Inhibition of calcite growth by alginate

The kinetics of calcite precipitation in the presence of alginate was investigated using the constant composition technique. In the concentration range investigated (0.0002-0.005 g L⁻¹), alginate inhibits calcite precipitation. The extent of inhibition increased with increased alginate concentration and decreased solution supersaturation. Alginate adsorption, derived from normalized calcite precipitation rates, is described satisfactorily by the Langmuir adsorption model. At lowest supersaturation, alginate adsorption onto calcite probably reaches its maximal uptake of 7.5E-4 g m⁻², corresponding to surface coverage of one molecule for each 200-300 nm², depending on the molecular mass of alginate. This means that one alginate molecule can be bound over 100-150 Ca surface sites. Initially, on the surface of the inhibited calcite, XPS identified alginate but after further time in solution, when the system had recovered, XPS demonstrated that it disappeared from the surface, presumably buried under the newly formed calcite. The alginate affinity constant decreases with increasing supersaturation, evidence for incomplete adsorption. A simple model based on competition between growth and desorption effectively describes the observed change in the adsorption constant.     

Kinetics of gypsum nucleation and crystal growth from Dead Sea brine

The Dead Sea brine is supersaturated with respect to gypsum (Ω = 1.42). Laboratory experiments and evaluation of historical data show that gypsum nucleation and crystal growth kinetics from Dead Sea brine are both slower in comparison with solutions at a similar degree of supersaturation. The slow kinetics of gypsum precipitation in the Dead Sea brine is mainly attributed to the low solubility of gypsum which is due to the high Ca²⁺/SO₄²⁻ molar ratio (115), high salinity (∼280 g/kg) and to Na⁺ inhibition.Experiments with various clay minerals (montmorillonite, kaolinite) indicate that these minerals do not serve as crystallization seeds. In contrast, calcite and aragonite which contain traces of gypsum impurities do prompt precipitation of gypsum but at a considerable slower rate than with pure gypsum. This implies that transportation inflow of clay minerals, calcite and local crystallization of minerals in the Dead Sea does not prompt significant heterogeneous precipitation of gypsum. Based on historical analyses of the Dead Sea, it is shown that over the last decades, as inflows to the lake decreased and its salinity increased, gypsum continuously precipitated from the brine. The increasing salinity and Ca²⁺/SO₄²⁻ ratio, which results from the precipitation of gypsum, lead to even slower kinetics of nucleation and crystal growth, which resulted in an increasing degree of supersaturation with respect to gypsum. Therefore, we predict that as the salinity of the Dead Sea brine continues to increase (accompanied by Dead Sea water level decline), although gypsum will continuously precipitate, the degree of supersaturation will increase furthermore due to progressively slower kinetics.     

Sr,Cd, Mn and Co distribution coefficients in calcite as a function of calcite precipitation rate

Distribution coefficients, as a function of precipitation rate, were determined for the metals Sr²⁺, Co²⁺, Mn²⁺ and Cd²⁺in calcite. A pH-stat was used to maintain a constant degree of-saturation, and hence precipitation rate, during each coprecipitation run. The precipitation rate was proportional to the degree of supersaturation and the mass of seed crystal introduced. Distribution coefficients (λ) as a function of rate were determined using radioactive isotopes for solutions with saturations $\text{Ω = 1}$ to $\text{Ω = 5.5}$. Strontium distribution coefficients increased with increasing precipitation rate, while Co, Mn and Cd distribution coefficients decreased with increasing precipitation rate. The following rate expressions (at 25°C) were derived: logλ_Sr = 0.249 log R ?1.57logλ_Mn = ?0.266 log R + 1.35logλ_Co = ?0.173 log R + 0.68logλ_Cd = ?0.194 log R + 1.46 where R is the observed precipitation rate in nmoles CaCO₃ per mg seed crystal per min.In separate experiments the uptake of radioactive isotopes was monitored during the recrystallization of calcite seed crystals. Rates of recrystallization were from 100 to 10, 000 times slower than the pH-stat experiments, but yielded distribution coefficients consistent with the above rate expressions.Using gross estimates of biogenic crystal growth rates, aragonite to calcite transformation rates, and the above Sr rate expression, biogenic calcite and diagenetic calcite Sr contents are estimated. These experiments indicate that in addition to solution composition, precipitation rate is a significant factor influencing the trace metal content of naturally occurring calcite.     

