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The stable carbon isotope composition (δ13C) of lacustrine sediments, which can record changes in past environmental conditions such as the C4/C3 terrestrial vegetation composition, has been widely used for the reconstruction of terrestrial ecosystems and global climate changes. It has also been widely used in paleolimnological studies in arid northwestern China. In recent years, however, an increasing number of studies have enriched the environmental significance reflected by the stable carbon isotope geochemistry of lakes; therefore, the interpretation of lake δ13C variations in paleoenvironmental reconstructions should be carefully evaluated from various aspects. In this paper, previous studies from the past several decades on the lacustrine δ13C variations in arid northwestern China were reviewed, and the possible mechanisms of the δ13C variations were discussed. Our study will provide a reference for the application of carbon isotope geochemistry in paleoenvironmental reconstructions. [ABSTRACT FROM AUTHOR]

Carbonate clumped isotope (Δ47) is a new and reliable geothermometer. Endogenic carbonates in lake sediments are good archives to reconstruct lake water temperature using clumped isotope thermometry. However, applications of carbonate clumped isotope thermometry to lacustrine carbonates are still scarce because the existed organic and/or sulfur contaminants in such samples interfere with clumped isotope analysis and cause notable temperature biases. Therefore, exploring an effective way to remove contamination is a prerequisite to widely applying carbonate clumped isotope thermometry to lake sediments. By pretreating fine-grained endogenic carbonates from Dali lake sediments with hydrogen peroxide of different concentrations for different lengths of time, we conducted a series of conditional experiments to seek the optimal pretreatment condition for clumped isotope analysis. Δ47, Δ48 offset and 49 parameter were obtained from clumped isotope measurements to assess the effect of contamination removal. Results showed that untreated samples ("0%–0") had a very high 49 parameter. After the treatment with hydrogen peroxide, the 49 parameter was significantly reduced while the Δ48 offset evidently increased. The Δ47, Δ48 offset and 49 parameter of samples pretreated under different conditions showed significant variability, even though the measured Δ47 temperatures changed within a relatively narrow range (i.e., from 17.0±1.3 to 22.8±1.6°C). Among various treated samples, the sample "3%–8" yielded a smaller Δ48 offset and 49 parameter, and lower Δ47 temperature of 17.0±1.3°C. Therefore, "reacting with 3% hydrogen peroxide for 8 h" was suggested to be the optimal condition for removing contaminants from lacustrine endogenic carbonates prior to clumped isotope analysis. At the time when the pretreatment condition was too intense (i.e., H2O2 concentration >3% and/or reaction time >8 h), secondary contaminants might have been generated in the closed reaction system, whose ultimately transformed state may interfere with masses 47, 48. Using the optimal pretreatment procedure, we obtained reasonable Δ47 temperature changes during the last deglaciation and found a temperature decrease of about 6°C in Northern China during the Younger Dryas period. Our study demonstrates a great potential of applying carbonate clumped isotope thermometry to lacustrine carbonates for paleo-temperature and paleo-elevation reconstructions in the future. [ABSTRACT FROM AUTHOR]

The Furongian Series of the Cambrian in the Tarim Craton (NW China) is mainly composed of crystalline dolostones, and is an important target for oil and gas exploration. The chronostratigraphic framework of the Furongian Series in the Tarim Craton has not yet been established due to the scarcity of fossils and the absence of the Steptoean Positive Carbon Isotope Excursion (SPICE), which is the primary global carbon isotope excursion of the Furongian Series. In this study, the SPICE was discovered by examining the carbon isotopes of the representative drilling wells in the West Platform and East Basin. A positive carbon isotope anomaly with δ13C values ranging from 1.5‰ to 1.9‰ (PDB), were found in the middle of the Xiaqiulitag Formation of the three drilling wells in the West Platform, and a positive excursion with δ13C values up to 4.0‰ (PDB), which is correlated with the SPICE, was found in the lower limestone of the Tuershaketag Formation in the East Basin. Based on the carbon isotopic data and chemostratigraphic correlation, the chronostratigraphic framework of the Furongian Series across different facies in the Tarim Craton is preliminarily established. The basal boundary of the Furongian Series was preliminarily defined by the occurrence of the SPICE. A weak positive anomaly of δ13C was observed at the base of the Penglaiba Formation of the three boreholes in the West Platform and the top of the Tuershaketag Formation of Tadong-2 well. We propose that the positive anomaly of the carbon isotope above the weak negative anomaly at the base of the Penglaiba Formation and the top of the Tuershaketag Formation can be used to define the Cambrian-Ordovician boundary in the Tarim Craton. [ABSTRACT FROM AUTHOR]

