Recent progress in the application of hydrogen isotopes from tree-ring lignin methoxy groups as a climate proxy
Authors: Wieland Anna , Greule Markus , Roemer Philipp , Esper Jan , Ladd Nemiah , Lehmann Marco , Schuler Philipp , Keppler Frank 
Published: February 2023

Stable hydrogen isotope values of tree-lignin methoxy groups (&#948;2HLM) are increasingly applied to reconstruct the stable hydrogen isotope composition of precipitation (&#948;2Hprecip) and mean annual temperatures in mid-latitude regions. The climate reconstructions are typically derived considering an isotope fractionation from -200 mUr to -216 mUr between lignin methoxy groups and tree source water (Keppler et al. 2007, Anh&#228;user et al. 2017, Greule et al., 2021, Porter et al. 2022, Wieland et al. 2022). This empirical relationship was derived from different tree species collected along a European north-south transect at elevations below 1000 m above sea level. However, it is so far unknown how environmental and physiological factors such as tree age, tree species, salinity, elevation, or precipitation amount influence the biochemical hydrogen isotope fractionation between lignin methoxy groups and precipitation.We here present several recent investigations that show how environmental and tree physiological factors might influence &#948;2HLM values. For example, potential amount effects of precipitation are analysed using tree cores from the Carpathians, where the mountain barrier led to large precipitation events at the Luv site. In addition, potential age trends are studied using trees from Greece that are over 500 years old, and the phylogenetical range of &#948;2HLM values is assessed by comparing 70 different tree species grown under uniform climatic conditions. Finally, the influence of salinity is evaluated by analysing different mangrove tree species from Australia.The improvements and limitations of &#948;2HLM values as a climate proxy at different spatial and temporal scales will be discussed. In order to better reconstruct long-term climate variations, additional gain is expected from the cross-comparison of multiple isotope proxies, including stable carbon isotope values of cellulose and lignin methoxy groups, as well as stable oxygen isotopes of cellulose.References:Anh&#228;user, T., Greule, M., Polag, D., Bowen, G. J., and Keppler, F.: Mean annual temperatures of mid-latitude regions derived from &#948;2H values of wood lignin methoxyl groups and its implications for paleoclimate studies, Sci. Total Environ., 574, 1276&#8211;1282, https://doi.org/10.1016/j.scitotenv.2016.07.189, 2017.Greule, M., Wieland, A., and Keppler, F.: Measurements and applications of &#948;2H values of wood lignin methoxy groups for paleoclimatic studies, Quaternary Sci. Rev., 268, 107107, https://doi.org/10.1016/j.quascirev.2021.107107, 2021.Keppler, F., Harper, D. B., Kalin, R. M., Meier-Augenstein, W., Farmer, N., Davis, S., Schmidt, H. L., Brown, D. M., and Hamilton, J. T. G.: Stable hydrogen isotope ratios of lignin methoxyl groups as a paleoclimate proxy and constraint of the geographical origin of wood, New Phytol., 176, 600&#8211;609, https://doi.org/10.1111/j.1469-8137.2007.02213.x, 2007.Porter, T. J., Anh&#228;user, T., Halfar, J., Keppler, F., Csank, A. Z., and Williams, C. J.: Canadian Arctic Neogene temperatures reconstructed from hydrogen isotopes of lignin&#8208;methoxy groups from sub&#8208;fossil wood, Paleoceanogr. Paleoclimatology, 37, https://doi.org/10.1029/2021pa004345, 2022.Wieland, A., Greule, M., Roemer, P., Esper, J., and Keppler, F.: Climate signals in stable carbon and hydrogen isotopes of lignin methoxy groups from southern German beech trees, Clim. Past, 18, 1849&#8211;1866, https://doi.org/https://doi.org/10.5194/cp-18-1849-2022, 2022.