Etch pit formation on iron silicate surfaces during siderophore-promoted dissolution

Understanding the effects of microbiota on mineral alteration requires the ability to recognize evidence of bacteria-promoted dissolution on mineral surfaces. Although siderophores are known to promote mineral dissolution, their effects on mineral surfaces are not well known. We have utilized atomic force microscopy (AFM), X-ray photoelectron spectroscopy (XPS), and Mirau vertical scanning interferometry (VSI) to investigate surfaces after incubation with the siderophore desferrioxamine-B mesylate (DFAM) and under colonies of bacteria. Iron-silicate glass planchets chemically similar to hornblende were incubated in buffered growth medium with siderophore-producing bacteria (Bacillus sp.) for 46 days with parallel abiotic experiments conducted with and without 240 μM DFAM, with and without 0.01 g l^− 1 of microbially produced extracellular polysaccharides (EPS, alginate or xanthan gum). Some glass planchets were protected by dialysis tubing from direct contact with the EPS. Weekly sampling and analysis of all filtered sample solutions showed negligible Fe and Al release in the control experiments and significant release of Fe and Al in the presence of DFAM, with negligible changes in pH. Concentration of Fe in the filtered solutions after incubation with bacteria was below detection, consistent with uptake of Fe by cells. Release of Fe, Al, and Si in control, xanthan-only, and alginate-only experiments was negligible. Release of these elements was enhanced in all experiments containing DFAM, and greatest in alginate + DFAM experiments.

AFM and VSI analyses reveal widespread, small etch pits and greater root mean squared roughness on siderophore-exposed surfaces and fewer, localized, larger etch pits on bacteria-exposed surfaces. This is the first documented case of etch pit development during siderophore-promoted dissolution. Roughness was not affected by the growth medium, alginate, or xanthan gum alone. The roughness trends among samples correlate with trends in Fe depletion documented by XPS. Enhanced dissolution and roughness cannot be attributed to direct contact with EPS because no significant chemical or physical differences were observed between surfaces directly exposed to EPS and those protected by dialysis tubing. Acetate released from the EPS may have enhanced the siderophore-promoted dissolution. Siderophores produced by Bacillus sp. may be responsible for some of the ‘biopits.’ The difference in size and distribution of the biopits may be related to colonization.     

Effect of solution saturation state and temperature on diopside dissolution

Steady-state dissolution rates of diopside are measured as a function of solution saturation state using a titanium flow-through reactor at pH 7.5 and temperature ranging from 125 to 175°C. Diopside dissolved stoichiometrically under all experimental conditions and rates were not dependent on sample history. At each temperature, rates continuously decreased by two orders of magnitude as equilibrium was approached and did not exhibit a dissolution plateau of constant rates at high degrees of undersaturation. The variation of diopside dissolution rates with solution saturation can be described equally well with a ion exchange model based on transition state theory or pit nucleation model based on crystal growth/dissolution theory from 125 to 175°C. At 175°C, both models over predict dissolution rates by two orders of magnitude indicating that a secondary phase precipitated in the experiments. The ion exchange model assumes the formation of a Si-rich, Mg-deficient precursor complex. Lack of dependence of rates on steady-state aqueous calcium concentration supports the formation of such a complex, which is formed by exchange of protons for magnesium ions at the surface. Fit to the experimental data yields where the Mg-H exchange coefficient, n = 1.39, the apparent activation energy, E _a = 332 kJ mol^-1, and the apparent rate constant, k = 10^41.2 mol diopside cm^-2 s^-1. Fits to the data with the pit nucleation model suggest that diopside dissolution proceeds through retreat of steps developed by nucleation of pits created homogeneously at the mineral surface or at defect sites, where homogeneous nucleation occurs at lower degrees of saturation than defect-assisted nucleation. Rate expressions for each mechanism (i) were fit to where the step edge energy (α) for homogeneously nucleated pits were higher (275 to 65 mJ m^-2) than the pits nucleated at defects (39 to 65 mJ m^-2) and the activation energy associated with the temperature dependence of site density and the kinetic coefficient for homogeneously nucleated pits (E_{b-homogeneous} = 2.59 × 10^-16 mJ K^-1) were lower than the pits nucleated at defects (E_{b-defect assisted} = 8.44 × 10^-16 mJ K^-1).     

