Dissolved Carbon Species Research Articles

Factors controlling the stable isotope composition and C:N ratio of seston and periphyton in shallow lake mesocosms with contrasting nutrient loadings and temperatures

Summary Carbon (C) and nitrogen (N) stable isotope composition (15N:14N, δ15N and 13C:12C, δ13C) have been widely used to elucidate changes in aquatic ecosystem dynamics created by eutrophication and climate warming, often, however, without accounting for seasonal variation. Here, we aim to determine the factors controlling the stable isotope composition and C:N ratio of seston and periphyton in shallow lakes with contrasting nutrient loadings and climate; for this purpose, we followed the monthly stable isotope composition (c. 1 year) of seston (SES) and periphyton (PER) in 24 mesocosms mimicking shallow lakes with two nutrient treatments (enriched and unenriched) and three temperature scenarios (ambient, +3 and +5°C). Nutrient enrichment and warming had a stronger impact on the δ15N and δ13C values of seston than on periphyton, and the temporal isotopic variability in both communities was large. δ15NPER did not differ markedly between nutrient treatments, whereas δ15NSES was lower in the enriched mesocosms, possibly reflecting higher N2‐fixation by cyanobacteria. δ15NSES was higher in winter in the heated mesocosms and its dynamics was linked with that of NH4‐N, whereas δ15NPER showed a stronger association with NO3‐N. δ15NSES demonstrated a positive relationship with mean monthly temperature, indicating less isotope fractionation among autotrophs when production increased. δ13CSES was lowest in the enriched mesocosms during winter, whereas δ13CPER did not differ between nutrient treatments. δ13CSES and δ13CPER were positively related to pH, likely reflecting a pH‐induced differential access to dissolved carbon species in the primary producers. The positive δ13C‐temperature relationship suggested less fractionation of CO2 and HCO3− and/or larger use of HCO3− at higher temperatures. The C:N ratios varied seasonally and the differences between the enriched and unenriched mesocosms were stronger for seston than for periphyton. Particularly, the C:NSES ratios did not indicate deficiencies in N as opposed to the C:NPER ratios, supporting the observed changes in δ15N and suggesting that seston and periphyton have access to different sources of nutrients. We did not observe any clear effect of temperature warming on the C:N ratios. Our study provides evidence of strong seasonality in the isotopic composition and C:N ratios of seston and periphyton across nutrient and temperature levels; also, we identified several factors that are likely to modulate the strength and variability in stable isotopes values and stoichiometry of sestonic and periphytic communities under these scenarios.

Groundwater Discharge as a Source of Dissolved Carbon and Greenhouse Gases in a Subtropical Estuary

Groundwater may be highly enriched in dissolved carbon species, but its role as a source of carbon to coastal waters is still poorly constrained. Exports of deep and shallow groundwater-derived dissolved carbon species from a small subtropical estuary (Korogoro Creek, Australia, latitude −31.0478°, longitude 153.0649°) were quantified using a radium isotope mass balance model (233Ra and 224Ra, natural groundwater tracers) under two hydrological conditions. In addition, air-water exchange of carbon dioxide and methane in the estuary was estimated. The highest carbon inputs to the estuary were from deep fresh groundwater in the wet season. Most of the dissolved carbon delivered by groundwater and exported from the estuary to the coastal ocean was in the form of dissolved inorganic carbon (DIC; 687 mmol m−2 estuary day−1; 20 mmol m−2 catchment day−1, respectively), with a large export of alkalinity (23 mmol m−2 catchment day−1). Average water to air flux of CO2 (869 mmol m−2 day−1) and CH4 (26 mmol m−2 day−1) were 5- and 43-fold higher, respectively, than the average global evasion in estuaries due to the large input of CO2- and CH4-enriched groundwater. The groundwater discharge contribution to carbon exports from the estuary for DIC, dissolved organic carbon (DOC), alkalinity, CO2, and CH4 was 22, 41, 3, 75, and 100 %, respectively. The results show that CO2 and CH4 evasion rates from small subtropical estuaries surrounded by wetlands can be extremely high and that groundwater discharge had a major role in carbon export and evasion from the estuary and therefore should be accounted for in coastal carbon budgets.