The kinetics of calcite precipitation from a high salinity water

Calcium carbonate is one of the most common and important scale-forming minerals in oilfield produced water, but the kinetics of CaCO₃ precipitation has been ignored in most scale prediction models because of the lack of reliable precipitation rate model. There are none in the open literature for oilfield conditions (temperature > 100°C, pressure > 200 bar and salinity > 0.5 mol kg− 1). In this work the kinetics of calcite (CaCO₃) precipitation from high salinity waters (up to 2 mol kg⁻¹) have been studied by a pH-free-drift method in a closed water system. This method. is much easier to operate than the often used steady-state method. The experimental results indicate that the calcite precipitation rate is not only affected by the solution CaCO₃ saturation level, but also by the solution pH, ionic strength and the concentration ratios of Ca to HCO₃₋ ions (C_Ca²⁺/C_HCO3⁻). When the concentration ratios of Ca to HCO₃⁻ ions are close to their chemical stoichiometric ratio of 0.5, the calcite growth from a supersaturated solution is believed to be surface reaction controlled. However, at higher C_Ca²⁺C_HCO3⁻ ratios, the transportation of the lattice ions to calcite crystal surface has to be considered.     

Comparison of two potential strategies of planktonic foraminifera for house building: Mg or H removal?

Marine organisms must possess strategies enabling them to initiate calcite precipitation despite the unfavorable conditions for inorganic precipitation in surface seawater. These strategies are poorly understood. Here we compare two potential strategies of marine calcifyers to manipulate seawater chemistry in order to initiate calcite precipitation: Removal of Mg²⁺ and H⁺ ions from seawater solutions. An experimental setup was used to monitor the onset of inorganic precipitation on seed crystals as a function of the Mg²⁺ concentration and pH in artificial seawater. We focused on precipitation rates typical for biogenic calcification in planktonic foraminifera (∼10⁻³ mol m⁻² h⁻¹) and time scales typical for the initiation of calcification in these organisms (minutes to hours). We find that the carbonate ion concentration has to increase by a factor of ∼13 when [Mg²⁺] increases from 0 to 53 mmol kg⁻¹ in order to maintain a typical biogenic precipitation rate. Model calculations for the energy requirement for various scenarios of Mg²⁺ and H⁺ removal including Ca²⁺ exchange and CO₂ diffusion are presented. We conclude that the more cost-effective strategy to initiate calcite precipitation in foraminifera is H⁺ removal, rather than Mg²⁺ removal.     