A calcite dissolution pulse in the Norwegian-Greenland Sea during the last deglaciation

Carbonate preservation profiles in the Atlantic Ocean and the Norwegian-Greenland Sea seem to be out-of phase during the last deglaciation. A possibly time transgressive deglacial preservation spike in the N-Atlantic overlaps with a major calcite dissolution pulse in the Norwegian Sea. Contemporaneously major changes in surface and bottom water circulation of the Norwegian-Greenland Sea occurred. Isotopic and sedimentological evidence suggests that bottom water formation in the Norwegian-Greenland Sea was almost shut down during the last maximum glaciation and probably also during the first part of the last deglaciation. During that time corrosive bottom waters might have filled the Norwegian Sea deep sea basins causing carbonate dissolution at the sea floor. Subsequently, the reinitiation of deep water formation could have been coupled with increased resuspension of organic matter and/or reworking of older organic matter rising the p CO₂ of bottom waters and contributing to carbonate dissolution at the sea floor. Additionally, large volumes of atmospheric CO₂ stored before the Younger Dryas might have been pumped into the deep sea possibly by downwelling of surface waters and been neutralized against carbonate at the sea floor.

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Zusammenfassung Erhaltungsprofile des Karbonatlösungszustandes, bestimmt an der planktonischen Foraminif ere N. pachyderma aus Sedimenten, die während der letzten Abschmelzphase der letzten Eiszeit abgelagert wurden, zeigen im Atlantik und in der Norwegisch-Grönländischen See eine deutliche Phasenverschiebung der Erhaltungszustände. Ein möglicherweise zeittransgressiver Erhaltungspeak im Atlantik tritt in zeitlicher überlappung mit einer Phase erhöhter Karbonatlösung in der Norwegischen See auf. Aufgrund verschiedener sedimentologischer Parameter (Korngrö\enspektren, C-org Gehalte, Sedimentstrukturen) und deutlicher Veränderungen der stabilen Sauerstoff- und Kohlenstoffisotopenverhältnisse benthonischer Foraminiferen erscheint es wahrscheinlich, da\ die Tiefenwasserproduktion in der Norwegisch-Grönländischen See während des Höchststandes der letzten Vereisung und möglicherweise auch während des ersten Abschnittes des Abschmelzvorgangs gestoppt war. Während dieses Zeitraumes war die vertikale Durchmischung der Wassermassen stark herabgesetzt und altes korrosives Bodenwasser füllte die Tiefseebecken der Norwegischen See und verursachte eine intensive Karbonatlösung am Meeresboden. Nachdem die Bodenwasserzirkulation und Tiefenwasserbildung später erneut einsetzte, wurde zunächst eine verstärkte Ventilation durch absinkendes, sauerstoffreiches Oberflächenwasser am Meeresboden verursacht. Die Oxidation von resuspendiertem und die Aufarbeitung von bereits abgelagertem organischem Material bedingte einen letzten starken Anstieg des CO₂ Partialdrucks im Bodenwasser und verstärkte die Karbonatlösung am Meeresboden. Eine mögliche zusätzliche Quelle liefert vor der Jüngeren Dryas in der Atmosphäre angereichertes CO₂, das möglicherweise durch »downwelling« von Oberflächenwasser zusätzlich in die Tiefsee gepumpt wurde.

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Résumé Au moyen den l'examen des foraminifères planctoniquesN. pachyderma, des profils de l'état de conservation des carbonates ont été établis dans les sédiments déposés dans l'Atlantique et dans la Mer de Norvège au cours de la phase de fusion de la dernière glaciation. Ces profils ne sont pas en corrélation: dans l'Atlantique apparaÎt un maximum de stabilité des carbonates, probablement transgressif dans le temps, qui correspond à une phase de forte dissolution dans la Mer de Norvège. Certains paramètres sédimentologiques (granularité, teneur en C organique, structures sédimentaires) ainsi que des variations de la composition isotopique de l'O et du C des foraminifères benthoniques permettent de penser que la production d'eau profonde s'est arrÊtée dans la Mer de Norvège au cours du maximum de la dernière glaciation et sans doute aussi pendant le début de la fusion qui a suivi. Pendant cette période, le mélange vertical des masses d'eau était fortement ralenti et une ancienne eau de fond à caractère corrosif occupait le bassin de la Mer de Norvège où elle induisait une forte dissolution des carbonates. Plus tard, lorsque se rétablirent la circulation de l'eau de fond et la formation d'eau profonde, on assita à une aération de la partie profonde de la mer, par l'effet de la descente d'eau superficielle riche en oxygène. L'oxydation de la matière organique provoqua un accroissement de la pression partielle de CO₂ dans l'eau de fond, ce qui y accentua la dissolution des carbonates. Une autre source possible pourrait Être le CO₂ accumulé dans l'atmosphère avant le Dryas supérieur et entraÎné vers la profondeur par la descente des eaux de surface.

