Early cambrian carbon isotope stratigraphy in the tarim basin and a correlation with the yangtze platform

Carbon isotope chemostratigraphy has been used worldwide for stratigraphic correlation.In this study,δ13Ccarb values are estimated for the Early Cambrian Sugaitebulake section in the Tarim Basin,Xinjiang Autonomous Region,China.As a result,one positive and two negative carbon isotope excursions in the studied section were iden-tified.The δ13Ccarb values reached the maximum negative excursion(N1:-12.39‰) at the basal of the Yuertusi For-mation,and then increased to P1.After P1,δ13Ccarb values sharply decreased to about-7.06‰(N2) in the studied section.The pattern of δ13Ccarb in the Early Cambrian is comparable to the synchronous records of other sections,such as the Laolin section,the Xiaotan section and the Anjiahe section of the Yangtze Platform.It is concluded that the Early Cambrian Yuertusi Formation from the Tarim Basin is within the Nemakit-Daldynian stage,and the lower strata of the Yuertusi Formation may belong to the Zhujiaqing Formation(Meishucun Formation) of the Yangtze Platform.The Ediacaran/Cambrian boundary of the Tarim Block should be located in cherts and phosphorites suc-cessions at the basal of the Yuertusi Formation.The δ13Ccarb negative excursion N1 is just across the PC/C boundary,and may be related to certain biomass extinction due to anoxic sedimentary environment,transgression and/or the oceanic overturn.The second δ13Ccarb negative excursion N2 may account for the sea-level falling in the Early Cam-brian.     

Atmospheric weathering of Scandinavian alum shales and the fractionation of C,N and S isotopes

Subaerial exposure and oxidation of organic carbon (C_org)-rich rocks is believed to be a key mechanism for the recycling of buried C and S back to Earth's surface. Importantly, processes coupled to microbial C_org oxidation are expected to shift new biomass δ¹³C_org composition towards more negative values relative to source. However, there is scarcity of information directly relating rock chemistry to oxidative weathering and shifting δ¹³C_org at the rock-atmosphere interface. This is particularly pertinent to the sulfidic, C_org-rich alum shale units of the Baltoscandian Basin believed to constitute a strong source of metal contaminants to the natural environment, following subaerial exposure and weathering. Consistent with independent support, we show that atmospheric oxidation of the sulfidic, C_org-rich alum shale sequence of the Cambrian-Devonian Baltoscandian Basin induces intense acid rock drainage at the expense of progressive oxidation of Fe sulfides. Sulfide oxidation takes priority over microbial organic matter decomposition, enabling quantitative massive erosion of C_org without producing a δ¹³C shift between acid rock drainage precipitates and shale. Moreover, ¹³C enrichment in inorganic carbon of precipitates does not support microbial C_org oxidation as the predominant mechanism of rock weathering upon exposure. Instead, a Δ³⁴S = δ³⁴S_shale − δ³⁴S_precipitates ≈ 0, accompanied by elevated S levels and the ubiquitous deposition of acid rock drainage sulfate minerals in deposited efflorescent precipitates relative to shales, provide strong evidence for quantitative mass oxidation of shale sulfide minerals as the source of acidity for chemical weathering. Slight δ¹⁵N depletion in the new surface precipitates relative to shale, coincides with dramatic loss of N from shales. Collectively, the results point to pyrite oxidation as a major driver of alum black shale weathering at the rock-atmosphere interface, indicating that quantitative mass release of C_org, N, S, and key metals to the environment is a response to intense sulfide oxidation. Consequently, large-scale acidic weathering of the sulfide-rich alum shale units is suggested to influence the fate and redistribution of the isotopes of C, N, and S from shale to the immediate environment.     

Identical carbon isotope trends of carbonate and organic carbon and their environmental significance from the Changhsingian (end-Permian), Meishan,South China

High resolution carbon isotope analyses of carbonate and organic carbon from Meishan, South China showed that the variation of δ13Ccarb is marked by three large positive excursions during the Changhsingian (end-Permian). Carbon isotope stratigraphy during this stage shows three cyclic intervals in δ13Ccarb, with two cycles corresponding to the lower (Paleofusulinid minima Zone) and one corresponding to the upper Changhsingian (P. sinensis Zone). The large positive δ13Ccarb excursions indicate episodes of enhanced burial of isotopically light or-ganic carbon, presumably in response to deep-water anoxia episodically extending into shallow water with the rise of sea level. The organic carbon during the Changhsingian is distinguished into two groups, and the δ13Corg of each group parallels (separately) the more detailed profile of δ13Ccarb, strongly showing that the values of fractionation Δ13Ccab-org remain relatively constant, with only two intervals with anomaly. The enhanced fractionation Δ13Ccab-org with large negative δ13Corg excursions apparently indicates significant inputs from sulfide-oxidizing bacteria and green sulfur bacteria, notably at bed 24 just predating mass extinction. Our evidence appears to support that the ex-tended euxinic water is possible for the main pulse of mass extinction at the end-Permian.     

