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The hydrogen isotopic composition (δD) 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 δD 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 δD values of C29n-alkane are correlated significantly with the lake water δD values both for algae (R2=0.85, p<0.01, n=9) and submerged plants (R2=0.90, p<0.01, n=25), indicating that δD values of these algae and submerged plants reflect the δD variation of lake water. We find that apparent hydrogen isotope fractionation factors between individual n-alkanes and water (εa/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 δD values. The δD 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<0.01), but the algae Chara in the Keluke Lake (an open lake) display similar δD values with grasses around the lake (one-way ANOVA, p=0.826>0.05), suggesting that the n-alkane δD 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 δD 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 δD 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). [ABSTRACT FROM AUTHOR]

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]

Stable carbon isotopes (δ13C) in lake sediments can record changes in CO 2 emissions from fossil fuels (FFs) combustion; however, the exact proportion of δ13C changes caused by FFs combustion remains unclear. Here, taking the Huguangyan Maar Lake (HGY) located in low latitude with high intensity of human activities in southern China as an example, we used the MixSIAR Bayesian stable isotope mixing model to trace the source change of δ13C in the HGY sediments over the past 130 years. The δ13C value, ranging from −20.23‰ to −24.29‰, with an average of −22.39 ± 1.5‰, displayed a continuous downward trend from the bottom to top throughout the core, with the fastest decline occurring between 1950AD and 1996AD. Quantitative source appointment indicates that the contribution of FFs sources to δ13C depletion in the HGY sediments has increased nearly three-fold over the past 130 years, from 18.6% to 67.8%, which may be related to the Suess Effect of the 13C-depleted FFs burning. The increase in δ13C in HGY sediments in the past decade was consistent with the increase in δ13C–CO 2 emitted by global fossil-fuel consumption, and was also related to the decrease in fossil fuels consumption caused by China's Clean Air Act. This study demonstrates that the isotope traceability model is helpful for source appointment of δ13C in lake sediments and that the δ13C in maar lake sediments can be used to trace CO 2 emissions from global FFs combustion. This therefore provides new insights on the carbon cycle during the Anthropocene. [Display omitted] • A Bayesian stable isotope mixing model was used to trace source change of δ13C. • δ13C has shown a continuous downward trend from bottom to top throughout the core. • The contribution of fossil fuels sources to δ13C depletion has doubled over 130 years. • Suess Effect from 13C-depleted fossil fuels burning may contribute to the δ13C depletion. • Decreased fossil fuels usage results in the rapid increase in δ13C over the past decade. [ABSTRACT FROM AUTHOR]

• Tetramethylated brGDGTs can also be produced along with 6-methyl brGDGTs. • This effect can potentially bias the MBT′ 5ME paleothermometer. • Temperature responses of 5- and 6- methyl brGDGT sets were separately quantified. • A new calibration of brGDGT paleothermometer mitigating the isomer effect was developed. Bacterial branched glycerol dialkyl glycerol tetraethers (brGDGTs) are emerging as one of the most promising tools for quantifying past temperatures across a large variety of continental settings. However, temperature-independent parameters such as the ratio of 6-methyl vs. 5-methyl brGDGTs and bacterial community composition, might contribute to the high uncertainty of the brGDGT paleothermometer. In this work, we investigated brGDGT distributions in surface soils surrounding Lake Qinghai, and reanalyzed previously published brGDGT data. The results show that tetramethylated brGDGTs can be produced along with 6-methyl brGDGTs in the absence of 5-methyl brGDGTs. The additional production of tetramethylated brGDGTs related to 6-methyl brGDGTs, in contradiction to the view that tetramethylated brGDGTs are only correlated with 5-methyl brGDGTs, can potentially bias the MBT′ 5ME paleothermometer. Accordingly, tetramethylated brGDGTs were hypothetically separated into the 5-methyl and 6-methyl sets, and the temperature responses of the two brGDGT sets were separately quantified. Based on theoretical calculation, we further developed a new temperature calibration model that effectively mitigates the mixing effect of the two sets likely associated with bacterial community shift. As demonstrated in downcore applications, the new calibration is promising for brGDGT-based growth temperature reconstruction in loess and lake sediments, especially when the relative abundance of 6-methyl vs. 5-methyl brGDGTs varies substantially. [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]