Rate-controlled calcium isotope fractionation in synthetic calcite

The isotopic composition of Ca (Δ⁴⁴Ca/⁴⁰Ca) in calcite crystals has been determined relative to that in the parent solutions by TIMS using a double spike. Solutions were exposed to an atmosphere of NH₃ and CO₂, provided by the decomposition of (NH₄)₂CO₃, following the procedure developed by previous workers. Alkalinity, pH and concentrations of CO₃²⁻, HCO₃⁻, and CO₂ in solution were determined. The procedures permitted us to determine Δ(⁴⁴Ca/⁴⁰Ca) over a range of pH conditions, with the associated ranges of alkalinity. Two solutions with greatly different Ca concentrations were used, but, in all cases, the condition [Ca²⁺]>>[CO₃²⁻] was met. A wide range in Δ(⁴⁴Ca/⁴⁰Ca) was found for the calcite crystals, extending from 0.04 ± 0.13‰ to −1.34 ± 0.15‰, generally anti-correlating with the amount of Ca removed from the solution. The results show that Δ(⁴⁴Ca/⁴⁰Ca) is a linear function of the saturation state of the solution with respect to calcite (Ω). The two parameters are very well correlated over a wide range in Ω for each solution with a given [Ca]. The linear correlation extended from Δ(⁴⁴Ca/⁴⁰Ca) = −1.34 ± 0.15‰ to 0.04 ± 0.13‰, with the slopes directly dependent on [Ca]. Solutions, which were vigorously stirred, showed a much smaller range in Δ(⁴⁴Ca/⁴⁰Ca) and gave values of −0.42 ± 0.14‰, with the largest effect at low Ω. It is concluded that the diffusive flow of CO₃²⁻ into the immediate neighborhood of the crystal-solution interface is the rate-controlling mechanism and that diffusive transport of Ca²⁺ is not a significant factor. The data are simply explained by the assumptions that: a) the immediate interface of the crystal and the solution is at equilibrium with Δ(⁴⁴Ca/⁴⁰Ca) ∼ −1.5 ± 0.25‰; and b) diffusive inflow of CO₃²⁻ causes supersaturation, thus precipitating Ca from the regions exterior to the narrow zone of equilibrium. The result is that Δ(⁴⁴Ca/⁴⁰Ca) is a monotonically increasing (from negative values to zero) function of Ω. We consider this model to be a plausible explanation of most of the available data reported in the literature. The well-resolved but small and regular isotope fractionation shifts in Ca are thus not related to the diffusion of very large hydrated Ca complexes, but rather due to the ready availability of Ca in the general neighborhood of the crystal-solution interface. The largest isotopic shift which occurs as a small equilibrium effect is then subdued by supersaturation precipitation for solutions where [Ca²⁺]>>[CO₃²⁻] + [HCO₃⁻]. It is shown that there is a clear temperature dependence of the net isotopic shifts that is simply due to changes in Ω due to the equilibrium “constants” dependence on temperature, which changes the degree of saturation and hence the amount of isotopically unequilibrated Ca precipitated. The effects that are found in natural samples, therefore, will be dependent on the degree of diffusive inflow of carbonate species at or around the crystal-liquid interface in the particular precipitating system, thus limiting the equilibrium effect.     

Interaction between dissolved silica and calcium carbonate: 1. Spontaneous precipitation of calcium carbonate in the presence of dissolved silica

The kinetics of spontaneous precipitation of CaCO₃ from aqueous solution in the presence of dissolved silica was investigated by recording pH as a function of time. The presence of dissolved silica, at concentrations below saturation with respect to the amorphous phase, decreases induction time for CaCO₃ nucleation, but does not affect CaCO₃ polymorphism. For a “pure” system without silica, the surface free energy, σ, determined from classical nucleation theory is 42 mJ m⁻². This agrees well with values reported in the literature for vaterite and indicates some degree of heterogeneous nucleation, which can occur because of the relatively low degree of supersaturation used for the experiments. In the presence of 1 and 2 mM silica, σ is 37 and 34 mJ m⁻², indicating an increasing degree of heterogeneous nucleation as the amount of polymeric silica increases. The ratio of Ca²⁺ to CO₃²⁻ activity was a governing parameter for determining which CaCO₃ polymorph precipitated. At high Ca²⁺ to CO₃²⁻ activity ratios, almost all initial solid was vaterite, whereas at low ratios, a mixture of vaterite and calcite was observed. In solutions with low Ca²⁺ to CO₃²⁻ activity ratios, the presence of silica at concentrations above saturation with respect to amorphous silica led to formation of only calcite and strongly influenced the crystalline structure and morphology of the precipitates. At high Ca²⁺ to CO₃²⁻ ratios, system behaviour did not differ from that without silica.     

