Net fractionation of hydrogen isotopes in n-alkanoic acids from soils in the northern boreal forest

Plant-derived n-alkyl lipids are well-preserved in sedimentary archives, and their stable hydrogen isotope ratio (δDwax) is a proxy for precipitation δD and climate. Net fractionation of H isotopes between plant source water and n-alkyl lipids (εwax/water) is the largest uncertainty for interpreting this proxy and depends on plant type and environment. Although popular proxies, n-alkanoic acids (n-acids) are less frequently calibrated in modern environments than n-alkanes. We constrain the net fractionation (εwax/MAP - Mean Annual Precipitation) for the northern boreal forest based on n-C24,26,28 acids in topsoils from a 13-site network in Yukon, Alaska and Northwest Territories (60-68°N). n-Acid homologue distributions for common boreal plants (gymnosperm trees, shrubs, forbs, C3 grasses and mosses) show that soil n-acids are dominated by mosses, but with a reduced carbon preference index compared to fresh mosses, possibly owing to post-depositional degradation. Regionally averaged εwax/MAP values are −93 ± 10‰, −101 ± 11‰ and −95 ± 11‰ for n-C24,26,28 acids, respectively, which are similar to values reported from low and high latitude regions. We find that εwax/MAP for n-C26,28 acids (but not n-C24) is significantly (p ≤ 0.05) smaller with increasing latitude and decreasing mean annual temperature, factors that covary across our network. Since latitude is typically known for ancient deposits, we parsed the dataset to obtain latitude-specific εwax/MAP values for 60-64°N (−107 ± 8‰ for n-C26; and −101 ± 8‰ for n-C28) and 65-68°N (−94 ± 11‰ for n-C26; and −89 ± 10‰ for n-C28). These soil-derived values are relevant for hydroclimate reconstructions using fossil n-acids in paleosols from a comparable paleoflora and latitude.