A numerical model of trace-element coprecipitation in a physicochemical calcification system: Application to coral biomineralization and trace-element ‘vital effects’

A mixed equilibrium/kinetic steady-state numerical model of coral calcification has been developed to test whether a physicochemical calcification mechanism is able to account for recent geochemical observations, in particular correlated trace-element variations presented in a companion paper [Sinclair, D.J., 2005. Correlated trace-element ‘vital effects’ in tropical corals: a new tool for probing biomineralization chemistry. Geochim. Cosmochim. Acta69 (13), 3265-3284]. The model simulates trace-element partitioning from a CaCO₃ supersaturated extracellular calcifying fluid (ECF) which has been modified by enzymatic input of Ca²⁺ and removal of 2H⁺ by CaATPase. CO₂ input is modelled as a diffusion process, while the ECF is continuously replenished by fresh seawater, which is the sole source of minor and trace-elements (TEs). Trace-element species fully equilibrate in the ECF, and selected trace-element species kinetically compete with Ca²⁺ or at the surface of the growing crystal. Each simulation is run to steady-state, and results are presented for a grid of CaATPase ion pumping rates and seawater replenishment rates. The dominant feature of the model output occurs when CaATPase ion pumping is high while seawater replenishment rates are low. At this point, CO₂ diffusion reaches its maximum, C input becomes limiting, buffering capacity is reduced and the pH of the system rises dramatically; significantly affecting the TE composition of the skeleton. At more modest pumping rates, the model reproduces the relative amplitudes of trace-element variations and slopes of the mutually positive correlations between B, Sr and U observed by Sinclair [Sinclair, D.J., 2005. Correlated trace-element ‘vital effects’ in tropical corals: a new tool for probing biomineralization chemistry. Geochim. Cosmochim. Acta69 (13), 3265-3284], but does not reproduce the negative correlations with Mg. The best fit between model and observation occurs when the coral simultaneously increases ion pumping and seawater replenishment rates: a strategy which allows rapid calcification while avoiding dangerously high pH variations. The model predicts that calcification occurs at only moderate pH elevations (8.3-8.4) with seasonal TE variations being explained by a shift of only 0.3 pH units. The model does not reproduce the full amplitude of diurnal pH variations observed recently. Sensitivity tests show that the model output is relatively insensitive to changes in the composition of the fluid from which the ECF is drawn (such as might occur if photosynthesis or active C transport mechanisms significantly modify the penultimate fluid source). Further research, however, is needed to establish the consequences of active transport of TEs and anions to the calcifying site.     

Clues from Current High CO2 Environments on the Effects of Ocean Acidification on CaCO3 Preservation

Acidification of surface seawater owing to anthropogenic activities has raised serious concerns on its consequences for marine calcifying organisms and ecosystems. To acquire knowledge concerning the future consequences of ocean acidification (OA), researchers have relied on incubation experiments with organisms exposed to future seawater conditions, numerical models, evidence from the geological record, and recently, observations from aquatic environments exposed to naturally high CO₂ and low pH, e.g., owing to volcanic CO₂ vents, upwelling, and groundwater input. In the present study, we briefly evaluate the distribution of dissolved CO₂–carbonic acid parameters at (1) two locations in the Pacific and the Atlantic Ocean as a function of depth, (2) a mangrove environment in Bermuda, (3) a seasonally stratified body of water in a semi-enclosed sound in Bermuda, and (4) in temporarily isolated tide pools in Southern California. We demonstrate that current in situ conditions of seawater pCO₂, pH, and CaCO₃ saturation state (Ω) in these environments are similar or even exceed the anticipated changes to these parameters in the open ocean over the next century as a result of OA. The observed differences between the Pacific and Atlantic Oceans with respect to seawater CO₂–carbonic acid chemistry, preservation of CaCO₃ minerals, and the occurrence and distribution of deep-sea marine calcifiers, support the hypothesized negative effects of OA on the production and preservation of CaCO₃ in surface seawater. Clues provided from shallow near-shore environments in Bermuda and Southern California support these predictions, but also highlight that many marine calcifiers already experience relatively high seawater pCO₂ and low pH conditions.     

