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Stable carbon isotopes of long-chain n-alkanes in lake sediments have been used for reconstructing past environmental changes. Gas chromatography isotope ratio mass spectrometry coupled to a combustion unit is the commonly applied technique for compound-specific carbon isotope analysis. This paper shows that sulfur in the lacustrine sediment can cause severe interference during the mass spectrometric measurement, leading to significant biases in δ13C of n-alkane. The δ13C deviation could be resulted from additional 12C33S+, 13C32S+ at mass-to-charge ratio (m/z) 45, and 12C34S+, H2C32S+at m/z 46, respectively. Such elemental sulphur interference can be eliminated by the activated copper in the pretreatment procedure. [ABSTRACT FROM AUTHOR]

Rationale: Gas chromatography‐combustion‐isotope ratio mass spectrometry (GC/C/IRMS) is widely used for compound‐specific carbon isotopic analysis. However, current isotopic analysis systems utilize the GC IsoLink combustion reactor, and independent reduction furnaces are not implemented. Therefore, whether this limitation in furnace use affects the precision of compound‐specific carbon isotopic analysis needs to be evaluated. Methods: We attempted to add a separate reduction furnace to the GC IsoLink interface and compared the δ13C values of n‐alkanes (including C and H elements), fatty acid methyl ester (including C, H, and O elements), caffeine (USGS61 and USGS62, including C, H, O, and N elements), and 9‐ethylcarbazole (including C, H, and N elements) before and after the addition of the reduction furnace using the GC IsoLink combustion reactor. Results: For n‐alkanes and fatty acid methyl esters, the δ13C differences between the measured values and their standard values were basically falling within 0.5‰ whether or not an independent reduction furnace was added. However, for the nitrogen‐containing compounds (caffeine and 9‐ethylcarbazole), the δ13C differences between the measured values and their standard values were much larger without an independent reduction furnace (1.0–3.71‰ for USGS61, 1.78–2.19‰ for USGS62, and 0.39–1.13‰ for 9‐ethylcarbazole) than with a reduction furnace (−0.31–0.68‰ for USGS61, −0.44–0.06‰ for USGS62, and −0.04–0.25‰ for 9‐ethylcarbazole). Conclusions: The addition of an independent reduction furnace had no significant effect on the δ13C of n‐alkanes and fatty acid methyl esters, but it had a significant effect on the δ13C of nitrogen‐containing compounds. It is suggested that GC IsoLink needs an independent reduction furnace to effectively eliminate the interference of NOx on CO2 isotopic determination to improve the accuracy of δ13C for nitrogen‐containing compounds. [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]

RATIONALE The condition of the pyrolysis reactor is very important for obtaining stable, precise hydrogen isotopic ratios using gas chromatography/thermal conversion/isotope ratio mass spectrometry (GC/TC/IRMS). However, few studies of the conditioning process have been conducted, and little is known about the best methods for high-precision hydrogen isotope analysis. METHODS We investigated δ2H variations and observed the changes in carbon coating using six different conditioning methods for the pyrolysis alumina tube: (i) no treatment; (ii) conditioning with 4 μL hexane; (iii) conditioning with 2 μL hexane; (iv) conditioning with 2 μL hexane followed by backflushing overnight; (v) conditioning with 10 s of backflushing with methane; (vi) conditioning with 3 s of backflushing with methane. RESULTS Conditioning the alumina tube can improve the pyrolysis efficiency of organic compounds because a moderate amount of carbon acts as a catalyst in high-temperature regions of the alumina tube. Carbon actually flows in the tube and is difficult to confine to the high-temperature region. Insufficient amounts of carbon in the high-temperature regions lead to incomplete pyrolysis of organic compounds and lower δ2H values due to kinetic fractionation of hydrogen isotopes. In contrast, excess hexane or methane can lead to higher δ2H values, probably due to enrichment of deuterium in the hydrocarbon residue. CONCLUSIONS The δ2H values obtained by Method 6 are closest to the TC/EA δ2H values and are more precise than those obtained by other methods, perhaps because this method introduces a moderate, consistent amount of carbon with each sample injection. Copyright © 2012 John Wiley & Sons, Ltd. [ABSTRACT FROM AUTHOR]

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]