Geochemical characteristics and possible origin of natural gas in the Taibei Depression,Turpan-Hami Basin,China

292 chemical composition data and 82 isotopic composition data of gas samples collected from the Taibei Depression of the Turpan-Hami Basin, West China, were used in the study of their origin. Non-hydrocarbon gas is poor in most samples whereas abundant nitrogen in some samples is positively correlated with δ13C1. Although methane is the main constituent, higher molecular gaseous hydrocarbons, from ethane to pentane, are detected in most samples, in accordance with the distribution of oil reservoirs. The stable carbon isotope ratios of methane, eth-ane and propane are defined as d13C1: -45.5‰ to -33.5‰, d13C2: -30.2‰ to -10.5‰, and d13C3: 27.6‰ to -11.2‰, respectively. According to the distribution of carbon isotope ratios, 2 families of gas can be grouped, most showing normal distribution of carbon isotopes, and others having obvious heavier carbon isotopes and being of abnormal distribution. Based on the isotopic composition, the disagreement between the relationship of Δ(d13C1-d13C2) and d13C2 and that of Δ(d13C1-d13C2) and d13C2, and the calculated Ro, there are oil-associated gas, coal-derived gas and mixture of them. Other samples with obviously heavier isotopic compositions from the Yanmuxi oilfield of the Taibei Depression have been degraded by organisms.     

Coincident negative shifts in sulfur and carbon isotope compositions prior to the end-Permian mass extinction at Shangsi Section of Guangyuan, South China

Sulfur isotope composition of carbonate-associated sulfate (δ³⁴S_CAS) and carbon isotope composition of carbonate (δ¹³C_carb) were jointly investigated on the Late Permian rocks at Shangsi Section, Guanyuan, Northeast Sichuan, South China. Both δ³⁴S_CAS and δ¹³C_carb show gradual decline trends in Late Permian strata, inferring the occurrence of the long-term variation of marine environmental conditions. Associated with the long-term variation are the two coincident negative shifts in δ³⁴S_CAS and δ¹³C_carb, with one occurring at the boundary between Middle Permian Maokou Formation and Late Permian Wujiaping Formation and another at Middle Dalong Formation. Of significance is the second shift which clearly predates the regression and the biotic crisis at the end of Permian at Shangsi Section, providing evidence that a catastrophic event occurred prior to the biotic crisis. The frequent volcanisms indicated by the volcanic rocks or fragments, and the upwelling are proposed to cause the second negative excursion. An abrupt extreme negative δ³⁴S_CAS (ca. −20‰) associated with a low relative concentration of CAS and total organic carbon without large change in δ¹³C_carb is found at the end of the second shift, which might arise from the short-term oxygenation of bottom waters and sediments that resulted from the abrupt sea level drop.     

Isotopic constraints on fluid evolution and precipitation mechanisms for the Boléo Cu–Co–Zn district, Mexico