Is Ocean Acidification an Open-Ocean Syndrome? Understanding Anthropogenic Impacts on Seawater pH

Ocean acidification due to anthropogenic CO₂ emissions is a dominant driver of long-term changes in pH in the open ocean, raising concern for the future of calcifying organisms, many of which are present in coastal habitats. However, changes in pH in coastal ecosystems result from a multitude of drivers, including impacts from watershed processes, nutrient inputs, and changes in ecosystem structure and metabolism. Interaction between ocean acidification due to anthropogenic CO₂ emissions and the dynamic regional to local drivers of coastal ecosystems have resulted in complex regulation of pH in coastal waters. Changes in the watershed can, for example, lead to changes in alkalinity and CO₂ fluxes that, together with metabolic processes and oceanic dynamics, yield high-magnitude decadal changes of up to 0.5 units in coastal pH. Metabolism results in strong diel to seasonal fluctuations in pH, with characteristic ranges of 0.3 pH units, with metabolically intense habitats exceeding this range on a daily basis. The intense variability and multiple, complex controls on pH implies that the concept of ocean acidification due to anthropogenic CO₂ emissions cannot be transposed to coastal ecosystems directly. Furthermore, in coastal ecosystems, the detection of trends towards acidification is not trivial and the attribution of these changes to anthropogenic CO₂ emissions is even more problematic. Coastal ecosystems may show acidification or basification, depending on the balance between the invasion of coastal waters by anthropogenic CO₂, watershed export of alkalinity, organic matter and CO₂, and changes in the balance between primary production, respiration and calcification rates in response to changes in nutrient inputs and losses of ecosystem components. Hence, we contend that ocean acidification from anthropogenic CO₂ is largely an open-ocean syndrome and that a concept of anthropogenic impacts on marine pH, which is applicable across the entire ocean, from coastal to open-ocean environments, provides a superior framework to consider the multiple components of the anthropogenic perturbation of marine pH trajectories. The concept of anthropogenic impacts on seawater pH acknowledges that a regional focus is necessary to predict future trajectories in the pH of coastal waters and points at opportunities to manage these trajectories locally to conserve coastal organisms vulnerable to ocean acidification.     

δB, Sr, Mg and B in a modern Porites coral: the relationship between calcification site pH and skeletal chemistry

Sr/Ca, B/Ca, Mg/Ca and δ¹¹B were determined at high spatial resolution across ∼1 year of a modern Hawaiian Porites lobata coral by secondary ion mass spectrometry (SIMS). We observe significant variations in B/Ca, Mg/Ca, Sr/Ca and δ¹¹B over short skeletal distances (nominally equivalent to periods of <20 days). This heterogeneity probably reflects variations in the composition of the extracellular calcifying fluid (ECF) from which the skeleton precipitates. Calcification site pH (total scale) was estimated from skeletal δ¹¹B and ranged from 8.3 to 8.8 (± ∼0.1) with a mean of ∼8.6. Sr/Ca and B/Ca heterogeneity is not simply correlated with calcification site pH, as might be expected if Ca-ATPase activity increases the pH and decreases the Sr/Ca and B(OH)₄⁻/CO₃²⁻ ratios of the ECF. We produced a simple model of the ECF composition and the skeleton deposited from it, over a range of calcium transport and carbonate scenarios, which can account for these observed geochemical variations. The relationship between the pH and Sr/Ca of the ECF is dependent on the concentration of DIC at the calcification site. At higher DIC concentrations the ECF has a high capacity to buffer the [H⁺] changes induced by Ca-ATPase pumping. Conversely, at low DIC concentrations, this buffering capacity is reduced and ECF pH changes more rapidly in response to Ca-ATPase pumping. The absence of a simple correlation between ECF pH and skeletal Sr/Ca implies that calcification occurred under a range of DIC concentrations, reflecting variations in the respiration and photosynthesis of the coral and symbiotic zooxanthellate in the overlying coral tissues. Our observations have important implications for the use of coral skeletons as indicators of palaeo-ocean pH.     

A spectrophotometric study of aqueous Au(III) halide-hydroxide complexes at 25-80 °C

