Changes in carbon and oxygen isotope composition during limestone diagenesis

The calcite fossils of the Derbyhaven Beds, Isle of Man, have δ¹³C values (+ 1·8 PDB) similar to modern, shallow-water marine skeletons, but the δ¹⁸O values (?6·1 PDB) are much lighter than modern skeletons. The light oxygen values indicate either re-equilibration with isotopically light water before cementation started, or Carboniferous sea water with δ¹⁸O of ?6‰. Aragonite dissolution was followed by precipitation of zoned calcite cement. In this cement, up to six intracrystalline zones, recognized in stained thin sections, show isotopic variation. Carbon varies from + 3-8 to + 1-2‰. and oxygen from ? 2-6 to ? 12-4‰. with decreasing age of the cement. This trend is attributed to increasing temperature and to isotopic evolution of the pore waters during burial. The zoned calcite is sequentially followed by dolomite and kaolinite cements which continue the trend towards light isotopic values. This trend is continued with younger, fault-controlled dolomite, and is terminated by vein-filling calcite and dolomite. The younger calcite, interpreted as a near-surface precipitate from meteoric waters, is unrelated to the older sequence of carbonates and has distinctly different carbon isotope ratios: δ¹³C ? 6-8‰.     

Morphology, stratigraphy and oxygen isotope composition of fossil glacier ice at Ledyanaya Gora, Northwest Siberia, Russia

Studies of the stratigraphy, sedimentology, structure and isotope composition of a buried massive ice body and its encompassing sediments at Ledyanaya Gora in northwestern Siberia demonstrate that the ice is relict glacier ice, probably emplaced during the Early Weichselian. Characteristics of this ice body should serve as a guide for the identification of other relict buried glacier ice bodies in permafrost regions.     

Fluid Composititon and Carbon ＆ Oxygen Isotope Geochemistry of Cenozoic Alkali Basalts in Eastern China

The fluid compositions of Cenozoic alkali basalts in eastern China have been determined by the pyrolysis-MS method,meanwhile the carbon and oxygen isotopic compositions of CO2 released from these samples at different heating temperatures have been analyzed by the vacuum step-heating method.The data show the volatiole heterogeneity in upper-mantle sources and different evolution trends of alkali basaltic magmas in eastern China,and these alkali basaltic magmas may be generated in the oxidizing milieu,as compared with mantle-derived xenoliths in these alkali basalts,and exotic volatile components were mixed into these magmas in the process of their formation and development.     

The sulfur and carbon isotope composition of scapolite-rich granulites from southern Tanzania

Sulfur and carbon isotope data are presented of 15 granulite samples from the Furua Complex, southern Tanzania, in which scapolite is a primary and major rock-forming constituent (up to 30 vol%). From these data, the isotopic composition is deduced of the sulfate and carbonate group in the scapolite structure. Subsequently, the composition and origin is discussed of the volatile species that are present in the deep crustal environment in which these scapolites formed.The ³⁴S-values show a narrow range from 0.3 to 3.6, consistent with a deep-seated (mantle) origin of the sulfur; the mean value of 1.9 is slightly higher than usually found in rocks of assumed mantle provenance. The results of the carbon isotope analyses are more difficult to interpret; they suggest that the granulites contain two different carbon components with different isotopic compositions. Firstly, one component, liberated by phosphoric acid at room temperature, has ¹³Cvalues between –3.8 and –11.2 and a mean value of –6.7. This carbon component is assumed to occur as finely dispersed, submicroscopic carbonate inclusions. The second carbon fraction is liberated by phosphoric acid treatment at temperatures between 200 and 400° C and has considerably lower ¹³Cvalues with a mean value of –14.1 This seems to represent the carbon isotope composition in the scapolite structure. Such low ¹³C-values do not agree with the generally accepted value of –7 for juvenile carbon, but they are comparable to those found in early, primary carbonic inclusions from various granulite regions. It is argued that these low ¹³C-values are typical for granulite-facies metamorphism and that they may characterize an important fluid phase of the lower crust.     