Intrashell oxygen isotope (δ18Os) analyses of terrestrial snails have been carried out over two decades. However, the intraspecies/interspecies differences are not yet well understood. Here, we conducted a high‐resolution intrashell δ18Os study on 43 shells from Cathaica fasciola and Bradybaena ravida, and 1449 δ18Os data were obtained. These large amounts of data demonstrate the reproducibility of intrashell δ18Os. Our results suggest that C. fasciola and B. ravida have similar growing seasons mainly from March to October in the studied region. The ecological habits of the two species could explain the distinctive variations in their δ18Os sequences. Therefore, our study may interpret the discrepancy between the studies of land snail species and the stable/clumped isotopes of their shells. Moreover, the calculated average growth rate is ∼290 μm/day (from 250 to 330 μm/day) for nonadult C. fasciola. Thus, the use of snail shells for reconstructing high‐resolution terrestrial climate changes is promising. Plain Language Summary: Land snails are sensitive to climatic and environmental changes. Therefore, the oxygen isotopic composition of snail shells (δ18Os) has often been used to study environmental changes. Researchers started to collect multiple samples from a single shell for δ18Os analyses about two decades ago. However, it is still unclear whether high‐resolution intrashell δ18Os results are repeatable and whether the influences of different individuals/species may exist. To address these issues, we collected 43 snail shells from Cathaica fasciola and Bradybaena ravida. In total, 1449 δ18Os data were obtained. The large amount of data showing similar variations in the intrashell δ18Os from different shells demonstrates the repeatability. Accordingly, our data suggest that the two species have similar growing seasons, mainly from March to October, although their ecological habits are different. In addition, the ecological habits of these two species could explain the subtle differences in their δ18Os. Taken together, our study could resolve the discrepancy between previous work on land snail species and stable/clumped isotopes. Furthermore, the estimated average growth rate is ∼290 μm/day (from 250 to 330 μm/day) for nonadult C. fasciola. Overall, snail shells are promising records for exploring high‐resolution terrestrial climate changes in the future. Key Points: The intrashell δ18Os of land snails is reproducible, which mainly records the δ18O of precipitationCathaica fasciola and Bradybaena ravida have different ecological habits but similar growing seasonEcological habits could explain the discrepancy between studies of land snail species and stable/clumped isotopes [ABSTRACT FROM AUTHOR]

Hydrogen isotopic compositions (δD) of fatty acids (FAs) in lake sediments have been widely used in palaeoenvironmental reconstruction, but investigations on FA δD values in algae and submerged plants from natural lakes are still limited, greatly hampering the application and interpretation of their δD values in lacustrine environments. Here, we systematically investigated δD values of short-chain (C 14 –C 18 ), mid-chain (C 20 –C 24 ), and long-chain (C 26 –C 30 ) FAs in four algae and submerged macrophytes, two algal genera Chara and Cladophora and two angiosperm genera Potamogeton and Ruppia , that commonly occur in six lakes on the north-eastern Tibetan Plateau. We obtained the following results: (i) C 20 –C 30 FAs in each algal and submerged plant sample have similar variation patterns in δD values, suggesting that δD values of C 20 –C 30 FAs have consistent responses to their source water δD. In C 4 -like taxa ( Potamogeton , Ruppia , and Chara ), the C 16 and C 26 FAs have minor differences in their δD values, while in the C 3 -like Cladophora , δD values of C 16 FAs are significantly more negative (avg. –48 ± 7‰) than that of the C 26 FAs. We propose that different photosynthetic mechanisms in these algae and submerged plants account for the different inter-molecular δD variations between C 16 and C 26 FAs; (ii) δD values of C 26 FAs in the carbonate-coated alga Chara are significantly correlated with the δ 18 O values of carbonate encrustations – such a co-variance suggests that in addition to lake water, other factors may also influence both lipid synthesis and carbonate encrustation building up in Chara , leading to a significantly positive apparent hydrogen isotope fractionation factors (ε FA-W ) between C 26 FAs of Chara and lake water; (iii) δD values of C 26 FAs from Potamogeton , Ruppia and Cladophora are correlated well with lake water δD values ( R 2 = 0.84, p  < 0.001, n = 19), suggesting that the δD values of long-chain FAs potentially record δD variations of lake water with the apparent hydrogen isotope fractionation factors (ε FA-W ) of C 26 FAs ranging from −153‰ to −134‰. Our results highlight the impact of different photosynthetic pathways on the inter-molecular difference of FA δD values in algae and submerged plants and illustrate both the potential and the challenge of applying their FA δD values as a palaeo-hydrology proxy. [ABSTRACT FROM AUTHOR]

The hydrogen isotopic composition of plant wax (δD wax ) inherits the isotopic values of the source water and isotopic fractionation that take place in lipid biosynthesis. The δD wax values of n -alkanes or n -acids in sediment records are considered to be effective paleo-δD for precipitation proxies for different timescales. However, the vegetation effect also contributes substantially to the apparent fractionation values (ε w-p ) between the precipitation and plant. Therefore, a method for evaluating the dominant plant source in the sediment records is key for reconstructing a reliable paleo-δD p record. In this study, based on field data from the extremely arid Qaidam Basin and the semi-arid central Chinese Loess Plateau (CLP), a plant type index (Pta), based on the traditional P aq and n -alkane C 31 /C 29 index, was used to evaluate the dominant plant type that contributed to the sediments. In the arid and semi-arid region, the Pta values of the aquatic plants were <90, while the shrub Pta ranged from 90 to 180 and the grass Pta was >180. A test on contemporary soils and surface lake sediments also showed that the Pta index could estimate the regional vegetation distribution in modern environments. We also evaluated the dominant n -alkane source for four sediment records in China during the last millennium and reconstructed a regional paleo-δD p by choosing a different ε w-p . The estimated paleo-δD p values showed a good correlation with the modern regional summer δD p . We suggest that a large number of field studies are needed to improve the Pta index for quantitatively calculating the precise ratios for various plant sources. Such data would play a key role in improving hydrogen isotopic hydroclimatic research. [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]

