Stable chlorine and carbon isotope measurements of selected chlorinated organic solvents

Stable Cl and C isotope ratio results for 3 selected chlorinated solvents, perchloroethylene (PCE), trichloroethylene (TCE) and 1,1,1-trichloroethane (TCA) provided by 4 different manufacturers are presented. The isotope ratio for all compounds range between −3.5 and +6.0‰ forδ³⁷Cl and from −37.2 to −23.3%. forδ¹³C. The greatest³⁷Cl difference between manufacturers is observed in the TCE samples which showδ³⁷CI values of −2.5%o for PPG, +2.43‰ for ICI and +4.4‰ for DOW. TCAs show a smaller range (−2.4 to +2.0‰), while the TCEs have slightly different³⁷Cl contents. The¹³C data show the most distinctδ¹³C values for PCEs (−23.3 for DOW, −24.1 for Vulcan, −33.8 for PPG and −37.2‰ for ICI) while both TCEs and TCAs show a smallerδ¹³C range, but still distinct differences. These preliminary data suggest that each manufacturer and solvent type may have distinctiveδ⁶³⁷Cl andδ¹³C values. These results show that by using a combination of³⁷Cl and¹³C, there is a potential to indicate a specific source of chlorinated solvents, as well as an ability to delineate contamination episodes caused by these compounds in groundwaters.     

Crassulacean acid metabolism influences D/H ratio of leaf wax in succulent plants

This study sought to characterize hydrogen isotopic fractionation during biosynthesis of leaf wax n-alkanes in succulent plants capable of crassulacean acid metabolism (CAM). The metabolic and physiological features of CAM represent crucial strategies for survival in hot and dry climates and have been hypothesized to impact hydrogen isotope fractionation. We measured the stable carbon and hydrogen isotopic compositions (δ¹³C and δD, respectively) of individual n-alkanes in 20 species of succulent plants from a global collection of the Huntington Botanical Gardens, San Marino, California. Greenhouse conditions and irrigation with water of constant δD value enabled determination of interspecies differences in net D/H fractionation between source water and leaf wax products. Carbon isotope ratios provide constraints on the extent of CAM vs. C₃ photosynthesis and indicate a wide range of CAM use, with δ¹³C values ranging from −33.01‰ to −18.54‰ (C₂₇–C₃₃ n-alkanes) and −26.66‰ to −17.64‰ (bulk tissue). Despite the controlled growth environment, we observed ca. 90‰ interspecies range in δD values from −193‰ to −107‰. A positive correlation between δ¹³C_bulk and δD_C31 values with R² = 0.60 (δ¹³C_C31 and δD_C31 values with R² = 0.41) implicates a metabolic isotope effect as the dominant cause of interspecies variation in the hydrogen isotopic composition of leaf wax n-alkanes in CAM-intermediate plants.     

Geochemical and boron,oxygen and hydrogen isotopic constraints on the origin of salinity in groundwaters from the crystalline basement of the Alpine Foreland

The B isotopic composition, in combination with O and H isotopes and hydrochemical tracers, is utilized to constrain the evolution of basement-hosted groundwaters via water–rock interactions and fluid infiltration from external (sedimentary) reservoirs. Two distinct groundwater types have been identified in the Central European crystalline basement (N Switzerland–SW Germany): (1) fresh groundwaters characterized by low values of δ¹¹B (−3.5 to −0.6‰), δ¹⁸O (−12.0 to −10.0‰), and δD (−86.8 to −71.9‰), and (2) brackish groundwaters with distinctly heavier B, O, and H isotopic compositions (δ¹¹B=+6.4 to +17.6‰, δ¹⁸O=−9.4 to −5.6‰, δD=−67.6 to −60.8‰). Fresh groundwaters show a systematic decrease in δ¹¹B, related to an increase in B concentrations (and degree of total mineralization), along the pathway of groundwater migration which can only be interpreted in terms of leaching of crystalline host rocks. A δ¹¹B value of −3.3‰ is inferred for the crustal B source (mainly Hercynian granites) involved in the leaching process, in agreement with the known δ¹¹B range of granitic rocks. The evolution of brackish groundwaters, derived from crystalline basement reservoirs with little water circulation, is more complex. As indicated by B–O–H stable isotope and hydrochemical (e.g. B/Cl, Na/Cl, and Br/Cl) constraints, brackish groundwaters from the study area are influenced by admixture of sediment-derived fluids which infiltrated from Late Paleozoic (Permo-Carboniferous) and Early Mesozoic (Lower Triassic) sedimentary strata. The data presented show that B isotopes are sensitive to mixing processes of fluids derived from different crustal reservoirs and, hence, may be utilized as a tracer for constraining the internal (autochthonous) vs external (allochthonous) origin of salinity in basement-hosted groundwaters.     

