THE EFFECT OF ATMOSPHERIC CO2 AND O2 CONCENTRATION ON HYDROGEN ISOTOPE VALUES OF EPICUTICULAR PLANT WAX COMPOUNDS

Hydrogen isotope (δD) values of epicuticular plant wax microfossils, such as n-alkanes and n-fatty acids, have become an important tool for paleoclimate and paleohydrological research. Despite their usefulness, a limited number of controlled studies have been done the effect of atmospheric gas concentrations on δD values measured in plant hydrocarbons. In this study, five different species of plants (Cyathea australis, Ginkgo biloba, Illicium simonsii, Magnolia delavyi, and Zantadeschia aethopica) were grown in controlled chambers with isotopically identical water (δD=-47‰) and two different atmospheric conditions: modern [CO₂] and [O₂], and conditions similar to the Devonian (high [CO₂], low [O₂]). Preliminary results from n-stearic acid (C_18:0) indicate the response of δD values to changed atmospheric conditions varies by species. Three species show no statically significant difference between measured δD values in these two environmental conditions (ΔD_D-A): Ginkgo biloba (ΔD_D-A=0‰, p=0.8881), Zantedeschia aethiopica (ΔD_D-A=-4‰, p=0.2617), and Illicium simonsii (ΔD_D-A=2‰, p=0.7375). Two other species showed opposite trends, with stearic acid δD values becoming more negative in Devonian conditions in Cyathea australis (ΔD_D-A=-25‰, p=0.0183) and more positive in Magnolia delavyi (ΔD_D-A=13‰, p=0.0247).