PLANT HYDROGEN ISOTOPE FRACTIONATION AT HIGH LATITUDES: IMPLICATIONS FOR PALEOGENE ARCTIC PALEOCLIMATE INTERPRETATIONS

Hydrogen isotopic compositions (δD) of leaf wax track the decreasing trend of precipitation δD with the increase of latitude. Greenhouse experiments using three deciduous conifers, Metasequoia glyptostroboides, Taxodium distichum, and Larix laricina, whose fossil counterparts were components of Paleogene Arctic floras, demonstrated the effect of low intensity continuous light, e.g., in High Arctic summer, on plant carbon and hydrogen isotope fractionations. Hydrogen isotope values of leaf n-alkanes under continuous light conditions resulted a D-enriched hydrogen isotope composition of up to 40‰ higher than in diurnal light conditions during a 24 hour transpiration cycle. Apparent hydrogen isotope fractionations between source water and individual lipids (ε_{lipid -water}) range from -62‰ (Metasequoia C₂₇ and C₂₉) to -87‰ (Larix C₂₉) in leaves under continuous light. Further tests using woody plants from high latitudes confirmed smaller apparent hydrogen isotope fractionations between leaf n-alkanes and environmental water than that of lower latitudes. To estimate δD values in ancient precipitation, we applied these hydrogen fractionation factors to hydrogen isotope compositions of in situ n-alkanes from well-preserved Paleogene deciduous conifer fossils from the Arctic region. Our data suggest that high-latitude summer precipitation was supplemented by moisture derived from regionally recycled transpiration of the polar forests that grew during the Paleogene warming.