The placement of the Ediacaran-Cambrian boundary in deep-water realm in South China and the high-precision temporal framework for the Ediacaran-Cambrian transition in this region have not yet been completely solved. Recently, we have found two K-bentonite beds in the top of the Liuchapo Formation at the Pingyin section, Guizhou Province. It provides an opportunity for constructing the temporal framework of the transitional strata on the Yangtze Platform in South China and for determining the Ediacaran-Cambrian boundary in this area. In this study, we conducted high-precision CA-ID-TIMS U-Pb dating on zircons from the two K-bentonites. The ages of the two K-bentonites were precisely constrained at 536.40±0.47/1.1/1.2 Ma (2σ, n=7, MSWD=0.92) and 541.48±0.46/1.1/1.2 Ma (2σ, n=8, MSWD=1.3). Combining the geochronological results with organic carbon isotope data of chert in the topmost Liuchapo Formation from the section, we suggest that the EdiacaranCambrian boundary should be consistent with a significant negative organic carbon isotope excursion between the two K-bentonites. The scheme of the Ediacaran-Cambrian boundary in this study is of great significance for global correlation, and further for unravelling the information of the terminal Ediacaran-early Cambrian ocean. [ABSTRACT FROM AUTHOR]

To date, few research have been reported on the evolution of C3/C4 vegetation in southern China, and the main mechanism influencing the evolution of C3/C4 vegetation is unclear. That makes it difficult for researchers to understand the competition mechanism of C3 and C4 plants under different climate environments and its relationship with the climate factors. It is also not conducive for researchers to assess the influence of future climate change on regional C3/C4 vegetation. Exactly, C3/C4 vegetation change in the regional-scale will have a significant impact on the global carbon cycle and agricultural production. Therefore, it is especially important to reconstruct the evolutionary history of C3/C4 vegetation in southern China and clarify the influencing mechanism of climate change. In this study, we systematically analyzed stable carbon isotope (δ13C) of the longchain n-alkanes in sediment samples from eight lakes and four peat profiles in southern China, to reconstruct the spatiotemporal evolution of C3/C4 vegetation in these regions since the Last Glacial Maximum (LGM) and to investigate the climate factors that affect the C4 plant abundance change in the research area. The integrated long-chain n-alkane (C27–C33) stable carbon isotope results of samples from Zhanjiang Huguangyan Maar Lake, Xingyun Lake, Lugu Lake and Dingnan peat showed that, from the LGM to the Early Holocene, C4 plant relative abundance exhibited a gradually increasing trend from 21% to 34%. In the Middle Holocene, the C4 plant abundance significantly declined and reached a lowest value of 10%. In space, the C4 plant abundance generally exhibited a gradually declining trend from south to north in the LGM and the Early Holocene, while it showed an opposite trend in the Holocene Climate Optimum (6.0 cal ka BP). It reflected that the main influencing factor on C4 plants spatial distribution was changing from temperature to precipitation. This study indicated that temperature was the dominant factor affecting C4 plant distribution in southern China, however, when the temperature condition was appropriate, an increase in precipitation (especially more spring precipitation) would reduce the competitive advantages of C4 plants. Therefore, the combination of temperature and seasonal precipitation was the important factor that determines the C3/C4 vegetation proportion change in the southern China. Under the premise that the temperature will rise and precipitation will increase in the future, the reduction of a competitive advantage for the C4 plants could affect agricultural production in China. [ABSTRACT FROM AUTHOR]

Neogene eolian successions are one of the most important terrestrial palaeoenvironmental archives in East Asia. However, they have received far less attention than Quaternary loess deposits, especially in the case of lipid biomarker analysis. In order to obtain a better insight into the early-middle Miocene palaeoenvironment, we conducted a study of n-alkane biomarkers from sediments of the QA-I section (Qinan) in the western Chinese Loess Plateau, and compared the results with those of previous n-alkane analyses of eolian and aquatic sediments of varying age. Our principal results are as follows: (1) All QA-I samples contain n-alkanes ranging from C to C, among which the relative content of short-chain n-alkanes (C-C) from microorganisms is significantly greater than that of long-chain n-alkanes (C-C) from the waxes of terrestrial higher plants; the main peak is at C-C. All samples have a relatively lower abundance of medium-chain n-alkanes (C21-C25) than that of long- and short-chain n-alkanes, similar to strongly weathered palaeosols in Quaternary loess and Late Miocene-Pliocene Hipparion Red-Earth; however, this distribution is significantly different from that in weakly-weathered loess of Quaternary loess and Late Miocene-Pliocene Hipparion Red-Earth, as well as from aquatic sediments. (2) Despite some odd-over-even carbon predominance of long-chain n-alkanes in the QA-I samples, the carbon preference index (CPI) values are significantly lower than those of most of the weakly-weathered sediments. Our results show that strong weathering and microbial processes have significantly altered the n-alkanes in the Miocene eolian deposits in Qinan, and led to a significant oxidation and degradation of long-chain n-alkanes and the predominance of short-chain n-alkanes from bacteria. Therefore, the contribution of microorganism to total organic carbon (TOC) and its resulting in carbon isotopic composition should be carefully assessed in future studies. [ABSTRACT FROM AUTHOR]

