Carbon isotopic fractionation between CO2 vapour,silicate and carbonate melts: an experimental study to 30 kbar

The carbon isotopic fractionation between CO₂ vapour and sodamelilite (NaCaAlSi₂O₇) melt over a range of pressures and temperatures has been investigated using solid-media piston-cylinder high pressure apparatus. Ag₂C₂O₄ was the source of CO₂ and experimental oxygen fugacity was buffered at hematite-magnetite by the double capsule technique. The abundance and isotopic composition of carbon dissolved in sodamelilite (SM) glass were determined by stepped heating and the ¹³C of coexisting vapour was determined directly by capsule piercing. CO₂ solubility in SM displays a complex behavior with temperature. At pressures up to 10 kbars CO₂ dissolves in SM to form carbonate ion complexes and the solubility data suggest slight negative temperature dependence. Above 20 kbars CO₂ reacts with SM to form immiscible Na-rich silicate and Ca-rich carbonate melts and CO₂ solubility in Na-enriched silicate melt rises with increasing temperature above the liquidus. Measured values for carbon isotopic fractionation between CO₂ vapour and carbonate ions dissoived in sodamelilite melt at 1200°–1400° C and 5–30 kbars average 2.4±0.2, favouring¹³C enrichment in CO₂ vapour. The results are maxima and are independent of pressure and temperature. Similar values of 2 are obtained for the carbon isotopic fractionation between CO₂ vapour and carbonate melts at 1300°–1400° C and 20–30 kbars.     

Carbon-oxygen isotopic covariation in hydrothermal calcite during degassing of CO2

The effect of the outgassing of CO₂ from a hydrothermal fluid on the C- and O-isotopic compositions of calcite, which is precipitated from this fluid, is quantitatively modelled in terms of batch and Rayleigh distillation equations. Both CO₂ degassing and calcite precipitation are considered to be the removal mechanisms for dissolved carbon species from the fluid. Combined degassing-precipitation models are then developed by taking H₂CO₃ and HCO ₃ ^– , respectively, as the dominant dissolved carbon species. A positive correlation array between ¹³C and ¹³O values of calcite can be yielded by the precipitation of calcite from a H₂CO ₃ ^– -dominant fluid, accompanied by a progressive decrease in temperature during CO₂ degassing, whereas calcite precipitated from a HCO ₃ ^– -dominant fluid under the same conditions tends to display much smaller variation in ¹³C values than in ¹⁸O values. The combined processes of CO₂ degassing and calcite precipitation result in lowering the ¹³C value of calcites with respect to those precipitated in a closed system simply due to temperature effect. Carbon and oxygen isotopic data for calcite from the Kushikino gold-mining area in Japan illustrate the application of quantitative modelling, and degassing of CO₂ is suggested as a more likely cause for the precipitation of the calcite and quartz in this mining area.     

Carbon biogeochemistry of the Betsiboka estuary (north-western Madagascar)

Madagascar’s largest estuary (Betsiboka) was sampled along the salinity gradient during the dry season to document the distribution and sources of particulate and dissolved organic carbon (POC, DOC) as well as dissolved inorganic carbon (DIC). The Betsiboka was characterized by a relatively high suspended matter load, and in line with this, low DOC/POC ratios (0.4–2.5). The partial pressure of CO₂ (pCO₂) was generally above atmospheric equilibrium (270–1530 ppm), but relatively low in comparison to other tropical and subtropical estuaries, resulting in low average CO₂ emission to the atmosphere (9.1 ± 14.2 mmol m⁻² d⁻¹). Despite the fact that C4 vegetation is reported to cover >80% of the catchment area, stable isotope data on DOC and POC suggest that C4 derived material comprises only 30% of both pools in the freshwater zone, increasing to 60–70% and 50–60%, respectively, in the oligohaline zone due to additional lateral inputs. Sediments from intertidal mangroves in the estuary showed low organic carbon concentrations (<1%) and δ¹³C values (average −19.8‰) consistent with important inputs of riverine imported C4 material. This contribution was reflected in δ¹³C signatures of bacterial phospholipid derived fatty acids (i + a15:0), suggesting the potential importance of terrestrial organic matter sources for mineralization and secondary production in coastal ecosystems.     

