Carbonate complexation of yttrium and the rare earth elements in natural waters

Potentiometric measurements of Yttrium and Rare Earth Element (YREE) complexation by carbonate and bicarbonate indicate that the quality of carbonate complexation constants previously obtained via solvent exchange analyses are superior to characterizations obtained using solubility and adsorptive exchange analyses. The results of our analyses at 25°C are combined with the results of previous solvent exchange analyses to obtain YREE carbonate complexation constants over a wide range of ionic strength (0 ≤ I ≤3 molal). YREE carbonate complexation constants are reported for the following equilibria, M³⁺+nHCO₃⁻?M(CO₃)_n³⁻²ⁿ+nH⁺, where n = 1 or 2. Formation constants written in terms of HCO₃⁻ concentrations require only minor corrections for ion pairing relative to the corrections required for constants expressed in terms of CO₃²⁻ concentrations. Formation constants for the above complexation equilibria, _CO3^Hβ₁=[MCO₃⁺][H⁺][M³⁺]⁻¹[HCO₃⁻]⁻¹ and _CO3^Hβ₂=[M(CO₃)₂⁻][H⁺]²[M³⁺]⁻¹[HCO₃⁻]⁻², have very similar dependencies on ionic strength because the reaction MCO₃⁺+HCO₃⁻?M(CO₃)₂⁻+H⁺ is isocoulombic. Potentiometric analyses indicate that the dependence of log_CO3^Hβ₁ and log_CO3^Hβ₂ on ionic strength at 25°C is given as

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Precise and accurate isotopic measurements using multiple-collector ICPMS

New techniques of isotopic measurements by a new generation of mass spectrometers equipped with an inductively-coupled-plasma source, a magnetic mass filter, and multiple collection (MC-ICPMS) are quickly developing. These techniques are valuable because of (1) the ability of ICP sources to ionize virtually every element in the periodic table, and (2) the large sample throughout. However, because of the complex trajectories of multiple ion beams produced in the plasma source whether from the same or different elements, the acquisition of precise and accurate isotopic data with this type of instrument still requires a good understanding of instrumental fractionation processes, both mass-dependent and mass-independent. Although physical processes responsible for the instrumental mass bias are still to be understood more fully, we here present a theoretical framework that allows for most of the analytical limitations to high precision and accuracy to be overcome. After a presentation of unifying phenomenological theory for mass-dependent fractionation in mass spectrometers, we show how this theory accounts for the techniques of standard bracketing and of isotopic normalization by a ratio of either the same or a different element, such as the use of Tl to correct mass bias on Pb. Accuracy is discussed with reference to the concept of cup efficiencies. Although these can be simply calibrated by analyzing standards, we derive a straightforward, very general method to calculate accurate isotopic ratios from dynamic measurements. In this study, we successfully applied the dynamic method to Nd and Pb as examples. We confirm that the assumption of identical mass bias for neighboring elements (notably Pb and Tl, and Yb and Lu) is both unnecessary and incorrect. We further discuss the dangers of straightforward standard-sample bracketing when chemical purification of the element to be analyzed is imperfect. Pooling runs to improve precision is acceptable provided the pooled measurements are shown to be part of a single population. Second-order corrections seem to be able to improve the precision on ¹⁴³Nd/¹⁴⁴Nd measurements. Finally, after discussing a number of potential pitfalls, such as the consequence of peak shape, correlations introduced by counting statistics, and the effect of memory on double-spike methods, we describe an optimal strategy for high-precision and accurate measurements by MC-ICPMS, which involves the repetitive calibration of cup efficiencies and rigorous assessment of mass bias combined with standard-sample bracketing. We suggest that, when these simple guidelines are followed, MC-ICPMS is capable of producing isotopic data precise and accurate to better than 15 ppm.     