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- , N. pachyderma , . . — , , — , - , , , , . , . , , . O₂ . O₂ .

     

The influence of grinding on the dissolution kinetics of calcite

This study of the dissolution of calcite in an acid environment demonstrates a significant dissolution anomaly for small particles. This cannot be attributed to very small, finely ground particles “stuck” to the grains, as there were too few to explain the magnitude of the observed anomaly. It is linked to the disruption resulting from the grinding. When observed in the electron microscope, the grains appear to be constructed of an aggregate of small crystals oriented in several principal directions. Their size increases progressively from the surface to the interior of the grain. An exothermic reaction is produced when the calcite is heated to 350° C. This causes a change in the microcrystalline structure leading to a nearly monocrystalline state. This recrystallisation retards the dissolution speed.     

The influence of experimental design on the rate of calcite dissolution

A rotating disc system has been constructed to study calcite dissolution kinetics. The disc system is shown to obey the Levich theory and to satisfy the stringent chemical controls required by the calcite dissolution system at neutral to alkaline pH values. The errors involved in using a CO₂-free atmosphere are shown to be insignificant. It is shown that the stirring dependence of systems which are not hydrodynamically well-defined is variable and dependent on the shape of the reaction vessel. Extrapolation of such results into natural systems involves very large uncertainties.     

Controls on Fe reduction and mineral formation by a subsurface bacterium

The reductive dissolution of FeIII (hydr)oxides by dissimilatory iron-reducing bacteria (DIRB) could have a large impact on sediment genesis and Fe transport. If DIRB are able to reduce FeIII in minerals of high structural order to carry out anaerobic respiration, their range could encompass virtually every O₂-free environment containing FeIII and adequate conditions for cell growth. Previous studies have established that Shewanella putrefaciens CN32, a known DIRB, will reduce crystalline Fe oxides when initially grown at high densities in a nutrient-rich broth, conditions that poorly model the environments where CN32 is found. By contrast, we grew CN32 by batch culture solely in a minimal growth medium. The stringent conditions imposed by the growth method better represent the conditions that cells are likely to encounter in their natural habitat. Furthermore, the expression of reductases necessary to carry out dissimilatory Fe reduction depends on the method of growth. It was found that under anaerobic conditions CN32 reduced hydrous ferric oxide (HFO), a poorly crystalline FeIII mineral, and did not reduce suspensions containing 4 mM FeIII in the form of poorly ordered nanometer-sized goethite (α-FeOOH), well-ordered micron-sized goethite, or nanometer-sized hematite (α-Fe₂O₃) crystallites. Transmission electron microscopy (TEM) showed that all minerals but the micron-sized goethite attached extensively to the bacteria and appeared to penetrate the outer cellular membrane. In the treatment with HFO, new FeII and FeIII minerals formed during reduction of HFO-Fe in culture medium containing 4.0 mmol/L P_i (soluble inorganic P), as observed by TEM with energy-dispersive X-ray spectroscopy, selected area electron diffraction, and X-ray diffraction. The minerals included magnetite (Fe₃O₄), goethite, green rust, and vivianite [Fe₃(PO₄)₂ · 8H₂O]. Vivianite appeared to be the stable end product and the mean coherence length was influenced by the rate of FeIII reduction. When P_i was 0.4 mol/L under otherwise identical conditions, goethite was the only mineral observed to form, and less Fe²⁺ was produced overall. Hence, the ability of DIRB to reduce Fe (hydr)oxides may be limited when the bacteria are grown under nutrient-limited conditions, and the minerals that result depend on the vigor of FeIII reduction.     