Nucleobase and amino acid formation through impacts of meteorites on the early ocean

The emergence of life's building blocks on the prebiotic Earth was the first crucial step for the origins of life. Extraterrestrial delivery of intact amino acids and nucleobases is the prevailing hypothesis for their availability on prebiotic Earth because of the difficulties associated with the production of these organics from terrestrial carbon and nitrogen sources under plausible prebiotic conditions. However, the variety and amounts of these intact organics delivered by meteorites would have been limited. Previous shock–recovery experiments have demonstrated that meteorite impact reactions could have generated organics on the prebiotic Earth. Here, we report on the simultaneous formation of nucleobases (cytosine and uracil) found in DNA and/or RNA, various proteinogenic amino acids (glycine, alanine, serine, aspartic acid, glutamic acid, valine, leucine, isoleucine, and proline), non-proteinogenic amino acids, and aliphatic amines in experiments simulating reactions induced by extraterrestrial objects impacting on the early oceans. To the best of our knowledge, this is the first report of the formation of nucleobases from inorganic materials by shock conditions. In these experiments, bicarbonate was used as the carbon source. Bicarbonate, which is a common dissolved carbon species in CO2-rich atmospheric conditions, was presumably the most abundant carbon species in the early oceans and in post-impact plumes. Thus, the present results expand the possibility that impact-induced reactions generated various building blocks for life on prebiotic Earth in large quantities through the use of terrestrial carbon reservoirs.

Mechanisms for mechanical trapping of geologically sequestered carbon dioxide.

Carbon dioxide (CO2) sequestration in subsurface reservoirs is important for limiting atmospheric CO2 concentrations. However, a complete physical picture able to predict the structure developing within the porous medium is lacking. We investigate theoretically reactive transport in the long-time evolution of carbon in the brine–rock environment. As CO2 is injected into a brine–rock environment, a carbonate-rich region is created amid brine. Within the carbonate-rich region minerals dissolve and migrate from regions of high-to-low concentration, along with other dissolved carbonate species. This causes mineral precipitation at the interface between the two regions. We argue that precipitation in a small layer reduces diffusivity, and eventually causes mechanical trapping of the CO2. Consequently, only a small fraction of the CO2 is converted to solid mineral; the remainder either dissolves in water or is trapped in its original form. We also study the case of a pure CO2 bubble surrounded by brine and suggest a mechanism that may lead to a carbonate-encrusted bubble owing to structural diffusion.

Magmatic gas emissions at Holocene volcanic features near Mono Lake, California, and their relation to regional magmatism

Silicic lavas have erupted repeatedly in the Mono Basin over the past few thousand years, forming the massive domes and coulees of the Mono Craters chain and the smaller island vents in Mono Lake. We report here on the first systematic study of magmatic CO2 emissions from these features, conducted during 2007–2010. Most notably, a known locus of weak steam venting on the summit of North Coulee is actually enclosed in a large area (~0.25km2) of diffuse gas discharge that emits 10–14t/d of CO2, mostly at ambient temperature. Subsurface gases sampled here are heavily air-contaminated, but after standard corrections are applied, show average δ13C-CO2 of −4.72‰, 3He/4He of 5.89RA, and CO2/3He of 0.77×1010, very similar to the values in fumarolic gas from Mammoth Mountain and the Long Valley Caldera immediately to the south of the basin. If these values also characterize the magmatic gas source at Mono Lake, where CO2 is captured by the alkaline lake water, a magmatic CO2 upflow beneath the lake of ~4t/d can be inferred. Groundwater discharge from the Mono Craters area transports ~13t/d of 14C-dead CO2 as free gas and dissolved carbonate species, and adding in this component brings the estimated total magmatic CO2 output to 29t/d for the two silicic systems in the Mono Basin. If these emissions reflect intrusion and degassing of underlying basalt with 0.5wt.% CO2, a modest intrusion rate of 0.00075km3/yr is indicated. Much higher intrusion rates are required to account for CO2 emissions from Mammoth Mountain and the West Moat of the Long Valley Caldera.