Stable and radiogenic isotope composition of stratiform Cu–Co–Zn mineralization and associated sedimentary rocks within the Boléo district of the Miocene Santa Rosalía basin, Baja California Sur, constrains the evolution of seawater and hydrothermal fluids and the mechanisms responsible for sulfide and oxide deposition. Stable isotope geochemistry of limestone and evaporite units indicates a strong paleogeographic influence on the chemistry of the water column. Near-shore limestone at the base of the Boléo Formation is characterized by modified marine carbon (δ ¹³C_PDB=−6.0 to +4.4‰) and oxygen (δ ¹⁸O_SMOW=+19.5 to +26.2‰) isotope composition due to the influx of ¹³C- and ¹⁸O-depleted fluvial water. Sulfate sulfur isotope composition (δ ³⁴S_CDT=+17.21 to +22.3‰ and δ ¹⁸O_SMOW=+10.7 to +13.1‰) for basal evaporite and claystone facies are similar to Miocene seawater. Strontium isotopes are less radiogenic than expected for Miocene seawater due to interaction with volcanic rocks. Low S/C ratios, high Mn contents and sedimentological evidence indicate the basin water column was oxidizing. The oxygenated basin restricted sulfide precipitation to within the sedimentary pile by replacement of early diagenetic framboidal pyrite and pore-space filling by Cu–Co–Zn sulfides to produce disseminated sulfides. Quartz–Mn oxide oxygen isotope geothermometry constrains mineralization temperature between 18 and 118°C. Sulfur isotopes indicate the following sources of sulfide: (1) bacterial sulfate reduction within the sedimentary pile produced negative δ ³⁴S values (<−20‰) in framboidal pyrite; and (2) bacterial sulfate reduction at high temperature (80–118°C) within the sedimentary pile during the infiltration of the metal-bearing brines produced Cu–Co–Zn sulfides with negative, but close to 0‰, δ ³⁴S values. Isotope modeling of fluid-rock reaction and fluid mixing indicates: (1) sedimentary and marine carbonates (δ ¹³C=−11.6 to −3.2‰ and δ ¹⁸O=+19.0 to +21.8‰) precipitated from basin seawater/pore water that variably mixed with isotopically depleted meteoric waters; and (2) hydrothermal calcite (δ ¹³C=−7.9 to +4.3‰ and δ ¹⁸O=+22.1 to +25.8‰) formed by dissolution and replacement of authigenic marine calcite by downward-infiltrating metalliferous brine and brine-sediment exchange, that prior to reaction with calcite, had mixed with isotopically depleted pore water. The downward infiltration of metalliferous brine is inferred from lateral and stratigraphic metal distributions and from the concentration of Cu sulfides along the upper contact of pyrite-bearing laminae. The co-existence and textural relationships among framboidal pyrite, base metal sulfides, carbonate and Mn–Fe oxides (including magnetite) within mineralized units are consistent with carbonate replacement and high-temperature bacterial reduction within the sedimentary pile occurring simultaneously below a seawater column under predominantly oxygenated conditions.     

Genesis of the Assif El Mal Zn–Pb (Cu,Ag) vein deposit. An extension-related Mesozoic vein system in the High Atlas of Morocco. Structural,mineralogical, and geochemical evidence

The Assif El Mal Zn–Pb (Cu–Ag) vein system, located in the northern flank of the High Atlas of Marrakech (Morocco), is hosted in a Cambro-Ordovician volcaniclastic and metasedimentary sequence composed of graywacke, siltstone, pelite, and shale interlayered with minor tuff and mudstone. Intrusion of synorogenic to postorogenic Late Hercynian peraluminous granitoids has contact metamorphosed the host rocks giving rise to a metamorphic assemblage of quartz, plagioclase, biotite, muscovite, chlorite, amphibole, chloritoid, and garnet. The Assif El Mal Zn–Pb (Cu–Ag) mineralization forms subvertical veins with ribbon, fault breccia, cockade, comb, and crack and seal textures. Two-phase liquid–vapor fluid inclusions that were trapped during several stages occur in quartz and sphalerite. Primary inclusion fluids exhibit T _h mean values ranging from 104°C to 198°C. Final ice-melting temperatures range from −8.1°C to −12.8°C, corresponding to salinities of ∼15 wt.% NaCl equiv. Halogen data suggest that the salinity of the ore fluids was largely due to evaporation of seawater. Late secondary fluid inclusions have either Ca-rich, saline (26 wt.% NaCl equiv.), or very dilute (3.5 wt.% NaCl equiv.) compositions and homogenization temperatures ranging from 75°C to 150°C. The δ¹⁸O and δD fluid values suggest an isotopically heterogeneous fluid source involving mixing between connate seawater and black-shale-derived organic waters. Low δ¹³C_VPDB values ranging from −7.5‰ to −7.7‰ indicate a homogeneous carbon source, possibly organic matter disseminated in black shale hosting the Zn–Pb (Cu–Ag) veins. The calculated δ³⁴S_H2S values for reduced sulfur (22.5‰ to 24.3‰) are most likely from reduction of SO₄ ²⁻ in trapped seawater sulfate or evaporite in the host rocks. Reduction of sulfate probably occurred through thermochemical sulfate reduction in which organic matter was oxidized to produce CO₂ which ultimately led to precipitation of saddle dolomite with isotopically light carbon. Lead isotope compositions are consistent with fluid–rock interaction that leached metals from the immediate Cambro-Ordovician volcaniclastic and metasedimentary sequence or from the underlying Paleo-Neoproterozoic crustal basement. Geological constraints suggest that the vein system of Assif El Mal formed during the Jurassic opening of the central Atlantic Ocean.     