Solubility of CO2 in a Ca-rich leucitite: effects of pressure,temperature, and oxygen fugacity

The solubility of carbon dioxide in a Ca-rich leucitite has been investigated as a function of pressure (0.1–2.0 GPa), temperature (1200–1600°C), and oxygen fugacity. The experiments were done in a rapid-quench internally-heated pressure vessel (0.1 GPa) and a piston cylinder (0.5–2.0 GPa). The leucitite glass, previously equilibrated at NNO, and silver oxalate were loaded in Fe-doped Pt capsules (oxidized conditions) and graphitelined Pt capsules (reduced conditions). Secondary Ion Mass Spectrometry and bulk carbon analyses were used to determine the amount of dissolved carbon. Speciation of carbon was characterized by Fourier transform microinfrared spectroscopy. At oxidized conditions, only CO₃ ^2- is observed as a dissolved species. The solubility is high with CO₂ contents in the melt attaining 6.2 wt% at 2.0 GPa and 1350°C. The solubility increases with pressure and shows a significant negative temperature dependence. An excellent correlation is obtained when the data are fit to a model, based on the simplified solubility reaction CO₂ ^(vapor)+O^2-(melt)CO₃ ^2-(melt), which describes the solubility of CO₂ as a function of pressure, temperature and fCO₂. At reduced conditions, the amount of carbon dissolved is significantly lower, and CO₃ ^2- is still the only species present in the melt. If the solubility model established at oxidized conditions is applied, the carbon dissolved appears to be essentially a function of fCO₂ alone although divergence increases in a consistent manner with pressure and temperature. This could suggest a low but significant solubility of CO with a positive temperature dependence or a departure of the calculated fluid compositions determined by the equation of state from the actual ones. The strong preferential solubility of carbon in its oxidized C⁴⁺ form, even at reduced conditions, implies that ascending melts with high CO₂ solubility can experience significant oxidation through degassing. This could reconcile the oxidized nature of some Ca-rich alkaline magmas with more reduced mantle source regions.     

Stable Isotope Fractionation during Experimental Formation of Norsethite (BaMg[CO3]2): A Mineral Analogue of Dolomite

Stable oxygen and carbon isotopefractionation during the experimental formation ofordered norsethite (BaMg[CO₃]₂) from thereaction of anhydrous BaCO₃ (witherite) withrelatively low concentrated sodium-magnesiumbicarbonate solutions has been studied between20° and 135 °C. In the investigatedtemperature range, ¹⁸O and ¹³C are enrichedin norsethite with respect to water and gaseous carbondioxide, respectively. Whereas ¹⁸O/¹⁶Opartitioning is intermediate between those of theBaCO₃–H₂O and MgCO₃–H₂O systems,¹³C/¹²C partitioning is more similar to thatfor BaCO₃–CO₂. Between 20° and90°C, the temperature dependences of the¹⁸O/¹⁶O and ¹³C/¹²C fractionationfactors are represented by the equations (T in °K):10³ ln _BaMg[CO₃]₂-H_2O = 2.83 10⁶T^--2.85, and 10³ln_BaMg[CO₃]₂-CO₂(gas) = 1.78 10⁶T^--10.16. The later equation considers carbon isotope fractionationbetween the dissolved carbonate ion and carbon dioxide measured by Halaset al. (1997). Under standard state conditions (25 °C) the fractionation factors in the system BaMg[CO₃]₂-CO₂-H₂O are: Oxygen isotopes: _BaMg(CO₃)₂-H_2O = 1.02941, _BaMg(CO₃)₂-OH-(aq) = 1.07059,_BaMg(CO₃)₂-CO₂(gas) = 0.98868, and_BaMg(CO₃)₂-H₂CO₃ ^* = 0.98843; carbon isotopes:_BaMg(CO₃)₂-CO₂(gas) = 1.00992,_BaMg(CO₃)₂-H₂CO₃ ^* = 1.01099,_BaMg(CO₃)₂-HCO₃ ^- = 1.00194,_BaMg(CO₃)₂-CO₃ ^2- = 1.00491 or 1.00150.The spontaneous precipitation of aBaMg[CO₃]₂ gel at 20 °C,followed by the alteration of the products at20° or 60°C for 31 days,demonstrated isotope exchange reactions betweensolids and mother solutions dueto recrystallization. Isotope equilibrium, wasnot reached within run time.     

