Methane and carbon dioxide flux from a macrotidal salt marsh,Bay of Fundy,New Brunswick

Fluxes of methane (CH₄) and carbon dioxide (CO₂) to the atmosphere at 52 sites within a salt marsh were measured by a dark static chamber technique from mid July to mid September. Mean CH₄ fluxes ranged from 0.2 mg m^?2 d^?1 to 11.0 mg m^?2 d^?1, with an overall average of 1.6 mg m^?2 d^?1. Flux of CH₄ was inversely correlated (r²=0.23, p = 0.001) with salinity of the upper porewater at the site, suggesting the dominant role of SO₄ ^2? in inhibiting methanogenesis in salt-marsh sediments. The combination of salinity and water table position was able to explain only 29% of the variance in CH₄ emission. Mean soil flux of CO₂ ranged from 0.3 g m^?2 d^?1 to 3.7 g m^?2 d^?1, with an overall average of 2.5 g m^?2 d^?1; it was correlated with aboveground biomass (positive, r²=0.38, p = 0.001) and position of the water table (negative, r² = 0.55, p = 0.001). The combination of biomass and water table position accounted for 63% of the variance in CO₂ flux. There were high variations in gas flux within the six plant communities. The sequences were CH₄: upland edge > panne > pool > middle marsh > low marsh > high marsh, and CO₂: middle marsh > low marsh > upland edge > high marsh > panne > pool. Compared to other salt-marsh systems, this Bay of Fundy marsh emits small amounts of CH₄ and CO₂.     

CO2 solubility in aqueous solutions containing Na+, Ca2+, Cl−, SO42− and HCO3-: The effects of electrostricted water and ion hydration thermodynamics

Dissolution of CO₂ into deep subsurface brines for carbon sequestration is regarded as one of the few viable means of reducing the amount of CO₂ entering the atmosphere. Ions in solution partially control the amount of CO₂ that dissolves, but the mechanisms of the ion's influence are not clearly understood and thus CO₂ solubility is difficult to predict. In this study, CO₂ solubility was experimentally determined in water, NaCl, CaCl₂, Na₂SO_4, and NaHCO₃ solutions and a mixed brine similar to the Bravo Dome natural CO₂ reservoir; ionic strengths ranged up to 3.4 molal, temperatures to 140 °C, and CO₂ pressures to 35.5 MPa. Increasing ionic strength decreased CO₂ solubility for all solutions when the salt type remained unchanged, but ionic strength was a poor predictor of CO₂ solubility in solutions with different salts. A new equation was developed to use ion hydration number to calculate the concentration of electrostricted water molecules in solution. Dissolved CO₂ was strongly correlated (R² = 0.96) to electrostricted water concentration. Strong correlations were also identified between CO₂ solubility and hydration enthalpy and hydration entropy. These linear correlation equations predicted CO₂ solubility within 1% of the Bravo Dome brine and within 10% of two mixed brines from literature (a 10 wt % NaCl + KCl + CaCl₂ brine and a natural Na⁺, Ca²⁺, Cl⁻ type brine with minor amounts of Mg²⁺, K⁺, Sr²⁺ and Br⁻).     

Anomalous carbon isotope fractionation between atmospheric CO2 and dissolved inorganic carbon induced by intense photosynthesis

A stable isotope mass-balance of dissolved inorganic carbon during a blue-green algae bloom in a softwater lake demonstrates that at low partial pressure of carbon dioxide there must be a large net negative carbon isotope fractionation between atmospheric CO₂ and the CO₂ absorbed by lake water at pH = 9.5. The net fractionation of CO₂(g) with respect to HCO⁻₃ was about −13%. compared with about +8%. for water at equilibrium with atmospheric CO₂ at pH ≈ 7. Chemical enhancement of CO₂ invasion at high pH by the reaction CO₂ + OH⁻→ HCO⁻₃ at large apparent film thicknesses may result in carbon isotope fractionation approaching that for a hydroxide solution. This phenomenon, coupled with a decrease in the photosynthetic fractionation, forced the surface water of a softwater lake to achieve increasingly negative δ¹³C values during an algal bloom, which is in the opposite sense to the trend that results from photosynthesis under less extreme conditions. This and other similar systems must operate under non-equilibrium (kinetic) conditions, causing a large kinetic fractionation during CO₂ invasion at pH > 8 and relatively large film thicknesses (i.e., low wind stress).     