Branched glycerol dialkyl glycerol tetraethers (brGDGTs) are emerging as a powerful tool for quantifying past terrestrial temperature variations. In lake sediments, mixed lacustrine vs. soil sources of brGDGTs pose a challenge to quantitative application of this paleothermometer, and this effect has not yet been well-constrained. In this work, we analyzed concentrations and distributions of brGDGTs along with two other types of typical lipid biomarkers (isoGDGTs and long-chain n -alkanes) in modern sediments and catchment soils from 26 park lakes across a large climatic gradient in China. BrGDGTs in these lakes may largely represent the lacustrine signal due to the relatively small catchment area and high eutrophic condition. This is confirmed by the combined use of crenarchaeol, its isomer crenarchaeol′, and C 33 n -alkane as tracers of soil input for brGDGTs. This novel method quantifies the upper limit of soil brGDGT contribution to be less than 10% for most investigated lakes, with an average value of 7% ± 10%. Subsequently, the brGDGT signal of the assumed pure lacustrine origin was constrained by the subtle difference between the original sedimentary brGDGTs and those corrected by the maximum possible soil brGDGT input for those park lakes. A compilation of global freshwater lakes further shows that the correlation between the methylation index of brGDGTs (MBT or MBT′ 5ME) of their sediments and temperature matches that for the assumed pure lacustrine signals. Hence, soil contribution might be generally minor and may not significantly impact the quantitative application of the brGDGT paleothermometer for worldwide lakes. This study reinforces a foundation for this novel lacustrine paleothermometer to be applied in paleoclimate reconstructions, although factors affecting lacustrine brGDGT distributions within lakes still need to be better constrained. • A new method for constraining soil brGDGT contribution to lakes was developed. • The upper limit of soil brGDGT contribution is generally <10% for park lakes. • The brGDGT signal of the assumed pure lacustrine origin was constrained. • The MBT′ 5ME -T relation for global lakes resembles that for pure lacustrine signals. [ABSTRACT FROM AUTHOR]

Eolian dust activity studies on the arid Central Asia (ACA), one of the world's largest dust sources, are of great importance to the global climatic system. Grain size distributions (GSDs) can provide information on sediment sources, transport processes and sedimentary environments. As a result, GSD is the most frequently applied proxy in reconstruction of past eolian dust activities of the ACA using lake sediments. The GSD dataset for a core spanning the last ~850 years from Kuhai Lake on the northeastern Tibetan Plateau was unmixed by end-member analysis (EMA) for reconstruction of past eolian dust activities on the southeastern margin of the ACA. The results suggest that EM 2 and EM 3, which were added together to form an eolian dust activity proxy, represent the proximal eolian suspension and saltation loads, respectively. The fractional abundances of EMs suggest that siliciclastic materials in the studied core were mainly from eolian contributions. The occurrence of enhanced eolian dust activities within the cold Little Ice Age period together with the GSDs of EM 2 and EM 3 indicate that eolian dust activities on the southeastern margin of the ACA were basically governed by the Siberian High-produced Asian Winter Monsoon over the last millennium. At decadal to centennial scales, enhanced eolian dust activities generally correspond to warm periods with high total solar irradiance (TSI). Increased temperatures as the result of high TSI would have caused larger increases in evaporation than in precipitation, and hence deteriorated vegetation cover and increased dust sources availability. • Eolian dust activity of the arid Central Asia was studied using a core of Kuhai Lake. • The HALS-NMF parametric algorithm of end-member analysis was applied. • Siliciclastic materials in the studied core are mainly from eolian contribution. • Eolian dust activity at centennial scale was governed by Siberian High. • Eolian dust activity at decadal scale was governed by total solar irradiance. [ABSTRACT FROM AUTHOR]

As a transitional region between arid and semi-arid areas, and the climatic junction where the Asian summer monsoon and the Westerlies interact strongly, the climatic pattern of northeastern Tibetan Plateau has always been the focus of paleoclimate research. In this study, the changes in the lake depth of Lake Qinghai on the northeastern Tibetan Plateau were reconstructed from analyses of biomarkers in sediment records from the center of the lake and aeolian deposits on its southern bank. On millennial scale, the lake depth showed a fluctuating upward trend from the minimum depth at 15.6 ka to the maximum depth at 5.9 ka, revealing an overall warming-wetting pattern since the last deglaciation. Compared to the typical monsoon-dominated areas, the relatively humid late Holocene of Lake Qinghai appears to have been related to a weakened evaporation on the long-term trend. We show that on a long-term scale, precipitation may control the rate of changes in lake depth. This reasonably explains the asynchrony of precipitation and lake depth over the northeastern Tibetan Plateau during the mid-Holocene. • Reconstruct lake depth change using P aq records in lacustrine and terrestrial records. • Precipitation controlling the rate of lake depth change. • Precipitation decreased and lake depth rising slowed in the mid-Holocene. [ABSTRACT FROM AUTHOR]