The mobility and transport of gold in low-temperature waters and brines is affected by the aqueous speciation of gold, which is sensitive in particular to pH, oxidation and halide concentrations. In this study, we use UV-Vis spectrophotometry to identify and measure the thermodynamic properties of Au(III) aqueous complexes with chloride, bromide and hydroxide. Au(III) forms stable square planar complexes with hydroxide and halide ligands. Based on systematic changes in the absorption spectra of solutions in three binary systems NaCl-NaBr, NaCl-NaOH and NaBr-NaOH at 25 °C, we derived log dissociation constants for the following mixed and end-member halide and hydroxide complexes: [AuCl₃Br]⁻, [AuCl₂Br₂]⁻, [AuBr₃Cl]⁻ and [AuBr₄]⁻; [AuCl₃(OH)]⁻, [AuCl₂(OH)₂]⁻, [AuCl(OH)₃]⁻ and [Au(OH)₄]⁻; and [AuBr₃(OH)]⁻, [AuBr₂(OH)₂]⁻ and [AuBr(OH)₃]⁻. These are the first reported results for the mixed chloride-bromide complexes. Increasing temperature to 80 °C resulted in an increase in the stability of the mixed chloride-bromide complexes, relative to the end-member chloride and bromide complexes. For the [AuCl_(4−n)(OH)_n]⁻ series of complexes (n = 0-4), there is an excellent agreement between our spectrophotometric results and previous electrochemical results of Chateau et al. [Chateau et al. (1966)]. In other experiments, the iodide ion (I⁻) was found to be unstable in the presence of Au(III), oxidizing rapidly to I₂(g) and causing Au to precipitate. Predicted Au(III) speciation indicates that Au(III) chloride-bromide complexes can be important in transporting gold in brines with high bromide-chloride ratios (e.g., >0.05), under oxidizing (atmospheric), acidic (pH < 5) conditions. Native gold solubility under atmospheric oxygen conditions is predicted to increase with decreasing pH in acidic conditions, increasing pH in alkaline conditions, increasing chloride, especially at acid pH, and increasing bromide for bromide/chloride ratios greater than 0.05. The results of our study increase the understanding of gold aqueous geochemistry, with the potential to lead to new methods for mineral exploration, hydrometallurgy and medicine.     

Evidence for ocean acidification in the Great Barrier Reef of Australia

Geochemical records preserved in the long-lived carbonate skeleton of corals provide one of the few means to reconstruct changes in seawater pH since the commencement of the industrial era. This information is important in not only determining the response of the surface oceans to ocean acidification from enhanced uptake of CO₂, but also to better understand the effects of ocean acidification on carbonate secreting organisms such as corals, whose ability to calcify is highly pH dependent. Here we report an ∼200 year δ¹¹B isotopic record, extracted from a long-lived Porites coral from the central Great Barrier Reef of Australia. This record covering the period from 1800 to 2004 was sampled at yearly increments from 1940 to the present and 5-year increments prior to 1940. The δ¹¹B isotopic compositions reflect variations in seawater pH, and the δ¹³C changes in the carbon composition of surface water due to fossil fuel burning over this period. In addition complementary Ba/Ca, δ¹⁸O and Mg/Ca data was obtained providing proxies for terrestrial runoff, salinity and temperature changes over the past 200 years in this region. Positive thermal ionization mass spectrometry (PTIMS) method was utilized in order to enable the highest precision and most accurate measurements of δ¹¹B values. The internal precision and reproducibility for δ¹¹B of our measurements are better than ±0.2‰ (2σ), which translates to a precision of better than ±0.02 pH units. Our results indicate that the long-term pre-industrial variation of seawater pH in this region is partially related to the decadal-interdecadal variability of atmospheric and oceanic anomalies in the Pacific. In the periods around 1940 and 1998 there are also rapid oscillations in δ¹¹B compositions equivalent changes in pH of almost 0.5 U. The 1998 oscillation is co-incident with a major coral bleaching event indicating the sensitivity of skeletal δ¹¹B compositions to loss of zooxanthellate symbionts. Importantly, from the 1940s to the present-day, there is a general overall trend of ocean acidification with pH decreasing by about 0.2-0.3 U, the range being dependent on the value assumed for the fractionation factor α_(B3-B4) of the boric acid and borate species in seawater. Correlations of δ¹¹B with δ¹³C during this interval indicate that the increasing trend towards ocean acidification over the past 60 years in this region is the result of enhanced dissolution of CO₂ in surface waters from the rapidly increasing levels of atmospheric CO₂, mainly from fossil fuel burning. This suggests that the increased levels of anthropogenic CO₂ in atmosphere has already caused a significant trend towards acidification in the oceans during the past decades. Observations of surprisingly large decreases in pH across important carbonate producing regions, such as the Great Barrier Reef of Australia, raise serious concerns about the impact of Greenhouse gas emissions on coral calcification.     