Kinetics of calcite precipitation induced by ureolytic bacteria at 10 to 20°C in artificial groundwater

The kinetics of calcite precipitation induced in response to the hydrolysis of urea by Bacillus pasteurii at different temperatures in artificial groundwater (AGW) was investigated. The hydrolysis of urea by B. pasteurii exhibited a temperature dependence with first order rate constants of 0.91 d⁻¹ at 20°C, 0.18 d⁻¹ at 15°C, and 0.09 d⁻¹ at 10°C. At all temperatures, the pH of the AGW increased from 6.5 to 9.3 in less than 1 d. Dissolved Ca²⁺ concentrations decreased in an asymptotic fashion after 1 d at 20°C and 15°C, and 2 d at 10°C. The loss of Ca²⁺ from solution was accompanied by the development of solid phase precipitates that were identified as calcite by X-ray diffraction. The onset of calcite precipitation at each temperature occurred after similar amounts of urea were hydrolyzed, corresponding to 8.0 mM NH₄⁺. Specific rate constants for calcite precipitation and critical saturation state were derived from time course data following a second-order chemical affinity-based rate law. The calcite precipitation rate constants and critical saturation states varied by less than 10% between the temperatures with mean values of 0.16 ± 0.01 μmoles L⁻¹ d⁻¹ and 73 ±3, respectively. The highest calcite precipitation rates (ca. 0.8 mmol L⁻¹ d⁻¹) occurred near the point of critical saturation. While unique time course trajectories of dissolved Ca²⁺ concentrations and saturation state values were observed at different temperatures, calcite precipitation rates all followed the same asymptotic profile decreasing with saturation state regardless of temperature. This emphasizes the fundamental kinetic dependence of calcite precipitation on saturation state, which connects the otherwise dissimilar temporal patterns of calcite precipitation that evolved under the different temperature and biogeochemical regimes of the experiments.     

Experimental study of nickel(II) interaction with calcite: Adsorption and coprecipitation

Partitioning of Ni in calcite, CaCO₃, was evaluated with the aim of collecting data on partition and distribution coefficients and to enhance understanding about the interaction of Ni with the calcite surface and further incorporation into the bulk. This information will aid in the interpretation of geological processes for safety assessment of waste repositories and contamination of groundwater. Coprecipitation experiments were carried out by the constant addition method at 25 °C and pCO₂ = 1 and 10^−3.5 atm. Ni was moderately partitioned from solution into calcite. For dilute solid solutions (X_Ni < 0.001), Ni partition coefficients were estimated to be ∼1 and found to be weakly dependent on calcite precipitation rate in the range of 3-230 nmol m⁻² s⁻¹. Ni molar fraction in the solid is directly proportional to Ni concentration in the solution. The fit of the data to such a model is good evidence that Ni is taken up as a true solid solution, not simply by physical trapping.     

Modeling of subsurface calcite dissolution, including the respiration and reoxidation processes of marine sediments in the region of equatorial upwelling off Gabon

Mineralization of organic matter and the subsequent dissolution of calcite were simulated for surface sediments of the upper continental slope off Gabon by using microsensors to measure O₂, pH, pCO₂ and Ca²⁺ (in situ), pore-water concentration profiles of NO₃⁻, NH₄⁺, Fe²⁺, and Mn²⁺ and SO₄²⁻ (ex situ), as well as sulfate reduction rates derived from incubation experiments. The transport and reaction model CoTReM was used to simulate the degradation of organic matter by O₂, NO₃⁻, Fe(OH)₃ and SO₄²⁻, reoxidation reactions involving Fe²⁺ and Mn²⁺, and precipitation of FeS. Model application revealed an overall rate of organic matter mineralization amounting to 50 μmol C cm⁻² yr⁻¹, of which 77% were due to O₂, 17% to NO₃⁻ and 3% to Fe(OH)₃ and 3% to SO₄²⁻. The best fit for the pH profile was achieved by adapting three different dissolution rate constants of calcite ranging between 0.01 and 0.5% d⁻¹ and accounting for different calcite phases in the sediment. A reaction order of 4.5 was assumed in the kinetic rate law. A CaCO₃ flux to the sediment was estimated to occur at a rate of 42 g m⁻² yr⁻¹ in the area of equatorial upwelling. The model predicts a redissolution flux of calcite amounting to 36 g m⁻² yr⁻¹, thus indicating that ∼90% of the calcite flux to the sediment is redissolved.     