Pyrite oxidation in moist air

The rate of pyrite oxidation in moist air was determined by measuring, over time, the pressure difference between a sealed chamber containing pyrite plus oxygen and a control. The experiments carried out at 25°C, 96.7% fixed relative humidity, and oxygen partial pressures of 0.21, 0.61, and 1.00 atm showed that the rate of oxygen consumption is a function of oxygen partial pressure and time. The rates of oxygen consumption (r, mol/m²sec) fit the expression

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Oxygen-exchange pathways in aluminum polyoxocations

Using molecular dynamics simulations and electronic structure methods, we postulate a mechanism to explain the complicated reactivity trends that are observed for oxygen isotope exchange reactions between sites in aluminum polyoxocations of the ε-Keggin type and bulk solution. Experimentally, the molecules have four nonequivalent oxygens that differ considerably in reactivity both within a molecule, and between molecules in the series: Al₁₃, GaAl₁₂, and GeAl₁₂ [MO₄Al₁₂(OH)₂₄(H₂O)₁₂ⁿ⁺(aq); with M = Al(III) for Al₁₃, n = 7; M = Ga(III) for GaAl₁₂, n = 7; M = Ge(IV) for GeAl₁₂, n = 8]. We find that a partly dissociated, metastable intermediate molecule of expanded volume is necessary for exchange of both sets of μ₂-OH and that the steady-state concentration of this intermediate reflects the bond strengths between the central metal and the μ₄-O. Thus the central metal exerts extraordinary control over reactions at hydroxyl bridges, although these are three bonds away.This mechanism not only explains the reactivity trends for oxygen isotope exchange in μ₂-OH and η-OH₂ sites in the ε-Keggin aluminum molecules, but also explains the observation that the reactivities of minerals tend to reflect the presence of highly coordinated oxygens, such as the μ₄-O in boehmite, α-, and γ-Al₂O₃ and their Fe(III) analogs. The partial dissociation of these highly coordinated oxygens, coupled with simultaneous activation and displacement of neighboring metal centers, may be a fundamental process by which metals atoms undergo ligand exchanges at mineral surfaces.     

Fe isotopic fractionation during mineral dissolution with and without bacteria

Fe released into solution is isotopically lighter (enriched in the lighter isotope) than hornblende starting material when dissolution occurs in the presence of the siderophore desferrioxamine mesylate (DFAM). In contrast, Fe released from goethite dissolving in the presence of DFAM is isotopically unchanged. Furthermore, Δ⁵⁶Fe_{solution-hornblende} for Fe released to solution in the presence of ligands varies with the affinity of the ligand for Fe. The extent of isotopic fractionation of Fe released from hornblende also increases when experiments are agitated continuously. The Fe isotope fractionation observed during hornblende dissolution with organic ligands is attributed predominantly to retention of ⁵⁶Fe in an altered surface layer, while the lack of isotopic fractionation during goethite dissolution in DFAM is consistent with the lack of an altered layer. When a siderophore-producing soil bacterium is added to the system (without added organic ligands), Fe released to solution from both hornblende and goethite differs isotopically from Fe in the bulk mineral: Δ⁵⁶Fe_{solution-starting material} = −0.56 ± 0.19 (hornblende) and −1.44 ± 0.16 (goethite). Increased isotopic fractionation is attributed in this case to the fact that as bacterial respiration depletes the system in oxygen and aqueous Fe is reduced, equilibration between aqueous ferrous and ferric iron creates a pool of isotopically heavy ferric iron that is assimilated by bacterial cells. Adsorption of isotopically heavy ferrous iron (Fe(II) enriched in the heavier isotope) or precipitation of isotopically heavy Fe minerals may also contribute to observed fractionations.To test whether these Fe isotope signatures are recorded in natural systems, we also investigated extractions of samples of soils from which the bacteria were isolated. These extractions show variability in the isotopic signatures of exchangeable Fe and Fe oxyhydroxide fractions from one soil sample to another, but exchangeable Fe is observed to be lighter than Fe in soil Fe oxyhydroxides and hornblende. This observation is consistent with isotopically light Fe-organic complexes in soil pore water derived from the Fe-silicate starting materials in the presence of growing microorganisms, as documented in experiments reported here. The contributions from phenomena including organic ligand-promoted nonstoichiometric dissolution of Fe silicates, uptake of ferric iron by organisms, adsorption of isotopically heavy ferrous iron, and precipitation of iron minerals should create complex isotopic signatures in soils. Better understanding of these processes and the timescales over which they contribute to fractionation is needed.     