Origin of formation waters of S-E parts of the Bohemian Massif and the Vienna basin

Deep formation waters were sampled from boreholes on the S-E slopes of the Bohemian Massif. They are NaClHCO₃ waters with TDS in the range 6–52 g L⁻¹. Some of them are associated with gas and oil deposits. The waters are rich in Br and I and their δDandδ¹⁸0 isotope compositions vary from −12 to −77‰ and + 4.6 to −10‰ respectively.The processes of concentration and dilution have been discussed based on deuterium and conservative element contents of the waters. Three regional groups can be identified in the plots Br vs I and Cl vs I: the Vienna Basin samples (VB), southern (S) and northern (N) flanks of the Bohemian Massif (BM). The VB samples have as an end member brackish water with about 7 g L⁻¹ Cl (about 40% marine component) enriched in Br and I. This water has been later diluted by meteoric water of recent isotopic composition. Only 3 VB samples can be considered as derived from the dissolution of evaporites. The salt content of the S and N end members is very close to or higher than sea water with an isotopic composition similar to the brackish water. Subaerial evaporation of diluted sea water is suggested as the process increasing salt content. The evaporative enrichment of primary brackish solution can be estimated from extrapolation of Cl vs I and Br vs I plots to zero I (about 25 mg L⁻¹ Br and 6 g L⁻¹ Cl for the southern flanks of the BM). Evaporated solutions were later diluted by meteoric waters with δD in the range from −50 to −80‰ (southern flanks) and about −80‰ (northern flanks).     

Chlorine stable isotope distribution of Michigan Basin formation waters

The Cl isotope ratio, mass³⁷Cl/³⁵Cl, was measured on 22 formation waters from Mississippian, Devonian, Silurian, Ordovician and Cambrian strata in the Michigan Basin. Because of its resistance to fractionation, the ratio was used to find evidence of mixing of formation waters within the Michigan Basin and between the Canadian Shield and the basin. The δ³⁷Cl composition of waters decreased from +0.05 to −0.55 (per mil difference from SMOC, precision of 0.16) with age of the strata among Devonian, Silurian, Ordovician and Cambrian samples from the basin margin. Mississippian samples from the middle of the basin were isotopically heaviest at +0.1. Ordovician samples, also from mid-basin, were isotopically lightest at −1.2. On plots of δ³⁷ Cl vs Cl/Br and δ³⁷ Cl vs⁸⁷Sr/⁸⁶Sr samples at the basin margin trend toward enrichment in³⁵Cl and⁸⁷Sr and increasing Cl/Br suggesting interformational mixing of the waters. On a δ³⁷ ClCl/Br plot, three samples not on this trend and tending toward high Cl/Br may reflect evaporite dissolution. Canadian Shield Waters were plotted with Michigan Basin waters on the graphs of δ³⁷ Cl vs Ca/Cl and δ³⁷ Cl vs K/Cl. On both plots data fall along linear trends of³⁵Cl depletion with Ca/Cl increase and with K/Cl decrease. Ordovician waters from the middle of the basin and shield waters are end members on the plots. The results suggest that despite water-rock interactions, δ³⁷ Cl data may be useful in studies of mixing relations in formation waters.     