Accounting for kinetic isotope effects in Soreq Cave (Israel) speleothems

There is growing evidence that speleothem calcite grows out of isotopic equilibrium with cave drip water, with clumped isotope analysis providing a sensitive indicator for disequilibrium. This disequilibrium is primarily the result of CO2 degassing from a thin film of water, leading to irreversible 13C enrichment and reversible 18O enrichment and Δ47 depletion. Here we examine isotopic disequilibrium in Soreq Cave (Israel) using multiple modern-day and late Holocene speleothems. The variability observed in Δ47 is small, within the analytical uncertainty, but the Δ47-derived temperature is offset from the modern cave temperature by ∼4°C, reflecting degassing related disequilibrium that is fairly constant spatially. δ18O is more heterogeneous, reflecting short-term variability in drip water δ18Ow combined with variability in the speleothem growth rates and related fractionation between dissolved carbonate species and the growing calcite mineral. This complexity, however, is markedly reduced by spatial or temporal averaging, enabling an interpretation of the cave paleoclimate record. We examine the Soreq Cave speleothems through a comparison with 2 types of thermometers: one is based on CaCO3 precipitation from a bulk solution and is typically used for calibration of the Δ47 and δ18O thermometers; a second that is based on CaCO3 precipitating at the surface of the solution thus focusing and amplifying the thin film characteristics of speleothem formation. Soreq Cave speleothems are intermediate between these thermometers, providing a cave-specific thermometer calibration.

Quantifying the kinetics of olivine dissolution in partially closed and closed batch reactor systems

This study quantified the rates of olivine dissolution under constant air injection, in partially closed batch reactors and in control closed batch reactors (in the absence of air flow), in the presence of 4g of olivine, and 0, 0.001 and 0.01M NaNO3. Experiments were performed for 40,440 and 36,048min in partially closed and closed reactors respectively. Under constant air injection, the solution pH ranged from 6.8 and 7.3 for all conditions at the start of the experiments, to 7.5 and 7.8 at 40,440min. In the absence of air flow, an alkaline pH of 9.6 to 9.3 was observed for 36,048min. This pH difference is attributed to the constant buffering the solution through the formation of dissolved carbonate species (H2CO3/HCO3−). The alkaline pH, measured in the presence of olivine, implied a mechanism of pH control caused by the presence of Si(OH)4 species from the mineral surface. This effect was suggested by a drop to near neutral pH of the same aliquots, upon filtration of the olivine through a N2 degassed filter. A decrease in total alkalinity was measured for experiments under constant air injection, as a function of increasing NaNO3, suggesting a mechanism of Na+/CO32− interaction is involved in the enhancement of olivine dissolution rates through proton/Mg exchange. In the absence of air flow, the total alkalinity reached a maximum of 0.25mM, compared to 0.9mM with air flow at 0.01M NaNO3. Magnesium concentration increased in the presence of NaNO3 and permanently dissolving CO2 in partially closed batch reactors. This strongly suggests a key role of Na+ (coupled to CO2 dissolution) for enhancing olivine dissolution. Mg/Si ratios for all conditions showed incongruent olivine dissolution, suggesting a combination of two dissolution mechanisms at alkaline and near neutral pH values, in closed and partially closed batch reactors. The rate of olivine dissolution (ROL) was calculated as a function of mineral surface area and forsterite equilibrium solution parameters. In the presence of constant air injection, highest log ROL values range from −11 to −10.7 at the acidic to near neutral conditions (pH=6.8 to 7.2). Slower dissolution rates, with log ROL values of −11.5 to −11.4 were determined for experiments in the absence of air flow at alkaline pH values (9.6 to 9.8). Constant air injection (i.e. constant dissolution of CO2 at atmospheric pressure) in the presence of Na+ has proven effective for enhancing olivine dissolution.