Oxygen isotope ratios in fossil wood cellulose: Isotopic composition of Eocene- to Holocene-aged cellulose

We measured the δ¹⁸O of cellulose (δ¹⁸O_cel) extracted from fossil wood collected at 9 sites in the northern and southern hemispheres as a potential source of information about precipitation δ¹⁸O (δ¹⁸O_ppt) in the past and paleotemperatures. The samples had been buried in fluvial sediments for periods of time ranging from ca. 45 million to 250 years. At the oldest localities (high latitude, Eocene- through Pliocene-age sites in Canada and Russia), mean annual temperature (MAT) estimates derived from the modern relationship between MAT and δ¹⁸O_cel are 6-16 °C lower than the MAT estimates derived from other biological proxies. Estimates of Pleistocene and Holocene mean annual temperatures are close to the modern values at those sites. These results are consistent with other recent findings that the MAT/δ¹⁸O_ppt relationship across North America was not constant throughout the Cenozoic. Paleo-δ¹⁸O_ppt estimates derived from fossil cellulose and the modern North American relationship between δ¹⁸O_cel and δ¹⁸O_ppt are within the current annual range of δ¹⁸O_ppt values at all locations. The middle Eocene δ¹⁸O_ppt we determined from arctic cellulose samples (−21.9‰) is consistent with river water δ¹⁸O determined in two other studies (−19.1‰ to −22‰). These findings provide some evidence that a precipitation δ¹⁸O signal may be retained in wood cellulose during millions of years of burial, and that latitudinal patterns in δ¹⁸O_ppt may not have changed much during the past 45 Ma. These interpretations depend, of course, on the assumption that the isotopic composition of the cellulose has not changed during burial, an assumption for which it is difficult to gather direct evidence. XRD analysis shows that the crystalline form of the fossil cellulose we used to estimate paleoprecipitation δ¹⁸O and paleo-MAT is the same as that of modern cellulose, and that the samples are free of quartz and iron oxide contaminants that result in negative errors in measured δ¹⁸O_cel.     

Precambrian sedimentary carbonates: carbon and oxygen isotope geochemistry and implications for the terrestrial oxygen budget

Carbon isotope values of 260 Precambrian limestones and dolomites (most of them being substantially unaltered) have yielded an overall mean of $\text{δ}{}^{13}\text{C}\text{= +0.4 ± 2.7‰}$ vs. PDB; the corresponding oxygen values average at $\text{δ}{}^{13}\text{O = +20.0 ± 4.2‰}$ vs. SMOW. Like the overall mean, the δ ¹³C values furnished by individual carbonate occurrences are, as a rule, fairly “modern” and almost constant as from the very beginning of the sedimentary record. A remarkable exception are the “heavy” dolomites of the Middle Precambrian Lomagundi Group, Rhodesia, with $\text{δ}{}^{13}\text{C}\text{= +9.4 ± 2.0‰}$ vs. PDB. As a result of our measurements, the sporadic occurrence in the geological past of anomalously heavy carbonates seems to be established.The approximate constancy around zero per mill of the δ ¹³C values of marine carbonates through geologic time would imply a corresponding constancy of the relative proportion of organic carbon in the total sedimentary carbon reservoir since about 3.3 · 10⁹ y ago (with C_org/C_total ? 0.2). Utilizing this ratio and current models for the accumulation of the sedimentary mass as a function of time, we get a reasonable approximation for the absolute quantity of organic carbon buried in sediments and, accordingly, of photosynthetic oxygen released. Within the constraints of our model (based on a terrestrial degassing constant λ = 1.16 · 10^?9 y^?1) close to 80% of the amount of oxygen contained in the present oxygen budget should have been released prior to 3 · 10⁹ y ago. Since geological evidence indicates an O₂-deficient environment during the Early and most parts of the Middle Precambrian, there is reason to believe that the distribution of this oxygen between the “bound” and the “molecular” reservoir was different from that of today (with effective O₂-consuming reactions bringing about an instantaneous transfer to the crust of any molecular oxygen released). Accordingly, the amount of C_org in the ancient sedimentary reservoir as derived from our isotope data is just a measure of the gross amount of photosynthetic oxygen produced, withholding any information as to how this oxygen was partitioned between the principal geochemical reservoirs. As a whole, the carbon isotope data accrued provide evidence of an extremely early origin of life on Earth since the impact of organic carbon on the geochemical carbon cycle can be traced back to almost 3.5 · 10⁹y.     