The CO2-He-Ar-H2O systematics of the manus back-arc basin: resolving source composition from degassing and contamination effects

We report new stepped heating He, Ar, CO₂ and water data on a petrogenetically diverse suite of lavas from the Manus back-arc basin, where a plume component has previously been identified. The aim of this study is to evaluate the superimposed effects of degassing and contamination in order to identify mantle source characteristics. CO₂ abundances and carbon isotopes in both the vesicle ([CO₂] up to 180ppm; δ¹³C as low as -33.6 ‰) and glass ([CO₂] up to 270ppm; δ¹³C as low as -34.3 ‰) phases reveal that samples have been modified by varying degrees of degassing. High water concentration samples (back-arc basin basalts (BABB) and arc type samples) show the highest degrees of degassing (i.e. lower δ¹³C values and lower CO₂ contents). The results are modelled for both the glass and vesicle phases using batch and fractional degassing models. Parental melt compositions can be constrained to show the following CO₂ concentration trend: arc-type > BABB s.r. (southern rift) > MORB-2, E-MORB, X-BABB (extreme BABB), BABB > MORB-1 and MORB-smt. ⁴He/⁴⁰Ar∗ ratios of samples (14.6-1100) are consistent with residual volatiles from a degassed source. Variations in CO₂/³He values are likely due to degassing, followed by contamination from a crustal source (either the subducting Solomon Sea Plate or the pre-existing crust through which the lavas erupt), as evidenced by high K₂O/TiO₂ ratios and low δ¹³C. The CO₂/³He of the Manus plume is best estimated by the MORB-smt and MORB-1 samples at 3.1 ± 0.6 x 10⁹. This value is similar to previous estimates of plume CO₂/³He values, which are either equal to or slightly greater than the upper mantle average of 2 x 10⁹.     

CO<Subscript>2</Subscript> flux variation and its contribution area in the debris-covered area of Koxkar Glacier,Mt. Tianshan in China

During the formation and development of glacial meltwater runoff, hydrochemical erosion is abundant, especially the hydrolysis of K/Na feldspar and carbonates, which can consume H⁺ in the water, promote the formation of bicarbonate by dissolving atmospheric CO₂, and affect the regional carbon cycle. From July 21, 2015, to July 18, 2017, the CO₂ concentration and flux were observed by the eddy covariance (EC) method in the relatively flat and open moraine cover area of Koxkar Glacier in western Mt. Tianshan, China. We found that: (1) atmospheric CO₂ fluxes ranged from ??408.95 to 81.58 mmol m^?2 day^?1 (average ? 58.68 mmol m^?2 day^?1), suggesting that the study area is a significant carbon sink, (2) the CO₂ flux footprint contribution areas were primarily within 150 m of the EC station, averaging total contribution rates of 93.30%, 91.39%, and 90.17% of the CO₂ flux in the snow accumulation, snow melting, and glacial melting periods, respectively. Therefore, the contribution areas with significant influences on CO₂ flux observed at EC stations were concentrated, demonstrating that grassland CO₂ flux around the glaciers had little effect at the EC stations, (3) in the predominant wind direction, under stable daytime atmospheric stratification, the measurement of CO₂ flux, as interpreted by the Agroscope Reckenholz Tanikon footprint tool, was 79.09% ± 1.84% in the contribution area. This was slightly more than seen at night, but significantly lower than the average under unstable atmospheric stratification across the three periods of interest (89%). The average distance of the farthest point of the flux footprint under steady state atmospheric conditions was 202.61?±?69.33 m, markedly greater than that under non-steady state conditions (68.55?±?10.34 m). This also indicates that the CO₂ flux observed using EC was affected primarily by hydrochemical erosion reactions in the glacier area, (4) a good negative correlation was found between net glacier exchange (NGE) of CO₂ and air temperature on precipitation-free days. Strong ice and snow ablation could promote hydrochemical reactions of soluble substances in the debris area and accelerated sinking of atmospheric CO₂. Precipitation events might reduce snow and ice melting, driven by reduced regional temperatures. However, a connection between NGE and precipitation, when less than 8.8 mm per day, was not obvious. When precipitation was greater than 8.8 mm per day, NGE decreased with increasing precipitation, (5) graphically, the slope of NGE, related to daily runoff, followed a trend: snow melting period?>?snow accumulation period?>?early glacial ablation period?>?late glacier ablation period?>?dramatic glacier ablation period. The slope was relatively large during snow melting, likely because of CO₂ sinking caused by water–rock interactions. The chemical reaction during elution in the snow layer might also promote atmospheric CO₂ drawdown. At the same time, the damping effect of snow cover and the almost-closed glacier hydrographic channel inhibited the formation of regional runoff, possibly providing sufficient time for the chemical reaction, thus promoting further CO₂ drawdown.     