The influence of kinetics on the oxygen isotope composition of calcium carbonate

Paleotemperature reconstructions rely on knowledge of the equilibrium separation of oxygen isotopes between aqueous solution and calcium carbonate. Although oxygen isotope separation is expected on theoretical grounds, the temperature-dependence remains uncertain because other factors, such as slow exchange of isotopes between dissolved CO2-species and water, can obscure the temperature signal. This is problematic for crystal growth experiments on laboratory timescales and for interpreting the oxygen isotope composition of crystals formed in natural settings. We present results from experiments in which inorganic calcite is precipitated in the presence of 0.25μM dissolved bovine carbonic anhydrase (CA). The presence of dissolved CA accelerates oxygen isotope equilibration between the dissolved carbon species CO2, H2CO3, HCO3−, CO32− and water, thereby eliminating this source of isotopic disequilibrium during calcite growth. The experimental results allow us to isolate, for the first time, kinetic oxygen isotope effects occurring at the calcite–water interface. We present a framework of ion-by-ion growth of calcite that reconciles our new measurements with measurements of natural cave calcites that are the best candidate for having precipitated under near-equilibrium conditions. Our findings suggest that isotopic equilibrium between calcite and water is unlikely to have been established in laboratory experiments or in many natural settings. The use of CA in carbonate precipitation experiments offers new opportunities to refine oxygen isotope-based geothermometers and to interrogate environmental variables other than temperature that influence calcite growth rates.

An experimental investigation of reaction zone controls on fluid flow and mass transport

This study examines homogeneous and heterogeneous mass transport in porous media featuring a solid-phase reaction zone that evolves and influences mass transport. It uses both flow-tank experiments and 2D reactive transport modeling to elucidate plume and reaction-zone behavior in situations with inherent complexity due to flow and transport and geochemical variability. The system is created by pumping a dilute Fe(ClO4)3 solution through a medium of glass beads and crushed calcite. As pH increases through the dissolution of calcite, amorphous ferric oxyhydroxide precipitates. The distribution of Fe, dissolved carbonate species, and pH are strongly coupled to the reaction zone because of the dissolution of CaCO3 and the precipitation of Fe(OH)3. Plugging influences the transport of dissolved constituents because it promotes fingering, hydraulic fracturing and flow bypassing. With the homogeneous systems, differences in the fingering produced by changing CaCO3 contents produced a heterogeneous distribution of Fe(OH)3(s) and dissolved Fe. As a result, a reaction zone with a complex shape developed due to the heterogeneity in hydraulic conductivity and CaCO3 content.

Chemical evolution of dissolved inorganic carbon species flowing in thin water films and its implications for (rapid) degassing of CO2 during speleothem growth

Rapid degassing of CO2 from a thin film of drip water on the surface of stalagmites is often considered to have a large effect on both speleothem growth and stable isotope values and is offered as an explanation for higher δ13C and δ18O values than expected under conditions of stable isotope equilibrium. However, the time constant for degassing of CO2 from the solution only depends on film thickness and the coefficient of molecular diffusion for CO2. Thus, for thin films, the time for degassing of CO2 is much shorter than the time for subsequent equilibration of the dissolved carbon species and precipitation of CaCO3. In this context, degassing of CO2 is always fast.Here we present three experiments that enable the determination of the time constants for degassing of CO2, τdeg, subsequent equilibration to a new pCO2, τeq, and precipitation of CaCO3, τpr, in a thin film of an H2O–CO2–CaCO3 solution flowing on a calcite surface. The experiments are performed under cave-analogue conditions.At a temperature of 20°C and for a film thickness of δ≈0.01cm, τdeg≈2s. τeq≈13s and, thus, one order of magnitude larger. Finally, τpr≈400s for δ≈0.01cm, again one order of magnitude larger. The experimentally determined values for τdeg, τeqτpr are in good agreement with the theoretical predictions.Our results confirm that the chemical evolution of the drip water proceeds in three subsequent major steps. During the first step of degassing of CO2, pH and Ca2+ concentration remain almost constant. During equilibration to the lower pCO2 of the solution, pH increases to about 8 whereas the Ca2+ concentration still remains constant. Finally, during precipitation of calcite, pCO2 remains at its low level and pH decreases slightly.These results suggest that the drip rate may have an important influence on the stable isotope signals recorded in speleothems. Stalagmites growing beneath drip sites with stable, intermediate drip rates (i.e., in the range of τeq) may be best suited for palaeoclimate reconstruction.