Low-density CO2-rich fluid inclusions from charnockites of southwestern Madurai Granulite Block, southern India; implications on graphite mineralization

Characterization of fluid inclusions in graphite-bearing charnockites from the southwestern part of the Madurai Granulite Block in southern India reveals a probable relation with the formation and break down of graphite during the high-grade metamorphism. The first-generation monophase pure CO₂ inclusions, the composition of which is confirmed by laser Raman spectroscopy, recorded moderate density (0.77–0.87 g/cc) corresponding to low tapping pressure (around 2 kb) than that of the peak granulite-facies metamorphism. The precipitation of graphite, as inferred from graphite inclusions and δ¹³C values of the graphite from the outcrops, is interpreted as the cause of this lowering of fluid density. An intermediate generation of pseudosecondary inclusions resulted from the re-equilibration or modification of the first-generation fluids and the CO₂ formed is interpreted to be the oxidation product from graphite. The youngest generation of fluids which caused widespread retrogression of the granulites is a low-temperature (350 °C) high-saline (32.4–52.0 wt% NaCl equivalent) brine. Carbon isotope data on the graphite from the charnockites show δ¹³C values ranging from −11.3 to −19.9‰, suggesting a possibility of mixing of carbon sources, relating to earlier biogenic and later CO₂ fluid influx. Combining the information gathered from petrologic, fluid inclusion and carbon stable isotope data, we model the fluid evolution in the massive charnockites of the southwestern Madurai Granulite Block.     

New density measurements on carbonate liquids and the partial molar volume of the CaCO<Subscript>3</Subscript> component

Density measurements on nine liquids in the CaCO₃–Li₂CO₃–Na₂CO₃–K₂CO₃ quaternary system were performed at 1 bar between 555 and 969 °C using the double-bob Archimedean method. Our density data on the end-member alkali carbonate liquids are in excellent agreement with the NIST standards compiled by Janz (1992). The results were fitted to a volume equation that is linear in composition and temperature; this model recovers the measured volumes within experimental error (±0.18% on average, with a maximum residual of ±0.50%). Our results indicate that the density of the CaCO₃ component in natrocarbonate liquids is 2.502 (±0.014) g/cm³ at 800 °C and 1 bar, which is within the range of silicate melts; its coefficient of thermal expansion is 1.8 (±0.5)×10^–4 K^–1 at 800 °C. Although the volumes of carbonate liquids mix linearly with respect to carbonate components, they do not mix linearly with silicate liquids. Our data are used with those in the literature to estimate the value of in alkaline silicate magmas (20 cm³/mol at 1400 °C and 20 kbar), where CO₂ is dissolved as carbonate in close association with Ca. Our volume measurements are combined with sound speed data in the literature to derive the compressibility of the end-member liquids Li₂CO₃, Na₂CO₃, and K₂CO₃. These results are combined with calorimetric data to calculate the fusion curves for Li₂CO₃, Na₂CO₃, and K₂CO₃ to 5 kbar; the calculations are in excellent agreement with experimental determinations of the respective melting reactions.Editorial responsibility: I Carmichael     

High integrated fluid/rock ratios during metamorphism at Naxos: evidence from carbon isotopes of calcite in schists and fluid inclusions

Calcite in schists of the metamorphic complex at Naxos is depleted both in ¹³C and in ¹⁸O with respect to massive marbles. This effect is attributed to isotope exchange with circulating CO₂-rich fluids, which had an >0.5 according to fluid inclusions. The carbon isotopic composition of the calcites is close to equilibrium with fluid inclusion CO₂ at metamorphic temperatures. Mass balance calculations assuming initial ¹³C values of 0 for calcite and –5 for the fluid, give integrated fluid/rock volume ratios between 0.1 and 2.0. Such high fluid/rock ratios are supported by observations on the distribution of CO₂/H₂O ratios of fluid inclusions, carbon isotopic compositions of fluid inclusion CO₂ and oxygen isotope systematics of silicates.     

Mobility of metamorphic fluids inferred from infrared microspectroscopy on natural fluid inclusions: the example of Tinos Island,Greece

OH structure of metamorphic fluids has been studied by high temperature infrared (IR) microspectroscopy on natural fluid inclusions contained in quartz veins, over the temperature range 25–370 °C. Blueschist-facies veins from Tinos island core complex (Cyclades, Greece) display H₂O–NaCl–CaCl₂–CO₂ inclusions whereas greenschist-facies veins contain H₂O–NaCl ± CO₂ inclusions. From 25 to 370 °C, peak positions of OH stretching IR absorption bands increase quasi-linearly with slopes of 0.25 and 0.50 cm^–1 °C^–1 for inclusions trapped under blueschist and greenschist conditions, respectively. Extrapolation to 400 °C yield peak positions of 3,475 cm^–1 for blueschist inclusions and 3,585 cm^–1 for greenschist inclusions. Because the smaller wave number indicates the shorter hydrogen-bond distance between water molecules, fluids involved in the greenschist event have a loose structure compared with blueschist fluids. We suggest that these properties might correspond to a low wetting angle of fluids. This would explain the high mobility of aqueous fluids suggested by structural observation and stable isotope analysis.Editorial responsibility: J. Hoefs     