The role of background electrolytes on the kinetics and mechanism of calcite dissolution

The influence of background electrolytes on the mechanism and kinetics of calcite dissolution was investigated using in situ Atomic Force Microscopy (AFM). Experiments were carried out far from equilibrium by passing alkali halide salt (NaCl, NaF, NaI, KCl and LiCl) solutions over calcite cleavage surfaces. This AFM study shows that all the electrolytes tested enhance the calcite dissolution rate. The effect and its magnitude is determined by the nature and concentration of the electrolyte solution. Changes in morphology of dissolution etch pits and dissolution rates are interpreted in terms of modification in water structure dynamics (i.e. in the activation energy barrier of breaking water-water interactions), as well as solute and surface hydration induced by the presence of different ions in solution. At low ionic strength, stabilization of water hydration shells of calcium ions by non-paired electrolytes leads to a reduction in the calcite dissolution rate compared to pure water. At high ionic strength, salts with a common anion yield similar dissolution rates, increasing in the order Cl⁻ < I⁻ < F⁻ for salts with a common cation due to an increasing mobility of water around the calcium ion. Changes in etch pit morphology observed in the presence of F⁻ and Li⁺ are explained by stabilization of etch pit edges bonded by like-charged ions and ion incorporation, respectively. As previously reported and confirmed here for the case of F⁻, highly hydrated ions increased the etch pit nucleation density on calcite surfaces compared to pure water. This may be related to a reduction in the energy barrier for etch pit nucleation due to disruption of the surface hydration layer.     

3种常见细菌胞外特征有机酸对方解石的溶蚀研究

为了更深入地了解微生物与大气可吸入矿物细颗粒的作用机理,实验以方解石(PM2.5)为研究对象,采用反相高效液相色谱(RP-HPLC)对硅酸盐细菌、金黄色葡萄球菌和大肠杆菌胞外有机酸主要组分进行定性定量分析,考察了细菌胞外有机酸对方解石的溶蚀效应。实验结果表明,3种常见细菌的胞外有机酸主要组分有草酸、乳酸、柠檬酸和琥珀酸;不同有机酸对方解石的溶蚀效应不同,但均能不同程度地促进Ca离子的释放;草酸、乳酸和琥珀酸对方解石发生作用后,表现为颗粒失重,而柠檬酸对方解石发生作用后,导致其发生重结晶行为,能改变方解石的表面形貌,使残余固体呈纤维状;在草酸和柠檬酸作用下,残余固体表现出了对有机酸基团(如C=O、C—C或C—H)的吸附,特别是柠檬酸在1 600~500 cm-1区域。     

Effects of magnesium ions on near-equilibrium calcite dissolution: Step kinetics and morphology

Dissolution kinetics at the aqueous solution-calcite interface at 50 °C were investigated using in situ atomic force microscopy (AFM) to reveal the influence of magnesium concentration and solution saturation state on calcite dissolution kinetics and surface morphology. Under near-equilibrium conditions, dissolved Mg²⁺ displayed negligible inhibitory effects on calcite dissolution even at concentrations of . Upon the introduction of , the solution saturation state with respect to calcite, , acted as a “switch” for magnesium inhibition whereby no significant changes in step kinetics were observed at Ω_calcite<0.2, whereas a sudden inhibition from Mg²⁺ was activated at Ω_calcite?0.2. The presence of the Ω-switch in dissolution kinetics indicates the presence of critical undersaturation in accordance with thermodynamic principles. The etch pits formed in solutions with exhibited a unique distorted rhombic profile, different from those formed in Mg-free solutions and in de-ionized water. Such unique etch pit morphology may be associated with the anisotropy in net detachment rates of counter-propagating kink sites upon the addition of Mg²⁺.     