Evolution of Radiocarbon in a Sandy Aquifer Across Large Temporal and Spatial Scales: Case Study from Southern Poland

We present the results of a comprehensive study aimed at tracing the evolution of carbon isotopic composition of the TDIC (total dissolved inorganic carbon) reservoir from the unsaturated zone down to the discharge area, in a sandy aquifer near Kraków, southern Poland. A multilevel well penetrating the unsaturated zone in the study area was equipped with horizontally mounted lysimeters with ceramic suction cups to collect samples of pore water and metal probes to collect soil air. Strong seasonal fluctuations were observed of soil pCO2 extending down to the water table, coupled with distinct, well-defined depth profiles of δ13CTDIC reaching approximately −10′ at 8 m depth and almost constant radiocarbon content in the TDIC pool, comparable to 14CO2 levels in the local atmosphere. Simple models (closed/open system) do not account for the observed depth variations of carbon isotopic composition of the TDIC pool. This suggests that the TDIC reservoir of pore waters is evolving under conditions gradually changing from an open towards a closed system. In order to explain 13C and 14C content of dissolved carbonates in groundwater in the recharge area of the studied aquifer, additional sources of carbon in the system are considered, such as organic matter decomposition accompanied by reduction of dissolved nitrates and sulfates. The piston-flow l4C ages of groundwater in the confined part of the studied system were calculated using 2 approaches: 1) the correction model proposed by Fontes and Garnier (1979) was used to calculate groundwater ages, utilizing the chemical and carbon isotopic data available for the sampled wells; and 2) inverse geochemical modeling was performed for selected pairs of wells using NETHPATH code. The calculated 14C ages of groundwater range from approximately 0.6 to 37.5 ka BP. Although both methods appeared to be in a broad agreement, NETHPATH calculations revealed that isotopic exchange processes between TDIC pool and solid carbonates present in relatively small amounts in the aquifer matrix play an important role in controlling the 13C and 14C signatures of the dissolved carbonate species in groundwater and should be taken into account when 14C ages are calculated.

Systematic review of forsterite dissolution rate data

This paper demonstrates a method for systematic analysis of published mineral dissolution rate data using forsterite dissolution as an example. The steps of the method are: (1) identify the data sources, (2) select the data, (3) tabulate the data, (4) analyze the data to produce a model, and (5) report the results. This method allows for a combination of critical selection of data, based on expert knowledge of theoretical expectations and experimental pitfalls, and meta-analysis of the data using statistical methods.Application of this method to all currently available forsterite dissolution rates (0<pH<14, and 0<T<150°C) normalized to geometric surface area produced the following rate equations:For pH<5.6 and 0°<T<150°C, based on 519 datalogrgeo=6.05(0.22)-0.46(0.02)pH-3683.0(63.6)1/T(R2=0.88)For pH>5.6 and 0°<T<150°C, based on 125 datalogrgeo=4.07(0.38)-0.256(0.023)pH-3465(139)1/T(R2=0.92)The R2 values show that ∼10% of the variance in r is not explained by variation in 1/T and pH. Although the experimental error for rate measurements should be±∼30%, the observed error associated with the logr values is ∼0.5logunits (±300% relative error). The unexplained variance and the large error associated with the reported rates likely arises from the assumption that the rates are directly proportional to the mineral surface area (geometric or BET) when the rate is actually controlled by the concentration and relative reactivity of surface sites, which may be a function of duration of reaction. Related to these surface area terms are other likely sources of error that include composition and preparation of mineral starting material.Similar rate equations were produced from BET surface area normalized rates. Comparison of rate models based on geometric and BET normalized rates offers no support for choosing one normalization method over the other. However, practical considerations support the use of geometric surface area normalization. Comparison of Mg and Si release rates showed that they produced statistically indistinguishable dissolution rates because dissolution was stoichiometric in the experiments over the entire pH range even though the surface concentrations of Mg and Si are known to change with pH. Comparison of rates from experiments with added carbonate, either from CO2 partial pressures greater than atmospheric or added carbonate salts, showed that the existing data set is not sufficient to quantify any effect of dissolved carbonate species on forsterite dissolution rates.

