CARBON AND HYDROGEN ISOTOPIC COMPOSITIONS OF DECIDUOUS CONIFERS UNDER A CONTINUOUS-LIGHT ENVIRONMENT: IMPLICATIONS FOR THE INTERPRETATION OF THE HIGH-LATITUDINAL PLANT ISOTOPE RECORD AT PETM

The exquisitely preserved early Tertiary deciduous coniferous floras in the polar region of the Canadian Arctic (paleolatitude ~80⁰N) yield bulk C isotopes of leaf tissues and molecular C and H isotopic compositions of leaf waxes. Interpretation of these isotopic values needs to take into account possible isotopic fractionations under the influence of the high latitudinal light regime. As these plants constitute major components of the polar flora across the PETM, a better understanding of the impact of continuous light on both bulk and molecular isotopic signals derived from terrestrial settings is critical, especially when these isotopic values are compared across a latitudinal gradient.

To test whether the pattern of plant carbon and hydrogen isotopic signals was influenced by the unique continuous light regime of the Arctic summer, we analyzed bulk carbon isotopes of leaf and wood tissues, and molecular carbon and hydrogen isotopes of homologous n-alkanes lipids, from living Metasequoia and Taxodium(Cupressaceae), and Larix(Pinaceae). We compared the results from plants grown under three months of continuous light (CL), a simulation of summer light regime in the high Arctic, with the same species grown under diurnal light (DL, 45⁰N Lat) in a greenhouse with identical controlled temperature, light intensity, and CO₂ concentration.

For all species bulk carbon isotopes were more negative under CL than under DL. In Larix, the offset was up to 4.6 ‰. In contrast, molecular carbon isotopic values of n-alkanes from leaves were 1.8‰ to 2.1‰ more positive under CL than under DL. Grown with source water of known &deltaD (-65.6‰), compound-specific &deltaD values of n-alkanes in leaves were, on average, 20‰ to 39‰ more enriched in CL than in DL. Depending upon individual compounds, the H isotopic fractionation factors between source water and n-alkanes ranged 43-87‰ in leaves grown in CL. Isotopic offsets between CL and DL plants, and variations observed among different taxa, are likely to be a consequence of adaptive physiological characteristics (gas exchanges, chlorophyll fluorescence, carbohydrate concentration, and stomatal density) and can be explained by the differences in photosynthetic capacity and water use efficiency of individual plant taxa under different light conditions.