Carbon and oxygen isotopic composition of car- bonate cements of different phases in terrigenous siliciclastic reservoirs and significance for their origin： A case study from sandstones of the Triassic Yanchang Formation, southwestern Ordos Basin, China

Early carbonate cements in the Yanchang Formation sandstones are composed mainly of calcite with relatively heavier carbon isotope （their δ^18O values range from -0.3‰- -0.1‰） and lighter oxygen isotope （their δ^18O values range from -22.1‰- -19.5‰）. Generally, they are closely related to the direct precipitation of oversaturated calcium carbonate from alkaline lake water. This kind of cementation plays an important role in enhancing the anti-compaction ability of sandstones, preserving intragranular volume and providing the mass basis for later disso- lution caused by acidic fluid flow to produce secondary porosity. Ferriferous calcites are characterized by relatively light carbon isotope with δ^13C values ranging from -8.02‰ to -3.23‰, and lighter oxygen isotope with δ^18O values ranging from -22.9‰ to -19.7‰, which is obviously related to the decarboxylation of organic matter during the late period of early diagenesis to the early period of late diagenesis. As the mid-late diagenetic products, ferriferous cal- cites in the study area are considered as the characteristic authigenic minerals for indicating large-scaled hydrocarbon influx and migration within the clastic reservoir. The late ankerite is relatively heavy in carbon isotope with δ^13C values ranging from -1.92‰ to -0.84‰, and shows a wide range of variations in oxygen isotopic composition, with δ^18O values ranging from -20.5‰ to -12.6‰. They are believed to have nothing to do with decarboxylation, but the previously formed marine carbonate rock fragments may serve as the chief carbon source for their precipitation, and the alkaline diagenetic environment at the mid-late stage would promote this process.     

Carbon and Oxygen Isotopic Compositions: How Lacustrine Environmental Factors Respond in Northwestern and Northeastern China

Surface lake sediments,28 from Hoh Xil,24 from northeastern China,99 from Lake Bosten,31 from Ulungur and 26 from Heihai were collected to determine δ13C and δ18O values.Considering the impact factors,conductivity,alkalinity,pH,TOC,C/N and carbonate-content in the sediments,Cl,P,S,and metal element ratios of Mg/Ca,Sr/Ca,Fe/Mn of bulk sediments as environmental variables enable evaluation of their influences on δ13C and δ18O using principal component analysis(PCA) method.The closure and residence time of lakes can influence the correlation between δ13C and δ18O.Lake water will change from fresh to brackish with increasing reduction and eutrophication effects.Mg/Ca in the bulk sediment indicates the characteristic of residence time,Sr/Ca and Fe/Mn infer the salinity of lakes.Carbonate formation processes and types can influence the δ13C–δ18O correlation.δ18O will be heavier from Mg-calcite and aragonite formed in a high-salinity water body than calcite formed in freshwater conditions.When carbonate content is less than 30%,there is no relationship with either δ13C or δ18O,and also none between δ13C and δ18O.More than 30%,carbonate content,however,co-varies highly to δ13C and δ18O,and there is also a high correlation between δ13C and δ18O.Vegetation conditions and primary productivity of lakes can influence the characteristics of δ13C and δ18O,and their co-variance.Total organic matter content(TOC) in the sediments is higher with more terrestrial and submerged plants infilling.In northeastern and northwestern China,when organic matter in the lake sediments comes from endogenous floating organisms and algae,the δ13C value is high.δ13C is in the range of 4‰ to 0‰ when organic matter comes mainly from floating organisms(C/N<6);in the range of 4‰ to 8‰ when organic matter comes from diatoms(C/N=6 to 8);and 8‰ to 4‰ when organic matter comes from aquatic and terrestrial plants(C/N>8).     

Mantle Degassing and Diamond Genesis：A Carbon Isotope Perspective

The effect of Co2 and CH4 degassing from the mantle on the carbon isotopic composition of diamond has been quantitatively modeled in terms of the principles of Rayleigh distillation.Assuming the δ^13 C value of -5‰ for the mantle,the outgassing of CO2 can result in the large negative δ^13 C values of diamond,whereas the outgassing of CH4 can drive the δ^13C values of diamond in the positive direction.The theoretical expectations can be used to explain the full range of δ^13 C values from-34.4‰5 to 5‰ observed for natural diamonds.It is possible that diamond formation was triggered by the degassing of Co2 and/or CH4 from the mantle and the associated fractional crystallization of carbonate-bearing melt.     