Comprehensive geochemical identification of highly evolved marine hydrocarbon source rocks： Organic matter, paleoenvironment and development of effective hydrocarbon source rocks

1 Introduction China’s widespread marine carbonate rock series are mostly characterized by intensive thermal evolution and low abundance of organic matter, especially the Lower Paleozoic carbonate rocks have experienced multi-episodes of tectonics and ap…     

Investigation of the pyrolytic liberation of molecular nitrogen from Palaeozoic sedimentary rocks

Open-system non-isothermal pyrolysis up to 1,200°C in combination with elemental analysis was used to study the thermal liberation of molecular nitrogen (N₂) from sedimentary rocks and kerogen concentrates of Palaeozoic age from the Central European Basin system and an Eocene shale (Liaohe Basin, China) with a high content (36%) of ammonium feldspar (buddingtonite). The N/C_org (atomic) ratios of the kerogen concentrates ranged from 0.005 to 0.014, which represents the range commonly observed for coals. Bulk N/C_org ratios of the Palaeozoic shales extended from 0.035 to 0.108, indicating the presence of significant amounts of inorganic nitrogen. Namurian A and A-B (CnA; CnA-B) samples typically exhibited the earliest onset of N₂ generation with intense, characteristic peaks around 600°C. N₂ liberation from the buddingtonite-rich sample occurred at higher temperatures, with a broad peak around 700°C. Pyrograms of the kerogen concentrates showed no or strongly reduced N₂ generation in the 500–700°C range. On-line isotope-specific analysis of the pyrolytically liberated N₂ on one sample revealed a variability of ∼10‰ in the δ¹⁵N values and a steady increase in δ¹⁵N with temperature during the main phase of N₂ generation.     

Genesis of organic and carbonate carbon in sediments of the North and Middle Caspian basins inferred from the isotope data

Isotopic compositions of organic (δ¹³C-C_org) and carbonate (δ¹³C-C_carb) carbon were analyzed in the particulate matter (hereafter, particulates) and sediments from the North and Middle Caspian basins. Isotopic composition of C_org was used for assessing proportions of the allochthonous and autochthonous organic matter in the particulates. Difference between the δ¹³C-C_org values in surface sediments and particulates is explained by the aerobic and anaerobic diagenetic transformations. Isotopic composition of C_org in sediments may be used as a tool for reconstructing the Quaternary transgressive-regressive history of the Caspian Sea.     

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 δ13C excursions spanning the Cambrian explosion to the Canglangpuian mass extinction in the Three Gorges area,South China

A remarkable increase of the animal genera and a subsequent mass extinction in the late Early Cambrian are well known as the “Cambrian explosion” and the “Botomian–Toyonian crisis.” A composite global curve of the carbon isotope ratios for inorganic carbon (δ¹³C_carb) shows multiple fluctuations during the evolution events, and it indicates significant changes of the oceanic carbon cycle at that time. This study reveals a new continuous isotopic chemostratigraphy for inorganic carbon (δ¹³C_carb) from the bottom of the Shipai to the base of the Shilongdong formations in Three Gorges area, South China. This section covers the Canglangpuian to the Longwangmiaoian stages in the Lower Cambrian. The δ¹³C_carb variation exhibits three negative excursions: a remarkably negative excursion down to ca. − 12‰ in the middle Canglangpuian stage, a negative excursion to ca. − 1.0‰ in the upper Canglangpuian stage, and a negative excursion to ca. − 1.0‰ in the Longwangmiaoian stage, respectively. The largest negative δ¹³C_carb excursion and a positive excursion before the excursion are definitely consistent with the δ¹³C_carb negative shift (AECE) during the mass extinction and the δ¹³C_carb positive values (MICE) during the increase of animal genera, respectively. However, the minimum values of the negative shifts among South China, Siberia, and Canada sections are different from each other. The positive δ¹³C_carb excursion at the bottom of the Canglangpuian stage indicates that primary productivities and organic carbon burial were enhanced. A sea level rise in the Qiongzhusian to bottom of the Canglangpuian stages in South China corresponds to the Sinsk transgression event in Siberia and Canada. A eutrophication due to higher continental weathering during the transgression after the long-term retrogression enhanced the high primary production and consequently promoted the significant increase of animal diversity.On the other hand, deposition of laminated black shales without bioturbation signatures and a decline of trilobite diversity are observed during the negative δ¹³C_carb excursion in the Canglangpuian stage, indicating that the shallow water environment became anoxic at that time. The negative δ¹³C_carb shift indicates an influx of abundant ¹²CO₂ due to oxidation of organic carbons in seawater. The difference of the minimum values among sections implies the local difference in size of the organic carbon reservoirs and extent of the degradation of the carbons. The largest δ¹³C anomaly in South China suggests the presence of the largest OCPs due to higher activity of primary production and high degree of oxidation of the OCPs because of higher activity of animals. The coincidence of the timing of the negative δ¹³C excursions in the Canglangpuian stage among the sections indicates a global event, and suggests that the onset was caused by increase of oxygen contents of seawater and atmosphere. Abundant oxygen yielded by the increased primary productivity in the Atdabanian and the Qiongzhusian stages caused onset of the oxidation of OCP, and possibly led to the shallow water anoxia and the mass extinction of benthic animals in the Botomian and the Canglangpuian stage.     