Enhancement in Electron Field Emission of Microcrystalline Diamond Films upon Iron Coating and Annealing Processes

The electron field emission (EFE) properties of diamond films were markedly improved by Fe-coating and postannealing processes. Transmission electron microscopy examination indicated that the possible mechanism of enhancing the EFE behavior is the reaction of the Fe layer with diamond and the reprecipitation of the dissolved carbon species to form nanographite. Scanning electron micrographs showed that the Fe coating first formed Fe particles at 700 °C and then reacted with diamond, forming iron carbide (Fe3C) at 800 °C. The dissolution and reprecipitation processes occurred simultaneously during the postannealing process at higher temperatures (800–950 °C), which leads to the formation of amorphous carbon when the postannealing temperature is low (800–850 °C) and to that of nanographite when the postannealing temperature is high (900–950 °C). The 900 °C-postannealed diamond films exhibit the best EFE properties, which can be turned on at a field of E0=2.8 V/µm, and attain an EFE current density of Je=21.4 µA/cm2 at 8 V/µm.

Geochemical evidence for and characterization of CO 2 rich gas sources in the epicentral area of the Abruzzo 2009 earthquakes

We describe the results of a detailed hydrogeochemical campaign on the groundwater circulating in two regional aquifers located in the area of the Abruzzo 2009 earthquakes. The influx of deeply derived CO 2 rich gases into the two aquifers is highlighted by the 13C isotopic composition of dissolved carbon species. The source of the gas is roughly localised beneath the epicentral area of the earthquakes where the presence of sources of fluids under high pressure is suggested by seismological investigations. The carbon isotopic-mass balance of the aquifers indicates that the amount of the deep CO 2 dissolved and transported by the groundwaters is ~ 530 t/day. The chemical and isotopic composition of the gas entering the aquifers, named Abruzzo gas, has been derived by comparing the data measured in the springs with the results of a gas–water–rock reaction model, that simulates the evolution of the chemical and isotopic composition of groundwater affected by the input of a deeply-derived CO 2 rich gas phase. The composition of Abruzzo gas is compared to that of 40 large gas emissions located in central Italy. The gas becomes progressively richer in radiogenic elements ( 4He and 40Ar) and in N 2, from the volcanic complexes in the west to the Apennines in the east. The Abruzzo gas, in agreement with its location, well matches the composition of the gases emitted in the pre-Apennine region. These geochemical features, consistent with the structural setting of the region, indicate increasing residence times of the gas in the crust moving from west to east. In particular we suggest that the strong increase in radiogenic crustal gases reflects the occurrence of deep traps where the gas is stored at high pressures for a long time and that such high pressure gas pockets play a major role in the generation of Apennine earthquakes.

On the molecular diffusion coefficients of dissolved [formula omitted], and [formula omitted] and their dependence on isotopic mass

The molecular diffusion coefficients of dissolved carbon dioxide ( CO 2 ) , bicarbonate ion ( HCO 3 - ) , and carbonate ion ( CO 3 2 - ) are fundamental physico-chemical constants and are of practical significance in various disciplines including geochemistry, biology, and medicine. Yet, very little experimental data is available, for instance, on the bicarbonate and carbonate ion diffusion coefficient. Furthermore, it appears that no information was hitherto available on the mass-dependence of the diffusion coefficients of the ionic carbonate species in water. Here I use molecular dynamics simulations to study the diffusion of the dissolved carbonate species in water, including their dependence on temperature and isotopic mass. Based on the simulations, I provide equations to calculate the diffusion coefficients of dissolved CO 2 , HCO 3 - , and CO 3 2 - over the temperature range from 0° to 100 °C. The results indicate a mass-dependence of CO 2 diffusion that is consistent with the observed 12 CO 2 / 13 CO 2 diffusion ratio at 25 °C. No significant isotope fractionation appears to be associated with the diffusion of the naturally occurring isotopologues of HCO 3 - and CO 3 2 - at 25 °C.

Microbially Enhanced Carbon Capture and Storage by Mineral-Trapping and Solubility-Trapping