Controls on pit lake water quality at sixteen open-pit mines in Nevada

Thirty-five mines in Nevada currently have, or will likely have, a pit lake. The large bulk mineable deposits in Nevada mined below the water table are of several types, including Carlin-type Au, quartz-adularia precious metal, quartz-alunite precious metal and porphyry-Cu (-Mo) deposits. Of the 16 past or existing pit lakes at 12 different Nevada mines, most had near neutral pH and low metal concentrations, yet most had at least one constituent (e.g., SO₄) which exceeded drinking water standards for at least one sampling event. Water quality data indicate that, in general, poor water quality will not develop in Carlin-type Au deposits. Wall rocks in the geologic environment typical of these deposits, and in the specific pits sampled, contain substantial amounts of carbonate, which buffers the pH at slightly basic conditions and thereby limits the solubility of most metals. Similarly, the quartz-adularia precious metal deposits generally have geologic conditions that buffer pH and naturally prevent the development of poor water quality. In both of these deposit types, certain elements such as As and Se that are mobile in neutral to basic waters may accumulate to levels near or exceeding drinking water standards. Pit lakes forming in quartz-alunite precious metal deposits hosted in volcanic rocks or in porphyry-Cu (-Mo) deposits in plutonic rocks are of greatest environmental concern in Nevada, as both deposit types have relatively high acid-generating potential and low buffering capacity. However, the sampled Nevada pits in these deposit types indicate that the water may not be of poor quality. In addition, water quality in some pits may actually improve with time due to the increased water-rock ratio as the pit fills with water, as suggested by pit waters at one mine in a Carlin-type deposit (Getchell) that improved between 1968 and 1982. Although water quality in pits in each deposit type is generally good, local, site specific conditions (e.g., surface water inflow) and variations (e.g., evaporation rates) result in some pit lakes (e.g., Boss) in the quartz-adularia deposit type being of substantially poorer water quality than other lakes (e.g., Tuscarora) in the same deposit type. Despite underlying geologic controls based on deposit type, site specific variations in hydrogeologic conditions and surface geologic features can result in differing water quality in pit lakes in the same deposit types, and these factors may, in some cases, provide an overriding control on the geochemical evolution of specific pit lakes.     

The dependence of albite feldspar dissolution kinetics on fluid saturation state at acid and basic pH: Progress towards a universal relation

Here we report on two separate ongoing, multi-year investigations on the dependence of the dissolution rate (R) of albite feldspar on fluid saturation state, as defined by the Gibbs free energy of reaction (ΔG_r) for dissolution. The investigations are based on dissolution at pH 9.2, 150 °C and pH 3.3, 100 °C. Both studies reveal that the R–ΔG_r relation is highly non-linear and sigmoidal. The kinetic data from the first study, being the most complete, can be fitted with a sigmoidal rate curve that is composed of two separate, parallel rate laws that represent distinct mechanisms of dissolution. The switch between one dominant mechanism and the other may be controlled by a critical free energy. The fact that in both studies the same type of sigmoidal R–ΔG_r relation exists for dissolution at different pH and temperature condition suggests that this behavior may be universal for albite and other feldspars. Moreover, the experimental data contradict the commonly used R–ΔG_r relation that is loosely based on transition state theory (TST). This has important implications with respect to the accuracy of geochemical codes that model water–rock interactions at near-equilibrium conditions.     

Sr/Ca and Ca/Ca fractionation during inorganic calcite formation: I. Sr incorporation

Partitioning of strontium during spontaneous calcite formation was experimentally studied using an advanced CO₂-diffusion technique. Results at different precipitation rates and T = 5, 25, and 40 °C show that at constant temperature Sr incorporation into calcite is controlled by the precipitation rate (R in μmol/m²/h) according to the individual expressions

     

Sr/Ca and Ca/Ca fractionation during inorganic calcite formation: II. Ca isotopes

Ca isotope fractionation during inorganic calcite formation was experimentally studied by spontaneous precipitation at various precipitation rates (1.8 < log R < 4.4 μmol/m²/h) and temperatures (5, 25, and 40 °C) with traces of Sr using the CO₂ diffusion technique.Results show that in analogy to Sr/Ca [see Tang J., Köhler S. J. and Dietzel M. (2008) Sr²⁺/Ca²⁺ and ⁴⁴Ca/⁴⁰Ca fractionation during inorganic calcite formation: I. Sr incorporation. Geochim. Cosmochim. Acta] the ⁴⁴Ca/⁴⁰Ca fractionation during calcite formation can be followed by the Surface Entrapment Model (SEMO). According to the SEMO calculations at isotopic equilibrium no fractionation occurs (i.e., the fractionation coefficient α_calcite-aq = (⁴⁴Ca/⁴⁰Ca)_s/(⁴⁴Ca/⁴⁰Ca)_aq = 1 and Δ^44/40Ca_calcite-aq = 0‰), whereas at disequilibrium ⁴⁴Ca is fractionated in a primary surface layer (i.e., the surface entrapment factor of ⁴⁴Ca, F_44Ca < 1). As a crystal grows at disequilibrium, the surface-depleted ⁴⁴Ca is entrapped into the newly formed crystal lattice. ⁴⁴Ca depletion in calcite can be counteracted by ion diffusion within the surface region. Our experimental results show elevated ⁴⁴Ca fractionation in calcite grown at high precipitation rates due to limited time for Ca isotope re-equilibration by ion diffusion. Elevated temperature results in an increase of ⁴⁴Ca ion diffusion and less ⁴⁴Ca fractionation in the surface region. Thus, it is predicted from the SEMO that an increase in temperature results in less ⁴⁴Ca fractionation and the impact of precipitation rate on ⁴⁴Ca fractionation is reduced.A highly significant positive linear relationship between absolute ⁴⁴Ca/⁴⁰Ca fractionation and the apparent Sr distribution coefficient during calcite formation according to the equation