Stable isotope geochemistry and diagenetic mineralization associated with the Tono sandstone-type uranium deposit in Japan

The Tono sandstone-type uranium mine area, middle Honsyu, Japan is composed of Miocene lacustrine sedimentary rocks in the lower part (18–22 Ma) and marine facies in the upper part (15–16 Ma). Calcite and pyrite occur as dominant diagenetic alteration products in these Neogene sedimentary rocks. The characteristics of calcite and pyrite differ significantly between lacustrine and marine facies. Abundant pyrite, calcite, organic matter, and small amounts of marcasite or pyrrhotite occur in the lacustrine facies, whereas small amounts of calcite and framboidal pyrite, organic matter and no marcasite or pyrrhotite are found within the marine units. The δ¹³C values of calcite in the lacustrine deposits are low (−19 to −6‰ PDB) but those in marine formation are high (−11 to +3‰). This implies that the contribution of marine carbonate is larger in upper marine sedimentary rocks, and carbon in calcite in the lower lacustrine formation was derived both from oxidation of organic matter and from dissolved marine inorganic carbon. The δ³⁴S values of framboidal pyrite in the upper marine formation are low (−14 to −8‰ CDT), indicating a small extent of bacterial seawater sulfate reduction, whereas those of euhedral-subhedral pyrite in the lower lignite-bearing arkose sandstone are high (+10 to +43‰), implying a large extent of closed-system bacterial seawater sulfate reduction. The δ³⁴S and δ¹³C data which deviate from a negative correlation line toward higher δ¹³C values suggest methanogenic CO₂ production. During diagenesis of the lacustrine unit, large amounts of euhedral-subhedral pyrite were formed, facilitated by extensive bacterial reduction of seawater sulfate with concomitant oxidation of organic matter, and by hydrolysis reactions of organic matter, producing CH₄ and CO₂. Uranium minerals (coffinite and uraninite) were also formed at this stage by the reduction of U⁶⁺ to U⁴⁺. The conditions of diagenetic alteration within the lacustrine deposits and uranium mineralization is characterized by low Eh in which nearly equal concentrations of CH₄ and HCO₃ ⁻ existed and reduced sulfur species (H₂S, HS⁻) are predominant among aqueous sulfur species, whereas diagenetic alteration of the marine formations was characterized by a predominance of SO₄ ²⁻ among dissolved sulfur species. Modern groundwater in the lacustrine formation has a low Eh value (−335 mV). Estimated and measured low Eh values of modern and ancient interstitial waters in lacustrine environments indicate that a reducing environment in which U⁴⁺ is stable has been maintained since precipitation of uranium minerals. Received: 9 February 1996 / Accepted: 11 April 1997     

Paleoceanographic Indicators for Early Cambrian Black Shales from the Yangtze Platform,South China: Evidence from Biomarkers and Carbon Isotopes