Solubility and equilibrium constants of lead in carbonate solutions (25°C,I = 0.3 mol dm−3)

The solubilities of PbCO₃(s), 2PbCO₂·Pb(OH)₂(s), and of 3PbCO₃ 2Pb(OH)₂(s) have been studied at 25°C ± 0.1°C in solutions of the constant ionic strength I = 0.3 mol/dm³, consisting primarily of sodium perchlorate. A few experiments with hydrocerussite were performed in solutions of 0.1 M KNO₃. The concentrations of lead and hydrogen ions have been determined in solution in contact with the solid phase. From experimental data the following values for equilibrium constants are obtained: log [Pb²⁺]·pCO₂·[H⁺]^?2 = 5.20log [Pb²⁺]·pCO^0.67₂·[H⁺]^?2 = 6.80log [Pb₂₊]³·[CO^2?₃]²·[OH^?]² = ?44.08 (and ?44.8 forI = 0.1 M)log [PbCO⁰₃]·[Pb²⁺]^?1·[CO^2?₃]^?1 = 5.40log [Pb(CO₃)^2?₂]·[Pb₂₊]^?1·[CO^2?₃]^?2 = 8.86 The data indicate that hydrocerussite is the most stable solid phase in natural waters. Comparison with the literature and needs for further research are also presented.     

Exploring aberrant bivalve shell ultrastructure and geochemistry as proxies for past sea water acidification

Throughout much of Earth's history, marine carbonates have represented one of the most important geological archives of environmental change. Several pivotal events during the Phanerozoic, such as mass extinctions or hyperthermal events have recently been associated with ocean acidification. Nevertheless, well‐defined geological proxies for past ocean acidification events are, at best, scarce. Here, experimental work explores the response of bivalve shell ultrastructure and isotope geochemistry (δ¹³C, δ¹⁸O and δ²⁶Mg) to stressful environments, in particular to sea water acidification. In this study, the common blue mussel, Mytilus edulis, was cultured (from early juvenile stages to one year of age) at four pH regimes (pH_NBS 7·2 to pH 8·0). Shell growth rate and ultrastructure of mainly the calcitic portion of the shells were compared between experimental treatments. Specimens exposed to low‐pH environments show patches of disordered calcitic fibre orientation in otherwise well‐structured shells. Furthermore, the electron backscattered diffraction analyses reveal that, under acidified conditions, the c‐axis of the calcite prisms exhibits a bimodal or multi‐modal distribution pattern. Similar shell disorder patterns have been reported from mytilids kept under naturally acidified sea water conditions. In contrast, this study found no evidence that different pH regimes affect shell carbon, oxygen or magnesium isotope ratios. Based on these observations, it is proposed that: (i) stressful environments, in this case low sea water pH, predictably affect bivalve biomineralization patterns; and (ii) these findings bear potential as a novel (petrographic) proxy for ancient sea water acidification. An assessment of the applicability of these data to well‐preserved fossil shell material from selected time intervals requires additional work.     

The study of soil acidification of paddy field influenced by acid mine drainage

The acidification of paddy fields was studied in Guizhou Province, China. Affected by acid mine drainage, the pH value of irrigation water was 2.9 with the concentrations of iron and aluminium above 40 mg/L. Based on the pH(H₂O) of topsoil, the paddy fields studied were classified spatially into three zones, the natural zone (pH value from 6.2 to 5.5), the acidified zone (pH value from 5.5 to 4.5), and the seriously acidified zone (pH value from 4.5 to 3.2), respectively. Comparing to the natural zone, the buffering processes for acidification of paddy soil were discussed by considering the changes of calcium, magnesium, potassium and aluminium in soils. The Ca, Mg and K were leached from the soil by the decomposition of carbonate and kaolinite. The leaching of Mg became less with the enrichment of iron in topsoil layer. When the soil pH was below 5.0, aluminium was leached from soil because of the dissolution of alumino silicate minerals. In addition, the hydrolysis of iron and aluminium in soil provided more protons to promote the soil acidification. Furthermore, the buffer capacity of paddy soil was discussed by the results of buffer experiment, based on which the pH buffer curve was drawn and the empirical formula for calculating the acidification rate was developed. Because pH buffer capacity of soil is about 2.78 cmol_c/kg pH for the pH(H₂O) value above 5.0, it is estimated that only another 50 years are needed for the pH(H₂O) of the paddy soil decrease to 3.5 in the acidified zone if the acid water is used for irrigation continuously.     

The adsorption of aquatic humic substances by iron oxides

The interactions of humic substances from Esthwaite Water with hydrous iron oxides (α-FeOOH, α-Fe₂O₃, amorphous Fe-gel) have been examined by measuring adsorption isotherms and by microelectrophoresis. In Na⁺-Cl^?-HCO₃^?at I = 0.002 M (medium I) the extent of adsorption decreases with increasing pH. The results are consistent with a mechanism involving ligand exchange of humic anionic groups with H₂O and OH^?of surface Fe-OH₂⁺and Fe-OH groups respectively, with an increasing degree of protonation of the adsorbed humics as the adsorption density increases at constant pH.At pH 7 in a medium containing Mg²⁺, Ca²⁺ and SO₄^2?, at their Esthwaite Water concentrations and at I= 0.002 M (medium II) the adsorption capacity of goethite (α-FeOOH) is approximately twice that in medium I. Electrophoresis experiments show that the extra capacity is associated with coadsorption of Mg²⁺ and/or Ca²⁺ ions.When the iron oxides are added to samples of Esthwaite Water itself they become negatively charged and plots of electrophoretic mobility against pH for the natural water are identical to those in medium II plus humics.     