碳还原法分析硫酸盐的氧同位素组成

硫酸盐矿物是自然界最常见矿物,也是自然界少数具有氧同位素非质量分馏效应的矿物之一.硫酸盐矿物的氧同位素组成可以为研究其形成过程和生成条件提供大量信息.目前,在国内外分析硫酸盐氧同位素的3种方法中,碳还原方法乃是分析硫酸盐中氧同位素组成的最精准方法.本次研究建立了分析硫酸盐中氧同位素组成的碳还原方法,介绍了硫酸钡的分析流...     

The oxygen isotope composition of nitrate generated by nitrification in acid forest floors

The oxygen isotope composition of nitrate is used increasingly for identifying the origin of nitrate in terrestrial and aquatic ecosystems. This novel isotope tracer technique is based on the fact that nitrate in atmospheric deposition, in fertilizers, and nitrate generated by nitrification in soils appear to have distinct oxygen isotope ratios. While the typical ranges of δ¹⁸O values of nitrate in atmospheric deposition and fertilizers are comparatively well known, few experimental data exist for the oxygen isotope composition of nitrate generated by nitrification in soils. The objective of this study was to determine δ¹⁸O values of nitrate formed by microbial nitrification in acid forest floors.Evidence from laboratory incubation experiments and field studies suggests that during microbial nitrification in acid forest floor horizons, up to two of the three oxygen atoms in newly formed nitrate are derived from water, particularly if ammonium is abundant and nitrification rates are high. It was, however, also observed that in ammonium-limited systems with low nitrification rates, significantly less than two thirds of the oxygen in newly formed nitrate can be derived from water oxygen, presumably as a result of heterotrophic nitrification. It can be concluded from the presented data that the δ¹⁸O values of nitrate formed by microbial nitrification in acid forest floors typically range between +2 and +14‰, assuming that soil water δ¹⁸O values vary between −15 and −5‰. Hence, oxygen isotope ratios of nitrate formed by nitrification in forest floors are usually distinct from those of other nitrate sources such as atmospheric deposition and synthetic fertilizers and, therefore, constitute a valuable qualitative tracer for distinguishing among these sources of nitrate. A quantitative source apportionment appears, however, difficult because of the wide range of δ¹⁸O values, particularly for atmospheric nitrate deposition and for nitrate from microbial nitrification.     

干旱化对成土碳酸盐碳同位素组成的影响

土壤碳酸盐的碳同位素组成可以作为古环境变化的指标.本文对黄土高原地区S1以来成土碳酸盐和红粘土中碳酸盐的碳同位素进行了研究.根据渭南、吉县、长武和会宁4个剖面末次间冰期以来土壤碳酸盐的碳同位素分析结果,探讨了不同气候条件下成土碳酸盐碳同位素组成的特征及其环境意义,指出气候的干湿程度可能是影响黄土地区成土碳酸盐δ13C值的主要原因;西峰红粘土序列碳酸盐的碳同位素记录表明,δ13C值在4.0Ma B.P.前后有一个明显增加的趋势,反映了我国西北地区上新世干旱化的发展,可能与青藏高原在这一时期发生较大规模的隆升有关.     

The oxygen isotope composition of granitoid and sedimentary rocks of the southern Snake Range,Nevada

Six diverse intrusive igneous types are exposed as discrete outcrops within an area of 900 km² in the southern Snake Range, White Pine County, Nevada. The previously recognized variety among these igneous types is reflected in the wide range of ¹⁸O values (–1.1 to 13.4 permil) found in these rocks. This range of ¹⁸O values probably results from differences in source material and post-crystallization history of the different intrusive types.The Jurassic intrusive of the Snake Creek-Williams Canyon area represents the chemical equivalent of a large part of a differentiation sequence, with the entire range of composition (63–76 percent SiO₂) exposed over a horizontal distance of about five km. The rather regular increase of ¹⁸O values from the most mafic to the most felsic parts of this pluton, together with ¹⁸O values determined for constituent minerals recovered from five of the samples, supports a fractional crystallization model. The high ¹⁸O values found (10.2–12.2 permil) indicate that the magma likely was derived from or assimilated sedimentary materials.Nine samples of the Cretaceous two-mica granite of the Pole Canyon-Can Young Canyon area have ¹⁸O values in the range 10.6–12.1 permil. These high ¹⁸O values, an initial⁸⁷Sr/⁸⁶Sr ratio of 0.7165, and the presence of muscovite along with an accessory mineral suite limited to monazite, apatite, zircon, and an allanite-like mineral, characterize this intrusive mass as an S-type granite. It probably formed through anatexis of late Precambrian pelitic rocks.The granitoid rock exposed in the Young Canyon-Kious Basin area is Tertiary (32 m.y.). Most of this intrusive has been cataclastically deformed as a result of late (18 m.y.) movement on the overlying Snake Range decollement. The undeformed portion of this intrusive has ¹⁸O values of 8.7–10.0 permil. However, the deformed portion of this intrusive has ¹⁸O values as low as –1.1 permil, apparently resulting from isotopic exchange between this rock and ground water at the time of cataclasis.Although the igneous types exposed in the southern Snake Range differ petrologically and range in age from Jurassic to Tertiary, most have relatively high ¹⁸O values compared with other granitoid rocks of the Basin-Range Province.     