The lower Cambrian Niutitang Formation, a widespread black shale deposition, is of geological interest because of its polymetallic beds, Cambrian explosion, depositional ages, dramatic environmental changes and so on. Previous study focused mainly on inorganic geochemistry and few studies have investigated the organic fractions of upper Neoproterozoic-lower Cambrian strata in South China. Here we report a study of biomarkers plus organic carbon isotopes for black shales from Ganziping, Hunan Province (China). All the saturated hydrocarbon fractions have a unimodal distribution of n-alkanes, a high content of short-chain alkanes and maximize at C 19 or C 20 (C 23 for sample Gzh00-1). The C 27 /C 29 sterane ratio ranges from 0.77 to 1.20 and 4-methylsteranes are in low abundance. These parameters indicate that algae and bacteria are the important primary producers. Furthermore, biomarker maturity proxies show the samples to be higher maturity. The low Pr/Ph values (<0.7) suggest that the samples were deposited under anoxic conditions and, likely, under stratified water columns. In addition, 25-norhopanes and gammacerane are present as diagnostic indicators of normal marine salinity and dysoxic to anoxic conditions. During the Early Tommotian, known to coincide with a transgression event, small shelly fossils increased in abundance and diversity. Moreover, positive δ 13 C org excursions close to 1.4‰ occur at the base of the Tommotian stage. In summary, the Early Cambrian black shales were deposited under dramatic paleoenvironmental changes, including oceanic anoxia, higher primary productivity and sea-level rise.     

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.     

Geochemistry of C,O, and Sr isotopes and chemostratigraphy of neoproterozoic rocks in the northern Yenisei Ridge

Isotopic compositions of C, O, and Sr in carbonates, as well as Rb-Sr systems in the silicate material from Upper Precambrian and Lower Cambrian rocks exposed by the Chapa River in the northern Yenisei Ridge, are studied. The Late Precambrian part of the section includes the following formations (from the bottom to top): Lopatinskaya (hereafter, Lopatino), Vandadykskaya (hereafter, Vandadyk) or Kar’ernaya, Chivida, Suvorovskaya (hereafter, Suvorovo), Pod”emskaya (hereafter, Podyom), and Nemchanka. They are characterized by alternation of horizons with anomalously high and low δ¹³C values (such alternation is typical of the ∼700–550 Ma interval). The lower, relatively thin (20 m), positive excursion (δ¹³C up to 4.3‰) was established in dolomites from the lower subformation of the Vandadyk (Kar’ernaya) Formation (hereafter, lower Vandadyk subformation). The upper positive excursion (δ¹³C = 2.2 ± 0.6‰) was recorded in the 3-km-thick Nemchanka Formation enriched in terrigenous rocks. The lower negative excursion stands out as uniform, moderately low δ¹³C values (−2 ± 1‰) and significant thickness. It comprises the upper part of the Vandadyk Formation, as well as Chivida and Podyom formations. The upper negative excursion is related to a thin (∼20 m) marker carbonate horizon of the upper Nemchanka subformation, in which δ¹³C values fall down to −8.3‰. The lower part of the Lebyazhinskaya (hereafter, Lebyazhino) Formation, which overlies the Nemchanka Formation, shows a step-by-step increase in δ¹³C from −2.2 to 2.5‰ typical of the Vendianto-Cambrian (Nemakit-Daldyn Horizon/Stage) transitional sequences. The absence of relationships between the carbon and oxygen isotope compositions and other parameters of postsedimentary alterations suggests that the excursions characterized above could form at the sedimentation stage and coincide in general with δ¹³C fluctuations in seawater. The value of ⁸⁷Sr/⁸⁶Sr = 0.7076−0.7078 in limestones of the Podyom Formation points to their early Ediacaran age. Values of ⁸⁷Sr/⁸⁶Sr = 0.70841 and 0.70845 in dolomites of the lower Lebyazhino subformation correspond to the Early Cambrian. The Rb-Sr systems of the clay material from the Vandadyk and Chivida formations are approximated by a straight line, parameters of which correspond to the age of 695 ± 20 Ma (⁸⁷Sr/⁸⁶Sr₀ = 0.7200 ± 0.0013) and probably characterize the epigenetic stage of older sedimentary rocks, which were subjected to very rapid exhumation and “polar” sulfuric acid weathering in the course of glacioeustatic regression.     

