Hydrogen isotope fractionation in modern plants along a boreal-tundra transect in Alaska

Abstract

An increasing number of studies from high latitude settings use the stable hydrogen isotope ratio (δ2H) of plant leaf waxes to quantify changes in past precipitation. Calibration of modern vegetation and source water use from these landscapes is important to more accurately reconstruct past hydroclimate variability using sedimentary leaf wax δ2H values. Here, we determine plant-water fractionations from 12 sites along a south-north transect in central Alaska, from Fairbanks to Deadhorse (64-70°N). We characterize the δ2H values of n-alkanes and n-alkanoic acids from modern plants and their xylem and leaf water δ2H values, as well as plant source water using surface soils, a soil core, and previously collected meteoric water data from the region. We find both transpiration (εLW/XW) and biosynthetic (εwax/LW) fractionation are sensitive to environmental controls with latitude, with increased leaf transpiration and biosynthetic fractionation to the north, potentially due to increased summer sunlight hours. Soil water δ2H values show near linear 2H enrichment toward the surface associated with evaporation, with the δ2H value ∼40 cm matching mean annual precipitation (MAP). Average net fractionation was calculated using xylem water and chain length weighted wax δ2H values (εwax/XW) and we find nearly the same mean values for both n-alkanes, −112 ± 27‰, and n-alkanoic acids, −110 ± 23‰, from 12 common high latitude vascular plants. To provide the range of likely net fractionations along this transect, we also calculate the average net fractionation using MAP, finding values are less negative than using xylem waters (εwax/MAP, −89 ± 28‰ for n-alkanes, and −86 ± 24‰ for n-alkanoic acids). To compare across studies, we determined the average εwax/MAP of n-alkanes from all available high latitude calibration studies and found more 2H enriched (smaller fractionations) for C27 (-87 ± 29‰), C29 (-87 ± 32‰), and C31 (-91 ± 31‰) than those of global εwax/MAP homolog averages. This new work in Alaska contributes to our growing understanding of plant water-wax fractionation in the high latitudes and is potentially important for the use of sedimentary δ2H values for paleoprecipitation estimates.