APPARENT FRACTIONATION OF HYDROGEN ISOTOPES IN N-ALKANOIC ACIDS IN A SOIL TRANSECT FROM BOREAL FORESTS IN EASTERN BERINGIA

Fossil plant waxes are well preserved in sedimentary archives (e.g., lake sediments, paleosols), and stable hydrogen isotope ratios (δD) of these waxes can provide constraints on past climates. Studies have shown that modern plant wax δD (δD_wax) reflects the δD of local precipitation (δD_precip), which correlates with air temperature in high latitude regions. However, δD_wax is depleted relative to δD_precip by the net (or apparent) fractionation (ε_wax/precip), the sum of environmental and biosynthetic fractionations. ε_wax/precip must be accounted for when using fossil plant waxes to estimate past δD_precip, but may vary regionally with climate and vegetation type and, thus, a local ε_wax/precip value may help to improve paleoclimate interpretations. Little is known about the ε_wax/precip in northern boreal regions which limits our ability to interpret fossil δD_wax archived in terrestrial Quaternary deposits in eastern Beringia (northwestern North America).

This study constrained the modern ε_wax/precip for high latitude boreal forest sites in 25 topsoil samples from a network of 12 sites spanning 58-68°N from Southern Yukon to the Mackenzie Delta region, and modern precipitation isotope data from the Online Isotopes in Precipitation Calculator. We measured the δD values of n-C₂₄, -C₂₆ and C₂₈ alkanoic acids from topsoil depths 0-20 cm, which represent an accumulation of Holocene plant lipids from all vegetation types at the site. The modal chain-length (C_max) was n-C₂₄, which indicates major contributions from Sphagnum (the primary ground vegetation) and secondary contributions from conifers and deciduous shrubs. Modern δD_precip ranges from -153 to -176‰ across the study region. δD values of the n-C₂₈ acid average -251‰ displaying variability within and between sites (1σ = 8‰, n = 25). Calculated ε_wax/precip for discrete soil samples across the transect display variability within and between sites. Values range from -80 to -136‰ (x̅ = -100‰, 1σ = 14‰, n = 25), which are within the range of published ε_wax/precip values for plants and sediments in the high latitudes and globally. The mean epsilon value, and the range of likely values from this large spatial survey, will assist future studies in using δD_wax archives to develop quantitative estimates of past δD_precip in this high latitude boreal region.