X-ray photoelectron spectroscopic studies of dolomite surfaces exposed to undersaturated and supersaturated aqueous solutions

Cleaved surfaces of dolomite were studied using ex-situ X-ray photoelectron spectroscopy (XPS) following exposure of the surfaces to various experimental conditions. Dolomite samples exposed to air, to a highly undersaturated solution (0.1 M NaCl, pH = 9), and to solution with a supersaturation (−Δμ/kT) of 5.5 (pH = 9) were investigated with semiquantitative methods of analysis to ascertain the degree of non-stoichiometry resulting at the dolomite surface from reactive conditions. It was found that the dolomite cleavage surface in undersaturated solution was not altered significantly from the stoichiometric surface termination. The composition of the cleaved surface after exposure to supersaturated solution, a surface known to have self-limiting growth characteristics under similar conditions, was found to be Ca²⁺ rich (Ca_xMg_2 − x(CO₃)₂, 1.7 > x > 1.3). The observations, while underscoring differences in hydration/dehydration kinetics of the two alkaline earth cations, suggest that achievement of equilibrium at dolomite-water interfaces may be subject to significant barriers from both undersaturated and supersaturated solutions.     

Geochemical controls on a calcite precipitating spring

A small spring fed stream was found to precipitate calcite by mainly inorganic processes and in a nonuniform manner. The spring water originated by rainwater falling in a 0.8 km² basin, infiltrating, and dissolving calcite and dolomite followed by dissolution of gypsum or anhydrite. The Ca²⁺/Mg²⁺ indicates that calcite is probably precipitated in the subsurface from a supersaturated solution. This water emerges from the spring still about 5 times supersaturated with respect to calcite and continues calcite precipitation. When 10 times supersaturation is reached, due to CO₂ degassing the precipitation is more rapid. The calcite accumulation from the stream with a flow of 5 l/s is calculated to be 12600 kg/yr with the highest rates in areas where CO₂ degassing is the greatest. The non-equilibrium, as shown by the high calcite supersaturation, is also reflected in a variable partitioning pattern for Sr²⁺ between the water and calcite.     

Effects of freshwater Synechococcus sp. cyanobacteria pH buffering on CaCO3 precipitation: Implications for CO2 sequestration

In the present study, a mixed-flow steady-state bio-reactor was designed to biomineralize CO₂ as a consequence of photosynthesis from active Synechococcus sp. Dissolved CO₂, generated by constant air bubbling of inorganic and cyanobacteria stock solutions, was the only source of inorganic carbon. The release of hydroxide ion by cyanobacteria from photosynthesis maintained highly alkaline pH conditions. In the presence of Ca²⁺ and carbonate species, this led to calcite supersaturation under steady state conditions. Ca²⁺ remained constant throughout the experiments showing the presence of steady state conditions. Similarly, the Synechococcus sp. biomass concentration remained stable within uncertainty. A gradual pH decrease was observed for the highest Ca²⁺ condition coinciding with the formation of CaCO₃. The high degree of supersaturation, under steady-state conditions, contributed to the stabilization of calcite and maintained a constant driving force for the mineral nucleation and growth. For the highest Ca²⁺ condition a fast crystal growth rate was consistent with rapid calcite precipitation as suggested further by affinity calculations. Although saturation state based kinetic precipitation models cannot accurately reflect the controls on crystal growth kinetics or reliably predict growth mechanisms, the relatively reaction orders obtained from modeling of calcite precipitation rates as function of decreasing carbonate concentration suggest that the precipitation occurred via surface-controlled rate determining reactions. These high reaction orders support in addition the hypothesis that crystal growth proceeded through complex surface controlled mechanisms. In conclusion, the steady state supersaturated conditions generated by a constant cyanobacteria biomass and metabolic activity strongly suggest that these microorganisms could be used for the development of efficient CO₂ sequestration methods in a controlled large-scale environment.     

