Search

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 C 16 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 C 16 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 C 16 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. • Only Cladophora showed significant depleted C 16 FA δD than other chain-length FAs. • C 16 FA δD values are more negative in surface and core sediments in Lake Qinghai. • Offering a δD record of lake water δD changes for Lake Qinghai since the LGM. • Lake water δD did not follow precipitation isotopes during the last deglaciation. [ABSTRACT FROM AUTHOR]

Lake Qinghai, the largest saltwater lake in China, is located on the northeastern Qinghai-Tibetan Plateau. The lake sediments record the climatic variation and response to global climate change on the plateau. Numerous studies of the lake have been conducted, but various opinions remain regarding the palaeoclimatic conditions for the lake since the Last Glacial Maximum, especially the palaeohydrological conditions for the northeastern plateau in the early Holocene. Here, we discuss the hydrogen isotopic composition (δD) of long-chain leaf wax n -alkanes for studying the hydrological changes in the lake since the Holocene. The results show that, on the northeastern Qinghai-Tibetan Plateau, C 31 δD values were quite negative (avg. −202 ± 4‰) in the early Holocene (8–12 ka), indicating a strong monsoon and greater precipitation during that period. During 8–4 ka, C 31 δD values gradually became more positive, indicating that the monsoon gradually weakened and precipitation gradually decreased. The C 31 δD values were more positive in the late Holocene, with a mean value of −186 ± 3‰, showing that the regional monsoon weakened and the amount of precipitation was low. In addition, C 31 δD values indicated large amounts of precipitation in the early Holocene, but the C 29 δD values during the same period were significantly more positive than C 31 due to a large contribution from aquatic plants. Therefore, the water level of the lake was low in this period, which could be related to strong evaporation of the lake water because of the high temperature in the early Holocene. Our study clarifies the changes in the monsoon and precipitation in the northeastern Qinghai-Tibetan Plateau and it provides a new estimate of the water level variation in Lake Qinghai in the early Holocene. [ABSTRACT FROM AUTHOR]

The hydrogen isotopic compositions (δD) of long-chain plant leaf waxes can reflect changes of continental hydrology and thus have been increasingly utilized for paleoclimate reconstruction. One of the unresolved major issues is whether variations of leaf wax δD (δD wax) signals along a precipitation gradient reflect changes of precipitation δD, precipitation amount, and/or evapo-transpiration. This ambiguity limits our interpretation of δD wax in geological records as well as the quantitative reconstruction of paleohydrological variability. Here we systematically investigated δD of soil water and waxes extracted from soil and plant leaves in the Chinese Loess Plateau (CLP) and its surrounding areas along a precipitation gradient with mean annual precipitation (MAP) varying from 140 mm to 676 mm. The results showed that while the variation of modeled precipitation δD has no significant correlation with MAP, soil water δD, soil δD wax , and individual plant δD wax exhibit negative correlations with MAP. In relatively arid areas, the δD values of soil water and plant leaf waxes are significantly more positive due to much lower precipitation relative to evapo-transpiration, suggesting that effective precipitation (P:E ratio) has played a crucial role in the D-enrichment of soil and plant leaf waxes in this region. While parallel decreasing trends in δD wax are found along the increase of precipitation gradient among dominant plants (including Artemisia spp., Aster hispidus , Stipa bungeana , and Cleistogenes squarrosa) in the region with similar slopes, the large δD offsets between plant groups suggest that plant type is an important factor in controlling plant hydrogen isotope fractionations. Our results indicate that δD wax preserved in paleosols can be used to infer past conditions of water availability in arid and semi-arid inland regions, but with a mechanism that is different from the influence of "amount effect" in humid areas. Moreover, vegetation changes should be constrained by independent paleobotanical data before monsoon related paleohydrology in the CLP and its surrounding areas can be quantitatively reconstructed using the plant wax δD proxy. • Modern precipitation δD has no significant correlation with mean annual precipitation amount on the Chinese Loess Plateau • Soil water δD, soil and plant leaf wax δD show negative correlations with mean annual precipitation • Precipitation-controlled evapotranspiration play a crucial role in the D-enrichment of soil and plant leaf waxes • Plant type is an important factor in controlling plant hydrogen isotope fractionations • Paleosols leaf wax δD can be used to infer past precipitation in Asian monsoon regions [ABSTRACT FROM AUTHOR]