Degradation of carbon disulphide (CS2) in soils and groundwater from a CS2-contaminated site

This study is the first investigation of biodegradation of carbon disulphide (CS₂) in soil that provides estimates of degradation rates and identifies intermediate degradation products and carbon isotope signatures of degradation. Microcosm studies were undertaken under anaerobic conditions using soil and groundwater recovered from CS₂-contaminated sites. Proposed degradation mechanisms were validated using equilibrium speciation modelling of concentrations and carbon isotope ratios. A first-order degradation rate constant of 1.25 × 10^?2 h^?1 was obtained for biological degradation with soil. Carbonyl sulphide (COS) and hydrogen sulphide (H₂S) were found to be intermediates of degradation, but did not accumulate in vials. A ¹³C/¹²C enrichment factor of ?7.5 ± 0.8 ‰ was obtained for degradation within microcosms with both soil and groundwater whereas a ¹³C/¹²C enrichment factor of ?23.0 ± 2.1 ‰ was obtained for degradation with site groundwater alone. It can be concluded that biological degradation of both CS₂-contaminated soil and groundwater is likely to occur in the field suggesting that natural attenuation may be an appropriate remedial tool at some sites. The presence of biodegradation by-products including COS and H₂S indicates that biodegradation of CS₂ is occurring and stable carbon isotopes are a promising tool to quantify CS₂ degradation.     

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.     

On leakage and seepage of CO2 from geologic storage sites into surface water

Geologic carbon sequestration is the capture of anthropogenic carbon dioxide (CO₂) and its storage in deep geologic formations. The processes of CO₂ seepage into surface water after migration through water-saturated sediments are reviewed. Natural CO₂ and CH₄ fluxes are pervasive in surface-water environments and are good analogues to potential leakage and seepage of CO₂. Buoyancy-driven bubble rise in surface water reaches a maximum velocity of approximately 30 cm s⁻¹. CO₂ rise in saturated porous media tends to occur as channel flow rather than bubble flow. A comparison of ebullition versus dispersive gas transport for CO₂ and CH₄ shows that bubble flow will dominate over dispersion in surface water. Gaseous CO₂ solubility in variable-salinity waters decreases as pressure decreases leading to greater likelihood of ebullition and bubble flow in surface water as CO₂ migrates upward.     

Quantifying inorganic sources of geochemical energy in hydrothermal ecosystems, Yellowstone National Park, USA

Combining analytical data from hot spring samples with thermodynamic calculations permits a quantitative assessment of the availability and ranking of various potential sources of inorganic chemical energy that may support microbial life in hydrothermal ecosystems. Yellowstone hot springs of diverse geochemical composition, ranging in pH from <2 to >9 were chosen for this study, and dozens of samples were collected during three field seasons. Field measurements of dissolved oxygen, nitrate, nitrite, total ammonia, total sulfide, alkalinity, and ferrous iron were combined with laboratory analyses of sulfate and other major ions from water samples, and carbon dioxide, hydrogen, methane, and carbon monoxide in gas samples to evaluate activity products for ∼300 coupled oxidation-reduction reactions. Comparison of activity products and independently calculated equilibrium constants leads to values of the chemical affinity for each of the reactions, which quantifies how far each reaction is from equilibrium. Affinities, in turn, show systematic behavior that is independent of temperature but can be correlated with pH of the hot springs as a proxy for the full spectrum of geochemical variability. Many affinities are slightly to somewhat dependent on pH, while others are dramatically influenced by changes in chemical composition. All reactions involving dissolved oxygen as the electron acceptor are potential energy sources in all hot spring samples collected, but the ranking of dominant electron donors changes from ferrous iron, and sulfur at high pH to carbon monoxide, hydrogen, and magnetite as pH decreases. There is a general trend of decreasing energy yields depending on electron acceptors that follows the sequence: O₂(aq) > NO₃⁻ ≈ NO₂⁻ ≈ S > pyrite ≈ SO₄⁻² ≈ CO(g) ≈ CO₂(g) at high pH, and O₂(aq) ≈ magnetite > hematite ≈ goethite > NO₃⁻ ≈ NO₂⁻ ≈ S ≈ pyrite ≈ SO₄⁻² at low pH. Many reactions that are favorable sources of chemical energy at one set of geochemical conditions fail to provide energy at other conditions, and vice versa. This results in energy profiles supplied by geochemical processes that provide fundamentally different foundations for chemotrophic microbial communities as composition changes.     