Contrasting moisture evolutions between westerlies‐dominated Central Asia and monsoonal Asia since the mid‐Holocene have been widely recognized. Yet, inconsistent hydrological records over the early Holocene from both regions hamper our understanding of hydroclimatic changes and associated mechanisms. Here we present isotopic and biomarker (alkenone) records from three Chinese lakes located at both regions to address a possible meltwater role in lake water budget. Our results show the occurrence of particularly depleted isotopic values of lake water associated with lake freshening within the deglaciation and early Holocene. Together with anomalously old 14C dates, which were also documented in many other Asian lakes, our observations likely indicate substantial meltwater contribution to mid‐latitude Asian lake water budget, along with the climatic warming. Hence, the potential meltwater effect should be considered when inferring Asian summer monsoon and westerlies behavior over this critical period, with important implications to Holocene hydrological reconstructions and future hydroclimatic projections. Plain Language Summary: Although meltwater events have been well documented in marine records, the effects of meltwater on Asian lake water budget and regional hydrological cycle during the deglaciation and warm early Holocene remain poorly investigated. Our isotopic and biomarker records from Lake Sayram, Yanhaizi, and Qinghai in northern China, together with anomalously old 14C dates from investigated lakes and other Asian lakes, likely indicate substantial meltwater contribution to lake water budget within the deglaciation and early Holocene, which appears to be a large‐scale phenomenon in mid‐latitude Asia. This suggests that the potential meltwater effect on Asian lake hydrology should be assessed in climatic reconstructions and simulations, which may shed light on accurate interpretation of westerlies and summer monsoon circulation over the early Holocene. Key Points: Isotopic records likely indicate meltwater contribution to Asian lake water budget within the deglaciation and early HoloceneAnomalously old 14C dates summarized from mid‐latitude Asian lakes also support substantial meltwater input during this periodMeltwater effects should be considered when inferring early Holocene Asian summer monsoon and westerlies behavior [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]

Record of long‐term land temperature changes remains ephemeral, discontinuous, and isolated, thus leaving the common view that Pleistocene land temperature evolution should have followed ocean temperatures unconfirmed. Here, we present a continuous land surface temperature reconstruction in the Asian monsoon region over the past 3.0 Myr based on the distribution of soil bacterial lipids from the Chinese Loess Plateau. The land temperature record indicates an unexpected warming trend over the Pleistocene, which is opposite to the cooling trend in Pleistocene ocean temperatures, resulting in increased land‐sea thermal contrast. We propose that the previously unrecognized increase of land‐sea thermal contrast during much of the Pleistocene is a regional climate phenomenon that provides a likely mechanism in favor of the long‐term enhancement of the Pleistocene East Asian summer monsoon. Plain Language Summary: Numerous sea surface temperature (SST) records indicate long‐term gradual cooling over the Pleistocene, whereas the East Asian summer monsoon (EASM) appears to have strengthened during this period. Land‐sea thermal contrast is thought to have an important influence on the intensity of the EASM, but the lack of land surface temperature (LST) records from monsoonal regions prevents the assessment of its role in driving monsoonal changes. Here, we present a 3‐Myr‐long LST record from the Chinese Loess Plateau, displaying a unique long‐term warming trend over the Pleistocene, different from the trajectories of SST records. We propose that the decoupling of long‐term land and sea temperatures, and thus the increase of land‐sea thermal contrast, provides a simple interpretation for the otherwise paradox, strengthening of the EASM over the Pleistocene. This study presents much needed original LST data that support novel interpretations on a mechanism for important paleoclimate phenomenon in East Asia with implications to the entire terrestrial ecosystem during the critical period of earth history. Key Points: Three million years of continuous land surface temperatures reconstructed based on soil bacterial lipidsThe land temperature record indicates an unexpected warming trend over the PleistoceneIncreasing land‐sea thermal contrast facilitated the long‐term intensification of Pleistocene East Asian summer monsoon [ABSTRACT FROM AUTHOR]

The chronology of geological records in lacustrine, peatland and marine sediments for the late glacial depends mainly on 14C dating technology, which provides the basic database for global paleoclimate research. However, 14C reservoir correction always challenges the accuracy of the 14C chronology of terrestrial and marine sediments and the uncertainty of the 14C chronology associated with the carbon reservoir effect becomes critical for high resolution paleoclimate studies. Here, based on the hypothesis of synchronization of precipitation isotopes, we verify and identify a series of in-phase points of precipitation isotopes (IPPIs) between sediment leaf-wax hydrogen and stalagmite-calcite oxygen isotopes. Because stalagmite oxygen isotope records are accurately dated by U–Th dating technology, the ages of stalagmite IPPIs could be used to improve 14C reservoir correction of lacustrine IPPIs. We found that reservoir-corrected 14C ages of lacustrine, peat, and marine IPPIs are scattered with an average uncertainty of 1 ka when compared with the corresponding IPPIs on U–Th age scales. We suggest that the reservoir age correction at different time intervals could be further adjusted by using the U–Th age-backed IPPIs, which largely reduces the uncertainty of the 14C chronology and thus provides more accurate paleoclimate records. • A series of in-phase points of precipitation isotopes (IPPIs) is identified between sediment δDwax and stalagmite δ18O. • IPPIs' U–Th ages could be used to correct 14C reservoir values in sediments. • The IPPIs correction in lacustrine records is promising for improving climate integration studies. [ABSTRACT FROM AUTHOR]