The carbon isotopic characteristics of dinosaur eggshell fossils in Late Cretaceous from Xixia Basin and their significance

This paper studies the Late Cretaceous dinosaur eggshell fossils from the Xixia Basin,Henan Province,by icroscopy and carbon isotope method.Careful observation under microscope revealed that all dinosaur eggshell fossils consist of primary calcite and secondary calcite.The content of the former is about 60.5% and the latter 39.5% according to image analysis.The δ13C values of secondary carbonate mineral filling within the dinosaur eggs in the strata range from-5.63‰ to -5.68‰,with an average value of -5.64‰,the δ^13 C values of sixteen dinosaur eggshell fossils are in the range from-5.88‰ to -7.79‰,Then we calculated the δ^13 C values of primary carbonate minerals,ranging from-6.03‰ to-9.19‰.Bsed on the δ^13C values of the primary carbonate ,the ancient food type and food proportion of dinosaurs were deduced.The dinosaru‘s food proportions of C3 plant and C4 plant are 61% and 39%,respectively.Finally,it is inferred that the palaeoclimate in the Xixia Basin should be a warm-sub-humid-sub-arid climate in the subtropical zone of the Xixia Basin.     

The Variation of Soil Organic Matter in a Forest-Cultivation Sequence Traced by Stable Carbon Isotopes

On the basis of different photosynthetic pathways.there is an obvious difference in δ^13C values between C3 and C4 plants,In terms of this characteristic,we analyzed the organic carbon content (forestlands:1.81%-16.00%;farmland:0.45%-2.22%) and δ^13C values(forestlands:-23.86‰--27.12‰;farmland:-19.66‰--23.26‰)of three profile-soil samples either in farmland or in forestland near the Maolan Karst virgin forest,where there were developed plant C3 plants previously and now are C4 plants.Results showed that the deforestation has accelerated the decomposition rate of soil organic matter and reduced the proportion of active components in soil organic matter and thus soil fertility.     

Geochemical Characteristics and Origin of Natural Gases in the Qaidam Basin, China

Sixty-five natural gas samples were collected from 19 oil-gasfields in the Qaidam basin, China. The chemical composition and carbon isotope values of the samples were measured, and the geochemical characteristics and origin of the natural gases were studied. The gases can be divided into biogenic gases, sapropelic oil-type gases, mixed type oil-type gases, coal-type gases and mixed gas. The δ13Ci values of the biogenic gases are very small and the C2+ contents of them are very low, ranging from -68.2‰ to -61.8‰ and 0.06% to 0.20% respectively. They have heavy δD and δ13Cco2> showing a CC>2 reduction pathway. They are distributed in the East depression region and derived from the Quaternary source rocks. The sapropelic oil-type gases have small δ13C2 values and high C2+ ranging from -36.6‰ to -28.6‰ and from 33.01% to 47.15% respectively. The mixed type oil-type gases have <5I3C2 values and C2+ contents varying from -28.6‰ to -24.8‰ and from 4.81% to 26.06% respectively. Both sapropelic oil-typ     

Organic Carbon Isotope Geochemistry of the Neoproterozoic Doushantuo Formation, South China

The Neoproterozoic Doushantuo Formation on the Yangtze Platform, South China, documents a sedimentary succession with different sedimentary facies from carbonate platform to slope and to deep sea basin, and hosts one of the world-class phosphorite deposits. In these strata, exquisitely preserved fossils have been discovered： the Weng＇an biota. This study presents carbon isotope geochemistry which is associated paired carbonate and organic matter from the Weng＇an section of a carbonate platform （shelf of the Yangtze Platform, Guizhou Province） from the Songtao section and Nanming section of a transition belt （slope of the Yangtze Platform, Guizhou Province） and from the Yanwutan section （basin area of the Yangtze Platform, Hunan Province）. Environmental variations and bio-events on the Yangtze Platform during the Late Neoproterozoic and their causal relationship are discussed. Negative carbon isotope values for carbonate and organic carbon （mean δ^13Corg = -35.0‰） from the uppermost Nantuo Formation are followed by an overall increase in δ^13C up-section. Carbon isotope values vary between -9.9‰ and 3.6‰ for carbonate and between -35.6‰ and -21.5‰ for organic carbon, respectively. Heavier δ^13Ccarb values suggest an increase in organic carbon burial, possibly related to increasing productivity （such as the Weng＇an biota）. The δ^13C values of the sediments from the Doushantuo Formation decreased from the platform via the slope to basin, reflecting a reduced environment with minor dissolved inorganic carbon possibly due to a lower primary productivity. It is deduced that the classical upwelling process, the stratification structure and the hydrothermal eruption are principally important mechanisms to interpret the carbon isotopic compositions of the sediments from the Doushantuo Formation.     