Source of 1,2,3,4-tetramethylbenzene in asphaltenes from the Tarim Basin

1-Alkyl-2,3,6-trimethylbenzenes and a high relative amount of 1,2,3,4-tetramethylbenzene (TTMB) have been previously detected in the marine oils and asphaltenes in the oils from the Tarim Basin. In the present study, the stable carbon isotopic compositions of TTMB and n-alkanes in the pyrolysates of asphaltenes in the marine oils from the northern Tarim Basin and Silurian tar sands from the Tarim Basin were determined. TTMB has stable carbon isotopic compositions in the range from −23‰ to −24‰ and are about 12‰ more enriched in ¹³C than concomitant n-alkanes (−35‰ to −37‰) in the pyrolysates. The results indicate a contribution from green sulfur bacteria (Chlorobiaceae) to TTMB. Thus, the depositional environments of the source rocks for the marine oils and the bitumen in tar sands from the Tarim Basin are characterized by periods of euxinic conditions within the photic zone.     

Characterization of the hydrogen isotopic composition of individual n-alkanes in terrestrial source rocks

In order to characterize the H isotopic compositions of individual lipid compounds from different terrestrial depositional environments, the δD values of C-bound H in individual n-alkanes from typical terrestrial source rocks of the Liaohe Basin and the Turpan Basin, China, were measured using gas chromatography–thermal conversion–isotope ratio mass spectrometry (GC–TC–IRMS). The analytical results indicate that the δD values of individual n-alkanes in the extracts of terrestrial source rocks have a large variation, ranging from −140‰ to −250‰, and are obviously lighter than the δD of marine-sourced n-alkanes. Moreover, a trend of depletion in ²H(D) was observed for individual n-alkanes from different terrestrial depositional environments, from saline lacustrine to freshwater paralic lacustrine, and to swamp. For example, the δD values of n-alkanes from a stratified saline lacustrine environment vary from −140‰ to −200‰, δD for n-alkanes from swamp facies range from −200‰ to −250‰, while those from freshwater paralic lacustrine–lacustrine environments fall between the δD values of the end members. The shift toward lighter δD from saltwater to freshwater environments indicates that the source water δD is the major controlling factor for the H isotopic composition of individual compounds. In addition, H exchange between formation water and sedimentary organic matter may possibly be important in regard to the δD of individual n-alkanes. Therefore, other lines of geochemical evidence must be considered when depositional paleoenvironments of source rocks are reconstructed based on the H isotopic composition of individual n-alkanes.     

Carbon and oxygen isotope study of the active water-carbonate system in a karstic Mediterranean cave: Implications for paleoclimate research in semiarid regions

In a semiarid climatic zone, such as the Eastern Mediterranean region, annual rainfall variations and fractionation processes in the epikarst zone exert a profound influence on the isotopic compositions of waters seeping into a cave. Consequently, the isotopic compositions of speleothems depositing from cave waters may show complex variations that need to be understood if they are to be exploited for paleoclimate studies. This is confirmed by a four-year study of the active carbonate-water system in the Soreq cave (Israel). The δ¹⁸O (SMOW) values of cave waters range from −6.3 to −3.5%.. The highest δ¹⁸O values occur at the end of the dry season in waters dripping from stalactites, and reflect evaporation processes in the epikarst zone, whereas the lowest values occur in rapidly dripping (fast-drip) waters at the peak of the rainy seasons. However, even fast-drip waters are about 1.5%. heavier than the rainfall above the cave, which is taken to reflect the mixing of fresh with residual evaporated water in the epikarst zone. δ¹³C (PDB) values of dissolved inorganic carbon (DIC) vary from −15.6 to −5.4%., with fast-drip waters having lower δ¹³C values (mostly −15.6 to −12%.) and higher DIC concentrations relative to pool and stalactite-drip water. The low δ¹³C values of fast-drip waters and their supersaturation with respect to calcium carbonate indicates that the seepage waters have dissolved both soil-CO₂ derived from overlying C₃-type vegetation and marine dolomite host rock.The δ¹⁸O (PDB) values of various types of present-day low-magnesium calcite (LMC) speleothems range from −6.5 to −4.3%. and δ¹³C values from −13 to −5.5%. and are not correlated with speleothem type. An analysis of δ¹⁸O values of present-day calcite rafts and pool waters shows that they form in oxygen isotope equilibrium. Similarly, the measured ranges of δ¹³C and δ¹⁸O values for all types of present-day speleothems are consistent with equilibrium deposition at cave temperatures. The δ¹³C–δ¹⁸O range of contemporary LMC thus reflects the variations in temperatures and isotopic compositions of the presentday cave waters. The 10%. variation in the δ¹³C values in waters can be modeled by a simple Rayleigh calculation of the carbon isotope fractionation accompanying CO₂-degassing and carbonate precipitation. These variations may obscure the differences in the carbon isotopic composition of speleothems that could arise when vegetation cover changes from C₃ to C₄-type plants. This consideration emphasizes that it is necessary to characterize the full range of δ¹³C values associated with contemporaneous speleothems in order to clarify the effects of degassing from those due to differing vegetation types.Isotopic studies of a number of different types of fossil LMC speleothems show many of them to exhibit isotopic trends that are similar to those of present-day LMC, but others show both higher and lower δ¹⁸O ranges. In particular, the higher δ¹⁸O range has been shown by independent age-measurements to be associated with a period of drier conditions. The results of the study thus indicate that it is necessary to work on a well calibrated cave system in semiarid climates and that the fossil speleothem record should be obtained from different types of contemporaneous deposit in order to fully characterize the δ¹⁸O–δ¹³C range representative of any given climatic period.     