Carbon isotope effects in carbonate systems

Global carbon cycle models require a complete understanding of the δ¹³C variability of the Earth’s C reservoirs as well as the C isotope effects in the transfer of the element among them. An assessment of δ¹³C changes during CO₂ loss from degassing magmas requires knowledge of the melt-CO₂ carbon isotope fractionation. In order to examine the potential size of this effect for silicate melts of varying composition, ¹³C reduced partition functions were computed in the temperature range 275 to 4000 K for carbonates of varying bond strengths (Mg, Fe, Mn, Sr, Ba, Pb, Zn, Cd, Li, and Na) and the polymorphs of calcite. For a given cation and a given pressure the ¹³C content increases with the density of the carbonate structure. For a given structure the tendency to concentrate ¹³C increases with pressure. The effect of pressure (‰/10 kbar) on the size of the reduced partition function of aragonite varies with temperature; in the pressure range 1 to 10⁵ bars the change is given by:

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Neodymium isotopes in the Mediterranean Sea: comparison between seawater and sediment signals

Nine depth-profiles of dissolved Nd concentrations and isotopic ratios (ε_Nd) were obtained in the Levantine Basin, the Ionian, the Aegean, the Alboran Seas and the Strait of Gibraltar. Thirteen core-top sediments and Nile River particle samples were also analyzed (leached with 1 N HCl, acetic acid or hydroxylamine hydrochloride). The seawater ε_Nd values become more radiogenic during the eastward circulation in the Mediterranean Sea. The relationship between salinity and the seawater ε_Nd shows that the Nd isotopic signature is more conservative than salinity in the Mediterranean Sea. The water mass with the highest ε_Nd (−4.8) is found at about 200 m in the easternmost Levantine basin. The average ε_Nd value for deep waters is −7.0 in the eastern basin, 2.5 ε-units higher than in the western basin. By examining the sensitivity of seawater ε_Nd to Nd inputs from the Nile, we conclude that the most significant radiogenic Nd source is partially dissolved Nile River particles. The Nd flux from the Nile River water has a minor influence on the Mediterranean seawater ε_Nd. Except for the easternmost Levantine Basin, the leachate ε_Nd values are consistent with the seawater values. In the easternmost Levantine Basin, the leachate ε_Nd values obtained with HCl leaching are systematically higher than the seawater values. The relationship between leachate and residual ε_Nd values indicates that the HCl leaching partially dissolves lithogenic Nd, so the dissolution of Nile River particles is the cause of the observed shift. Some ε_Nd values obtained with hydroxylamine hydrochloride leaching are higher than those obtained with HCl leaching. Although the reason for this shift is not clear, ⁸⁷Sr/⁸⁶Sr successfully detects the presence of a nonmarine component in the leachate. Our results suggest that leaching performance may vary with the mineralogy of marine sediments, at least in the case of the Mediterranean Sea.     

Manganese-chromium isotope systematics of enstatite meteorites

We present results of a study of the ⁵³Mn-⁵³Cr isotope systematics in the enstatite chondrites and achondrites (aubrites). The goal of this study was to explore the capabilities of this isotope system to obtain chronological information on these important classes of meteorites and to investigate the original distribution in the inner solar system of the short-lived radionuclide ⁵³Mn. Our earlier work (Lugmair and Shukolyukov, 1998; Shukolyukov and Lugmair, 2000a) has shown that the asteroid belt bodies are characterized by essentially the same initial ⁵³Mn abundance. However, we have found the presence of a gradient in the abundance of the radiogenic ⁵³Cr between the earth-moon system, Mars, and the asteroid Vesta. If this gradient is considered as a function of the heliocentric distance a linear radial dependence is indicated. This can be explained either by an early, volatility controlled Mn/Cr fractionation in the nebula or by an original radially heterogeneous distribution of ⁵³Mn. The enstatite chondrites are suggested to form in the inner zones of the solar nebula, much closer to the Sun than the ordinary chondrites. Therefore, their investigation may be an important test on the hypothesis on a radial heterogeneity in the initial ⁵³Mn.We have studied the bulk samples of the EH4-chondrites Indarch and Abee and the EL6-chondrite Khairpur. Although these meteorites have essentially the same Mn/Cr ratio as the ordinary chondrites, the relative abundance of the radiogenic ⁵³Cr is three times smaller than in the ordinary chondrites. Because these meteorites are primitive (undifferentiated) and no Mn/Cr fractionation had occurred within their parent bodies, this difference is a strong argument in favor of an initially heterogeneous distribution of ⁵³Mn in the early inner solar system. This finding is also consistent with formation of the enstatite chondrites in the inner zones of the solar nebula. Using the characteristic ⁵³Cr excess of the enstatite chondrites and the observed gradient, their place of origin falls at about 1.4 AU or somewhat closer to the Sun (i.e. >1.0-1.4 AU).We also present chronological results for the enstatite chondrites and achondrites. The ‘absolute’ ⁵³Mn-⁵³Cr ages of the EH4-chondrites are old: ∼4565 Ma. The EL6-chondrite Khairpur is ∼4.5 Ma younger, which is in good agreement with the ¹²⁹I-¹²⁹Xe data from the literature. The age of the aubrite Peña Blanca Spring appears to be similar to those of the enstatite chondrites while that of the aubrite Bishopville is at least ∼10 Ma younger, which is also in agreement with the ¹²⁹I-¹²⁹Xe data. The results from bulk samples of aubrites indicate that the last Mn/Cr fractionation in their parent body occurred ∼ 4563 Ma ago and imply an evolution of the Mn-Cr isotope system in an environment with an higher than chondritic Mn/Cr ratio for several millions of years.     

