Lake water δD in Lake Qinghai decoupled from precipitation isotope during the last deglaciation

Abstract

Hydrogen isotope compositions (δD) are considered as reliable indicators for tracking changes in lake hydrological environments. However, lipids in lake sediments are influenced by multiple sources, and no biomarkers have been found to fully record lake water δD changes, limiting the applications of lipid δD values for paleohydrological reconstructions. In this study, we investigated fatty acid (FA) δD values with different chain lengths from various sources in 8 lakes on the northeastern Tibetan Plateau, including internal sources (aquatic submerged plants, emergent plants and algae) and external sources (terrestrial plants and soil). We found that only the algae (Cladophora) showed significant depleted C16 FA δD values (avg. –51‰) compared to other chain-length FAs, which can be used to identify the influence of algae (Cladophora) on organic matter in lake sediments. Furthermore, we observed that C16 FA δD values are consistently more negative than other chain-length FAs in both surface sediments (avg. –23‰) and core sediments since the Last Glacial Maximum (avg. –58‰) in Lake Qinghai, consistent with the Cladophora δD feature. This indicates that the C16 FAs in Lake Qinghai sediments (including surface and core sediments) are primarily contributed by algae (Cladophora). As lipid δD values of algae record lake water δD changes, this study offers a δD record of lake water δD changes for Lake Qinghai since the Last Glacial Maximum. Notably, lake water δD values in this lake did not follow precipitation isotopes during the last deglaciation, likely due to the stronger influence of glacier meltwater on its water δD values during the period. This highlights that the hydrogen and oxygen isotopes of lake water and precipitation could differ, and that the difference reflects hydrological changes.