Fluid infiltration effects on stable isotope systematics of the Susurluk skarn deposit,NW Turkey

A calcic skarn deposit occurs along the contact zone between Oligo-Miocene Çatalda? Granitoid and Mesozoic limestones in Susurluk, northwestern Turkey. The skarn zone with little or no retrograde stage is represented by fluid inclusions with high homogenization temperatures (up to >600 °C) and a wide range of salinity (12 to >70 wt.% NaCl). Pluton-derived fluids facilitated occurrence of continuous prograde reactions in the country rocks (particularly in the proximal zone) and oxygen isotopic depletion in calc-silicate and calcite minerals. δ¹⁸O of anhydrous minerals within proximal and distal zones indicate that skarn-forming fluids had a magmatic origin. The δ¹⁸O values are 5.93–9.08‰ (mean 6.8‰) for garnet, 4.08–9.94‰ (mean 6.4‰) for pyroxene, 4.89–7.92‰ (mean 6.4‰) for wollastonite and 6.65–8.28‰ (mean 7.5‰) for vesuvianite. Temperatures estimated by isotopic compositions of mineral pairs are significantly lower than those measured from the fluid inclusions, indicating that isotopic equilibrium is not preserved between the skarn minerals. δ¹⁸O and δ¹³C values are systematically depleted from marbles to skarn carbonates. Calc-silicate forming reactions and permeability increase triggered by volatilization and consequent strong infiltration of H₂O-rich siliceous fluids into the system promoted fluid–rock interaction causing isotopic resetting and isotopic depletion of silicates (e.g. pyroxene and wollastonite) and skarn calcites.     

The application of C isotope measurements to the identification of the sources of C in diamonds: a review

In the past decade, the isotopic compositions of C in > 600 inclusion-bearing diamonds have been determined. Such analyses have revealed the following isotopic characteristics: (1) peridotitic diamonds, which typically contain garnet, chromite, olivine and/or orthopyroxene inclusions with refractory compositions (high Mg, Cr), have δ¹³C values predominantly between −10 and −1‰, with a sharp peak in the distribution near −5‰; (2) eclogitic diamonds, which commonly contain inclusions of omphacitic clinopyroxene, Cr-poor pyrope, and/or eclogitic accessory minerals such as rutile, kyanite, coesite or sanidine, have δ¹³C values between −34 and +3‰, with a smaller peak near −5‰; (3) the isotopic compositions obtained for suites of diamonds from individual occurrences are, in general, unique and do not resemble the range and distribution obtained by amalgamating the diamond isotope data from a number of localities; (4) isotopic zoning patterns and heterogeneities are found in some diamonds; cores of coated diamonds tend to be depleted in¹³C relative to the rims, and within single octahedral diamonds δ¹³C variations of nearly 6‰ have been reported.Because expected C isotope fractionations at mantle temperatures are small, attempts to model the full range of diamond isotope values through fractionating a homogeneous mantle C source have been unsuccessful. Nevertheless, fractionation is probably responsible for some of the observed variation in δ¹³C values. Two other models have also been proposed to account for the diamond characteristics outlined above. The “primordial model” suggests that the range and distribution of C isotope compositions are inherited from primordial C in the mantle which has an inhomogeneous isotopic composition, such as that found in meteorites. The “subduction model” suggests that subducted, crustal C is the source of C in diamonds, as organic and inorganic C compounds in the crust exhibit a range of δ¹³C values similar to that observed in diamonds. This paper reviews the C isotope characteristics of diamonds and compares the models which have been proposed to explain the origins of these characteristics.     