Δ^44/40Ca_calcite-aq=(−1.90±0.26)·logD_Sr−2.83±0.28     

The behaviour of Li and Mg isotopes during primary phase dissolution and secondary mineral formation in basalt

This study presents lithium (Li) and magnesium (Mg) isotope data from experiments designed to assess the effects of dissolution of primary phases and the formation of secondary minerals during the weathering of basalt. Basalt glass and olivine dissolution experiments were performed in mixed through-flow reactors under controlled equilibrium conditions, at low pH (2-4) in order to keep solutions undersaturated (i.e. far-from equilibrium) and inhibit the formation of secondary minerals. Combined dissolution-precipitation experiments were performed at high pH (10 and 11) increasing the saturation state of the solutions (moving the system closer to equilibrium) and thereby promoting the formation of secondary minerals.At conditions far from equilibrium saturation state modelling and solution stoichiometry suggest that little secondary mineral formation has occurred. This is supported by the similarity of the dissolution rates of basalt glass and olivine obtained here compared to those of previous experiments. The δ⁷Li isotope composition of the experimental solution is indistinguishable from that of the initial basalt glass or olivine indicating that little fractionation has occurred. In contrast, the same experimental solutions have light Mg isotope compositions relative to the primary phases, and the solution becomes progressively lighter with time. In the absence of any evidence for secondary mineral formation the most likely explanation for these light Mg isotope compositions is that there has been preferential loss of light Mg during primary phase dissolution.For the experiments undertaken at close to equilibrium conditions the results of saturation state modelling and changes in solution chemistry suggest that secondary mineral formation has occurred. X-ray diffraction (XRD) measurements of the reacted mineral products from these experiments confirm that the principal secondary phase that has formed is chrysotile. Lithium isotope ratios of the experimental fluid become increasingly heavy with time, consistent with previous experimental work and natural data indicating that ⁶Li is preferentially incorporated into secondary minerals, leaving the solution enriched in ⁷Li. The behaviour of Mg isotopes is different from that anticipated or observed in natural systems. Similar to the far from equilibrium experiments initially light Mg is lost during olivine dissolution, but with time the δ²⁶Mg value of the solution becomes increasingly heavy. This suggests either preferential loss of light, and then heavy Mg from olivine, or that the secondary phase preferentially incorporates light Mg from solution. Assuming that the secondary phase is chrysotile, a Mg-silicate, the sense of Mg fractionation is opposite to that previously associated with silicate soils and implies that the fractionation of Mg isotopes during silicate precipitation may be mineral specific. If secondary silicates do preferentially remove light Mg from solution then this could be a possible mechanism for the relatively heavy δ²⁶Mg value of seawater. This study highlights the utility of experimental studies to quantify the effects of natural weathering reactions on the Li and Mg geochemical cycles.     

Production of a molybdophore during metal-targeted dissolution of silicates by soil bacteria

Although many bioessential metals are scarce in natural water and rock systems, microbial secretion of high-affinity ligands for metal extraction from solid phases has only been documented for Fe. However, we have discovered that Mo is extracted from a silicate by a high-affinity ligand (a possible “molybdophore”) secreted by an N₂-fixing soil bacterium. The putative molybdophore, aminochelin, is secreted as a siderophore under Fe-depleted conditions, but is also secreted under Fe-sufficient, Mo-depleted conditions. Presumably, molybdophore production facilitates uptake of Mo for use in Mo enzymes. In contrast, an Fe-requiring soil bacterium without a special Mo requirement only enhances the release of Fe from the silicate. Fractionation of Mo stable isotopes during uptake to cells may provide a “fingerprint” for the importance of chelating ligands in such systems. Many such metal-specific ligands secreted by prokaryotes for extraction of bioessential metals, their effects on Earth materials, and their possible utility in the recovery of economic metals remain to be discovered.     