Experimental reconstruction of sodic dolomitic carbonatite melts from metasomatised lithosphere

 Investigations of peridotite xenolith suites have identified a compositional trend from lherzolite to magnesian wehrlite in which clinopyroxene increases at the expense of orthopyroxene and aluminous spinel, and in which apatite may be a minor phase. Previous studies have shown that this trend in mineralogy and chemical composition may result from reaction between sodic dolomitic carbonatite melt and lherzolite at pressures around 1.7 to 2 GPa. This reaction results in decarbonation of the carbonatite melt, releasing CO₂-rich fluid. In this study, we have experimentally reversed the decarbonation reaction by taking two natural wehrlite compositions and reacting them with CO₂ at a pressure of 2.2 GPa and temperatures from 900 to 1150° C. Starting materials were pargasite-bearing wehrlites, one with minor apatite (composition 71001^*) and one without apatite (composition 70965^*). At lower temperatures (900° C) the products were apatite+pargasite+magnesite harzburgite for runs using composition 71001^*, and pargasite+dolomite lherzolite for runs using composition 70965^*. At and above 1000° C, carbonatite melt with harzburgite residue (olivine+orthopyroxene+spinel) and with lherzolite residue (olivine+orthopyroxene+clinopyroxene+ spinel) were produced respectively. Phase compositions in reactants and products are consistent with the documented carbonatite/lherzolite reactions, and also permit estimation of the carbonatite melt compositions. In both cases the melts are sodic dolomitic carbonatites. The study supports the hypothesis of a significant role for ephemeral, sodic dolomitic melts in causing metasomatic changes in the lithosphere at P≤2 GPa. The compositions of wehrlites imply fluxes of CO₂, released by metasomatic reactions, which are locally very large at around 5 wt% CO₂. Received: 15 December 1995/Accepted: 14 February 1996     

Early oxidation of organic matter in pelagic sediments of the eastern equatorial Atlantic: suboxic diagenesis

Pore water profiles of total-CO₂, pH, PO^3?₄, NO^?₃ plus NO^?₂, SO^2?₄, S^2?, Fe²⁺ and Mn²⁺ have been obtained in cores from pelagic sediments of the eastern equatorial Atlantic under waters of moderate to high productivity. These profiles reveal that oxidants are consumed in order of decreasing energy production per mole of organic carbon oxidized (O₂ > manganese oxides ~ nitrate > iron oxides > sulfate). Total CO₂ concentrations reflect organic regeneration and calcite dissolution. Phosphate profiles are consistent with organic regeneration and with the effects of release and uptake during inorganic reactions. Nitrate profiles reflect organic regeneration and nitrate reduction, while dissolved iron and manganese profiles suggest reduction of the solid oxide phases, upward fluxes of dissolved metals and subsequent entrapment in the sediment column. Sulfate values are constant and sulfide is absent, reflecting the absence of strongly anoxic conditions.     

The cost of meeting increased cooling-water demands for CO2 capture and storage utilizing non-traditional waters from geologic saline formations

Deep (> ～800 m) saline water-bearing formations in the United States have substantial pore volume that is targeted for storage of carbon dioxide (CO₂) and the associated saline water can be extracted to increase CO₂ storage efficiency, manage pressure build up, and create a new water source that, once treated, can be used for power-plant cooling or other purposes. Extraction, treatment and disposal costs of saline formation water to meet added water demands from CO₂ capture and storage (CCS) are discussed. This underutilized water source may be important in meeting new water demand associated with CCS. For a representative natural gas combined-cycle (NGCC) power plant, simultaneous extraction of brine from the storage formation could provide enough water to meet all CCS-related cooling demands for 177 out of the 185 (96 %) saline formations analyzed in this study. Calculated total cost of water extraction, treatment and disposal is less than 4.00 US Dollars (USD) m^?3 for 93 % of the 185 formations considered. In 90 % of 185 formations, treated water costs are less than 10.00 USD tonne^?1 of CO₂ injected. On average, this represents approximately 6 % of the total CO₂ capture and injection costs for the NGCC scenario.     