Temperature dependence of calcite dissolution kinetics between 1 and 62°C at pH 2.7 to 8.4 in aqueous solutions

The kinetics of calcite dissolution in aqueous KCl-solutions far from equilibrium, between 1 and 62°C in the pH-range 2.7 to 8.4 have been investigated using a rotating disc apparatus. At neutral and alkaline pH in the mixed kinetic regime the empirical apparent activation energy (EAAE) for the surface chemical reaction rate constant is 54 ± 4 kJ mole^?1 for Carrara marble and 46 ± 4 kJ mole^?1 for Iceland spar. Under similar conditions the EAAE of the transport rate constant increases with decreasing temperature, but has a mean value of 27 ± 2 kJ mole^?1. The corresponding diffusion coefficient has a mean EAAE of 37 ± 3 kJ mole^?1 and this high EAAE is consistent with transport dependence on product diffusion in this H⁺-independent regime.In contrast, in acid solutions, where the rate approaches end-member transport control, the EAAE of the diffusion coefficient is 16 kJ mole^?1, also decreasing with increasing temperature. This is compatible with H⁺-diffusion to the surface being rate-controlling.In inhibitor-free natural systems, calcite dissolution kinetics far from equilibrium can be described in terms of three regimes: an H⁺-dependent regime (pH < 4 at 25°C), a transition regime (4 < pH < 5.5 at 25°C) and an H⁺-independent regime (pH > 5.5 at 25°C). At lower temperatures these boundaries move to higher pH values. The presence of inhibitors in natural systems may enhance surface controlled kinetics.     

Mechanism of kaolinite dissolution at room temperature and pressure Part II: kinetic study

Studies on the dissolution kinetics of kaolinite were performed using batch reactors at 25°C and in the pH range from 1 to 13. A rapid initial dissolution step was first observed, followed by a linear kinetic stage reached after approximately 600 hr of reaction during which the kaolinite dissolves congruently at pH < 4 and pH > 11. The apparent incongruency between pH 5 and 10 was due to the precipitation of an Al–hydroxide phase. The true dissolution rates were computed from the amount of Si released into solution. The rate dependence on pH can be described by: r = 10−12.19aH+0.55 + 10−14.36 + 10−10.71aOH−0.75Between pH 5 and 10, the rate is approximately constant, although a smooth minimum was observed at pH close to 9. mAn attempt was made to obtain a general rate law based on the coordination theory, which was first applied to the mineral dissolution studies by Stumm and co-workers. The kinetic data were combined with the results obtained for the surface speciation by Huertas et al. (1998). It is possible to express the linear dissolution rate as a simple power function of the concentration of the surface sites active in various pH ranges: r = 10−8.25 [>Al2OH2+] + 10−10.82 [>AlOH2+]0.5 + 10−9.1 [>Al2OH + >AlOH + >SiOH] + 103.78 [>Al2O− + >AlO−]3This equation assumes that the dissolution mechanism is mainly controlled by the two Al surface sites (external and internal structural hydroxyls, and aluminol at the crystal edges) under both acidic and alkaline conditions. The model reflects well the important contribution of the crystal basal planes to the dissolution of kaolinite.     

Solubility of Ca6[Al(OH)6]2(CrO4)3·26H2O,the chromate analog of ettringite; 5–75°C

