Hydrogen isotope fractionation variations of n-alkanes and fatty acids in algae and submerged plants from Tibetan Plateau lakes: Implications for palaeoclimatic reconstruction

摘要

The hydrogen isotope compositions (6D) of n-alkanes and fatty acids (FAs) are widely applied in palaeoclimatic reconstructions, and the determinations of their hydrogen isotope fractionation factor values (epsilon) are vital for quantitatively reconstructing past precipitation variations. Currently, studies on n-alkane and FA epsilon values focus on terrestrial plants, which, however, show large uncertainties because of the influence of evapotranspiration. Therefore, in this study, we analysed the c values of algae and submerged plants immersed in lakes, which are not affected by evapotranspiration, to understand the hydrogen isotope fractionation of plant lipid synthesis. By investigating the delta D values of lipids (n-alkanes and Elks) in algae and submerged plants and the delta D values of co-existing water (including lake bottom water, surface sediment water, and leaf water of algae and submerged plants) from five Tibetan Plateau lakes, we find that the n-alkane c values of algae and submerged plants show narrow changes, ranging from 176 to 159%. and 167 to 142%, respectively. The FA c values of algae and submerged plants also show small variations, ranging from -160 to -121 parts per thousand. (except Chara) and -161 to -138 parts per thousand., respectively. Therefore, the average biosynthetic hydrogen isotope fractionation of these plants is -162 parts per thousand. for n-alkanes and -145 parts per thousand. for FAs, and the small epsilon differences between FAs and n-alkanes can be related to the different magnitudes of FA utilization in n-alkane synthesis. Finally, we find that the biosynthetic hydrogen isotope fractionation factors of aquatic plants are close to those of terrestrial grasses but slightly more negative than those of terrestrial woody plants. Thus, our results are helpful for understanding the hydrogen isotope fractionation variations in terrestrial plant lipids, which is beneficial for palaeohydrological reconstructions. (C) 2019 Elsevier B.V. All rights reserved.