Stable isotope and fluid inclusion studies of gem-bearing granitic pegmatite-aplite dikes,San Diego Co., California

Late Cretaceous, granitic pegmatite-aplite dikes in southern California have been known for gem-quality minerals and as a commercial source of lithium. Minerals, whole-rock samples, and inclusion fluids from nine of these dikes and from associated wall rocks have been analyzed for their oxygen, hydrogen, and carbon isotope compositions to ascertain the origins and thermal histories of the dikes. Oxygen isotope geothermometry used in combination with thermometric data from primary fluid inclusions enabled the determination of the pressure regime during crystallization.Two groups of dikes are evident from their oxygen isotope compositions (¹⁸O_qtz+10.5 in Group A, and +8.5 in Group B). Prior to the end of crystallization, Group A pegmatites had already extensively exchanged oxygen with their wall rocks, while Group B dikes may represent a closer approximation to the original isotopic composition of the pegmatite melts. Oxygen isotope fractionations between minerals are similar in all dikes and indicate that the pegmatites were emplaced at temperatures of about 730 ° to 700 ° C. Supersolidus crystallization began with the basal aplite zone and ended with formation of quench aplite in the pocket zone, nearly to 565 ° C. Subsolidus formation of gem-bearing pockets took place over a relatively narrow temperature range of about 40 ° C (approximately 565–525 ° C). Nearly closed-system crystallization is indicated.Hornblende in gabbroic and noritic wall rocks (D_w.r. = –90 to –130) in the Mesa Grande district crystallized in the presence of, or exchanged hydrogen with, meteoric water (D –90) prior to the emplacement of the pegmatite dikes. Magmatic water was subsequently added to the wall rocks adjacent to the pegmatites.Groups A and B pegmatites cannot be distinguished on the basis of their hydrogen isotope compositions. A decrease in D of muscovite inward from the walls of the dikes reflects a decrease in temperature. D values of H₂O from fluid inclusions are: –50 to –73 (aplite and pegmatite zones); –62 to –75 (pocket quartz: Tourmaline Queen and Stewart dikes); and –50 ± 4 (pocket quartz from many dikes). The average ¹³C of juvenile CO₂ in fluid inclusions in Group B pegmatites is –7.9. In Group A pegmatities, ¹³C of CO₂ is more negative (–10 to –15.6), due to exchange of C with wall rocks and/or loss of ¹³C-enriched CO₂ to an exsolving vapor phase.Pressures during crystallization of the pockets were on the order of 2,100 bars, and may have increased slightly during pocket growth. A depth of formation of at least 6.8 km (sp. gr. of over burden = 3.0, and P _fiuid=P _load) is indicated, and a rate of uplift of 0.07 cm/yr. follows from available geochronologic data.     

Stable carbon-isotopic compositions of lipids isolated from the ammonia-oxidizing chemoautotroph Nitrosomonas europaea

For the ammonia-oxidizing bacterium Nitrosomonas europaea, grown autotrophically using semicontinuous culturing, average biomass was depleted in ¹³C relative to CO₂ dissolved in the medium by ca. 20‰ and the total-lipid extract was depleted in ¹³C relative to biomass by 3.7‰. The n-alkyl lipids (weighted average of fatty acids) and isoprenoid lipids (weighted average of hopanoids) were both depleted in ¹³C relative to biomass by about 9‰. The large depletion in the isoprenoid lipids seems to indicate that isotopic fractionations associated with the biosynthesis of methylerythritol phosphate (MEP) affected at least two carbon positions in each isoprene unit. Among the fatty acids, trans-9-hexadecenoic acid was most depleted (13.0‰ relative to biomass), followed by cis-9- hexadecenoic acid (9.6‰) and hexadecanoic acid (6.9‰). Isotopic relationships between the three acids suggest that significant isotope effects were associated with the desaturation and cis to trans isomerization of fatty acids. Given these observations, hopanoids produced by ammonia-oxidizing bacteria growing in natural waters are likely to be depleted in ¹³C by 26–30‰ relative to dissolved CO₂. Since CO₂ at aquatic oxyclines is often depleted in ¹³C, the range of δ values expected for hopanoids is ca. −34‰ to −55‰. The δ values of geohopanoids observed in numerous studies and attributed to unspecified chemoautotrophs fall within this range.     