Paleoclimatic interpretation of the past 30 ka from isotopic studies of the deep confined aquifer of the North China plain

The δ¹⁸O and δD values in the deep confined aquifer beneath the North China Plain which is located at 112°30′E–119°30′E and 34°46′N–40°25′N, reflect differences in paleoclimatic conditions between the Holocene and the late Pleistocene. Groundwater samples whose ¹⁴C ages are between 12 and 25 ka B.P have ranges of −9.4 to −11.7‰ for δ¹⁸O and −76‰ to −85‰ for δD values. These very negative δ¹⁸O and δD values reflect the cold and arid climate in the last glacial period. The temperature estimated in this period is 6–9 °C cooler than that of the present. The entire ranges of δ¹⁸O and δD values for samples with ¹⁴C dating from 7 ka B.P to present are −7.7‰ to −10.2‰ and −63‰ to −73‰, respectively. The greater δ¹⁸O and δD enrichments of these samples indicate a period of relatively humid and warm climate in the Holocene. However, the wide ranges of δ¹⁸O (−9.0‰ to −11.1‰) and δD (−66‰ to −80‰) values for samples with ¹⁴C age ranging from 12 to 7 ka B.P. imply an unstable climatic condition of rapidly increasing temperature, which marks the transition from the Pleistocene to the Holocene.     

Chlorine stable isotope studies of old groundwater,southwestern Great Artesian Basin,Australia

Stable Cl isotope ratios (³⁷Cl/³⁵Cl) were measured in groundwater samples from the southwestern flow system of the Great Artesian Basin, Australia to gain a better understanding of the Cl⁻ sources and transport mechanisms. δ³⁷Cl values range from 0‰ to −2.5‰ (SMOC), and are inversely correlated with Cl⁻ concentration along the inferred flow direction. The Cl isotopic compositions, in conjunction with other geochemical parameters, suggest that Cl⁻ in groundwaters is not derived from salt dissolution. Mixing of the recharge water with saline groundwater cannot explain the relationship between δ³⁷Cl and Cl⁻ concentration measured. Marine aerosols deposited via rainfall and subsequent evapotranspiration appear to be responsible for the Cl⁻ concentrations observed in wells that are close to the recharge area, and in groundwaters sampled along the southern transect. δ³⁷Cl values measured in the leachate of the Bulldog shale suggest that the aquitard is the subsurface source of Cl⁻ for the majority of groundwater samples studied. Diffusion is likely the mechanism through which Cl⁻ is transported from the pore water of the Bulldog shale to the aquifer. However, a more detailed study of the aquitard rocks is required to verify this hypothesis.     

Determining the origin of carbon dioxide and methane in the gaseous emissions of the San Vittorino plain (Central Italy) by means of stable isotopes and noble gas analysis

The chemistry and isotope ratios of He, C (δ¹³C) and H (δD) of free gases collected in the San Vittorino plain, an intramontane depression of tectonic origin, were determined to shed light on mantle degassing in central Italy. The C isotopic composition of CO₂ (δ¹³C–CO₂ −2.0‰ to −3.8‰) and He isotope ratios (R/R_A 0.12–0.27) were used to calculate the fraction of CO₂ originating from mantle degassing vs. sedimentary sources. The results show that CO₂ predominantly (average of 75%) derives from the thermo-metamorphic reaction of limestone. Between 6% and 22% of the CO₂ in the samples derives from organic-rich sedimentary sources. The mantle source accounts for 0–6% of the total CO₂; however, in two samples, located in proximity to the most important faults of the plain, the mantle accounts for 24% and 42%. The presence of faults and fractures allows upward gas migration from a deep source to the Earth’s surface, not only in the peri-Tyrrhenian sector, as generally reported by studies on natural gas emissions in central Italy, but also in the pre-Apennine and Apennine belts. Isotope ratios of CH₄ (δ¹³C–CH₄ −6.1‰ to −22.7‰; δD–CH₄ −9‰ to −129‰) show that CH₄ does not appear to be related to mantle or magma degassing, but it is the product of thermal degradation of organic matter (i.e. thermogenic origin) and/or the reduction of CO₂ (i.e. geothermal origin). Most of the samples appear to be affected by secondary microbial oxidation processes.     