In saline lakes, the paleoenvironmental interpretation of long-chain alkenones (LCAs) is difficult due to the inputs of Isochrysidales Group 2i, which are ice-related bloomers. Here, we perform detailed analysis of LCAs and long-chain alkenoates in sediment cores from Lake Daihai in north China. During the late Holocene, alkenone-inferred temperatures and the Group 2i species abundance indicate a long-term warming trend in cold seasons (winter and early spring), possibly driven by regional insolation and by atmospheric CO 2 concentrations. Notably, our records indicate cold climate conditions during the period of 4.3–3.0 cal ka BP, possibly linked to the abrupt expansion of Arctic winter sea ice in a given of low insolation and atmospheric CO 2 concentration conditions. The abrupt cooling may have led to the collapse of the East Aian Summer Monsoon (EASM) and the deterioration of the ecosystems in northern China during the middle to late Holocene transition, and thus likely accounted for the nearly synchronous Chinese Neolithic cultural collapses. • C 36 OEt/C 37 Me ratio is a reliable indicator of March–June warming rate. • Late Holocene cold season warming trend controlled by insolation and CO 2. • The ice-related Group 2i species rapidly increased at 4.3–3.0 cal ka BP. • Abrupt cooling at 4.3–3.0 cal ka BP linked to Arctic winter sea-ice expansion. • The cold-dry and unstable climate led to cultural interruption at 4.3–4.0 cal ka BP. [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]

Abstract: Pleistocene vegetation history on the Chinese Loess Plateau has been traditionally investigated using palynological methods, and questions remain regarding whether an extensive broadleaf deciduous forest ever developed on the loess table under favorable climatic conditions. The authors have employed a C isotope approach to address this question by comparing δ13C values in soil organic matter from different loess ecological domains with known source vegetation to the C isotope values obtained from a paleosol section that can be dated back to 130 ka. The C isotopic compositions of modern soils from the loess table and the loess–desert transition gave δ13C values of −24.5‰ to −18.2‰ and −25.7‰ to −20.7‰, respectively. These C isotopic ratios are consistent with the standing modern vegetation that is dominated by a mixture of C3 and C4 plants and can be distinguished from that in the patchy forest areas where exclusive C3 trees yield a narrow δ13C value range from −26.9‰ to −25‰ (average −26.1‰). Obtained δ13C compositions from paleosols and loess sediments in the Lantian and the Luochuan profiles vary from −24‰ to −16.9‰, indicating a grass-dominated steppe with shifting C3 and C4 contributions controlled mainly by paleoclimatic changes during the late Pleistocene. The present results suggest no extensive forest coverage on the loess table during the past 130 ka even under the most suitable conditions for forest development. This conclusion supports the explanation of natural causes for the development of only patchy forests on the modern loess table and provides critical historical information toward the vegetation restoration project that is currently underway on the Chinese Loess Plateau. [Copyright &y& Elsevier]

• Salinity affects the relative abundance of 5-methyl brGDGTs versus their isomers. • Salinity proxies based on the isomerization of brGDGTs were developed. • The isomerization of brGDGTs can bias MBT′ 5ME towards higher values. • Temperature can be extracted out of MBT′ 5ME after correcting the salinity effect. Branched glycerol dialkyl glycerol tetraethers (brGDGTs) are microbial molecular fossils ubiquitous in natural environments. The correlation between the Methylation Index of brGDGTs (the M B T 5 M E ' paleothermometer) and temperature offers an important tool for reconstructing past terrestrial temperatures. However, factors other than temperature could also affect the distribution of brGDGTs in lacustrine systems, hampering the quantitative application of this paleothermometer. Here we investigated brGDGT distributions in contemporary sediments collected from 52 lakes in mid-latitude Asia. Combined with published brGDGT data from other lakes across the globe, we have found a strong salinity control on the relative abundance of 5-methyl brGDGTs versus their late-eluting isomers (including 6-methyl, 7-methyl, and unknown isomers). This allows the development of novel indices based on the isomerization of brGDGTs for tracing past lake water salinity. We also demonstrate that salinity-controlled isomerization of pentamethylated and hexamethylated brGDGTs can significantly impact the M B T 5 M E ' paleothermometer, potentially leading to an overestimation of past temperature, but the temperature signal can be extracted out of the M B T 5 M E ' index after correcting the salinity effect. As demonstrated in application to a Lake Qinghai (China) sediment core spanning the last 18 kyr, our finding could facilitate the simultaneous retrieval of reliable temperature and salinity records using brGDGTs in lacustrine settings, in particular for lakes that have experienced large salinity changes during the geological past. [ABSTRACT FROM AUTHOR]

Precipitation isotopic composition variations that are directly recorded in geological samples play an important role in understanding past hydroclimatic changes. In arid regions, the hydrogen isotopes of leaf wax (δD wax) in loess sections can provide records of summer precipitation isotopes. However, previous studies have primarily focused on the orbital scale, and there was still a lack of clear understanding of variations in precipitation isotopes on shorter timescales and their implications for climate change. In this study, we report centennial-resolution loess δD wax records from the central Chinese Loess Plateau (CLP) from the last deglaciation to the early Holocene. We compared the loess optically stimulated luminescence chronology and U-Th ages conveyed by linking stalagmite δ18O and loess δD wax to validate the δD wax method as a potential tool for assessing loess chronology on the suborbital scale. Our findings also indicated that regional precipitation isotopes experienced a depletion of approximately 24‰ for δD and around 3.2‰ for δ18O during the last deglaciation. The overall precipitation isotope amplitude in the central CLP was similar to that of typical East Asian monsoon regions and lower than that of the Indian monsoon and westerly regions. However, the precipitation isotope oscillations on the millennial scale in the central CLP were significantly weaker than those in eastern to central China. The finding affirmed that although the summer precipitation in the CLP was primarily influenced by the East Asian monsoon system at the orbital scale, the northern boundary of the summer rain belt remained a distance away from the interior of the CLP during the last deglaciation. As a result, the short-term oscillations of atmospheric circulation were not strongly reflected in the precipitation isotopes of the central CLP. This result highlighted spatial and temporal differences in precipitation isotope variability between the dominant and marginal regions of the East Asian monsoon system. • The Chinese Loess Plateau (CLP) centennial-resolution δDwax record was assessed. • Similar precipitation isotopes amplitudes in the central CLP and East Asian monsoon regions during the last deglaciation. • Weak millennium-scale oscillations were found in the precipitation isotopes of the central CLP. • The potential application of δDwax for short-scale loess chronology was validated. [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]