Pyrite,marcasite, and arsenopyrite type minerals: Crystal chemical and structural principles

Quantitative molecular orbital (MO) calculations and qualitative perturbational MO arguments are used to interpret the spectra and structure of transition metal dichalcogenides and related compounds. Competition between pyrite (FeS₂), marcasite (FeS₂) and loellingite (FeAs₂) structure types is explained in terms of the number of electrons occupying a set of MO's obtained from the mixing of dianion (A ₂) orbitals and metal (M) dσ orbitals. Direct metal-metal d orbital interaction is argued to be small. Attention is focused upon the M - A - M angles which differ substantially among the three structure types as a consequence of varying numbers of electrons in orbitals which closely resemble the pπ* orbitals of the dianions. Variations in M - A and A - A distances can also be understood in terms of the occupations of this set of MO's. Disulfide valence region photo-emission spectra are interpreted in terms of calculations on MS₆ and S₆ molecular clusters. M3d orbitals are found to progressively approach the S3p orbitals with increasing atomic number of M from Fe to Ni. For CuS₂ comparison of calculation and experiment supports an approximate electron configuration of Cu⁺¹ S ₂ ^?1 .     

Genesis of fumarolic emissions as inferred by isotope mass balances: CO2 and water at Vulcano Island, Italy

We have developed a quantitative model of CO₂ and H₂O isotopic mixing between magmatic and hydrothermal gases for the fumarolic emissions of the La Fossa crater (Vulcano Island, Italy). On the basis of isotope balance equations, the model takes into account the isotope equilibrium between H₂O and CO₂ and extends the recent model of chemical and energy two-end-member mixing by Nuccio et al. (1999). As a result, the H₂O and CO₂ content and the δD, δ¹⁸O, and δ¹³C isotope compositions for both magmatic and hydrothermal end-members have been assessed. Low contributions of meteoric steam, added at a shallow depth, have been also recognized and quantified in the fumaroles throughout the period from 1988 to 1998. Nonequilibrium oxygen isotope exchange also seems to be occurring between ascending gases and wall rocks along some fumarolic conduits.The δ¹³C_CO2 of the magmatic gases varies around −3 to 1‰ vs. Peedee belemnite (PDB), following a perfect synchronism with the variations of the CO₂ concentration in the magmatic gases. This suggests a process of isotope fractionation because of vapor exsolution caused by magma depressurization. The hydrogen isotopes in the magmatic gases (−1 to −‰ vs. standard mean ocean water [SMOW]), as well as the above δ¹³C_CO2 value, are coherent with a convergent tectonic setting of magma generation, where the local mantle is widely contaminated by fluids released from the subducted slab. Magma contamination in the crust probably amplifies this effect.The computed isotope composition of carbon and hydrogen in the hydrothermal vapors has been used to calculate the δD and δ¹³C of the entire hydrothermal system, including mixed H₂O-CO₂ vapor, liquid water, and dissolved carbon. We have computed values of about 10‰ vs. SMOW for water and −2 to −6.5‰ vs. PDB for CO₂. On these grounds, we think that Mediterranean marine water (δD_H2O ≈ 10‰) feeds the hydrothermal system. It infiltrates at depth throughout the local rocks, reaching oxygen isotope equilibrium at high temperatures. Interaction processes between magmatic gases and the evolving seawater also seem to occur, causing the dissolution of isotopically fractionated aqueous CO₂ and providing the source for hydrothermal carbon. These results have important implications concerning fluid circulation beneath Vulcano and address the more convenient routine of geochemical surveillance.     