The potential of microorganisms for enhancing carbon capture and storage (CCS) via mineral-trapping (where dissolved CO(2) is precipitated in carbonate minerals) and solubility trapping (as dissolved carbonate species in solution) was investigated. The bacterial hydrolysis of urea (ureolysis) was investigated in microcosms including synthetic brine (SB) mimicking a prospective deep subsurface CCS site with variable headspace pressures [p(CO(2))] of (13)C-CO(2). Dissolved Ca(2+) in the SB was completely precipitated as calcite during microbially induced hydrolysis of 5-20 g L(-1) urea. The incorporation of carbonate ions from (13)C-CO(2) ((13)C-CO(3)(2-)) into calcite increased with increasing p((13)CO(2)) and increasing urea concentrations: from 8.3% of total carbon in CaCO(3) at 1 g L(-1) to 31% at 5 g L(-1), and 37% at 20 g L(-1). This demonstrated that ureolysis was effective at precipitating initially gaseous [CO(2)(g)] originating from the headspace over the brine. Modeling the change in brine chemistry and carbonate precipitation after equilibration with the initial p(CO(2)) demonstrated that no net precipitation of CO(2)(g) via mineral-trapping occurred, since urea hydrolysis results in the production of dissolved inorganic carbon. However, the pH increase induced by bacterial ureolysis generated a net flux of CO(2)(g) into the brine. This reduced the headspace concentration of CO(2) by up to 32 mM per 100 mM urea hydrolyzed because the capacity of the brine for carbonate ions was increased, thus enhancing the solubility-trapping capacity of the brine. Together with the previously demonstrated permeability reduction of rock cores at high pressure by microbial biofilms and resilience of biofilms to supercritical CO(2), this suggests that engineered biomineralizing biofilms may enhance CCS via solubility-trapping, mineral formation, and CO(2)(g) leakage reduction.

Dissolved carbon species in associated diffuse and focused flow hydrothermal vents at the Main Endeavour Field, Juan de Fuca Ridge: Phase equilibria and kinetic constraints

Despite significant advances in the understanding of hydrothermal processes at mid‐ocean ridges, the linkage between focused flow and diffusively flowing vent fluids has remained elusive. Here, we report the distribution of dissolved carbon species in fluids issuing from associated low‐, moderate‐, and high‐temperature vent systems, collected in 2005 at the Main Endeavour Field (MEF), Juan de Fuca Ridge. Excess CO(aq) measured in the moderate‐temperature diffuse flow fluids indicates redox disequilibria between CO(aq) and CO2(aq), suggesting hydrothermal circulation of short residence time in near‐seafloor mixing zones. According to geochemical modeling, the chemical variability of MEF fluids is consistent with a combination of conductive cooling, reaction transport, and subsequent seawater mixing of a single source fluid. The similar distribution of C1‐C3 alkanes observed in diffuse and focused flow fluids likely indicates a minimal fingerprint of biological and methanogenic metabolism on organic fluxes, consistent with a possible short‐lived hydrothermal near‐seafloor circulation. Accordingly, dissolved carbon species in low‐temperature vent fluids can serve as geochemical proxies to distinguish the extent of compositional change in subsurface microbial habitats in the MEF hydrothermal fluids and elsewhere. Part of this reflects conditions unique to the near‐surface, diffuse flow environment, but the flux of components from deeper‐seated hydrothermal processes is important as well. In comparison with earlier measurements (1999 and 2000) of high‐temperature vent fluids at MEF, the 2005 data set exhibits lower Li/Cl ratios and H2(aq) concentrations indicative of higher fluid/rock mass ratios and more oxidizing and/or lower‐temperature conditions, respectively.

Sequestration of CO2 in Mixtures of Caustic Byproduct and Saline Waste Water

Ex-situ carbonation of mixtures of caustic byproduct materials and produced oil-field brine provides a niche opportunity to sequester anthropogenic CO2, while concomitantly reducing the basicity of the reactive slurry. A series of tests were conducted to investigate a novel reaction concept designed to achieve neutralization of mixtures of acidic oil field produced brine and caustic industrial byproducts while sequestering substantial quantities of anthropogenic carbon dioxide (CO2) in a mixed-flow reactor. Experiments were conducted to determine the CO2-bearing capacity of reactive mixtures of brine from the Oriskany Sandstone Formation with three caustic industrial byproducts: flue gas desulfurization (FGD) spray dryer ash, Class C fly ash subbituminous coal combustion byproduct, and bauxite residue slurry from the alumina production process. Reactions were conducted in a closed, well-mixed (1,500 rpm) reactor with gas composed of 29.46% vol./vol. CO2 balanced by nitrogen gas (N2) fed at a rate of 300 mL/min. Reactions were carried out at ambient conditions. Results show linear relationships between caustic byproduct addition and CO2-bearing capacity, with relatively small impact of brine addition as compared to deionized water addition. FGD spray dryer ash/brine mixtures exhibited higher CO2 reactivity than those using Class C fly ash (0.759 moles CO2, at 23.6% solids by weight and 0.036 moles CO2 at 23.3% solids by weight, respectively). Bauxite residue exhibited moderate capacities in mixtures with higher percent solids (0.335 moles CO2 in 40% solids bauxite residue slurry). Carbonation capacity of caustic byproduct/acidic brine mixtures was shown to increase linearly with respect to percent caustic byproduct addition, but enhanced mineral carbonate precipitation resulting from synergistic reaction of brine cations with increased dissolved carbonate species was not observed in the short term.