Kohlenstoff- und Sauerstoff-Isotopenuntersuchungen an Karbonatkonkretionen und umgebendem Gestein

The composition of the carbon and oxygen isotopes has been determined in about 40 carbonate concretions and surrounding clays and shales of different geological ages. Two different areas and stratigraphic levels in Northwestern Germany have been sampled: 1. concretions in shales of Lower Cretaceous age fromt he area between Hildesheim and Hannover; 2. concretions in shales of Devonian age from the Harz mountains (and the foreland).While the concretions of Group 1 generally are enriched in the light isotope ¹²C (¹³C values from –3.3 to –43.2 relative to PDB), compared to the surrounding shales (0.9 to –5.3), no significant differences could be observed between concretions and shales of Group 2 (concretions: 2.0 to –7.0; shales: –0.3 to –6.2).The average ¹⁸O/¹⁶O ratios of the Devonian samples are lower than those from the Cretaceous, because the probability of an exchange with light meteoric water in diagenetic reactions increases with geologic age.Formed under special conditions of the microenvironment, such as the presence of organic material and local alkalinity during the early stages of diagenesis, the carbon isotopic composition of concretions will probably have preserved some characteristic properties of this mioroenvironment.It is assumed that concretions with the heavy carbon contain carbon from CO₂ which was in isotope equilibrium with CH₄, both of them liberated during the decay of organic material. The light carbon from concretions of Group 1 is explained as fixed CO₂, originating from microbiological or inorganic oxidation of organic substances, which was not in isotope equilibrium with methane (if this was present at all).After precipitation of the concretionary carbonates, no significant carbon isotope exchange seems to have occurred, otherwise the pattern of a heterogeneous carbon isotope composition found in several concretions could not be explained.Strontium concentrations (see Appendix) range from those of primary calcite precipitated in sea water to diagenetic carbonates formed from solutions with a high Ca/Sr ratio. They indicate that during the formation of concretions in abundant cases the system was closed to ocean water.     

Origin of methane in high-arsenic groundwater of Taiwan – Evidence from stable isotope analyses and radiocarbon dating

Groundwaters in the confined aquifers of the Chianan and Ilan coastal plains of Taiwan are rich in dissolved methane (CH₄). Serious endemic “blackfoot disease”, which occurred in the Chianan plain, especially during AD1950-1970, has been demonstrated to have arisen from drinking highly reducing groundwater with abnormal arsenic and humic substance levels. In order to explore the origin of CH₄ and its hydrological implications, stable carbon isotope ratios (δ¹³C) and radiocarbon (¹⁴C) ages of exsolved CH₄, dissolved inorganic carbon (DIC), and sedimentary biogenic sediments from a total of 34 newly completed water wells at 16 sites were determined. The main results obtained are as follows: (1) The δ¹³C_CH4 (−65‰ to −75‰) values indicate that, except for one thermogenic sample (δ¹³C_CH4=38.2‰) from the Ilan plain, all CH₄ samples analyzed were produced via microbially mediated CO₂ reduction. Many δ¹³C_DIC values are considerably greater than −10‰ and even up to 10‰ due to Rayleigh enrichment during CO₂ reduction. (2) Almost all the ¹⁴C ages of CH₄ samples from the shallow aquifer (I) (<60 m depth) are greater than the ¹⁴C ages of coexisting DIC and sediments, suggesting the presence of CH₄ from underlying aquifers. (3) The ¹⁴C ages of coexisting CH₄, DIC and sediments from aquifer (II) of the Chianan plain are essentially equal, reflecting in-situ generation of CH₄ and DIC from decomposition of sedimentary organic matter and sluggishness of the groundwater flow. On the other hand, both CH₄ and DIC from each individual well of the relatively deep aquifers (III) and (IV) in the Chianan plain are remarkably younger than the deposition of their coexisting sediments, indicating that current groundwaters entered these two aquifers much later than the deposition of aquifer sediments. (4) Each CH₄ sample collected from the Ilan plain is older than coexisting DIC, which in turn is distinctly older than the deposition of respective aquifer sediments, demonstrating the presence of much older CO₂ and CH₄ from underlying strata.     