Estimation of dolomite formation: Dolomite precipitation and dolomitization

Reactive-transport models are developed here that produce dolomite via two scenarios: primary dolomite (no CaCO₃ dissolution involved) versus secondary dolomite (dolomitization, involving CaCO₃ dissolution). Using the available dolomite precipitation rate kinetics, calculations suggest that tens of meters of thick dolomite deposits cannot form at near room temperature (25-35°C) by inorganic precipitation mechanism, though this mechanism will provide dolomite aggregates that can act as the nuclei for dolomite crystallization during later dolomitization stage. Increase in supersaturation, Mg⁺²/Ca⁺² ratio and CO₃^-2 on the formation of dolomite at near room temperature are subtle except for temperature.This study suggests that microbial mediation is needed for appreciable amount of primary dolomite formation. On the other hand, reactive-transport models depicting dolomitization (temperature range of 40 to 200°C) predicts the formation of two adjacent moving coupled reaction zones (calcite dissolution and dolomite precipitation) with sharp dolomitization front, and generation of >20% of secondary porosity. Due to elevated temperature of formation, dolomitization mechanism is efficient in converting existing calcite into dolomite at a much faster rate compared to primary dolomite formation.     

Experimental study of brucite dissolution and precipitation in aqueous solutions: surface speciation and chemical affinity control

Dissolution and precipitation rates of brucite (Mg(OH)₂) were measured at 25°C in a mixed-flow reactor as a function of pH (2.5 to 12), ionic strength (10⁻⁴ to 3 M), saturation index (−12 < log Ω < 0.4) and aqueous magnesium concentrations (10⁻⁶ to 5·10⁻⁴ M). Brucite surface charge and isoelectric point (pH_IEP) were determined by surface titrations in a limited residence time reactor and electrophoretic measurements, respectively. The pH of zero charge and pH_IEP were close to 11. A two-pK, one site surface speciation model which assumes a constant capacitance of the electric double layer (5 F/m²) and lack of dependence on ionic strength predicts the dominance of >MgOH₂⁺ species at pH < 8 and their progressive replacement by >MgOH° and >MgO⁻ as pH increases to 10-12. Rates are proportional to the square of >MgOH₂⁺ surface concentration at pH from 2.5 to 12. In accord with surface speciation predictions, dissolution rates do not depend on ionic strength at pH 6.5 to 11. Brucite dissolution and precipitation rates at close to equilibrium conditions obeyed TST-derived rate laws. At constant saturation indices, brucite precipitation rates were proportional to the square of >MgOH₂⁺ concentration. The following rate equation, consistent with transition state theory, describes brucite dissolution and precipitation kinetics over a wide range of solution composition and chemical affinity:

     

Magnesium-isotope fractionation during low-Mg calcite precipitation in a limestone cave - Field study and experiments

