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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]

High-resolution carbon and oxygen isotope records in stalagmites provide significant information for quantitative paleo-reconstructions of past temperature and precipitation. This paper describes results of isotopic analysis of dissolved inorganic carbon (DIC) in Panlong Cave water (from drips and shallow pools). Cave water was sampled monthly throughout 2012 and analyzed for δC composition. The values of δC in drip water were in the range of −10.42 to −16.07 ‰, while the values of δC in pool water were in the range of −1.88 to −6.94 ‰. Dissolved inorganic carbon isotope records showed a gradual stepwise negative excursion from January to August, and a gradual stepwise positive excursion from September to December. δC in cave water showed remarkable seasonal variations, which were negative excursion from 2 to 5 ‰ in rainy season than in dry season. A negative δC excursion in summer was not the result of vegetation types, which was contributed to enhance root respiration and decomposition of soil organic matter. A correlation between δC values of perennial drip water and pool water samples with air temperature and precipitation amount suggests that these negative excursions were caused by increasing air temperature during rainy season. Negative excursions in δC values of seasonal drip water were correlated with monthly precipitation, caused by the influx of soil organic matter into the cave. Hence, this paper provides an important framework and an empirical basis for reconstructing temperature from stalagmites and drip water in this region. [ABSTRACT FROM AUTHOR]

Three positive carbon isotope excursions are reported from Middle-Upper Ordovician in Tahe oil-gas field, northern Tarim Basin. Based on conodont biostratigraphy, the Middle Darriwilian Isotope Carbon Excursion (MDICE) and the Guttenberg Carbon Isotope Excursion (GICE) are identified from Darriwilian to Early Katian by the aid of whole-rock carbon isotope data from two well cores. Positive excursion within conodont Pygodus anserinus zone is developed in Early Sandbian, and the fluctuation range is no less than MDICE. Because the range of this excursion in the generalized global carbon isotope curve is short, previous studies paid little attention to it, and named Early Sandbian Isotope Carbon Excursion (ESICE) in this paper. Furthermore, these positive excursions are not directly related to sea level fluctuations and the MDICE and GICE identified in northern Tarim can be globally correlated to that in southern China, North America, South America, and Europe. The Saergan Fm. source rocks of Middle-Upper Ordovician in Kalpin Dawangou outcrop are in accord with the geologic time of MDICE and ESICE, and GICE have strong ties to the source rock of Lianglitag Fm. in basin. Abundant organic carbon burial is an important factor in genesis of positive isotope carbon excursions. Positive oxygen isotope excursion, conodont fauna turnover, decreased conodont total diversity, and the change of sedimentary facies indicated that dramatic changes of paleoceanographic environment of Early-Middle Ordovician in Tarim Basin started from the end of Darriwillian, and an obvious icehouse climate of Late Ordovician occurred in ESICE. [ABSTRACT FROM AUTHOR]

The crude oils typically from the Cambrian-Lower Ordovician source rocks of Tarim Basin, NW China, such as TD2Є and TZ62S, are C-enriched with the stable carbon isotopic ratios (VPDB) approaching −28 ‰. In this paper, the main research viewpoints on this issue are summarized, and combined with results from organic and inorganic carbon isotope stratum curves of the outcrop at the Ya'erdang Mountain in Tarim Basin. In addition, more alternative interpretations are discussed. On one hand, the inverse fractionation features of stable carbon and hydrogen isotopes of these crude oils may imply their protogenous nature. On the other hand, the anisotropy of source rocks and contribution from older stratum source rocks need verifying as well. For the sake of the final resolution of this issue, some further study topics are recommended. [ABSTRACT FROM AUTHOR]

Based on petrography and geochemistry, formation phases of carbonate cements and their sedimentary–diagenetic environments in Jurassic clastic rocks from the Lenghu V tectonic belt are determined. The results demonstrate that studied successions are mainly dark gray or black mudstone with interbedded thin gray siltstone and a few greywacke layers. Sandstones are lithic and feldspathic lithic greywacke with high kaolinite content. Calcite and ferrocalcite are the main carbonate cements. The carbon isotope values(δ13C) of the carbonate cements range from − 15.6 to 9.2‰ (average − 3.2‰) with a bimodal distribution. One peak is 4‰, which includes samples from the upper Xiaomeigou Formation (Fm) and the other one is − 12‰, which mainly represents the lower section. The oxygen isotope values(δ18O) have a wide range from − 18.5 to − 8.3‰ (average − 13.31‰). Combined with microscopic observations, two phases of carbonate cements are distinguished according to their isotopic characteristics. The earlier carbonate cements are mainly calcite and ferrocalcite, and they were precipitated in eodiagenesis stage. The carbon source of calcite was inorganic, and ferrocalcite was mixed with carbon from methanogenesis. This mixture led to a positive bias in the δ13C curve, which artificially inflated the paleosalinity. The later carbonate cements, formed in mesodiagenesis stage, were ferrocalcite. Decarboxylation of organic matters led to lower δ13C values. The major and trace elements data show that Sr, Ba, Fe and Mn have weak correlation with Al, Ti. So, they are effective to interpret the sedimentary environment. The Sr/Ba ratio < 1 indicates fresh water environment, Sr/Ca, Sr/Cu and Fe/Mn ratios suggest humid climate, and V/(V + Ni) ratio shows suboxic–anoxic environment. The carbon and oxygen isotopes, together with elemental geochemistry, show that the climate during the Early Jurassic was warm and wet and the studied successions were deposited in fresh water. The variations in elemental ratio curves indicate that climate became wetter and warmer later in the Early Jurassic. [ABSTRACT FROM AUTHOR]

Niutuozhen geothermal field is located in the Jizhong graben, belonging to the northern part of Bohai Bay Basin in North China. Chemical and isotopic analyses were carried out on 14 samples of the geothermal fluids discharged from Neogene Minghuazhen (Nm), Guantao (Ng), and Jixianian Wumishan (Jxw) formations. The δ2H and δ18O in water, δ13C in CH4, δ13C in CO2, and 3He/4He ratio in the gases were analyzed in combination with chemical analyses on the fluids in the Niutuozhen geothermal field. The chemical and isotopic compositions indicate a meteoric origin of the thermal waters. The reservoir temperatures estimated by chemical geothermometry are in the range between 60 and 108 °C. The results show that the gases are made up mainly by N2 (18.20–97.42 vol%), CH4 (0.02–60.95 vol%), and CO2 (0.17–25.14 vol%), with relatively high He composition (up to 0.52 vol%). The chemical and isotopic compositions of the gas samples suggest the meteoric origin of N2, predominant crustal origins of CH4, CO2, and He. The mantle-derived He contributions are calculated to be from 5 to 8% based on a crust–mantle binary mixing model. The deep temperatures in the Jxw reservoir were evaluated based on gas isotope geothermometry to be in the range from 141 to 165 °C. The mantle-derived heat fraction in the surface heat flow is estimated to be in the range of 48–51% based on 3He/4He ratios. [ABSTRACT FROM AUTHOR]