Influence of aquatic plants on the hydrogen isotope composition of sedimentary long-chain n-alkanes in the Lake Qinghai region, Qinghai-Tibet Plateau

The hydrogen isotopic composition (delta D) of leaf wax long-chain n-alkanes (C-27, C-29, and C-31) from lacustrine sediments has been widely applied to reconstruct terrestrial paleoclimatic and paleohydrological changes. However, few studies have addressed whether the aquatic-derived n-alkanes can affect the delta D values of lake sedimentary long-chain n-alkanes, which are usually regarded as a recorder of the terrestrial hydrological signals. Here we systematically investigated delta D values of long-chain n-alkanes from modern aquatic plants, both near-shore and off-shore surface sediments, surrounding terrestrial plant litters, as well as river water and lake water in Lake Qinghai and its satellite lakes on the northeastern Qinghai-Tibet Plateau. Our data showed that (i) delta D values of long-chain n-alkanes from aquatic plants varied from -184aEuro degrees to -132aEuro degrees for n-C-27, from -183aEuro degrees to -138aEuro degrees for n-C-29, and from -189aEuro degrees to -130aEuro degrees for n-C-31, respectively, with no significant differences among the three n-alkanes homologues; (ii) delta D values of long-chain n-alkanes from aquatic plants were generally more positive than those from surrounding terrestrial plants, possibly because that they recorded the D-enrichment of lake water in this semi-arid region; (iii) delta D values of long-chain n-alkanes from surface sediments showed significant differences among the three n-alkanes homologues, due to the larger aquatic input of n-C-27 to the sedimentary lipid pool than that of n-C-31, and (iv) n-C-27 delta D values of near-shore aquatic plants and near-shore sediments are more negative than those from off-shore as a result of lower delta D values of near-shore lake water. Our findings indicate that in this region (i) the offset between sedimentary n-C-27 and n-C-31 delta D values (Delta delta DC27-C31) could potentially be used to evaluate if sedimentary long-chain n-alkanes are derived from a single source; (ii) while delta D values of n-C-27 may be influenced by lake water hydrological changes, sedimentary n-C-31 is derived predominantly from terrestrial plants and thus its delta D can serve as a relatively reliable indicator for terrestrial paleoclimatic and paleohydrological reconstructions.