Influence of physiology and climate on δD of leaf wax n-alkanes from C 3 and C 4 grasses

Abstract

We measured hydrogen isotope compositions ( δD) of high-molecular-weight n-alkanes (C 27–C 33) from grasses grown in greenhouses and collected from the US Great Plains. In both cases, n-alkanes from C 4 grasses are enriched in D by more than 20‰ relative to those from C 3 grasses. The apparent enrichment factor ( ε C 29 - GW ) between C 29 n-alkane and greenhouse water is −165 ± 12‰ for C 3 grasses and −140 ± 15‰ for C 4 grasses. For samples from the Great Plains, δD values of C 29 n-alkanes range from −280 to −136‰, with values for C 4 grasses ca. 21‰ more positive than those for C 3 grasses from the same site. Differences in C 3 and C 4 grass n-alkane δD values are consistent with the shorter interveinal distance in C 4 grass leaves, and greater back-diffusion of enriched water from stomata to veins, than in C 3 grass leaves. Great Plains’ grass n-alkane isotopic ratios largely reflect precipitation δD values. However, the offset or apparent fractionation between n-alkanes and precipitation is not uniform and varies with annual precipitation and relative humidity, suggesting climatic controls on lipid δD values. The dryer sites exhibit smaller absolute apparent fractionation indicative of D-enrichment of source waters through transpiration and/or soil evaporation. To explore the relationship between climate and n-alkane δD values, we develop three models. (1) The ‘direct analog’ model estimates δ D C 29 values simply by applying the apparent enrichment factors, ε C 29 - GW , observed in greenhouse grasses to precipitation δD values from the Great Plains. (2) The ‘leaf-water’ model uses a Craig–Gordon model to estimate transpirational D-enrichment for both greenhouse and field sites. The transpiration-corrected enrichment factors between C 29 and bulk leaf-water, ε C 29 - GW , calculated from the greenhouse samples (−181‰ for C 3 and −157‰ for C 4) are applied to estimate δ D C 29 values relative to modeled bulk leaf-water δD values. (3) The ‘soil- and leaf-water’ model estimates the combined effects of soil evaporation, modeled by analogy with a flow-through lake, and transpiration on δ D C 29 values. Predictions improve with the addition of the explicit consideration of transpiration and soil evaporation, indicating that they are both important processes in determining plant lipid δD values. D-enrichment caused by these evaporative processes is controlled by relative humidity, suggesting that important climatic information is recorded in leaf wax n-alkane δD values. Calibration studies such as this one provide a baseline for future studies of plant-water–deuterium systematics and form the foundation for interpretation of plant wax hydrogen isotope ratios as a paleo-aridity proxy.