Compound-specific δD and its hydrological and environmental implication in the lakes on the Tibetan Plateau

Abstract

The hydrogen isotopic composition ( δ D) of n -alkanes in lacustrine sediments is widely used in palaeoenvironmental studies, but the heterogeneous origins and relative contributions of these lipids provide challenges for the interpretation of the increasing dataset as an environment and climatic proxy. We systematically investigated n -alkane δ D values from 51 submerged plants (39 Potamogeton , 1 Myriophyllum , and 11 Ruppia ), 13 algae (5 Chara , 3 Cladophora , and 5 Spirogyra ) and 20 terrestrial plants (10 grasses and 10 shrubs) in and around 15 lakes on the Tibetan Plateau. Our results demonstrate that δ D values of C29 n -alkane are correlated significantly with the lake water δ D values both for algae ( R 2=0.85, p n =9) and submerged plants( R 2=0.90, p n =25), indicating that δ D values of these algae and submerged plants reflect the δ D variation of lake water. We find that apparent hydrogen isotope fractionation factors between individual n -alkanes and water ( e a/w) are not constant among different algae and submerged plants, as well as in a single genus under different liminological conditions, indicating that the biosynthesis or environmental conditions (e.g. salinity) may affect their δ D values. The δ D values of submerged plant Ruppia in the Xiligou Lake (a closed lake) are significant enriched in D than those of terrestrial grasses around the lake (one-way ANOVA, p Chara in the Keluke Lake (an open lake) display similar δ D values with grasses around the lake (one-way ANOVA, p =0.826>0.05), suggesting that the n -alkane δ D values of the algae and submerged plants record the signal of D enrichment in lake water relative to precipitation only in closed lakes in arid and semi-arid area. For each algae and submerged plant sample, we find uniformed δ D values of different chain length n -alkanes, implying that, in combination with other proxies such as Paq and Average Chain Length, the offset between the δ D values of different chain length n -alkanes can help determine the source of sedimentary n -alkanes as well as inferring the hydrological characteristics of an ancient lake basin (open vs closed lake).