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

An increasing number of studies from high latitude settings use the stable hydrogen isotope ratio (delta H-2) 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 delta H-2 values. Here, we determine plant-water fractionations from 12 sites along a south-north transect in central Alaska, from Fairbanks to Deadhorse (64-70 degrees N). We characterize the delta H-2 values of n-alkanes and n-alkanoic acids from modern plants and their xylem and leaf water delta H-2 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 (epsilon(LW/XW)) and biosynthetic (epsilon(wax/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 delta H-2 values show near linear H-2 enrichment toward the surface associated with evaporation, with the delta H-2 value similar to 40 cm matching mean annual precipitation (MAP). Average net fractionation was calculated using xylem water and chain length weighted wax delta H-2 values (epsilon(wax/XW)) and we find nearly the same mean values for both n-alkanes, -112 +/- 27 parts per thousand, and n-alkanoic acids, -110 +/- 23 parts per thousand, 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 (epsilon(wax/MAP), -89 +/- 28 parts per thousand for n-alkanes, and -86 +/- 24 parts per thousand for n-alkanoic acids). To compare across studies, we determined the average epsilon(wax/MAP) of n-alkanes from all available high latitude calibration studies and found more H-2 enriched (smaller fractionations) for C-27 (-87 +/- 29 parts per thousand), C-29 (-87 +/- 32 parts per thousand), and C-31 (-91 +/- 31 parts per thousand) than those of global epsilon(wax/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 delta H-2 values for paleoprecipitation estimates. (C) 2020 Elsevier Ltd. All rights reserved.