Carbon and nitrogen pools in Padanian soils (Italy): Origin and dynamics of soil organic matter

Carbon and nitrogen elemental (C-N, wt%) and isotopic (δ¹³C-δ¹⁵N, ‰) investigation has been carried out on alluvial and deltaic soils from the Padanian plain (northern Italy), an area interested by intensive agricultural activities, to refine previous inferences on depositional facies, pedogenetic processes and anthropogenic influences. Soil analysis, carried out by EA-IRMS, have been focused on inorganic and organic fractions properly speciated by a thermally-based method, whereas further insights on the organic matter constituents have been obtained by sequential fractionation. The bulk EA-IRMS analyses reveal a remarkable compositional heterogeneity of the investigated soils (TC 0.89 to 11.93?wt%, TN 0.01 to 0.78?wt%, δ¹³C_TC -1.2 to -28.2‰, δ¹⁵N -1.2 to 10.0‰) that has to be explained as an integration between inorganic and organic pools. The latter have been subdivided in Non-Extractable Organic Matter (NEOM, δ¹³C -16.3 to -28.6‰) and in extractable fractions as Fulvic (FA, δ¹³C -24.7 to -27.5‰, δ¹⁵N 0.6 to 5.7‰) and Humic (HA, δ¹³C -24.6 to -27.0‰, δ¹⁵N 1.0 to 9.7‰) Acids, which have been used to infer soil dynamics and Soil Organic Matter (SOM) stability processes. Results indicate that SOM at depth of 100?cm was generally affected by microbial reworking, with the exception of clayey and peaty deposits in which biological activity seems inhibited. Peaty and clayey soils display an organic fraction loss of ca. 20% toward the surface, suggesting deterioration possibly induced by intensive agricultural activities. These latter may be the cause of the ubiquitous losses of organic fraction throughout the investigated area over the last seventy years, evaluated by the comparison with historical data on corresponding topsoils. The obtained insights are very important because these soils are carbon (and nitrogen) sinks that are vulnerable and can be degraded, loosing agricultural productivity and potentially contributing to greenhouse gases fluxes.     

Carbon isotope fractionation by the marine ammonia-oxidizing archaeon Nitrosopumilus maritimus

Ammonia-oxidizing archaea (AOA) are abundant and widely distributed microorganisms in aquatic and terrestrial habitats. By catalyzing the first and rate limiting step in nitrification, these chemolithoautotrophs play a significant role in the global nitrogen cycle and contribute to primary production. Here, the carbon isotopic fractionation relative to inorganic carbon source was determined for bulk biomass, biphytanes and polar lipid bound sugars of a marine AOA pure culture. Bulk biomass and biphytanes from Nitrosopumilus maritimus showed identical carbon isotope fractionation (ε_DIC/bulk and ε_{DIC/byphytanes}) of ca. −20‰. The glycoside head groups were mainly glucose, mannose and inositol, and exhibited different carbon isotopic composition. In general, these monosaccharides were enriched in ¹³C (ε −6.1‰ to −13.8‰) relative to bulk biomass and biphytanes. The fact that the carbon isotope composition of the biphytanes reflected that of the bulk biomass of N. maritimus suggests that the depletion of ¹³C in both biomass and biphytanes resulted mainly from the carbon isotope discrimination by the bicarbonate-fixing enzyme in the autotrophic hydroxypropionate/hydroxybutyrate cycle. Our results further revealed that lipid compounds represent suitable biomarkers for determining δ¹³C values of archaeal ammonia oxidizers without biosynthetic correction.     

