperms are on average 1.9‰ more enriched in ^13C than the angiosperms. This isotopic offset likely stems from physiological differences that led to greater water-use efficiency in the gymnosperms, however, future δD measurements will shed more light on issues of stomatal conductance and water-use. In addition, soil-extracted n-alkane δ^13C range between -36.2 to -30.7‰ and correlate positively with latitude (R^2=0.60). Furthermore, soil n-alkane distributions demonstrate a wider range of saturated compounds (n-C23 to n-C33) than either present in the red maple (n-C25 to n-C33) or redcedar (n-C31 to n-C35), indicative of other leaf wax inputs to the soils. Notably, J. virginiana-specific waxes make up minor measurable component of the n-alkanes extracted from topsoils and therefore contribute very little to the wax input in Eastern hardwood soils. Finally, surface water δD and δ^18O were highly correlated and very similar to the meteoric water line with a relation of δD = 7.4453 δ^18O + 4.3016, laying the groundwork for future detailed leaf wax and stem water dD analysis.