Ecological and environmental controls on plant wax production and stable isotope fractionation in modern terrestrial Arctic vegetation

Abstract

Terrestrially-derived plant waxes and their compound-specific stable carbon (δ13C) and hydrogen (δ2H) isotope ratios are valuable tools for inferring past changes in vegetation and hydrology in sedimentary archives. Such inferences require knowing the ecological (i.e., plant growth form) and environmental (i.e., latitude, temperature, precipitation, relative humidity, elevation) mechanisms that govern the production of different plant wax carbon chain-lengths and the fractionation of their stable isotopes. These mechanisms, however, are uncertain in the Arctic, limiting our ability to infer past vegetation and hydrology changes. To address this, we produced terrestrial plant n-alkanoic acid and n-alkane abundance data along with n-alkanoic acid δ13C and δ2H data from a latitudinal environmental gradient along the Eastern Canadian Arctic (105 individuals), which we combined with published data from across the Arctic (additional 281 individuals). We compared this plant wax dataset to environmental parameters, including temperature, precipitation amount, relative humidity, latitude, and elevation, to assess the mechanisms that govern plant-wax production and isotope fractionation. We found that total plant wax concentrations and Average Chain-Length (ACL) were statistically different between vascular (trees, shrubs, forbs, ferns, graminoids) and non-vascular plants (mosses, liverworts) and lichens, whereas δ13C values and δ2H apparent fractionation relative to growing season precipitation δ2H often did not differ significantly between plant growth forms. Correlations between plant wax indices and mean of the months above freezing (MAF) environmental parameters were generally weak (r ≤ 0.4) and/or not significant (p ≥ 0.05). These results suggest that a fundamental assumption to paleoclimate research holds in the Arctic: for individual terrestrial plant taxa and plant communities, the abundance, ACL, and δ13C / δ2H isotopic fractionation of both n-alkanoic acids and n-alkanes is independent of local environmental parameters. Instead, changes in terrestrially-derived sedimentary plant wax distributions reflect changes in plant taxa present through time, and changes in terrestrially-derived plant wax δ2H reflect changes in precipitation δ2H. Therefore, plant waxes can be used to infer past changes in climate and ecology.