The oxygen isotopic composition of sedimentary carbonate (δ18O carb) provides information on the regional water balance between precipitation and evaporation in a lacustrine system. A volume effect was proposed to explain the abnormally low δ18O carb values in Lake Qinghai on the northern Tibetan Plateau, under the conditions of high evaporation and low water levels, as observed during the early Holocene. Volume effect theory states that the small volume of the lake and its sensitivity to isotopic signals from freshwater discharge control the δ18O carb values. In this study, we present 7.6-kyr records of carbonate content and oxygen isotopes from Lake Gahai, a small and saline satellite lake near Lake Qinghai. The records demonstrate that the water level and lake size of Lake Gahai considerably changed over the past 7.6 kyr, and the volume effect was responsible for the δ18O carb variations. The records also suggest contrasting responses of carbonate precipitation to water depth across different timescales. For example, over the past 7.6 kyr, the carbonate content decreased with increasing lake water levels. This can be explained by gradual water freshening under low evaporation conditions and enhanced connection between Lake Qinghai and Lake Gahai. On the centennial scale, however, the carbonate content varied significantly and was positively correlated with Lake Gahai's water level. Such a variation is probably due to the periodically lower lake water levels and a stronger influence of terrestrial clastic material, which contributed to the dilution of the carbonate content under centennial cool and dry periods. Therefore, in order to advance our understanding of past hydroclimatic variations in the climatically sensitive region, information across different timescales should be separated. • 7.6-kyr carbonate records from Lake Gahai, a satellite lake near Lake Qinghai. • Lake level is the major contributor for the δ18O carb variation in Lake Gahai. • Contrasting responses of carbonate precipitation to water depth in different scales. • Satellite lakes benefit from their vulnerability and sensitivity to climatic changes. [ABSTRACT FROM AUTHOR]

Reconstruction of the paleoaltitude history of the Tibetan Plateau is critical for understanding the linkage between tectonics and its effect on regional and global climate change. Presently, most of the paleoaltitude studies are concentrated on the southern and southeastern part of the Tibetan Plateau, and few studies have been conducted in the central‐northern part. In this paper we focused on the Wudaoliang Formation in the Tuotuohe Basin, central to northern Tibetan Plateau, to reconstruct paleoaltitude based on carbonate oxygen isotopes. The carbonate samples are primary or have experienced an early stage of digenesis. Based on the thermodynamic and empirical model results, the paleoaltitude of the Wudaoliang Formation is found to be around 2700–3260 m (average of 2980 ± 280 m) in the early Miocene (∼24 Ma). Integrating paleoaltitude results from Wudaoliang Basin and our results, we conclude that crustal shortening and tectonic activity were strong during the late Eocene to late Oligocene‐early Miocene and relatively weak during the early Miocene in the central‐northern Tibetan Plateau. [ABSTRACT FROM AUTHOR]

Terrestrial snails are sensitive to climate changes and their shell's oxygen isotope composition (δ18O shell) is widely used in studies of paleoclimate reconstructions. However, the interpretation of δ18O shell remains complex due to the combined effects from different factors such as precipitation δ18O (δ18O p), relative humidity (RH), and temperature. Furthermore, no systematic studies have addressed the transformation of oxygen isotope signals from rainfall to the aragonite shell via snail body water. Here we present the results of Cathaica fasciola body water δ18O (δ18O BW) and its content, which were sampled every two days throughout the growing season in 2021 in Xi'an. Meanwhile, the soil water δ18O (δ18O SW), δ18O p , rainfall amount, RH and temperature were also simultaneously monitored, and two high-resolution (at ∼0.3 mm interval) δ18O shell series were obtained from two live snails collected in September and October 2021, respectively. These data reveal that the δ18O BW mimic the variations of δ18O p , and they are positively correlated (r2 = 0.84) for precipitation events and the correlation coefficient only increases slightly (r2 = 0.88) when RH is included in the multivariate analysis. During non-precipitation intervals, there is an obvious negative correlation between δ18O BW and RH. Moreover, the variations of two δ18O shell series are broadly consistent with the theoretically calculated δ18O shell from δ18O BW /δ18O p at seasonal scale. Collectively, this study demonstrates that δ18O p is the first-order control on the seasonal variations of δ18O BW and δ18O shell , while RH play its role during non-precipitation periods. Importantly, this study lays good foundation for reconstructing terrestrial seasonal distribution of precipitation and extreme rainfall events using fossil shells in the geological past. • Precipitation δ18O paced the seasonal variations of snail body water δ18O. • Relative humidity plays its role on body water δ18O during non-rainfall periods. • High-resolution snail shell δ18O recorded seasonal precipitation δ18O signals. [ABSTRACT FROM AUTHOR]