Chemostratigraphy and lithological characters of Neoproterozoic cap carbonates from the Kuruktag Mountain, Xinjiang, western China

The Neoproterozoic Era includes some of the most largest ice ages in the geological history. The exact number of glaciations is unknown, though there were at least two events of global glaciation. Neoproterozoic glacial deposits in the Kuruktag Mountain, Xinjiang, western China have proven that there had occurred three discrete Neoproterozoic glaciations. Diamictite units occurred in the Bayisi, Tereeken, and Hankalchough formations, carbonate units were recognized among the diamictites and immediately overlied the Bayisi, Tereeken and Hankalchough diamictites. Carbonates at the top of the Bayisi Formation are characterized by the dolo-sility stones with negative δ13C values ranging from -4.10‰ to -8.17‰ (PDB), comparable to the Sturtian cap carbonates that overlie the Sturtian glacial deposits from other Neoproterozoic sequences. Carbonates overlying the Tereeken Formation are characterized by the pinkish cap dolostones (ca. 10 m thick) with negative δ13C values ranging from -2.58‰ to -4.77‰ (PDB), comparable to the Marinoan cap carbonates. The cap is also characterized by tepee-like structures, barite precipitates and pseudomorphous aragonite crystal fan limestones. Carbonates at the top of the Hankalchough Formation are characterized by subaerial exposure crust (vadose pisolite structure, calcareous crust structure) dolostones with negative δ13C values ranging from -4.56‰ to -11.45‰ (PDB) and the calcareous crust dolostones, implying that the Hankalchough cap carbonates differ from either the Sturtian or Marinoan cap carbonates in sedimentary environment and carbon isotopic composition. In addition, it is suggested the Hankalchough glaciation belongs to a terrestrial glaciation and it is the third largest glaciation during the Neoproterozoic period on the Tarim platform.     

Carbon and nitrogen isotope records in sediments of Lake Taihu, China, and their paleoenvironmental significance

Lake sediments can provide important historical information on records of paleoenvironments and paleoclimates and their changes.This study deals with the sedimentary history of the westem Taihu Lake based on seven geochemical indices measured in an 89-cm long sediment core. The core, corresponding to a time period from 6870 a B.P. to the present, was analyzed for δ^13Corg, δ^15N, TOC, TN, TP, C/N ratio and radiocarbon dates. Comparison of these multiple geochemical tracers helps to improve interpretations of the paleoenvironmental changes. All of the geochemical proxies used in the study change regularly, and show four major time scales that suggest different environments. During 6870-6532 a B.P., the values of all parameters slightly varied. δ^13Corg values increased in a wave pattern from -25.9‰ to -20.7‰. Similarly, δ^15N values increased from 1.6‰ to 4.5‰. TOC, TN and TP concentrations remained around 0.8%, 0.1%-3.2% and 0.5%, respectively. C/N ratios varied from 20.6 to 6.6. At 6370 a B.P., the sediment record profile showed dramatic variations in all parameters. δ^13Corg and δ^15N values dropped to -26.9‰ and 1.3‰, respectively.     

Stable isotopes （S, Cl） and hydrochemical variations in a karstic ground water system, Guiyang, SW China

A groundwater system in a karstic terrain is easily subject to pollution and its remediation is difficult once contaminated by human activities. The groundwater samples collected in both winter and summer seasons at Guiyang, SW China, show a wide range of variations in chemical composition and dominance of Ca^2＋, Mg^2＋, SO4^2-, and HCO3^- ions. The anthropogenic inputs include Cl^-, NO3^-, SO4^2-, Na^＋, and K^＋, as judged from the relationships between different elemental ratios and sewage samples. Cl^- concentrations of all water samples in the study area vary from 0 to 3.9 mmol/L in winter and are higher than in summer. In most of the samples, SO4^2- concentrations are high, ranging from 0.1 to 11 mmol/U The studies show that the cycling of sulfur plays an important role in controlling water chemistry and fate of contaminants in groundwater. In order to understand the source of SO4^2- and the geochemical cycling of some elements in the karstic groundwater environment, we have carried out a study on the variations of S and Cl isotope compositions, in addition to other isotopic tracers such as ^87Sr/^86Sr and δ^13C studied previously. The δ^37Cl values of the groundwater samples present a seasonal variation, ranging from 0.00‰ to ＋2.03‰ in winter and from -1.46‰ to ＋0.29‰ in summer. The surface water samples show a similar range of variations as observed for groundwater. Two rainwater samples collected at Guiyang have a distinct isotopic signature （mean value is -3‰） compared to the δ^37Cl values reported in literature. These results indicate that ground water is mainly derived from precipitation and suggest a fast recharge of the karst system by surface water.     

Genetic identification of natural gases from shallow reservoirs in some oil- and gas-bearing basins of China

Natural gases of shallow reservoirs with the carbon isotopic compositions of methane ranging from -50‰ to -60‰ (PDB) were considered as mixed gases of biogenic and thermogenic origins previously and some of them were considered as low-mature (or low temperature thermogenic) gases lately. In this paper natural gases with the carbon isotopic compositions of methane in the above range were identified using the molecular and stable carbon isotopic compositions of methane, ethane and propane. The mixed gases of biogenic and mature thermogenic origins display the characteristics of δ 13 C1 ranging from -50‰to -60‰,δ13C2 ＞ -35‰,Δvalues (δ13C3 -δ13C2) ＜ 5‰ and C1/∑C2 ratios ＜ 40. Immature to low-mature gases display the characteristics of δ 13 C1 ranging from - 50‰ to - 60‰, δ13 C2 ＜- 40‰,Δ values (δ13C3 -δ13C2) ＞7‰, and C1/∑C 2 ratios ＞60.     