In order to explore subsistence patterns in northern Shaanxi Province around 4,000 yr BP, 28 human and 24 animal bones from the Shengedaliang site were sampled for stable carbon and nitrogen isotope ratio analysis. The results show that most people primarily subsisted on C resources, e.g. millet and millet-related animal products, despite the fact that there was some intake of C plants by some individuals. Stable nitrogen isotope values indicate that there were differences in meat consumption between individuals at the site. Pigs were mainly foddered with millet and millet byproducts, as well as some cattle, according to their high δ C values. However, most cattle and the sheep/goats consumed wild C plants at Shengedaliang. Our above findings indicates that subsistence patterns in northern Shaanxi around 4,000 yr BP were characterized by millet farming, while the grassland animal husbandry, e.g. cattle and sheep/goats raising, displayed very little contribution to local economy. The intensive millet farming in northern Shaanxi provided enough food for population growth, ensured the accumulation of wealth, and consequently accelerated social differentiation and complexity. [ABSTRACT FROM AUTHOR]

The Early Triassic was a period of ecological restoration for the earth's system after the end-Permian mass extinction. Geochemical records linked to the variations in marine ecosystems during this period have attracted the interest of geologists for many years. Based on petrographic analysis of samples and evaluations of their reliability as proxies for original seawater, this study investigated the carbon and oxygen isotopic compositions of 350 carbonate rock samples from the Lower Triassic (and adjacent strata) in the southern Longmenxia section of Guang'an, eastern Sichuan Basin. The results indicate that the Triassic carbonate rocks from the southern Longmenxia section favorably preserved the original seawater's δ C signal. Furthermore, carbon and oxygen isotopic compositions are found to be poorly correlated, with a determination coefficient as low as 0.0205 and only 44 rock samples show results of Mn/Sr>2 and/or δ O<-6.5‰. The complete carbon isotopic curve for the Lower Triassic is established using the data from the remaining 306 samples with Mn/Sr<2 and/or δ O>-6.5‰. This curve presents favorable comparability on a global scale, specifically in the δ C minima near the Permian-Triassic boundary, at the top of the Jia1 and within the the Jia3, as well as in the δ C maxima at the tops of the Ye1 and Ye4, at the base of the Jia2 and at the top of the Ye3. The peaks and troughs corresponding to these maxima and minima all reflect global signals. By comparing these results to previous research on coeval carbon isotopic curves established within the chronostratigraphic framework, the ages of these critical evolution points are determined. The results show that the Ye1 roughly corresponds to the Griesbachian substage; the Ye2 through Ye4 correspond to the Dienerian substage; Jia1 corresponds to the Smithian substage; from the Jia2 to the lower part of the Jia4 correspond to the Spathian substage; and the middle and upper parts of the Jia4 belongs to the Aegean Substage of the Middle Triassic. Around the boundary between the Jia1 and Jia2 (which represents the Smithian-Spathian boundary (SSB)), the value of δ C increases rapidly from -0.911‰ to 3.679‰. The span during which the seawater's carbon isotope experiences this drastic change may be less than 36 kyr. The oxygen isotope, which is more sensitive to sedimentary environmental changes, exhibits changes prior to the carbon isotope near the SSB, indicating a significant increase in the salinity of the seawater before a sharp rise in the carbon isotope; this event leads to the formation of evaporites and dolomites. [ABSTRACT FROM AUTHOR]

