中国科学院南京地质古生物研究所机构知识库

The effect of low intensity continuous light, e.g., in the High Arctic summer, on plant carbon and hydrogen isotope fractionations is unknown. We conducted greenhouse experiments to test the impact of light quantity and duration on both carbon and hydrogen isotope compositions of three deciduous conifers whose fossil counterparts were components of Paleogene Arctic floras: Metasequoia glyptostroboides, Taxodium distichum, and Larix laricina. We found that plant leaf bulk carbon isotopic values of the examined species were 1.75-4.63aEuro degrees more negative under continuous light (CL) than under diurnal light (DL). Hydrogen isotope values of leaf n-alkanes under continuous light conditions revealed a D-enriched hydrogen isotope composition of up to 40aEuro degrees higher than in diurnal light conditions. The isotope offsets between the two light regimes is explained by a higher ratio of intercellular to atmospheric CO(2) concentration (C (i)/C (a)) and more water loss for plants under continuous light conditions during a 24-h transpiration cycle. Apparent hydrogen isotope fractionations between source water and individual lipids (epsilon(lipid-water)) range from -62aEuro degrees (Metasequoia C(27) and C(29)) to -87aEuro degrees (Larix C(29)) in leaves under continuous light. 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 to estimate the delta D value in ancient precipitation. Precipitation in the summer growing season yielded a delta D of -186aEuro degrees for late Paleocene, -157aEuro degrees for early middle Eocene, and -182aEuro degrees for late middle Eocene. We propose that high-latitude summer precipitation in this region was supplemented by moisture derived from regionally recycled transpiration of the polar forests that grew during the Paleogene warming.