Textural and isotopic variations in graphite from plutonic rocks,South-Central New Hampshire

Graphite occurs in two distinct textural varieties in syntectonic granitoids of the New Hampshire Plutonic Series and in associated metasedimentary wall rocks. Textural characteristics indicate that coarse graphite flakes were present at an early stage of crystallization of the igneous rocks and thus may represent xenocrystic material assimilated from the wall rocks. The range of ¹³C values determined for flake graphite in the igneous rocks (–26.5 to –13.8) overlaps the range for flake graphite in the wall rocks (–26.0 to –16.7), and spatial correlation of some ¹³C values in the plutons and wall rocks supports the assimilation mechanism. The textures of fine-grained irregular aggregates or spherulites of graphite, on the other hand, indicate that they formed along with secondary hydrous silicates and carbonates during retrograde reactions between the primary silicates and a carbon-bearing aqueous fluid phase. Relative to coexisting flake graphite, spherulitic graphite shows isotopic shifts ranging from 1.9 higher to 1.4 lower in both igneous and metasedimentary samples.The observed isotopic shifts and the association of spherulitic graphite with hydrous silicates are explained by dehydration of C-O-H fluids initially on or near the graphite saturation boundary. Hydration of silicates causes dehydration of the fluid and drives the fluid composition to the graphite saturation surface. Continued dehydration of the fluid then requires coprecipitation of secondary graphite and hydrous silicates and drives the fluid toward either higher or lower CO₂/CH₄ depending upon the inital bulk composition. Isotopic shifts in graphite formed at successive reaction stages are explained by fractionation of ¹³C between secondary graphite and the evolving fluid because ¹³C is preferentially concentrated into CO₂ relative to CH₄.Epigenetic graphite in two vein deposits assiciated with the contacts of these igneous rocks is generally enriched in ¹³C (–15.7 to –11.6) relative to both the igneous and wall-rock ¹³C values. Values of ¹³C vary by up to 3.4 within veins, with samples taken only 3 cm apart differing by 2.0 These variations in ¹³C correlate with textural evidence showing sequential deposition of different generations of graphite in the veins from fluids which differed in proportions of carbon species or isotopic composition (or both).     

Stable Carbon and Nitrogen Isotopic

The perennially ice-covered lakes in the McMurdo Dry Valleys, Antarctica, are part of the coldest and driest ecosystem on earth. To understand lacustrine carbon and nitrogen cycling in this end-member ecosystem, and to define paleolimnological proxies for ice-covered lakes, we measured the stable carbon and nitrogen isotopic composition of particulate organic matter (POM) and benthic organic matter (BOM) within the lakes of Taylor Valley. The ¹³C compositions of seasonally ice-free edges of the lakes (moats) are enriched relative to under-ice organic matter. Thus, the organic carbon isotopic composition of buried sediments may be a proxy for sample position within the lake. In the moats, ¹³C values are governed by limited CO₂diffusion across benthic cyanobacterial cell membranes. During a high glacial melt (2001–2002) season, both ¹³C_POM and ¹³C_BOM in the moats were more depleted than during previous low melt years. We propose that this occurred in response to higher [CO₂]_(aq) and/or reduced growth rates resulting from turbidity-induced light limitation. Though moats and under-ice environments are usually poorly connected, during the 2001–2002 season, the enrichment of the ¹³C_POM values at 6 m depth in the stream-proximal sites relative to deep-profile sites implies enhanced connectivity between these environments. The ¹³C compositions of BOM and POM profiles in Lake Hoare and Lake Fryxell indicate that these lakes are dominated by benthic productivity. In contrast, in Lake Bonney, the similarity of the ¹³C values of BOM and POM indicates the pelagic component dominance in the carbon cycle.     

Sources and transport of carbon and nitrogen in the River Sava watershed, a major tributary of the River Danube

