Stable isotope (carbon, hydrogen) variation in plants and lake surface sediments from northwestern Canada

Abstract

<p>Hydrogen isotope ratios of leaf waxes are used to reconstruct past hydroclimate because they are correlated to meteoric/growth water hydrogen isotopes. The interpretation of these signatures from ancient sedimentary archives relies on a thorough understanding of the drivers of modern isotope variability. Studies in the high latitudes, regions that are particularly valuable in light of their vulnerability to rapid climate change, are scarce. We studied modern vegetation (22 plants) in two areas in the Northwestern Territories (Canada): Herschel Island and Peel Plateau, to understand the stable isotope variability found in plants of Arctic regions. Bulk biomass stable carbon isotope and radiocarbon composition have been measured as well as fatty acids (wax lipids coating the plant leaves) stable carbon and hydrogen isotopes. Furthermore, lake surface sediments and river bank sediments from the Mackenzie River Delta (surrounded by the same plants) have been similarly studied. Bulk carbon isotope composition of the plants show strong difference between plant type, i.e. herbs, shrubs, lichen and moss, as shown in previous studies. Whereas the commonly used average chain length (ACL) is not useful to differentiate the plants. In term of compound-specific isotope ratios, herbs are generally <sup>2</sup>H-enriched in comparison to shrubs as shown in other regions of the world, and the C<sub>28</sub> fatty acid present the most differences amongst plant type (from ~ -207‰ for herbs to ~ – 240‰ for shrubs). No major difference between the areas is noted indicating that the ~ 250 km (Herschel Island 69.5⁰N and Peel Plateau 67.3⁰N) have no impact on the hydrogen isotope composition of the fatty acids. As such we decided to compare the plant with the lake surface sediments (from the Mackenzie Delta, located between Herschel Island and the Peel Plateau). Short-chain fatty acids, sourced from organisms growing in the lake, from isolated lakes shows <sup>2</sup>H-enriched isotopic values indicating the effect of increased evaporation in the lake during summer plant growth. Whereas long-chain fatty acids do not show any differences and are enriched compared to the shorter-chain (~ -260‰ for long-chain vs ~ -260‰ to - 280‰ for short-chain). In conclusion, differences between plant fatty acids seems to be best represented by the C<sub>28</sub> fatty acid, indicating the potential to reconstruct past vegetation and hydrological conditions in the region using lacustrine archives.</p>