碳酸盐中碳、氧同位素快速制样装置研制

介绍一种碳酸盐中碳、氧同位素快速制样装置。实验证明,该装置具有真空度好、使用方便、无污染、制样效率高、成本低的优点,所生成的二氧化碳具有高纯、分析结果准确、可靠等特点,完全能满足碳酸盐中碳、氧同位素分析的制样要求。     

Annual fluctuation in the stable carbon isotope ratio of coral skeletons: The relative intensities of kinetic and metabolic isotope effects

Stable carbon and oxygen isotope measurements of biogenic carbonate provide information for reconstructing past oceanic environments. In particular, ¹⁸O/¹⁶O ratios correlate with the temperature and salinity of seawater and ¹³C/¹²C is a proxy for dissolved inorganic carbon in seawater and symbiont photosynthesis. Here, we report ¹³C/¹²C and ¹⁸O/¹⁶O values for skeletons of corals (genus Porites) with various growth rates. In faster-growing corals, ¹³C/¹²C and ¹⁸O/¹⁶O showed out-of-phase annual fluctuations. In slower-growing corals, the isotopes fluctuated in phase. We developed a simple vector notation to show two patterns of ¹³C/¹²C annual fluctuation, each with a different offset in relation to ¹⁸O/¹⁶O annual fluctuation. The phase offset between ¹³C/¹²C and ¹⁸O/¹⁶O annual fluctuations depends on the relative intensities of kinetic isotope effects on calcification and metabolic isotope effects such as photosynthesis. This model might improve our ability to infer past climate and oceanographic conditions from coral skeletons.     

The oxygen isotope composition of Hercynian granites and pre-Hercynian gneisses from the Schwarzwald,SW Germany

The oxygen isotope composition of 56 Hercynian granites and 42 pre-Hercynian gneisses has been investigated. In addition some mineral δ ¹⁸O data and 5 δD values of whole rocks have been obtained. The granites from the N-Schwarzwald show, in general, relatively uniform δ ¹⁸O values between 11.5 and 13.5‰, those from the S-Schwarzwald are less uniform and lighter in ¹⁸O and range from 2.3 to 11.5‰. The gneisses from the pre-Hercynian basement exhibit more or less the same variation and range from 1.7 to 10.4‰. δ ¹⁸O values <6‰ only occur in the S-Schwarzwald and obviously indicate hydrothermal interactions of meteoric waters, which probably took place after the emplacement and solidification of the granites and which equally affected granites and gneisses. Due to the nearly identical ¹⁸O/¹⁶O ratios of S-Schwarzwald granites and gneisses, it is proposed that such gneisses in the pre-Hercynian basement qualify as the precursor rocks of the S-Schwarzwald, granites whereas for the N-Schwarzwald granites crustal rocks with heavier δ ¹⁸O values, unknown from the present surface, have to be postulated. This distribution is also reflected on a ⁸⁷Sr/⁸⁶Sr-¹⁸O/¹⁶O diagram.     