The stable carbon isotope biogeochemistry of acetate and other dissolved carbon species in deep subseafloor sediments at the northern Cascadia Margin

Ocean drilling has revealed the existence of vast microbial populations in the deep subseafloor, but to date little is known about their metabolic activities. To better understand the biogeochemical processes in the deep biosphere, we investigate the stable carbon isotope chemistry of acetate and other carbon-bearing metabolites in sediment pore-waters. Acetate is a key metabolite in the cycling of carbon in anoxic sediments. Its stable carbon isotopic composition provides information on the metabolic processes dominating acetate turnover in situ. This study reports our findings for a methane-rich site at the northern Cascadia Margin (NE Pacific) where Expedition 311 of the Integrated Ocean Drilling Program (IODP) sampled the upper 190 m of sediment. At Site U1329, δ 13C values of acetate span a wide range from −46.0‰ to −11.0‰ vs. VPDB and change systematically with sediment depth. In contrast, δ 13C values of both the bulk dissolved organic carbon (DOC) (−21.6 ± 1.3‰ vs. VPDB) and the low-molecular-weight compound lactate (−20.9 ± 1.8‰ vs. VPDB) show little variability. These species are interpreted to represent the carbon isotopic composition of fermentation products. Relative to DOC, acetate is up to 23.1‰ depleted and up to 9.1‰ enriched in 13C. Broadly, 13C-depletions of acetate relative to DOC indicate flux of carbon from acetogenesis into the acetate pool while 13C-enrichments of pore-water acetate relative to DOC suggest consumption of acetate by acetoclastic methanogenesis. Isotopic relationships between acetate and lactate or DOC provide new information on the carbon flow and the presence and activity of specific functional microbial communities in distinct biogeochemical horizons of the sediment. In particular, they suggest that acetogenic CO 2-reduction can coexist with methanogenic CO 2-reduction, a notion contrary to the hypothesis that hydrogen levels are controlled by the thermodynamically most favorable electron-accepting process. Further, the isotopic relationship suggests a relative increase in acetate flow to acetoclastic methanogenesis with depth although its contribution to total methanogenesis is probably small. Our study demonstrates how the stable carbon isotope biogeochemistry of acetate can be used to identify pathways of microbial carbon turnover in subsurface environments. Our observations also raise new questions regarding the factors controlling acetate turnover in marine sediments.

An expression for the overall oxygen isotope fractionation between the sum of dissolved inorganic carbon and water

Stable oxygen isotope fractionation factors (α's) between individual dissolved carbonate species (CO2(aq.), H2CO3, HCO3−, CO32−) and water, and their sum ( = [CO2(aq.)] + [H2CO3] + [HCO3−] + [CO32−]) and water are fundamental geochemical parameters. While values of , α, and have recently been examined, an overall mass balance for stable oxygen isotopes in the carbonate system still needs to be derived. The theoretical treatment of this mass balance is not trivial (in contrast to, e.g., stable carbon isotopes) because (1) oxygen is exchanged with H2O and (2) complications arise from the nature of the oxygen mole fractions. I use the recently reported individual α's to derive an expression for the overall oxygen isotope fractionation between and water. This allows the calculation of α(−H2O) at any given pH of the solution. The result differs from a relationship published earlier, and implications for previous studies are discussed. Numerical routines in MATLAB and Excel for the present calculation have been developed and are available as auxiliary material (Software S1). Applications of the expression include, for example, examination of biomineralization mechanisms and corrections to isotopic paleotemperature equations. The latter application can significantly alter reconstructions of Phanerozoic climate change.