Conditions of meteoric calcite formation along a Variscan fault and their possible relation to climatic evolution during the Jurassic–Cretaceous

Two calcite cements, filling karst cavities and replacing Lower Carboniferous limestones at the Variscan Front Thrust, were precipitated after mid-Jurassic Cimmerian uplift and subsequent erosion but before late Cretaceous strike-slip movement. The first calcite (stage A) is nonferroan and crystals are coated by hematite and/or goethite. These minerals also occur as inclusions along growth zones. The calcite lattice contains < 0·07 mol.% Fe, but Mn concentrations can be as high as 0·72 mol.% in bright yellow luminescent zones. Primary, originally one-phase, all-liquid, aqueous inclusions have a final melting temperature between ?0·2° and +0·2 °C, indicating a meteoric origin of the ambient water. The δ¹³C and δ¹⁸O values of the calcites are between ?7·3‰ and ?6·3‰, ?7·8‰ and ?5·5‰ on the Vienna PeeDee Belemnite (VPDB) scale, respectively. The second calcite (stage B) consists of ferroan (0·13–0·84 mol.% Fe) blocky crystals with Mn concentrations between 0·34 and 0·87 mol.%. Primary, single-phase aqueous fluid inclusions indicate precipitation from a meteoric fluid below 50 °C . The δ¹³C values of stage B calcites vary between ?7·3‰ and ?2·1‰ VPDB and the δ¹⁸O values between ?7·9‰ and ?7·2‰ VPDB. A precipitation temperature below 50 °C for the stage A calcites and the presence of iron oxide/hydroxide inclusions in the crystals indicate near-surface precipitation conditions. Within this setting, the geochemistry of the nonferroan stage A calcites reflects precipitation under oxic to suboxic conditions. The ferroan stage B calcites precipitated in a reducing environment. The evolution from the stage A to stage B calcites and the associated geochemical changes are interpreted to be related to the change from semiarid to humid conditions in western Europe during late Jurassic–Cretaceous times. A change in humidity can explain the evolution of groundwater from oxic/suboxic to reducing conditions during calcite precipitation. The typically higher δ¹³C values of the stage B compared to the stage A calcites can be explained by a smaller contribution of carbon derived from soil-zone processes than from carbonate dissolution in the groundwater under humid conditions. The small shift to lower δ¹⁸O between stage A and B calcites may be caused by a higher precipitation temperature or a decrease in the δ¹⁸O value of the meteoric water. This decrease could have been caused by a change in the source of the air masses or by an increase in the amount of rainfall during the early mid-Cretaceous. Although the latter interpretation is preferred, it cannot be proven.     

Specific effects of background electrolytes on the kinetics of step propagation during calcite growth

The mechanisms by which background electrolytes modify the kinetics of non-equivalent step propagation during calcite growth were investigated using Atomic Force Microscopy (AFM), at constant driving force and solution stoichiometry. Our results suggest that the acute step spreading rate is controlled by kink-site nucleation and, ultimately, by the dehydration of surface sites, while the velocity of obtuse step advancement is mainly determined by hydration of calcium ions in solution. According to our results, kink nucleation at acute steps could be promoted by carbonate-assisted calcium attachment. The different sensitivity of obtuse and acute step propagation kinetics to cation and surface hydration could be the origin of the reversed geometries of calcite growth hillocks (i.e., rate of obtuse step spreading < rate of acute step spreading) observed in concentrated (ionic strength, IS = 0.1) KCl and CsCl solutions. At low IS (0.02), ion-specific effects seem to be mainly associated with changes in the solvation environment of calcium ions in solution. With increasing electrolyte concentration, the stabilization of surface water by weakly paired salts appears to become increasingly important in determining step spreading rate. At high ionic strength (IS = 0.1), overall calcite growth rates increased with increasing hydration of calcium in solution (i.e., decreasing ion pairing of background electrolytes for sodium-bearing salts) and with decreasing hydration of the carbonate surface site (i.e., increasing ion pairing for chloride-bearing salts). Changes in growth hillock morphology were observed in the presence of Li⁺, F⁻ and , and can be interpreted as the result of the stabilization of polar surfaces due to increased ion hydration. These results increase our ability to predict crystal reactivity in natural fluids which contain significant amounts of solutes.