The chemical and isotopic composition of speleothem calcite and particularly that of stalagmites and flowstones is increasingly exploited as an archive of past environmental change in continental settings. Despite intensive research, including modelling and novel approaches, speleothem data remain difficult to interpret. A possible way foreword is to apply a multi-proxy approach including non-conventional isotope systems. For the first time, we here present a complete analytical dataset of magnesium isotopes (δ²⁶Mg) from a monitored cave in NW Germany (Bunker Cave). The data set includes δ²⁶Mg values of loess-derived soil above the cave (−1.0 ± 0.5‰), soil water (−1.2 ± 0.5‰), the carbonate hostrock (−3.8 ± 0.5‰), dripwater in the cave (−1.8 ± 0.2‰), speleothem low-Mg calcite (stalactites, stalagmites; −4.3 ± 0.6‰), cave loam (−0.6 ± 0.1‰) and runoff water (−1.8 ± 0.1‰) in the cave, respectively. Magnesium-isotope fractionation processes during weathering and interaction between soil cover, hostrock and solute-bearing soil water are non-trivial and depend on a number of variables including solution residence times, dissolution rates, adsorption effects and potential neo-formation of solids in the regolith and the carbonate aquifer. Apparent Mg-isotope fractionation between dripwater and speleothem low-Mg calcite is about 1000lnα_Mg-cc-Mg(aq) = −2.4‰. A similar Mg-isotope fractionation (1000lnα_Mg-cc-Mg(aq) ≈ −2.1‰) is obtained by abiogenic precipitation experiments carried out at aqueous Mg/Ca ratios and temperatures close to cave conditions. Accordingly, ²⁶Mg discrimination during low-Mg calcite formation in caves is highly related to inorganic fractionation effects, which may comprise dehydration of Mg²⁺ prior to incorporation into calcite, surface entrapment of light isotopes and reaction kinetics. Relevance of kinetics is supported by a significant negative correlation of Mg-isotope fractionation with the precipitation rate for inorganic precipitation experiments.     

Role of molecular oxygen in the dissolution of siderite and rhodochrosite

The dissolution of siderite (FeCO₃) and rhodochrosite (MnCO₃) under oxic and anoxic conditions is investigated at 298 K. The anoxic dissolution rate of siderite is 10^−8.65 mol m⁻² s⁻¹ for 5.5 < pH < 12 and increases as [H⁺]^0.75 for pH < 5.5. The pH dependence is consistent with parallel proton-promoted and water hydrolysis dissolution pathways. Atomic force microscopy (AFM) reveals a change in pit morphology from rhombohedral pits for pH > 4 to pits elongated at one vertex for pH < 4. Under oxic conditions the dissolution rate decreases to below the detection limit of 10⁻¹⁰ mol m⁻² s⁻¹ for 6.0 < pH < 10.3, and hillock precipitation preferential to steps is observed in concurrent AFM micrographs. X-ray photoelectron spectroscopy (XPS) and thermodynamic analysis identify the precipitate as ferrihydrite. At pH > 10.3, the oxic dissolution rate is as high as 10^−7.5 mol m⁻² s⁻¹, which is greater than under the corresponding anoxic conditions. A fast electron transfer reaction between solution O₂ or [Fe³⁺(OH)₄]⁻ species and surficial >Fe^II hydroxyl groups is hypothesized to explain the dissolution kinetics. AFM micrographs do not show precipitation under these conditions. Anoxic dissolution of rhodochrosite is physically observed as rhombohedral pit expansion for 3.7 < pH < 10.3 and is chemically explained by parallel proton- and water-promoted pathways. The dissolution rate law is 10^−4.93[H⁺] + 10^−8.45 mol m⁻² s⁻¹. For 5.8 < pH < 7.7 under oxic conditions, the AFM micrographs show a tabular precipitate growing by preferential expansion along the a-axis, though the macroscopic dissolution rate is apparently unaffected. For pH > 7.7 under oxic conditions, the dissolution rate decreases from 10^−8.45 to 10^−9.0 mol m⁻² s⁻¹. Flattened hillock precipitates grow across the entire surface without apparent morphological influence by the underlying rhodochrosite surface. XPS spectra and thermodynamic calculations implicate the precipitate as bixbyite for 5.8 < pH < 7.7 and MnOOH (possibly feitnkechtite) for pH >7.7.     