Modelling the long-term carbon cycle,atmospheric CO2, and Earth surface temperature from late Neoproterozoic to present day

Over geological timescales, CO₂ levels are determined by the operation of the long term carbon cycle, and it is generally thought that changes in atmospheric CO₂ concentration have controlled variations in Earth's surface temperature over the Phanerozoic Eon. Here we compile independent estimates for global average surface temperature and atmospheric CO₂ concentration, and compare these to the predictions of box models of the long term carbon cycle COPSE and GEOCARBSULF.We find a strong relationship between CO₂ forcing and temperature from the proxy data, for times where data is available, and we find that current published models reproduce many aspects of CO₂ change, but compare poorly to temperature estimates. Models are then modified in line with recent advances in understanding the tectonic controls on carbon cycle source and sink processes, with these changes constrained by modelling ⁸⁷Sr/⁸⁶Sr ratios. We estimate CO₂ degassing rates from the lengths of subduction zones and rifts, add differential effects of erosion rates on the weathering of silicates and carbonates, and revise the relationship between global average temperature changes and the temperature change in key weathering zones.Under these modifications, models produce combined records of CO₂ and temperature change that are reasonably in line with geological and geochemical proxies (e.g. central model predictions are within the proxy windows for >~75% of the time covered by data). However, whilst broad long-term changes are reconstructed, the models still do not adequately predict the timing of glacial periods. We show that the ⁸⁷Sr/⁸⁶Sr record is largely influenced by the weathering contributions of different lithologies, and is strongly controlled by erosion rates, rather than being a good indicator of overall silicate chemical weathering rates. We also confirm that a combination of increasing erosion rates and decreasing degassing rates over the Neogene can cause the observed cooling and Sr isotope changes without requiring an overall increase in silicate weathering rates.On the question of a source or sink dominated carbon cycle, we find that neither alone can adequately reconstruct the combination of CO₂, temperature and strontium isotope dynamics over Phanerozoic time, necessitating a combination of changes to sources and sinks. Further progress in this field relies on >10⁸ year dynamic spatial reconstructions of ancient tectonics, paleogeography and hydrology. Whilst this is a significant challenge, the latest reconstruction techniques, proxy records and modelling advances make this an achievable target.     

CO2-water-basalt interaction. Numerical simulation of low temperature CO2 sequestration into basalts

The interaction between CO₂-rich waters and basaltic glass was studied using reaction path modeling in order to get insight into the water-rock reaction process including secondary mineral composition, water chemistry and mass transfer as a function of CO₂ concentration and reaction progress (ξ). The calculations were carried out at 25-90 °C and pCO₂ to 30 bars and the results were compared to recent experimental observations and natural systems. A thermodynamic dataset was compiled from 25 to 300 °C in order to simulate mineral saturations relevant to basalt alteration in CO₂-rich environment including revised key aqueous species for mineral dissolution reactions and apparent Gibbs energies for clay and carbonate solid solutions observed to form in nature. The dissolution of basaltic glass in CO₂-rich waters was found to be incongruent with the overall water composition and secondary mineral formation depending on reaction progress and pH. Under mildly acid conditions in CO₂ enriched waters (pH <6.5), SiO₂ and simple Al-Si minerals, Ca-Mg-Fe smectites and Ca-Mg-Fe carbonates predominated. Iron, Al and Si were immobile whereas the Mg and Ca mobility depended on the mass of carbonate formed and water pH. Upon quantitative CO₂ mineralization, the pH increased to >8 resulting in Ca-Mg-Fe smectite, zeolites and calcite formation, reducing the mobility of most dissolved elements. The dominant factor determining the reaction path of basalt alteration and the associated element mobility was the pH of the water. In turn, the pH value was determined by the concentration of CO₂ and extent of reaction. The composition of the carbonates depended on the mobility of Ca, Mg and Fe. At pH <6.5, Fe was in the ferrous oxidation state resulting in the formation of Fe-rich carbonates with the incorporation of Ca and Mg. At pH >8, the mobility of Fe and Mg was limited due to the formation of clays whereas Ca was incorporated into calcite, zeolites and clays. Competing reactions between clays (Ca-Fe smectites) and carbonates at low pH, and zeolites and clays (Mg-Fe smectites) and carbonates at high pH, controlled the availability of Ca, Mg and Fe, playing a key role for low temperature CO₂ mineralization and sequestration into basalts. Several problems of the present model point to the need of improvement in future work. The determinant factors linking time to low temperature reaction path modeling may not only be controlled by the primary dissolving phase, which presents challenges concerning non-stoichiometric dissolution, the leached layer model and reactive surface area, but may include secondary mineral precipitation kinetics as rate limiting step for specific reactions such as retrieved from the present reaction path study.     