The multi-proxy approach is becoming an effective means to reduce uncertainties and errors compared to the single proxy approach. In this study, we investigated leaf wax biomarker and brGDGTs in topsoils along a semiarid elevation transect on the Chinese Loess Plateau. We found that both leaf wax n -alkane δ2H wax values and brGDGTs-derived M B T 5 M E ′ decreased significantly with elevation, and thus the coupled δ2H wax - M B T 5 M E ′ proxy can improve correlation coefficients (R2 = 0.96) compared to a single proxy (R2 = 0.82 and 0.90 for δ2H wax and M B T 5 M E ′ , respectively) and reduce the uncertainty (RMSE = 230 m) relative to a single proxy (RMSE = 275 m and 360 m for δ2H wax and M B T 5 M E ′ , respectively) in elevation estimates. Then we tested the effectiveness and advantages of coupled δ2H wax - M B T 5 M E ′ proxy from surface soils on the Tibetan Plateau compared to a single proxy. Collectively, the coupled δ2H wax - M B T 5 M E ′ proxy can be used as a potentially advantageous proxy for reconstructing paleoelevation in semiarid conditions. • A seasonal investigation of δ2H wax and brGDGTs was performed along a semiarid elevation transect. • The coupled δ2H wax - M B T 5 M E ′ proxy shows improved accuracy compared to a single proxy approach. • The benefit of using the coupled δ2H wax - M B T 5 M E ′ proxy is tested for paleoaltimetry in semiarid conditions. [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 human society is facing growing flood risks due to the rapidly-changing global climate. Understanding how early human coped with flooding will shed light on the increase of contemporary social resilience to projected intensified extreme climate change. However, assessment of such ancient human response to hydrological changes remains difficult due to the paucity of records for flood risk coupled with ancient human activities. Here, we present a two-millennium continuous reconstruction of water level changes for Poyang Lake, China's largest freshwater body, using microbial tetraethers extracted from a well-dated sedimentary core. The rising trend of lake water level over the past 2000 years indicates an increase in flood risks for this fertile region that has long been populated by ancient human settlements. A comparison with spatial-temporal distribution of ancient cities and towns in the lake basin reveals that, during the first millennium, human settlements passively retreated outwards and upwards away from the lake to minimize flood risk due to the rising lake water. However, a reversal pattern occurred at ∼1000 CE, witnessing human settlements to moving closer to lakeshore during the second millennium despite of the high water table. The strategic shift, accompanied by a booming economy in southern China during the Song Dynasty, was possibly facilitated by technology innovations, such as intensified water conservancy construction and enhanced shipbuilding, in and around the lake basin. Such an enhanced social resilience in responding to flood risks as early as 1000 years ago remains as a common practice today although the unprecedented scale of anthropogenic climate change in the near future may post additional and unexpected challenges. • A 2000-year lake-level record for China's largest freshwater lake was established. • The reconstructed lake-level record was supported by historical documentation. • Fluctuations of its water level were affected by both climatic and tectonic factors. • We observed enhanced social resilience to flood risk around the lake since 1000 CE. [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]

The influence of Holocene East Asian Summer Monsoon (EASM) rainfall on ecosystems over northern China remains controversial: delay or synchronization. Here we present Holocene quantitatively reconstructed temperature and salinity records based on alkenone proxies in the Dali Lake from northern China. Our data indicate that the Dali Lake was significantly diluted to a freshwater lake (0.8 ± 0.4 g/L) during ∼11.4–3.8 cal ka BP, relative to the modern brackish state. From the early Holocene (before 8.0 cal ka BP) to the middle Holocene (8.0–5.0 cal ka BP), the average salinity value of Dali Lake decreased by ∼0.3 g/L (ca. 30%), but our reconstructed temperature record shows an increase of ∼2.1 °C. In the context of climate warming, the evaporation was enhanced, and thus the decline in salinity is attributed to the increased precipitation, indicating a mid-Holocene EASM rainfall maximum. After comparing our reconstruction with vegetation data, we proposed that the stable and abundant EASM rainfall promoted the forest expansion and vegetation recovery over northern China during the mid-Holocene, suggesting the coupling of ecosystems and EASM rainfall in North China. • Group I Isochrysidales identified in Dali Lake during the early and middle Holocene. • Dali Lake was a freshwater lake during the early and middle Holocene. • Quantitative salinity and temperature reconstructions based on alkenone proxies. • A 3000-yr delay between temperature and moisture optimum and local summer insolation maximum. • Lake and terrestrial ecosystems and Holocene EASM rainfall were coupled in North China. [ABSTRACT FROM AUTHOR]