Ca isotopes in carbonate sediment and pore fluid from ODP Site 807A: The Ca(aq)-calcite equilibrium fractionation factor and calcite recrystallization rates in Pleistocene sediments

The calcium isotopic compositions (δ⁴⁴Ca) of 30 high-purity nannofossil ooze and chalk and 7 pore fluid samples from ODP Site 807A (Ontong Java Plateau) are used in conjunction with numerical models to determine the equilibrium calcium isotope fractionation factor (α_s−f) between calcite and dissolved Ca²⁺ and the rates of post-depositional recrystallization in deep sea carbonate ooze. The value of α_s−f at equilibrium in the marine sedimentary section is 1.0000 ± 0.0001, which is significantly different from the value (0.9987 ± 0.0002) found in laboratory experiments of calcite precipitation and in the formation of biogenic calcite in the surface ocean. We hypothesize that this fractionation factor is relevant to calcite precipitation in any system at equilibrium and that this equilibrium fractionation factor has implications for the mechanisms responsible for Ca isotope fractionation during calcite precipitation. We describe a steady state model that offers a unified framework for explaining Ca isotope fractionation across the observed precipitation rate range of ∼14 orders of magnitude. The model attributes Ca isotope fractionation to the relative balance between the attachment and detachment fluxes at the calcite crystal surface. This model represents our hypothesis for the mechanism responsible for isotope fractionation during calcite precipitation. The Ca isotope data provide evidence that the bulk rate of calcite recrystallization in freshly-deposited carbonate ooze is 30-40%/Myr, and decreases with age to about 2%/Myr in 2-3 million year old sediment. The recrystallization rates determined from Ca isotopes for Pleistocene sediments are higher than those previously inferred from pore fluid Sr concentration and are consistent with rates derived for Late Pleistocene siliciclastic sediments using uranium isotopes. Combining our results for the equilibrium fractionation factor and recrystallization rates, we evaluate the effect of diagenesis on the Ca isotopic composition of marine carbonates at Site 807A. Since calcite precipitation rates in the sedimentary column are many orders of magnitude slower than laboratory experiments and the pore fluids are only slightly oversaturated with respect to calcite, the isotopic composition of diagenetic calcite is likely to reflect equilibrium precipitation. Accordingly, diagenesis produces a maximum shift in δ⁴⁴Ca of +0.15‰ for Site 807A sediments but will have a larger impact where sedimentation rates are low, seawater circulates through the sediment pile, or there are prolonged depositional hiatuses.     

Carbonate and silicate weathering in two presently glaciated, crystalline catchments in the Swiss Alps

We present a weathering mass balance of the presently glaciated Rhône and Oberaar catchments, located within the crystalline Aar massif (central Switzerland). Annual chemical and physical weathering fluxes are calculated from the monthly weighted means of meltwater samples taken from July, 1999 to May, 2001 and are corrected for precipitation inputs. The meltwater composition issuing from the Oberaar and Rhône catchments is dominated by calcium, which represents 81% and 55% of the total cation flux respectively (i.e. 555 and 82-96 keq km⁻² yr⁻¹). The six to seven times higher Ca²⁺ denudation flux from the Oberaar catchment is attributed to the presence of a strongly foliated gneissic zone. The gneissic zone has an elevated calcite content (as reflected by the 4.6 times higher calcite content of the suspended sediments from Oberaar compared to Rhône) and a higher mechanical erosion rate (resulting in a higher flux of suspended sediment). The mean flux of suspended calcite of the Oberaar meltwaters during the ablation period is 7 times greater than that of the Rhône meltwaters. Taking the suspended calcite as a proxy for the total (including sub-glacial sediments) weathering calcite surface area, it appears that the available surface area is an important factor in controlling weathering rates. However, we also observe an increased supply of protons for carbonate dissolution in the Oberaar catchment, where the sulphate denudation flux is six times greater. Carbonic acid is the second important source of protons, and we calculate that three times as much atmospheric CO₂ is drawn down (short term) in the Oberaar catchment. Silica fluxes from the two catchments are comparable with each other, but are 100 kmol km² yr⁻¹ lower than fluxes from physically comparable, non-glaciated basins.