2H/1H ratio of hopanes,tricyclic and tetracyclic terpanes in oils and source rocks from the Potiguar Basin,Brazil

We report the hydrogen isotope ratio ²H/¹H (expressed as δ²H values) of 8 selected hopanes, tricyclic and tetracyclic terpanes from oils and source rocks in the Potiguar Basin, and of associated formation water. Hopanes ranged in δ²H value from −79‰ to −142‰, whereas tri- and tetracyclic terpanes (TTTs) ranged from −137‰ to −225‰. Formation water δ²H values ranged from −23‰ to −32‰. The most significant pattern in the data is the systematic ²H enrichment of hopanes relative to TTTs, by an average of 45‰ in oils and 78‰ in source rock extracts. The hopanes appear close to hydrogen isotopic equilibrium with coeval formation water, whereas TTTs are significantly more ²H depleted. Given the similarities in structure between the two compound classes, it is unlikely that hopanes would be exchanged completely while the others would not. More likely, both classes have undergone a limited extent of exchange, but with the hopanes being biosynthesized with δ²H values closer to equilibrium. Our data suggest that at least some primary environmental and/or biotic information can be retained in the δ²H values of biomarkers in oils and extracts, and is not completely obscured by hydrogen exchange.     

An integrated thermal and isotopic study of the diagenesis of the Brent group,Alwyn South,U.K. North Sea

The petrography, fluid inclusion thermometry and isotope geochemistry of diagenetic cements are used to reconstruct the pore-fluid history of the Middle Jurassic Brent Group reservoir sandstones in the Alwyn South area of the U.K. North Sea. The study focuses on a relatively limited area of three adjacent reservoir compartments at successively higher structural levels. The cement assemblage of kaolinite, quartz and illite has resulted in severe deterioration of otherwise good reservoir quality. Early precipitation of vermiform and late blocky kaolinite was succeeded by a period of relatively intense illite precipitation. Temperature estimates for kaolinite precipitation of 80°C andδ¹⁸O of ≈ + 15‰ (±3‰) suggest co-existing fluids ofδ¹⁸O ≈ −3‰. Quartz cementation overlapped both kaolinite and illite formation. Fluid inclusion data indicate that quartz cementation took place at temperatures of 109±7°C. Pore fluid salinities were ≈4 wt% NaCl with a H₂OO isotopic composition of ≈ -1 %o ± 0.5‰ SMOW. The fluids which precipitated coexisting illite were compositionally homogeneous with equilibriumδ¹⁸O water compositions of +0.5‰ SMOW. Illite SD values range from −33 to −50‰ SMOW. These fluid inclusion and isotopic data suggest that both quartz and illite were precipitated from pore waters with a uniform, marine signature. This is consistent with the predominantly marine to paralic depositional context of the Brent Group in Alywn South. Illite precipitation was followed by hydrocarbon emplacement between the Middle Eocene and Lower Oligocene.     

A secondary isotopic reference material of chlorine from selected seawater

A secondary isotopic reference material of chlorine as NaCl collected from a sea water, located at 4°18′N, 161°08′E and collected on October 22, 1994, was prepared using ion exchange technique. The δ³⁷Cl of an NaCl product named ISL 354 was measured to be −0.39±0.05‰ (2σ) related to NIST SRM 975, an isotopic standard reference material with an absolute ³⁷Cl/³⁵Cl ratio of 0.31977. No contamination or isotopic fractionation was observed due to a complete recovery of chlorine and the avoidance of chemical reagents during preparation of ISL 354 from selected seawater. The isotopic composition of chlorine in ISL 354 does not change on long-term storage. So ISL 354 can be used as a secondary isotopic reference material of chlorine.     

Nanhuan manganese deposits within restricted basins of the southeastern Yangtze Platform,China: Constraints from geological and geochemical evidence