• δ13C of minute land snail shell could record the local vegetation signals. • Shell δ13C has the potential to be a reliable precipitation proxy. • Spatially scattered shell δ18O reflects the dominant control of precipitation δ18O. Land snails are abundant and well preserved in loess-paleosol sequences on the Chinese Loess Plateau (CLP) and are often regarded as valuable climatic indicators in paleoclimate reconstruction. To date, many stable isotope investigations have been carried out on land snail shells (e.g., δ13C shell and δ18O shell) from diverse geographical and climatic regions. However, few studies have been conducted on modern minute land snails (2–10 mm) on the CLP, and their climatic significance has not been adequately understood. Here, we present new δ13C shell and δ18O shell data from four minute modern land snails (Pupilla aeoli , Gastrocopta armigerella , Opeas striatissimum , Vallonia tenera), and analyses of their correlations with climatic factors (growing season temperature, precipitation and relative humidity) were conducted to examine their climatic significance across the CLP. The results show that δ13C shell can record local vegetation information and has the potential to be a reliable precipitation proxy. Interestingly, δ18O shell exhibits spatially scattered values in the studied region, and these data yielded poor correlations with various climatic parameters, such as precipitation amount, temperature and relative humidity. Given the similar spatial characteristics of observed precipitation δ18O (δ18O p), these phenomena may support the dominant control of δ18O p on δ18O shell. [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]

• Varied lake area was reconstructed over the past ∼500 years in Lake Tuosu. • Westerly-forced Winter-Spring Precipitation controlled lake area changes before 1850. • Mountain-river-basin hydrological system in ACA varied in the industrial era. Investigations of the evolution of naturally and anthropogenically driven hydrology in arid regions are of great significance for hydrological forecasts. Here, we built the past ∼500-year history of lake area and discharge of Lake Tuosu in the northeastern Qaidam Basin. We found that the lake area and inflow variations were sensitive to westerly-forced winter-spring precipitation and showed long-term decreasing trends after 1850. Enhanced local human activities (e.g., agricultural irrigation) led to further rapid shrinkage of Lake Tuosu during 1990–2000, and the lake size reached its minimum in the past 500 years in approximately 2000. Our integrated moisture indices for mountains and basins in southern arid Central Asia showed opposing trends, and the humidity gradient from mountains to basins successively increased after 1850, which may be linked to the migration of the boreal westerlies, global warming, and human activities. After 1950, the humidity increased further, reaching its maximum value in approximately 2000, which likely resulted from the excessive utilization of water resources from anthropogenic activities. Therefore, the mountain-river-basin hydrological system in arid Central Asia has changed significantly, beyond the natural variation pattern, after CE 1950. Therefore, integrated management and rational utilization of water resources are important for ecological protection and sustainable development in the future. [ABSTRACT FROM AUTHOR]

[Display omitted] • δ13C of CO 2 in lake water was in situ measured from three Tibetan Plateau lakes. • δ13C of CO 2 in lake water at different water depths show small variations. • δ13C of CO 2 /HCO 3 – cannot fully explain δ13C difference of measured aquatic plants. The carbon isotope values (δ13C) of aquatic plants are of great significance for recognizing carbon cycles and tracing environmental changes in lakes. Yet, the influence of δ13C values of their utilizing carbon sources (especially in the case of CO 2 separated from the HCO 3 – in water) on their δ13C differences have never been evaluated, which is mainly due to the lack of CO 2 δ13C investigations in lake water. In this study, we used a newly developed fast-response automated gas equilibrator for real-time concentration and δ13C determination of dissolved CO 2 in the surface and bottom lake water of Lake Tuosu, Lake Qinghai and Lake Keluke, China. Moreover, we determined the corresponding δ13C values of total organic carbon (TOC) of a submerged plant (Potamogeton) and an alga (Cladophora), as well as the δ13C values of dissolved inorganic carbon (DIC) in bottom lake water. The results showed that the CO 2 δ13C values of bottom and surface water in these three lakes did not show significant differences and the δ13C values of CO 2 at different water depths also did not show significant differences. However, Potamogeton had higher TOC δ13C values (average 14–16‰) than Cladophora in these lakes. Although Potamogeton and Cladophora could utilize HCO 3 – as an additional carbon source whose δ13C values were more positive (ca. 7–8‰) than CO 2 , this effect on their δ13C values was no >4‰ after evaluation. Thus, our results show that the δ13C values of carbon sources (including CO 2 and HCO 3 –) are not the main factors in determining the TOC δ13C difference between Potamogeton and Cladophora , which could be primarily related to the biosynthetic fractionation for different aquatic plant species. Our results benefit further understanding and interpretations of δ13C variations in carbon cycles and paleoenvironment reconstructions for lakes. [ABSTRACT FROM AUTHOR]

RATIONALE In recent experiments, we found that compound-specific δ2H values can vary as a result of changing the gas chromatography temperature program under common pyrolysis conditions. To achieve better precision, it is necessary to examine the details and find a solution to this problem when using gas chromatography/thermal conversion/isotope ratio mass spectrometry (GC-TC-IRMS) for hydrogen isotope analysis. METHODS A test was designed to find the possible temperature effect under four different GC temperature ramp rates using n-alkanes ( n-C21, n-C27, and n-C31) and fatty acids ( n-C12, n-C18, and n-C24). The common 'hexane' method was used initially to condition the pyrolysis reactor. Experiments were then carried out using the 'methane condition' method because it was considered to improve pyrolysis efficiency. RESULTS Under the 'hexane condition' the measured hydrogen isotope ratios of the n-alkanes and n-fatty acids became more positive with increasing GC temperature ramp rate. The ion current intensity of hydrogen also generally increased. However, when the 'methane condition' method was used, the measured δ2H values of the n-alkanes and n-fatty acids showed little change under different GC temperature ramp rates. CONCLUSIONS Higher pyrolysis efficiency could reduce the tailing of the H2 peak and the related isotopic variations at increased GC temperature ramp rates. In addition, too slow a temperature ramp rate could broaden the peak width and thus increase the background effect and possible isotopic fractionations in the split interface; this could also influence the hydrogen isotope values. We therefore suggest that the appropriate temperature ramp rate is an important factor in improving the precision in analyzing compound-specific hydrogen isotopes. Copyright © 2012 John Wiley & Sons, Ltd. [ABSTRACT FROM AUTHOR]