Ca₆[Al(OH)₆]₂(CrO₄)₃·26H₂O, the chromate analog of the sulfate mineral ettringite, was synthesized and characterized by X-ray diffraction, Fourier transform infra-red spectroscopy, thermogravimetric analyses, energy dispersive X-ray spectrometry, and bulk chemical analyses. The solubility of the synthesized solid was measured in a series of dissolution and precipitation experiments conducted at 5–75°C and at initial pH values between 10.5 and 12.5. The ion activity product (IAP) for the reaction Ca₆[Al(OH)₆]₂(CrO₄)₃·26H₂O⇌6Ca²⁺+2Al(OH)⁻₄+3CrO²⁻₄+4OH⁻+26H₂O varies with pH unless a CaCrO_4(aq) complex is included in the speciation model. The log K for the formation of this complex by the reaction Ca²⁺+CrO²⁻₄=CaCrO_4(aq) was obtained by minimizing the variance in the IAP for Ca₆[Al(OH)₆]₂(CrO₄)₃·26H₂O. There is no significant trend in the formation constant with temperature and the average log K is 2.77±0.16 over the temperature range 5–75°C. The log solubility product (log K_SP) of Ca₆[Al(OH)₆]₂(CrO₄)₃·26H₂O at 25°C is −41.46±0.30. The temperature dependence of the log K_SP is log K_SP=A−B/T+D log(T) where A=498.94±48.99, B=27,499±2257, and D=−181.11±16.74. The values of ΔG⁰_r,298 and ΔH⁰_r,298 for the dissolution reaction are 236.6±3.9 and 77.5±2.4 kJ mol⁻¹. the values of ΔC⁰_P,r,298 and ΔS⁰_r,298 are −1506±140 and −534±83 J mol⁻¹ K⁻¹. Using these values and published standard state partial molal quantities for constituent ions, ΔG⁰_f,298=−15,131±19 kJ mol⁻¹, ΔH⁰_f,298=−17,330±8.6 kJ mol⁻¹, ΔS⁰₂₉₈=2.19±0.10 kJ mol⁻¹ K⁻¹, and ΔC⁰_Pf,298=2.12±0.53 kJ mol⁻¹ K⁻¹, were calculated.     

Dissolution of illite in saline-acidic solutions at 25 °C

The dissolution rate of illite, a common clay mineral in Australian soils, was studied in saline-acidic solutions under far from equilibrium conditions. The clay fraction of Na-saturated Silver Hill illite (K_1.38Na_0.05)(Al_2.87Mg_0.46Fe³⁺_0.39Fe²⁺_0.28Ti_0.07)[Si_7.02Al_0.98]O₂₀(OH)₄ was used for this study. The dissolution rates were measured using flow-through reactors at 25 ± 1 °C, solution pH range of 1.0-4.25 (H₂SO₄) and at two ionic strengths (0.01 and 0.25 M) maintained using NaCl solution. Illite dissolution rates were calculated from the steady state release rates of Al and Si. The dissolution stoichiometry was determined from Al/Si, K/Si, Mg/Si and Fe/Si ratios. The release rates of cations were highly incongruent during the initial stage of experiments, with a preferential release of Al and K over Si in majority of the experiments. An Al/Si ratio >1 was observed at pH 2 and 3 while a ratio close to the stoichiometric composition was observed at pH 1 and 4 at the higher ionic strength. A relatively higher K⁺ release rate was observed at I = 0.25 in 2-4 pH range than at I = 0.01, possibly due to ion exchange reaction between Na⁺ from the solution and K⁺ from interlayer sites of illite. The steady state release rates of K, Fe and Mg were higher than Si over the entire pH range investigated in the study. From the point of view of the dominant structural cations (Si and Al), stoichiometric dissolution of illite occurred at pH 1-4 in the higher ionic strength experiments and at pH ?3 for the lower ionic strength experiments. The experiment at pH 4.25 and at the lower ionic strength exhibited lower R_Al (dissolution rate calculated from steady state Al release) than R_Si (dissolution rate calculated from steady state Si release), possibly due to the adsorption of dissolved Al as the output solutions were undersaturated with respect to gibbsite. The dissolution of illite appears to proceed with the removal of interlayer K followed by the dissolution of octahedral cations (Fe, Mg and Al), the dissolution of Si is the limiting step in the illite dissolution process. A dissolution rate law showing the dependence of illite dissolution rate on proton concentration in the acid-sulfate solutions was derived from the steady state dissolution rates and can be used in predicting the impact of illite dissolution in saline acid-sulfate environments. The fractional reaction orders of 0.32 (I = 0.25) and 0.36 (I = 0.01) obtained in the study for illite dissolution are similar to the values reported for smectite. The dissolution rate of illite is mainly controlled by solution pH and no effect of ionic strength was observed on the dissolution rates.     