In 2013, a great breakthrough of deep petroleum exploration was achieved in the Cambrian pre-salt intervals of Wells Zhongshen1 (ZS1) and Zhongshen1C (ZS1C), Tazhong Uplift. However, the hydrocarbon discovery in the Cambrian pre-salt intervals has triggered extensive controversy regarding the source of marine oils in the Tarim Basin. The geochemistry and origin of the Cambrian pre-salt hydrocarbons in Wells ZS1 and ZS1C were investigated using GC, GC-MS and stable carbon isotope technique. These hydrocarbons can be easily distinguished into two genetic families based on their geochemical and carbon isotopic compositions. The oil and natural gases from the Awatage Formation of Well ZS1 are derived from Middle- Upper Ordovician source rocks. In contrast, the condensate and gases from the Xiaoerbulake Formation of Wells ZS1 and ZS1C probably originate from Cambrian source rocks. The recent discovery of these hydrocarbons with two different sources in Wells ZS1 and ZS1C suggests that both Middle-Upper Ordovician-sourced hydrocarbons and Cambrian-sourced petroleums are accumulated in the Tazhong Uplift, presenting a great exploration potential. [ABSTRACT FROM AUTHOR]

To apply carbon isotope composition ( δC) analyses of C plants to the quantitative reconstruction of paleoclimate, the functional mechanism linking plant δC ( δC) to the environment, which is based on the plants' physiological characteristics and morphological adaptability, must be thoroughly understood. Foxtail millet ( Setaria italic) and common millet ( Panicum miliaceum), as C plants, are representative crops of the rain-fed agriculture present in northern China. Fossil millets are ideal for paleoclimatic studies because of the ease of acquisition and identification to the species level. Modern seeds of foxtail and common millet collected from different habitats of the Chinese Loess Plateau, and their carbon isotope compositions, were analyzed and correlated with environmental factors, such as latitude, altitude, temperature, precipitation, water availability, and relative humidity. The results showed that the δC of foxtail millet had a significantly negative correlation with latitude ( R=-0.46), which may indicate the influence of light. The effect of light on the δC of foxtail millet accounted for only 21% of variability, while other climatic factors did not exert significant influences. Thus, the δC of foxtail millet was not suitable for extracting climatic information. The δC of common millet was significantly and positively correlated with precipitation during the growing period ( R=0.75), explaining 56% of variability. The functional mechanisms analyzed, using the plants' physiological characteristics and morphological adaptability, indicated that common millet can adapt to environmental changes because of stomatal sensitivity and some non-stomatal factors. Therefore, the δC of common millet can record precipitation during growth and is a promising factor for paleoclimatic reconstruction. [ABSTRACT FROM AUTHOR]

A detailed correlation of Hetianhe condensates versus typical source rocks from the Tarim Basin was established. Moreover, the genetic relationship between the condensates and their associated gases was also studied based on their geochemical compositions and fluid inclusion data. Hetianhe condensates are characterized by high pristine/phytane (Pr/Ph) ratios, high relative abundances of C regular steranes, C-C triaromatic steroids, and triaromatic dinosteranes, and relatively heavy stable carbon isotopic compositions. They geochemically correlate well with the Cambrian source rocks, indicating that these condensates are derived from the Cambrian rocks. Based on several molecular maturity parameters, the condensates are recognized as being moderately to highly mature, which are too low in maturity to extensively crack into gases. The gases and condensates of the Hetianhe Field are derived from the same source rocks and the gases are from oil-cracking; however, clear evidence indicates that the gases were not cracked from their associated condensates and that the gas generation from oil-cracking did not occur in the present reservoirs. The liquid condensates produced from wellheads were likely dissolved in their associated gases underground and carried into the reservoirs by the gases. [ABSTRACT FROM AUTHOR]

In this study, to further promote the application of the stable carbon isotope natural abundance (SCINA) method to the study of CH in paddy fields in China, field experiments were carried out to investigate carbon isotope fractionation during CH transport in both rice- and non-rice-growing seasons. More importantly, two new methods for the measurement of the CH transport fractionation factor ( ɛ) in paddy fields were introduced. The results indicated that the closed chamber + syringe method was much better for the determination of ɛ during the non-rice-growing season. Presently, ɛ was calculated using the δC value of the CH emitted from a rice field minus that of the CH in the floodwater (−6.7‰ to −3.0‰). In addition, there were three methods available for estimating ɛ during the rice-growing season: deduction of the δC value of the CH in the floodwater from that of the CH emitted from the field (−16.6‰ to −15.2‰); deduction of the δC value of the CH in the soil pore water from that of the CH emitted from the field (−13.2‰ to −1.1‰); and deduction of the δC value of the CH in the aerenchyma of plants from that of the CH emitted from plants (−16.3‰ to −10.9‰). Unfortunately, the first two methods showed relatively large uncertainties. Only the last one, the dividing + cutting method, was not only scientific and reliable but also provided accurate measurements. [ABSTRACT FROM AUTHOR]