Paired δ13Ccarb and δ13Corg records of the Ordovician on the Yangtze platform,South China

During the Ordovician, huge biological revolutions and environmental changes happened in Earth’s history, including the Great Ordovician Biodiversification Event, global cooling and so on, but the cause of these events remains controversial. Herein, we conducted a paired carbon isotopic analysis of carbonate (δ¹³C_carb) and organic matter (δ¹³C_org) through the Ordovician in the Qiliao section on the Yangtze platform of South China. Our results showed that the δ¹³C_carb trend of the Qiliao section can be correlated with local and global curves. The δ¹³C_org trend seems is less clear than the δ¹³C_carb trend for stratigraphic correlations, but some δ¹³C_org positive excursions in the Middle and Upper Ordovician may be used for correlation studies. These carbon isotopic records may have global significance rather than local significance, revealing several fluctuations to the global carbon cycle during the Ordovician. Several known δ¹³C_carb and δ¹³C_org negative and positive excursions have been recognised in this study, including the early Floian Negative Inorganic Carbon (δ¹³C_carb) Excursion (EFNICE), as well as the early Floian Positive Organic Carbon (δ¹³C_carb) Excursion, the mid-Darriwilian Inorganic Carbon (δ¹³C_carb and δ¹³C_org) Excursion (MDICE), and the early Katian Guttenberg Inorganic Carbon (δ¹³C_carb and δ¹³C_org) Excursion (GICE). These positive excursions and a smooth decline trend of δ¹³C_org values during the early to mid-Floian may imply multiple episodes of enhanced organic carbon burial that began at the early Floian stage, probably resulting in further decline in atmospheric pCO₂ and then global cooling.     

Carbon isotope geochemistry of the 3.7 × 109-yr-old Isua sediments,West Greenland: implications for the Archaean carbon and oxygen cycles

One hundred and twenty-four carbonate samples from the meta-sedimentary sequence of the 3.7 × 10⁹ yr old Isua supracrustal belt (W-Greenland) have yielded a δ¹³C_carb average of ?2.5 ± 1.7%. vs PDB and a δ¹⁸O_carb average of +13.0 ± 2.5%. vs SMOW. The oxygen mean comes fairly close to the averages of other early Precambrian carbonates. The carbon average, however, is some 2%. more negative than those of younger marine carbonates. In terms of a simple terrestrial ¹³C mass balance, if δ¹³C_carb values are original sedimentary values, this more negative δ¹³C average would imply a considerably smaller $\text{C}{}_{\mathrm{org}}\text{C}{}_{\mathrm{carb}}$ ratio in the sedimentary shell during Isua times, and would thus support the concept of a gradual buildup of a sedimentary reservoir of organic carbon during the early history of the Earth. Since, however, the Isua supracrustal rocks have experienced amphibolite-grade metamorphism, which in other areas has been shown to lower δ¹³C_carb values, it is most likely that the original values of these rocks were approx 0%.. This indicates that C_orx and C_carb were present in the ancient carbon reservoir in about ‘modern’ proportions. Unless this early stabilization of the terrestrial carbon cycle in terms of a constant partitioning of carbon between the reduced and oxidized species is shown to have been caused by some inorganic geochemical process, a considerably earlier start of chemical evolution and spontaneous generation of life must be considered than is presently accepted.     

A fluid inclusion and stable isotope study of 200 Ma of fluid evolution in the Galway Granite, Connemara, Ireland