Solubility of water and carbon dioxide in an icelandite at 1400 °C and 10 kilobars

The concentrations of water and carbon dissolved in an icelandite glass quenched from 1400 °C and 10 kbar were measured using Fourier transform infra-red spectroscopy and elemental analyses of carbon and hydrogen. Only carbon dioxide and water were observed in the fluid phase as analysed after quenching with a qudrupole mass analyser. The mole fraction of carbon dioxide in the fluid phase ranged from 0.36 to 0.95. Carbon is dissolved as carbonate except at the highest CO₂ fluid fugacity, where a small amount of molecular CO₂ is observed. Dissolved carbon in the glasses, calculated as CO₂, remained constant at approximately 1 wt %, in spite of the different CO₂ fluid fugacities. Water was dissolved as molecular water and as hydroxyl groups, the hydroxyl concentration in the quenched glasses remaining almost constant over the whole interval, whereas the molecular water dissolves in accordance with Henry's law. Molecular water peaks at 5200␣cm⁻¹ and 1630 cm⁻¹, the hydroxyl peak at 4500␣cm⁻¹, and the carbonate peaks at 1400 cm⁻¹–1550 cm⁻¹ have been calibrated using elemental analyses of C and H in the quenched glasses. As molecular water decreases in the melt the higher wavenumber carbonate peak is observed to move towards the molecular water peak at 1630 cm⁻¹ causing a split of the carbonate peaks, ranging from 45 cm⁻¹ to 100 cm⁻¹. Received: 15 November 1995 / Accepted: 21 September 1996     

A survey of oil and gas wells in the Texas Gulf Coast,USA, and implications for geological sequestration of CO<Subscript>2</Subscript>

Subsurface sequestration of CO₂ in oil and gas provinces where permanence of hydrocarbon accumulations has proven the reliability of potential traps is rightly seen as a solid option for containment of CO₂ atmospheric concentrations. However, one of the most promising provinces for carbon storage in North America, the Texas Gulf Coast, has also been heavily drilled for more than a century, puncturing many otherwise perfectly sound seals (>125,000 wells over ~50,000 km²). As a result, boreholes and, in particular, older abandoned wells could be major leakage pathways for sequestered CO₂. This article presents statistics on well spatial and depth distribution that have been drawn from public domain sources and relates these data to historical plugging and abandonment regulations in the Texas Gulf Coast. Surface-well density averages of 2.4 wells/km² can be locally much higher—but also much lower in larger areas. Average well penetration density drops to 0.27 and 0.05 well/km² below a depth of 2,440 and 3,660 m, respectively. Natural mitigating factors such as thief zones and heaving “shales” could also play a role in limiting the impact of these direct conduits to the shallow subsurface and surface.     

Soil microbial community changes as a result of long-term exposure to a natural CO2 vent

The capture and geological storage of CO₂ can be used to reduce anthropogenic greenhouse gas emissions. To assess the environmental impact of potential CO₂ leakage from deep storage reservoirs on the abundance and functional diversity of microorganisms in near-surface terrestrial environments, a natural CO₂ vent (>90% CO₂ in the soil gas) was studied as an analogue. The microbial communities were investigated using lipid biomarkers combined with compound-specific stable carbon isotope analyses, the determination of microbial activities, and the use of quantitative polymerase chain reactions (Q-PCR). With this complementary set of methods, significant differences between the CO₂-rich vent and a reference site with a normal CO₂ concentration were detected. The δ¹³C values of the plant and microbial lipids within the CO₂ vent demonstrate that substantial amounts of geothermal CO₂ were incorporated into the microbial, plant, and soil carbon pools. Moreover, the numbers of Archaea and Bacteria were highest at the reference site and substantially lower at the CO₂ vent. Lipid biomarker analyses, Q-PCR, and the determination of microbial activities showed the presence of CO₂-utilising methanogenic Archaea, Geobacteraceae, and sulphate-reducing Bacteria (SRB) mainly at the CO₂ vent, only minor quantities were found at the reference site. Stable carbon isotopic analyses revealed that the methanogenic Archaea and SRB utilised the vent-derived CO₂ for assimilatory biosynthesis. Our results show a shift in the microbial community towards anaerobic and acidophilic microorganisms as a consequence of the long-term exposure of the soil environment to high CO₂ concentrations.