Land–sea carbon and nutrient fluxes and coastal ocean CO2 exchange and acidification: Past,present, and future

Epochs of changing atmospheric CO₂ and seawater CO₂–carbonic acid system chemistry and acidification have occurred during the Phanerozoic at various time scales. On the longer geologic time scale, as sea level rose and fell and continental free board decreased and increased, respectively, the riverine fluxes of Ca, Mg, DIC, and total alkalinity to the coastal ocean varied and helped regulate the C chemistry of seawater, but nevertheless there were major epochs of ocean acidification (OA). On the shorter glacial–interglacial time scale from the Last Glacial Maximum (LGM) to late preindustrial time, riverine fluxes of DIC, total alkalinity, and N and P nutrients increased and along with rising sea level, atmospheric PCO₂ and temperature led, among other changes, to a slightly deceasing pH of coastal and open ocean waters, and to increasing net ecosystem calcification and decreasing net heterotrophy in coastal ocean waters. From late preindustrial time to the present and projected into the 21st century, human activities, such as fossil fuel and land-use emissions of CO₂ to the atmosphere, increasing application of N and P nutrient subsidies and combustion N to the landscape, and sewage discharges of C, N, P have led, and will continue to lead, to significant modifications of coastal ocean waters. The changes include a rapid decline in pH and carbonate saturation state (modern problem of ocean acidification), a shift toward dissolution of carbonate substrates exceeding production, potentially leading to the “demise” of the coral reefs, reversal of the direction of the sea-to-air flux of CO₂ and enhanced biological production and burial of organic C, a small sink of anthropogenic CO₂, accompanied by a continuous trend toward increasing autotrophy in coastal waters.     

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.     

Microbial dissolution of calcite at T = 28 °C and ambient pCO2

This study used batch reactors to quantify the mechanisms and rates of calcite dissolution in the presence and absence of a single heterotrophic bacterial species (Burkholderia fungorum). Experiments were conducted at T = 28°C and ambient pCO₂ over time periods spanning either 21 or 35 days. Bacteria were supplied with minimal growth media containing either glucose or lactate as a C source, NH₄⁺ as an N source, and H₂PO₄⁻ as a P source. Combining stoichiometric equations for microbial growth with an equilibrium mass-balance model of the H₂O-CO₂-CaCO₃ system demonstrates that B. fungorum affected calcite dissolution by modifying pH and alkalinity during utilization of ionic N and C species. Uptake of NH₄⁺ decreased pH and alkalinity, whereas utilization of lactate, a negatively charged organic anion, increased pH and alkalinity. Calcite in biotic glucose-bearing reactors dissolved by simultaneous reaction with H₂CO₃ generated by dissolution of atmospheric CO₂ (H₂CO₃ + CaCO₃ → Ca²⁺ + 2HCO₃⁻) and H⁺ released during NH₄⁺ uptake (H⁺ + CaCO₃ → Ca²⁺ + HCO₃⁻). Reaction with H₂CO₃ and H⁺ supplied ∼45% and 55% of the total Ca²⁺ and ∼60% and 40% of the total HCO₃⁻, respectively. The net rate of microbial calcite dissolution in the presence of glucose and NH₄⁺ was ∼2-fold higher than that observed for abiotic control experiments where calcite dissolved only by reaction with H₂CO₃. In lactate bearing reactors, most H⁺ generated by NH₄⁺ uptake reacted with HCO₃⁻ produced by lactate oxidation to yield CO₂ and H₂O. Hence, calcite in biotic lactate-bearing reactors dissolved by reaction with H₂CO₃ at a net rate equivalent to that calculated for abiotic control experiments. This study suggests that conventional carbonate equilibria models can satisfactorily predict the bulk fluid chemistry resulting from microbe-calcite interactions, provided that the ionic forms and extent of utilization of N and C sources can be constrained. Because the solubility and dissolution rate of calcite inversely correlate with pH, heterotrophic microbial growth in the presence of nonionic organic matter and NH₄⁺ appears to have the greatest potential for enhancing calcite weathering relative to abiotic conditions.     

The effect of ionic interactions on the oxidation of metals in natural waters

The effect of ionic interactions of the major components of natural waters on the oxidation of Cu(I) and Fe(II) has been examined. The various ion pairs of these metals have been shown to have different rates of oxidation. For Fe(II), the chloride and sulfate ion pairs are not easily oxidized. The measured decrease in the rate constant at a fixed pH in chloride and sulfate solutions agrees very well with the values predicted. The effect of pH (6 to 8) on the oxidation of Fe(II) in water and seawater have been shown to follow the rate equation