The Nanhuan manganese deposits in the southeastern Yangtze Platform occur in the black shale series in the lower part of the Datangpo Formation. In order to constrain the genesis of the deposits, a detailed study was undertaken that involved field observations, major and trace element analyses, organic carbon analyses, and isotope analyses (C, O, S). The major findings are as follows. (1) The ore-bearing rock series, morphology of the ore bodies, and characteristics of ores in several deposits are similar. The ore minerals are rhodochrosite and manganocalcite. The gangue minerals are mainly quartz, feldspar, dolomite, and illite. Minor apatite and bastnaesite occur in the manganese ores. (2) The ores are enriched in Ca and Mg, whereas they are depleted in Si, Al, K, and Ti compared to wall rocks. The ores normalized to average Post-Archean Australian shale (PAAS) are enriched in Co, Mo, and Sr. The chondrite-normalized rare earth element (REE) patterns for ores and wall rocks are between those of typical hydrogenous and hydrothermal type manganese deposits. Additionally, the ores have positive Ce anomalies with an average Ce/Ce* of 1.23 and positive Eu anomalies with an average Eu/Eu* of 1.18 (normalized to PAAS). (3) The average content of organic carbon is 2.21% in the samples, and the average organic carbon isotopic value (δ¹³C_V-PDB) is − 33.44‰. The average inorganic carbon isotopic value (δ¹³C_V-PDB) of carbonates in Gucheng is − 3.07‰, while the values are similar in the other deposits with an average of − 8.36‰. The oxygen isotopic compositions (δ¹⁸O_V-PDB) are similar in different deposits with an average of − 7.72‰. (4) The sulfur isotopic values (δ³⁴S_V-CDT) of pyrite are very high and range from + 37.9‰ to + 62.6‰ (average of 52.7‰), which suggests that the pyrite was formed in restricted basins where sulfate replenishment was limited. The sulfate concentrations in the restricted basins were extremely low and enriched in δ³⁴S, which resulted in the very high δ³⁴S values for the pyrite that formed in the manganese deposits. Therefore, a terrigenous weathering origin for manganese can be excluded; otherwise, the sulfate would have been introduced into the basins together with terrigenous manganese, which would have decreased the δ³⁴S values of pyrites. The manganese, which originated from hydrothermal processes, was enriched in the restricted and anoxic basins, and then, it was oxidized to manganese oxyhydroxide in the overlying oxic waters whereby the products precipitated into the sediments. The manganese oxyhydroxide in the sediment was then reduced to Mn^2 + and released to the pore waters during the process of diagenesis. Some organic carbon was oxidized to CO₃^2 −, which made the depletion of ¹³C in manganese carbonates. Therefore, we suggest that the Nanhuan manganese deposits are hydrothermal–sedimentary/diagenetic type deposits.     

Stable isotopic signatures of the modern land snail Eremina desertorum from a low-latitude (hot) dry desert—A study from the Petrified Forest,New Cairo,Egypt

This study was conducted on recent desert samples—including (1) soils, (2) plants, (3) the shell, and (4) organic matter from modern specimens of the land snail Eremina desertorum—which were collected at several altitudes (316–360 m above sea level) from a site in the New Cairo Petrified Forest. The soils and shell_{E. desertorum} were analyzed for carbonate composition and isotopic composition (δ¹⁸O, δ¹³C). The plants and organic matter_{E. desertorum} were analyzed for organic carbon content and δ¹³C. The soil carbonate, consisting of calcite plus minor dolomite, has δ¹⁸O values from −3.19 to −1.78‰ and δ¹³C values −1.79 to −0.27‰; covariance between the two values accords with arid climatic conditions. The local plants include C3 and C4 types, with the latter being dominant. Each type has distinctive bulk organic carbon δ¹³C values: −26.51 to −25.36‰ for C3-type, and −13.74 to −12.43‰ for C4-type plants.The carbonate of the shell_{E. desertorum} is composed of aragonite plus minor calcite, with relatively homogenous isotopic compositions (δ¹⁸O_mean = −0.28 ± 0.22‰; δ¹³C_mean = −4.46 ± 0.58‰). Most of the δ¹⁸O values (based on a model for oxygen isotope fractionation in an aragonite-water system) are consistent with evaporated water signatures. The organic matter_{E. desertorum} varies only slightly in bulk organic carbon δ¹³C values (−21.78 ± 1.20‰) and these values suggest that the snail consumed more of C3-type than C4-type plants. The overall offset in δ¹³C values (−17.32‰) observed between shell_{E. desertorum} carbonate and organic matter_{E. desertorum} exceeds the value expected for vegetation input, and implies that 30% of carbon in the shell_{E. desertorum} carbonate comes from the consumption of limestone material.