Source enrichment processes responsible for isotopic anomalies in oceanic island basalts

A new method has been developed to separate the compositional variations in ocean island basalts into those that result from variations in source composition and from the melting process itself. The approach depends on correlations between isotope ratios, which can only come from source inhomogeneities, and elemental concentrations. Analysis of three data sets shows that the inhomogeneities beneath Theistareykir, in NE Iceland, Kilauea and Pitcairn can be produced by subduction of oceanic islands and volcanic ridges. The thicknesses of the lithosphere on which such islands were constructed and potential temperatures of the plumes that produced them can be estimated from the geochemical observations. Model ages are harder to determine, though simple assumptions give about 400 Ma for the Theistareykir source and 1.2 Ga for Kilauea. The model may also provide a physical explanation for the commonly used isotopic classification of ocean island basalts, with the isotopic composition changing from HIMU through EMII to EMI as the melt fraction increases. These results have been obtained from a small number of data sets obtained from ocean island basalts erupted in small areas during short time intervals. More such observations are needed to discover whether geochemical observations from other islands are consistent with the same model.     

A stable isotope-based approach to tropical dendroclimatology

We describe a strategy for development of chronological control in tropical trees lacking demonstrably annual ring formation, using high resolution δ¹⁸O measurements in tropical wood. The approach applies existing models of the oxygen isotopic composition of alpha-cellulose (Roden et al., 2000), a rapid method for cellulose extraction from raw wood (Brendel et al., 2000), and continuous flow isotope ratio mass spectrometry (Brenna et al., 1998) to develop proxy chronological, rainfall and growth rate estimates from tropical trees lacking visible annual ring structure. Consistent with model predictions, pilot datasets from the temperate US and Costa Rica having independent chronological control suggest that observed cyclic isotopic signatures of several permil (SMOW) represent the annual cycle of local rainfall and relative humidity. Additional data from a plantation tree of known age from ENSO-sensitive northwestern coastal Peru suggests that the 1997-8 ENSO warm phase event was recorded as an 8‰ anomaly in the δ¹⁸O of α-cellulose. The results demonstrate reproducibility of the stable isotopic chronometer over decades, two different climatic zones, and three tropical tree genera, and point to future applications in paleoclimatology.     

Transient dissolution patterns on stressed crystal surfaces

We present an experimental investigation on the dissolution of uniaxially stressed crystals of NaClO₃ in contact with brine. The crystals are immersed in a saturated fluid, stressed vertically by a piston and monitored constantly in situ with a CCD camera. The experiments are temperature-controlled and uniaxial shortening of the sample is measured with a high-resolution capacitance analyzer. Once the crystal is stressed it develops dissolution grooves on its free surface. The grooves are oriented with their long axis perpendicular to the direction of compressive stress and the initial distance between the parallel grooves is in accordance with the Asaro-Tiller-Grinfeld instability. We observe a novel, transient evolution of this roughness: The grooves on the crystal surface migrate upwards (against gravity), grow in size and the inter-groove distance increases linearly with time. During the coarsening of the pattern this switches from a one-dimensional geometry of parallel grooves to a two-dimensional geometry with horizontal and vertical grooves. At the end of the experiment one large groove travels across the crystal and the surface becomes smooth again. Uniaxial shortening of the crystal by pressure solution creep decays exponentially with time and shows no long term creep within the range of the resolution of the capacitance analyzer (accuracy of 100nm over a period of 14 days). This indicates that, while active, the fast transient processes on the free surface increase the solution concentration and thereby significantly slow down or stop pressure solution at the top of the crystal. This novel feedback mechanism can explain earlier results of cyclic pressure solution creep and demands development of a more complex theory of pressure-solution creep including processes that act on free surfaces.     