$\text{-d in [Fe(II)]/dt = k}{}_1\text{β}{}_1\text{α}{}_{\mathrm{Fe}}\text{/[H}{}^{\mathrm{+}}\text{] + k}{}_2\text{β}{}_2\text{α}{}_{\mathrm{Fe}}\text{/[H}{}^{\mathrm{+}}\text{]}{}^2$

where k₁ and k₂ are the pseudo first order rate constants, β₁ and β₂ are the hydrolysis constants for Fe(OH)⁺ and Fe(OH)⁰. The value of α_FE is the fraction of free Fe²⁺. The value of k₁ (2.0 ±0.5 min^?1) in water and seawater are similar within experimental error. The value of k₂ (1.2 × 10⁵ min^?1) in seawater is 28% of its value in water in reasonable agreement with predictions using an ion pairing model.For the oxidation of Cu(I) a rate equation of the form

$\text{?d ln [Cu(I)]/dt = k}{}_0\text{α}{}_{\mathrm{Cu}}\text{+ k}{}_1\text{β}{}_1\text{α}{}_{\mathrm{Cu}}\text{[Cl]}$

was found where k₀ (14.1 sec^?1) and k₁ (3.9 sec^?1) are the pseudo first order rate constants for the oxidation of Cu⁺ and CuCl⁰, β₁ is the formation constant for CuCl⁰ and α_Cu is the fraction of free Cu⁺. Thus, unlike the results for Fe(II), Cu(I) chloride complexes have measurable rates of oxidation.     

Evaluation of aluminum speciation in surface waters in China and its environmental risk assessment

As the ongoing global research on acid precipitation is developing in depth, more and more attention has been paid to the ecological effects of aluminum (Al) due to its toxicity to plants and animals, which is caused by acid precipitation. As a very serious problem of terrestrial and aquatic environmental acidification occurs in China, especially in southwestern China, a systematic investigation of Al speciation in these regions is very important. In this paper, the Al speciation results of surface waters in China are reported and its ecological impacts is evaluated. More than 100 water samples were collected from about twenty provinces of China. Driscoll's Al speciation scheme combined with the modified MINQEL computer model is used for speciation of Al. This study shows that the ecological impacts of acidification are quite different between China and Western countries, because of different geographical environments and geological settings. In Western countries, acidification is mainly caused by NO₂^-. Due to low concentrations of K⁺, Na⁺, Ca²⁺, Mg²⁺, the buffer capacities of soil and water are weak. Therefore, natural waters can be acidified to pH<5 very easily, resulting in a considerable mobilization of Al and worsening of the ecological environment. In China, acid precipitation is mainly in the form of sulfuric acid. In northwestern China, concentrations of K⁺, Na⁺, Ca²⁺, Mg²⁺ are high in soil and surface waters. This leads to much higher capacity and a high resistance ability to acidification. The pH values of waters in this region are high (around 7) and no serious Al toxicity is found at present. However, in northeastern and southeastern China, the soil is rich in Al (unsaturated aluminosilicates in northeastern China, saturated aluminosilicates in north and central China, aluminum-rich soil in southeastern and southwestern China). The concentrations of K⁺, Na⁺, Ca²⁺, Mg²⁺ in soil and waters are lower than those of northwestern China. Therefore the buffer capacity is limited. Numerous surface waters have already been acidified and pH values declined to 5. The impacts of Al toxicity on ecological systems in these regions are very serious, especially in Jiangxi, Hubei Provinces and Chongqing Municipality.     

Theoretical and experimental investigation of nickel isotopic fractionation in species relevant to modern and ancient oceans

Nickel plays a central role as an enzyme co-factor in the metabolism of methanogenic Archaea. Methanogens can fractionate Ni isotopes during assimilation, opening the possibility of using the stable isotopic composition of Ni as a biomarker. However, other sources of Ni isotopic variations need to be evaluated before one can establish Ni isotopes as an unambiguous tracer of methanogenesis in the rock record. Equilibrium exchange of Ni between the different species present in the ocean is a potential source of isotopic fractionation. Through controlled laboratory experiments and theoretical calculations, we quantify equilibrium Ni isotope fractionation between different species relevant to the modern and ancient ocean: Ni(H₂O)₆²⁺, Ni(H₂O)₁₈²⁺, NiOH(H₂O)₅⁺, Ni(OH)₂(H₂O)₄, NiCl(H₂O)₅⁺, cis-NiCl₂(H₂O)₄, trans-NiCl₂(H₂O)₄, NiHS(H₂O)₅⁺, Ni(HS)₂(H₂O)₄, NiSO₄(H₂O)₄, NiHCO₃(H₂O)₄⁺, NiCO₃(H₂O)₄, and organic ligands (crown ether and oxalic acid). The magnitude of ligand-controlled Ni isotopic fractionation, approximately 1.25‰/amu (2.5‰ for the ⁶⁰Ni/⁵⁸Ni ratio), is similar to that previously measured in cultures of methanogenic Archaea.