Carbon and nitrogen dynamics were examined throughout the River Sava watershed, a major tributary of the River Danube, in 2005 and 2006. The River Sava exported 2.1 × 10¹¹ mol C/yr as dissolved inorganic carbon (DIC), and emitted 2.5 × 10¹⁰ mol C/yr as CO₂ to the atmosphere. Stable carbon isotope ratios indicate that up to 42% of DIC originated from carbonate weathering and 23% from degradation of organic matter. Loads of dissolved and particulate organic carbon increased with discharge and export rates were calculated to be 2.1 × 10¹⁰ mol C/yr and up to 4.1 × 10⁹ mol C/yr, respectively. Isotopic compositions (δ¹³C and δ¹⁵N) and C/N ratios indicated that soil organic matter was the dominant source of particulate organic matter for 59% of the samples. Eighteen percent of the samples were dominated by plankton, 12% by periodic inputs of fresh terrestrial plant detritus with C/N > 15, and about 11% of the samples were dominated by the contribution of aquatic vascular plants. Nitrate inputs were controlled by land use in the River Sava watershed. δ¹⁵N_NO3 values <6‰ were found in predominantly forested watersheds, while values >6‰ typically represented watersheds with a higher percentage of agricultural and/or urban land use. Elevated δ¹⁵N_NO3 values (up to +25.5‰) at some sites were probably due to the combined effects of low-flow and inputs from sewage and/or animal waste.     

Fluid inclusions as petrogenetic indicators in granulite xenoliths,Pali-Aike volcanic field,Chile

Granulite xenoliths within alkali olivine basalts of the Pali-Aike volcanic field, southern Chile, contain the mineral assemblage orthopyroxene + clinopyroxene + plagioclase + olivine + green spinel. These granulites are thought to be accidental inclusions of the lower crust incorporated in the mantle-derived basalt during its rise to the surface. Symplectic intergrowths of pyroxene and spinel developed between olivine and plagioclase imply that the reaction olivine+plagioclase = Al-orthopyroxene + Al-clinopyroxene + spinel (1) occurred during subsolidus cooling and recrystallization of a gabbroic protolith of the granulites.Examination of fluid inclusions in the granulites indicates the ubiquitous presence of an essentially pure CO₂ fluid phase. Inclusions of three different parageneses have been recognized: Type I inclusions occur along exsolution lamellae in clinopyroxene and are thought to represent precipitation of structurally-bound C or CO₂ during cooling of the gabbro. These are considered the most primary inclusions present. Type II inclusions occur as evenly distributed clusters not associated with any fractures. These inclusions probably represent entrapment of a free fluid phase during recrystallization of the host grains. IIa inclusions are found in granoblastic grains and have densities of 0.68–0.88 g/cm³. Higher density (=0.90–1.02 g/cm³) IIb inclusions occur only in symplectite phases. Secondary Type III CO₂+glass inclusions with =0.47–0.78 g/cm³ occur along healed fractures where basalt has penetrated the xenoliths. Type III inclusions appear related to exsolution of CO₂ from the host basalt during its ascent to the surface. These data suggest that CO₂ is an important constituent of the lower crust under conditions of granulite facies metamorphism, indicated by Type I and II fluid inclusions, and of the mantle, as indicated by Type III inclusions.Correlation of fluid inclusion densities with P-T conditions calculated from both two-pyroxene geothermometry and reation (1) indicate emplacement of a gabbroic pluton at 1,200–1,300° C, 4–6 kb; cooling was accompanied by a slight increase in pressure due to crustal thickening, and symplectite formation occurred at 850±35° C, 5–7 kb. Capture of the xenoliths by the basalt resulted in heating of the granulites, and CO₂ from the basalt was continuously entrapped by the xenoliths over the range 1,000–1,200° C, 4–6 kb. Examination of fluid inclusions of different generations can thus be used in conjunction with other petrologic data to place tight constraints on the specific P-T path followed by the granulite suite, in addition to indicating the nature of the fluid phase present at depth.     

Groundwater quality and its suitability for drinking and agricultural use in Chithar River Basin, Tamil Nadu, India

Hydrochemistry of groundwater in Chithar Basin, Tamil Nadu, India was used to assess the quality of groundwater for determining its suitability for drinking and agricultural purposes. Physical and chemical parameters of groundwater such as electrical conductivity, pH, total dissolved solids (TDS), Na⁺, K⁺, Ca²⁺, Mg²⁺, Cl^–, HCO₃^–, CO₃^2–, SO₄^2–, NO^–₃, F^–, B^– and SiO₂ were determined. Concentrations of the chemical constituents in groundwater vary spatially and temporarily. Interpretation of analytical data shows that mixed Ca–Mg–Cl, Ca–Cl and Na–Cl are the dominant hydrochemical facies in the study area. Alkali earths (Ca²⁺, Mg²⁺) and strong acids (Cl^–, SO₄^2–) are slightly dominating over alkalis (Na⁺, K⁺) and weak acids (HCO₃^–, CO₃^2–). The abundance of the major ions is as follows: Na⁺ Ca²⁺ Mg²⁺ > K⁺ = Cl^– > HCO₃^–> SO₄^2– > NO₃^– > CO₃^2– . Groundwater in the area is generally hard, fresh to brackish, high to very high saline and low alkaline in nature. High total hardness and TDS in a few places identify the unsuitability of groundwater for drinking and irrigation. Such areas require special care to provide adequate drainage and introduce alternative salt tolerance cropping. Fluoride and boron are within the permissible limits for human consumption and crops as per the international standards.     