The expansion of plants using the C4 photosynthetic pathway is one of the most prominent changes in the global ecosystem during the Cenozoic Era. A significant late Miocene expansion is well documented. However, the existence and cause of subsequent expansions are still not clear, owing in part to the lack of long, continuous climate-vegetation records. Here we present two high-resolution carbon isotope time series, covering the past 7 m.y., derived from eolian deposits on the Chinese Loess Plateau. The data indicate three intervals of significant C4 plant expansions within the semiarid monsoonal region of East Asia (ca. 2.9-2.7 Ma, 1.3-0.9 Ma, and 0.6 Ma-present). These expansions covary with strengthened East Asian summer monsoon circulation. We conclude that in East Asia, large-scale late Miocene C4/C3 vegetation changes in semiarid areas have been primarily driven by warm seasonal precipitation and temperature variations associated with changes in monsoon circulation. [ABSTRACT FROM AUTHOR]

We have investigated carbon isotopic compositions of four plant genus/species, Bothriochloa ischaemum (C4), Stipa bungeana (C3), Lespedeza sp. (C3) and Heteropappus less (C3), along a precipitation gradient in northwest China in order to assess the impact of water availability on the carbon isotopic discrimination against 13C during carbon assimilation in this area. This information is necessary for reconstruction of paleovegetation, particularly paleo-C3/C4 plant ratios using δ13C value of organic matter in loess and paleosols in the Chinese Loess Plateau. The δ13C of C3 plants, as a group, exhibits a negative correlation with the annual precipitation amount with a total change and sensitivity of 5‰ and −1.1‰/100 mm, respectively, for the precipitation range from 200 to 700 mm. The C4 grass, B. ischaemum responds to aridity by decreasing 1.7‰ for over the precipitation range from 350 to 700 mm; the plant δ13C is significantly correlated with annual precipitation with a slope −0.61‰/100 mm. This result implies that without considering the effect of water availability on the plant δ13C values, reconstruction of percent C4 vegetation during the last glaciation can be overestimated by about a factor of two. [ABSTRACT FROM AUTHOR]

Methods were developed to determine the mass ratios of carbon isotopes in trace amounts of aerosol carbonate. A Finnigan MAT 252 mass spectrometer fitted with an on-line Kiel device was to determine the 13C/12C ratio in CO2 produced from the carbonate. A study using these methods was conducted to characterize the carbonate carbon isotopes in aerosol samples collected in Xi'an on dusty and normal days during March and April 2002. Results of the study demonstrate that insights into the origin of the dust can be deduced from its isotopic composition. That is, the δ 13C of carbonate for dust storm samples ranged from -1.4%o to -4.2‰, and this is consistent with sandy materials in dust source regions upwind. In contrast, for non-dusty days δ 13C ranged from -7.5 % to -9.3‰, which is more similar to fine particles emitted from local surface soils. Comparisons of dust storm aerosols with surface soils from source regions and with aerosol samples collected downwind indicate that the δ 13C values did not change appreciably during long-range transport. Therefore, carbon isotopes have the potential for distinguishing among source materials, and this approach provides a powerful new tool for identifying dust provenance. [ABSTRACT FROM AUTHOR]

The concentrations of total suspended matter (TSM) and the compositions of organic stable carbon isotopes of TSM and bottom sediments were analyzed to study the sources of TSM and sediments and the transportation processes. For this study, 284 TSM samples and 64 sediment ones taken from 67 stations along 7 transects and in 5 layers were collected in the southern Yellow Sea on the cruise in May, 1998. The main sediment transportation pattern in the southern Yellow Sea was obtained by analyzing the distribution characteristics of TSM concentration and particulate organic carbon δ13C values. It was confirmed from the pattern that the bottom layer plays a more important role than the surface one in the transportation processes of terrigenous material to the central deep-water area of the southern Yellow Sea. The Yellow Sea circulation is an important control factor in determining the sediment transportation pattern in the southern Yellow Sea. The carbon isotope signals of sedimentary organic matter confirmed that the main material in sediments with high sedimentation rate in the Shandong subaqueous delta originated from the modern Yellow River. The terrigenous sediments in the deep-water area of the southern Yellow Sea are mainly from the abandoned Yellow River and the modern Yellow River, and a small portion of them are from the modern Yangtze material. The amount of terrigenous material from Korean Peninsula and its influence range are relatively limited. The conclusions derived from TSM and stable carbon isotopes were further confirmed by another independent material source tracer-PAHs. [ABSTRACT FROM AUTHOR]

Examines the oxygen isotope compositions of a stalagmite from Qixing Cave, Guizhou Province, China. Chronology of the stalagmite; Variations of the &delta[sup 18]O of the stalagmite; Indication of climate changes during the Holocene epoch.

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]

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]