Stable carbon isotope characteristics of different plant species and surface soil in arid regions

The stable carbon isotope composition in surface soil organic matter (δ¹³Csoil) contains integrative information on the carbon isotope composition of the standing terrestrial plants (δ¹³Cleaf). In order to obtain valuable vegetation information from the δ¹³C of terrestrial sediment, it is necessary to understand the relationship between the δ¹³C value in modern surface soil and the standing vegetation. In this paper, we studied the δ¹³C value in modern surface soil organic matter and standing vegetation in arid areas in China, Australia and the United States. The isotopic discrepancy between δ¹³Csoil and δ¹³Cleaf of the standing dominant vegetation was examined in those different arid regions. The results show that the δ¹³Csoil values were consistently enriched compared to the δ¹³Cleaf. The δ¹³Cleaf values were positively correlated with δ¹³Csoil, which suggests that the interference of microorganisms and hydrophytes on the isotopic composition of surface soil organic matter during soil organic matter formation could be ignored in arid regions. The averaged discrepancy between δ¹³Csoil and δ¹³Cleaf is about 1.71%in Tamarix L. in the Tarim Basin in China, 1.50% in Eucalytus near Orange in Australia and 1.22% in Artemisia in Saratoga in the United States, which are different from the results of other studies. The results indicate that the discrepancies in the δ¹³C value between surface soil organic matter and standing vegetation were highly influenced by the differences in geophysical location and the dominant species of the studied ecosystems. We suggest that caution should be taken when organic matter δ¹³C in terrestrial sediment is used to extract paleovegetation information (C₃/C₄ vegetation composition), as the δ¹³C in soil organic matter is not only determined by the ratio of C₃/C₄ species, but also profoundly affected by climate change induced variation in the δ¹³C in dominant species.     

Carbon isotopic characteristics of hydrocarbon gases from coal-measure source rocks——A thermal simulation experiment

1 IntroductionThe compositional characteristics of natural gasesand their carbon isotopic composition are controlledmainly by the type of organic matter and the degree ofits thermal evolution in the source rocks (Song Yan,1995; Xu Yongchang et al., 1994; …     

The dynamics of organic matter in soil size and density fractions traced by stable carbon isotopes

1IntroductionSoilorganicmatter (SOM)isakeysourceofnutrientsforplantgrowth ,itisessentialforthemaintenanceofsoilstructureanditcontributestotheabilityofsoiltoretainnutrientsandwater.AnimprovedunderstandingofSOMdynamicsiscentraltothedevelopmentofmoreenvironmentallysoundandsustainablepracticesofagriculturalmanagement (Collinsetal.,2 0 0 0 ) .Avarietyofcon ceptualmethodshavebeenusedtodescribetheprocessesofSOMaccumulationandturnover (Jen kinsonandRayner,1 977;Duxburyetal.1 989;Partonetal.,1 99…     

东海泥质区单体正构烷烃的碳同位素组成及物源分析 *

利用气相色谱(GC)和气相色谱/同位素比值质谱(GC/IRMS)对东海近岸泥质区、济州岛西南泥质区和冲绳海槽北部表层沉积物中正构烷烃的单体碳同位素组成及分布进行了分析。结果显示东海不同泥质区典型海洋藻类源正构烷烃C₁₉同位素组成基本相似,在-27.4 ‰ ~-28.0 ‰ 之间,平均为-27.7 ‰ 。典型海洋水生植物源C₂₃同位素组成在-28.5 ‰ ~-31.6 ‰ 之间,平均为-30.5 ‰ ,碳同位素组成从近岸泥质区到冲绳海槽北部逐渐变重,表明海槽区与陆架区海洋水生植物种类有所不同。陆架区长链正构烷烃(C₂₅~C₃₁)部分随着碳数的增加,其同位素组成逐渐变轻,但海槽区这一变化不大,显示陆架区的陆源高等植物蜡具有相似的物源,而冲绳海槽北部由于黑潮主干区和黑潮分支(对马暖流)对陆架沉积物进入深海的控制性阻隔作用,其物源与陆架区区别较大。现代输入东海的陆源植物以C₃植物为显著优势,C₃植物对近岸泥质区北部、近岸泥质区南部、远端济州岛西南泥质区和冲绳海槽北部陆源植物的贡献分别为83 % ,95 ％,75 % 和70 % 。