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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]

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]

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]

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]

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]

Nitrate (NO3−) contamination has become a dominant international problem in the aquatic environment, so identifying the sources and transformations of NO3− is the basis for improving water quality. Since the Jing River is the largest tributary of the Wei River, to understand its water quality, this study collected surface water samples from the Shaanxi section of the Jing River during the dry season. The potential sources of NO3− were analyzed by hydrochemical and bi-isotopic methods, and the SIAR model was used to estimate the proportional contribution of each source. Results indicated that NO3−-N was the main form of inorganic nitrogen in this area, and the average total nitrogen content was 10.23 mg·L−1, which showed that nitrogen pollution was highly serious; the transformation process of nitrogen in this study area was mainly nitrification; The results of Bayesian model showed that manure and sewage contributed to the most NO3− (64.39%) in the dry season, followed by soil nitrogen, which was 26.35%. These results help to adopt better nitrogen management measures to meet the national environmental quality standards for surface water. [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]

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]

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]

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]

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]

The relationship between the carbon isotopic composition of paleosols and pale-ovegetation on the Loess Plateau is still unclear. One of the main reasons is that we are short of knowledge about the characteristics of the carbon isotopic composition of modern soil in this area. A preliminary investigation of the carbon isotopic compositions of the modern soil and the loess/paleosol sequence on the Loess Plateau shows that the carbon isotopic composition of modern soil is consistent with the distribution of modern plants on the Loess Plateau, where the ecosystem is dominated by a mixture of C4 and C3 plants. Comparing the δ 13C values of modern soil and loess-paleosol sequences from the Xunyi profile, we conclude that C3 plants dominated the landscape during loess sediment stages, while C4 plants expanded during paleosol stages. [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.