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The effect of plant type on n-alkane biomarker has been widely studied, but the influence of plant type on n-alkanoic acids (n-FAs) has received less attention. Understanding the effect of the distribution of n-FAs and their hydrogen isotope composition (δ2HFA) is critical for interpreting sedimentary δ2HFA values as proxies of hydroclimate and/or vegetation changes. In this study, we systematically investigated the distribution of n-FAs and δ2HFA values across seasons on the Chinese Loess Plateau (CLP). Our results showed that there were no significant seasonal differences in n-FAs distribution and δ2HFA values, but there were significant differences in δ2HFA values across plant types. The δ2HFA values ranged from −185‰ to −125‰ in dicots (−161‰±28‰), and from −215‰ to −170‰ in monocots (−196‰±22‰) on the CLP. Isotope fractionation between δ2HFA values and precipitation δ2H (εFA-P) was also 2H-enriched in dicots relative to monocots. At the regional scale of the CLP, δ2HFA values were not correlated with δ2H values of precipitation. However, δ2HFA values from the CLP were similar to expectations from a global comparison, both in terms of their mean values and high variability among plants growing in sites with similar δ2H values of precipitation. As some of this variability in εFA-P is due to the effect of plant type, a careful assessment of the likely sources of waxes in terrestrial sediments is critical when using δ2HFA values for paleohydroclimate reconstruction. [ABSTRACT FROM AUTHOR]

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]

The hydrogen isotopic composition ( δD) of leaf wax long-chain n-alkanes (C, C, and C) 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 δD values of lake sedimentary long-chain n-alkanes, which are usually regarded as a recorder of the terrestrial hydrological signals. Here we systematically investigated δD 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) δD values of long-chain n-alkanes from aquatic plants varied from -184‰ to -132‰ for n-C, from -183‰ to -138‰ for n-C, and from -189‰ to -130‰ for n-C, respectively, with no significant differences among the three n-alkanes homologues; (ii) δD 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) δD 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 to the sedimentary lipid pool than that of n-C, and (iv) n-C δD values of near-shore aquatic plants and near-shore sediments are more negative than those from off-shore as a result of lower δD values of near-shore lake water. Our findings indicate that in this region (i) the offset between sedimentary n-C and n-C δD values (Δ δD) could potentially be used to evaluate if sedimentary long-chain n-alkanes are derived from a single source; (ii) while δD values of n-C may be influenced by lake water hydrological changes, sedimentary n-C is derived predominantly from terrestrial plants and thus its δD can serve as a relatively reliable indicator for terrestrial paleoclimatic and paleohydrological reconstructions. [ABSTRACT FROM AUTHOR]

Lipid compound-specific hydrogen isotopes (δD) have been widely applied in studies of biogeochemistry and paleoclimatology. δD values of lipid homologues with different chain lengths from a single analysis are often utilized together in order to make full use of the δD information. However, this approach could be potentially impacted by the large analytical error due to the inappropriate amount injected for some homologues. Here, we systematically investigate compound-specific δD deviations with varying contents injected on the isotope ratio mass spectrometry, expressed as amplitudes, using both lab working standards and natural samples. The n -alkane δD values of lab working standards (mixed C 21 , C 25 , C 27 , C 29 , C 31 , and C 33) could vary by 40‰ to 70‰ when the amplitudes of n -alkanes change from 0.5 V to 10 V. For natural samples (27 n -alkane and 77 fatty acid samples), we have made repeated analyses of the same samples with different homologues targeted for the optimum range. The measured δD values are higher by 20‰ to 40‰ with the amplitudes lower than the optimum range, and lower by 10‰ to 20‰ with higher amplitudes. All the results consistently show higher δD values with decreasing amplitudes, and larger deviations occurring in low amplitude range, implying that special caution should be taken with the δD values measured at low amplitude range. We have attempted amplitude-based correction of lipid δD values, however, this approach should be cautious owing to their large residual errors. Hence, δD values of different homologues in the same samples have to be measured in separate analysis if they could not fall within the optimum range at the same time. [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]

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]