南昌市PM2.5中稳定碳同位素组成特征

近几年,PM2.5浓度上升导致灰霾事件频繁发生,已经引起了广泛的关注。碳组分是PM2.5中的重要组分,被认为是灰霾形成和转化的重要因素,因此,研究PM2.5中含碳组分的来源及其化学过程具有重要的意义。本研究于2016年12月至2017年8月期间在南昌地区共采集105个PM2.5样品,分析了PM2.5样品中总碳(TC)浓度及其碳同位素(δ^13C)。结果表明,采样期间TC的年平均浓度为(12.1±2.1)μg/m^3,总体上呈现冬季高、夏季低的变化趋势,可能是受不同季节气象因素和来源变化的影响。δ^13C的年平均值为(?26.1±0.2)‰,总体上呈现冬季高、春季低的变化趋势,可能是受不同来源的影响。利用贝叶斯模型计算南昌地区PM2.5中TC主要来源于C3植物燃烧和机动车尾气,年源贡献分别为49.3%和28.7%;其次是煤燃烧和C4植物燃烧,年源贡献分别为17.7%和4.2%。春季δ^13C值偏低是由于C3植物燃烧贡献相对较高,而冬季δ^13C值偏高则是煤燃烧贡献增加。     

The non-mass-dependent oxygen isotope effect in the electrodissociation of carbon dioxide: A step toward understanding NoMaD chemistry

A non-mass dependent (NoMaD) oxygen isotope effect is demonstrated in the dissociation of CO₂ similar to that observed in the electrosynthesis of ozone. The molecular oxygen produced carries the signature of two separate isotopic fractionation processes; a mass-dependent fractionation probably due to CO₂ + O isotopic exchange, and a secondary NoMaD fractionation (δ¹⁷O = 0.97 ± 0.09δ¹⁸O, with the O₂ depleted in ¹⁷O and ¹⁸O). It is suggested that the effect is due to either the formation or relaxation of ozone in an excited electronic state. This represents the latest advance in the understanding of chemical NoMaD effects which may be essential to the explanation of non-mass-dependent fractionations observed in meteorites.     

Oxygen isotope salt effects at high pressure and high temperature and the calibration of oxygen isotope geothermometers

The influence of NaCl, CaCl₂, and dissolved minerals on the oxygen isotope fractionation in mineral-water systems at high pressure and high temperature was studied experimentally. The salt effects of NaCl (up to 37 molal) and 5-molal CaCl₂ on the oxygen isotope fractionation between quartz and water and between calcite and water were measured at 5 and 15 kbar at temperatures from 300 to 750°C. CaCl₂ has a larger influence than NaCl on the isotopic fractionation between quartz and water. Although NaCl systematically changes the isotopic fractionation between quartz and water, it has no influence on the isotopic fractionation between calcite and water. This difference in the apparent oxygen isotope salt effects of NaCl must relate to the use of different minerals as reference phases. The term oxygen isotope salt effect is expanded here to encompass the effects of dissolved minerals on the fractionations between minerals and aqueous fluids. The oxygen isotope salt effects of dissolved quartz, calcite, and phlogopite at 15 kbar and 750°C were measured in the three-phase systems quartz-calcite-water and phlogopite-calcite-water. Under these conditions, the oxygen isotope salt effects of the three dissolved minerals range from ∼0.7 to 2.1‰. In both three-phase hydrothermal systems, the equilibrium fractionation factors between the pairs of minerals are the same as those obtained by anhydrous direct exchange between each pair of minerals, proving that the use of carbonate as exchange medium provides correct isotopic fractionations for a mineral pair.When the oxygen isotope salt effects of two minerals are different, the use of water as an indirect exchange medium will give erroneous fractionations between the two minerals. The isotope salt effect of a dissolved mineral is also the main reason for the observation that the experimentally calibrated oxygen isotope fractionations between a mineral and water are systematically 1.5 to 2‰ more positive than the results of theoretical calculations. Dissolved minerals greatly affect the isotopic fractionation in mineral-water systems at high pressure and high temperature. If the presence of a solute changes the solubility of a mineral, the real oxygen isotope salt effect of the solute at high pressure and high temperature cannot be correctly derived by using the mineral as reference phase.     

川东地区石炭系碳酸盐岩碳、氧、锶同位素特征及其成因分析

利用微量元素和碳、氧同位素特征对样品有效性作出了检验.在证明样品未受明显蚀变的前提下对川东地区石炭系碳酸盐岩碳、氧、锶同位素进行了分析.在层序地层格架中对比不同体系域、不同岩性的碳、氧同位素特征,87Sr/86Sr比值特征,Z值与古温度特征,分析了碳酸盐岩的成岩环境.低位体系域以膏盐湖及萨巴哈环境为主,炎热干旱,陆源淡...