The speciation of carbon dioxide in sodium aluminosilicate glasses

Infrared spectroscopy has been used to study the speciation of CO₂ in glasses near the NaAlO₂-SiO₂ join quenched from melts held at high temperatures and pressures. Absorption bands resulting from the antisymmetric stretches of both molecular CO₂ (2,352 cm^–1) and CO ₃ ^2– (1,610 cm^–1 and 1,375 cm^–1) are observed in these glasses. The latter are attributed to distorted Na-carbonate ionic-complexes. Molar absorptivities of 945 liters/mole-cm for the molecular CO₂ band, 200 liters/mole-cm for the 1,610 cm^–1 band, and 235 liters/mole-cm for the 1,375 cm^–1 band have been determined. These molar absorptivities allow the quantitative determination of species concentrations in the glasses with a precision on the order of several percent of the amount present. The accuracy of the method is estimated to be ±15–20% at present.The ratio of molecular CO₂ to CO ₃ ^2– in sodium aluminosilicate glasses varies little for each silicate composition over the range of total dissolved CO₂ content (0–2%), pressure (15–33 kbar) and temperature (1,400–1,560° C) that we have studied. This ratio is, however, a strong function of silicate composition, increasing both with decreasing Na₂O content along the NaAlO₂-SiO₂ join and with decreasing Na₂O content in peraluminous compositions off the join.Infrared spectroscopic measurements of species concentrations in glasses provide insights into the molecular level processes accompanying CO₂ solution in melts and can be used to test and constrain thermodynamic models of CO₂-bearing melts. CO₂ speciation in silicate melts can be modelled by equilibria between molecular CO₂, CO ₃ ^2– , and oxygen species in the melts. Consideration of the thermodynamics of such equilibria can account for the observed linear relationship between molecular CO₂ and carbonate concentrations in glasses, the proposed linear relationship between total dissolved CO₂ content and the activity of CO₂ in melts, and observed variations in CO₂ solubility in melts.     

Open to closed system transition traced through the TDIC isotopic signature at the aquifer recharge stage, implications for groundwater C dating

¹⁴C dating models are limited when considering recent groundwater for which the carbon isotopic signature of the total dissolved inorganic carbon (TDIC) is mainly acquired in the unsaturated zone. Reducing the uncertainties of dating thus implies a better identification of the processes controlling the carbon isotopic composition of the TDIC during groundwater recharge. Geochemical interactions between gas, water and carbonates in the unsaturated zone were investigated for two aquifers (the carbonate-free Fontainebleau sands and carbonate-bearing Astian sands, France) in order to identify the respective roles of CO₂ and carbonates on the carbon isotopic signatures of the TDIC; this analysis is usually approached using open or closed system terms. Under fully open system conditions, the seasonality of the ¹³C values in the soil CO₂ can lead to important uncertainties regarding the so-called “initial ¹⁴C activity” used in ¹⁴C correction models. In a carbonate-bearing unsaturated zone such as in the Astian aquifer, we show that an approach based on fully open or closed system conditions is not appropriate. Although the chemical saturation between water and calcite occurs rapidly within the first metre of the unsaturated zone, the carbon isotopic contents (δ¹³C) of the CO₂ and the TDIC evolve downward, impacted by the dissolution-precipitation of the carbonates. In this study, we propose a numerical approach to describe this evolution. The δ¹³C and the A¹⁴C (radiocarbon activity) of the TDIC at the base of the carbonate-bearing unsaturated zone depends on (i) the δ¹³C and the A¹⁴C of the TDIC in the soil determined by the soil CO_2, (ii) the water’s residence time in the unsaturated zone and (iii) the carbonate precipitation-dissolution fluxes. In this type of situation, the carbonate δ¹³C-A¹⁴C evolutions indicate the presence of secondary calcite and permit the calculation of its accretion flux, equal to . More generally, for other sites under temperate climate and with similar properties to the Astian sands site, this approach allows for a reliable determination of the carbon isotopic composition at the base of the unsaturated zone as the indispensable “input function” data of the carbon cycle into the aquifer.