Fluid inclusions in granite quartz and three generations of veins indicate that three fluids have affected the Caledonian Galway Granite. These fluids were examined by petrography, microthermometry, chlorite thermometry, fluid chemistry and stable isotope studies. The earliest fluid was a H₂O-CO₂-NaCl fluid of moderate salinity (4–10 wt% NaCl eq.) that deposited late-magmatic molybdenite mineralised quartz veins (V₁) and formed the earliest secondary inclusions in granite quartz. This fluid is more abundant in the west of the batholith, corresponding to a decrease in emplacement depth. Within veins, and to the east, this fluid was trapped homogeneously, but in granite quartz in the west it unmixed at 305–390 °C and 0.7–1.8 kbar. Homogeneous quartz δ¹⁸O across the batholith (9.5 ± 0.4‰n = 12) suggests V₁ precipitation at high temperatures (perhaps 600 °C) and pressures (1–3 kbar) from magmatic fluids. Microthermometric data for V₁ indicate lower temperatures, suggesting inclusion volumes re-equilibrated during cooling. The second fluid was a H₂O-NaCl-KCl, low-moderate salinity (0–10 wt% NaCl eq.), moderate temperature (270–340 °C), high δD (−18 ± 2‰), low δ¹⁸O (0.5–2.0‰) fluid of meteoric origin. This fluid penetrated the batholith via quartz veins (V₂) which infill faults active during post-consolidation uplift of the batholith. It forms the most common inclusion type in granite quartz throughout the batholith and is responsible for widespread retrograde alteration involving chloritization of biotite and hornblende, sericitization and saussuritization of plagioclase, and reddening of K-feldspar. The salinity was generated by fluid-rock interactions within the granite. Within granite quartz this fluid was trapped at 0.5–2.3 kbar, having become overpressured. This fluid probably infiltrated the Granite in a meteoric-convection system during cooling after intrusion, but a later age cannot be ruled out. The final fluid to enter the Granite and its host rocks was a H₂O-NaCl-CaCl₂-KCl fluid with variable salinity (8–28 wt% NaCl eq.), temperature (125–205 °C), δD (−17 to −45‰), δ¹⁸O (−3 to + 1.2‰), δ¹³C_CO2 (−19 to 0‰) and δ³⁴S_sulphate (13–23‰) that deposited veins containing quartz, fluorite, calcite, barite, galena, chalcopyrite sphalerite and pyrite (V₃). Correlations of salinity, temperature, δD and δ¹⁸O are interpreted as the result of mixing of two fluid end-members, one a high-δD (−17 to −8‰), moderate-δ¹⁸O (1.2–2.5‰), high-δ¹³C_CO2 (> −4‰), low-δ³⁴S_sulphate (13‰), high-temperature (205–230 °C), moderate-salinity (8–12 wt% NaCl eq.) fluid, the other a low-δD (−61 to −45‰), low-δ¹⁸O (−5.4 to −3‰), low-δ¹³C (<−10‰), high-δ³⁴S_sulphate (20–23‰) low-temperature (80–125 °C), high-salinity (21–28 wt% NaCl eq.) fluid. Geochronological evidence suggests V₃ veins are late Triassic; the high-δD end-member is interpreted as a contemporaneous surface fluid, probably mixed meteoric water and evaporated seawater and/or dissolved evaporites, whereas the low-δD end-member is interpreted as a basinal brine derived from the adjacent Carboniferous sequence. This study demonstrates that the Galway Granite was a locus for repeated fluid events for a variety of reasons; from expulsion of magmatic fluids during the final stages of crystallisation, through a meteoric convection system, probably driven by waning magmatic heat, to much later mineralisation, concentrated in its vicinity due to thermal, tectonic and compositional properties of granite batholiths which encourage mineralisation long after magmatic heat has abated. Received: 3 April 1996 / Accepted: 5 May 1997     

C-S-Fe relationships and the isotopic composition of pyrite in the New Albany Shale of the Illinois Basin,U.S.A.

The relationship between pyritic sulfur content (S_pyr) and organic carbon content (C_org) of shales analyzed from the New Albany Group depends upon C_org. For samples of <6 wt.% C_org, S_pyt, and C_org are strongly correlated (r = 0.85). For C_org-“rich” shales (>6 wt.%), no S_pty-C_org, correlation is apparent. The degree of Fe pyritization (DOP) shows similar relationships to C_org. These C-S-Fe relationships suggest that pyrite formation was limited by the availability of metabolizable organic carbon in samples where C_org < 6 wt.% and by the availability of reactive Fe for samples where C_org > 6 wt.%. Apparent sulfur isotope fractionations relative to contemporaneous seawater sulfate (Δ³⁴S) for pyrite formation average −40% for non-calcareous shales and −25%. for calcareous shales. Δ³⁴S values become smaller with increasing C_org, S_pyt, and DOP for all C_org-“poar” (<6 wt%) and some C_org-“nch” (<6 wt.%) shales. These trends suggest that pyrite formation occurred in a closed system or that instantaneous bacterial fractionation for sulfate reduction decreased in magnitude with increasing organic carbon content. The isotopic trends observed in the New Albany Group are not necessarily representative of other shales having a comparable range of organic carbon content, e.g. Cretaceous shales and mudstones from the western interior of North America (GAUTIER, 1986). Δ³⁴S values in the remainder of the C_org-rich New Albany Group shales are relatively large (−38 to −47%.) and independent of C_org, S_pyr, and DOP, which suggests that pyrite in these shales formed mostly at or above the sediment-water interface by precipitation from an isotopically uniform reservoir of dissolved H₂S.