Thermochemistry of jarosite-alunite and natrojarosite-natroalunite solid solutions

The thermochemistry of jarosite-alunite and natrojarosite-natroalunite solid solutions was investigated. Members of these series were either coprecipitated or synthesized hydrothermally and were characterized by XRD, FTIR, electron microprobe analysis, ICP-MS, and thermal analysis. Partial alkali substitution and vacancies on the Fe/Al sites were observed in all cases, and the solids studied can be described by the general formula K_1-x-yNa_y(H₃O)_xFe_zAl_w(SO₄)₂(OH)_6-3(3-z-w)(H₂O)_3(3-z-w). A strong preferential incorporation of Fe over Al in the jarosite/alunite structure was observed. Heats of formation from the elements, ΔH°_f, were determined by high-temperature oxide melt solution calorimetry. The solid solutions deviate slightly from thermodynamic ideality by exhibiting positive enthalpies of mixing in the range 0 to +11 kJ/mol. The heats of formation of the end members of both solid solutions were derived. The values ΔH°_f = −3773.6 ± 9.4 kJ/mol, ΔH°_f = −4912.2 ± 24.2 kJ/mol, ΔH°_f = −3734.6 ± 9.7 kJ/mol and ΔH°_f = −4979.7 ± 7.5kJ/mol were found for K_0.85(H₃O)_0.15Fe_2.5(SO₄)₂(OH)_4.5(H₂O)_1.5, K_0.85(H₃O)_0.15Al_2.5(SO₄)₂(OH)_4.5(H₂O)_1.5, Na_0.7(H₃O)_0.3Fe_2.7(SO₄)₂(OH)_5.1(H₂O)_0.9, and Na_0.7(H₃O)_0.3Al_2.7(SO₄)₂(OH)_5.1(H₂O)_0.9 respectively. To our knowledge, this is the first experimentally-based report of ΔH°_f for such nonstoichiometric alunite and natroalunite samples. These thermodynamic data should prove helpful to study, under given conditions, the partitioning of Fe and Al between the solids and aqueous solution.     

Oxygen isotope fractionation in synthetic magnesian calcite

Mg-bearing calcite was precipitated at 25°C in closed system free-drift experiments from solutions containing NaHCO₃, CaCl₂ and MgCl₂. The chemical and isotope composition of the solution and precipitate were investigated during time course experiments of 24-h duration. Monohydrocalcite and calcite precipitated early in the experiments (<8 h), while Mg-calcite was the predominant precipitate (>95%) thereafter. Solid collected at the end of the experiments displayed compositional zoning from pure calcite in crystal cores to up to 23 mol% MgCO₃ in the rims. Smaller excursions in Mg were superimposed on this chemical record, which is characteristic of oscillatory zoning observed in synthetic and natural solid-solution carbonates of differing solubility. Magnesium also altered the predominant morphology of crystals over time from the {104} to {100} and {110} growth forms.The oxygen isotope fractionation factor for the magnesian-calcite-water system (as 10³lnα_Mg-cl-H2O) displayed a strong dependence on the mol% MgCO₃ in the solid phase, but quantification of the relationship was difficult due to the heterogeneous nature of the precipitate. Considering only the Mg-content and δ¹⁸O values for the bulk solid, 10³lnα_Mg-cl-H2O increased at a rate of 0.17 ± 0.02 per mol% MgCO₃; this value is a factor of three higher than the single previous estimate (Tarutani T., Clayton R.N., and Mayeda T. K. (1969) The effect of polymorphims and magnesium substitution on oxygen isotope fractionation between calcium carbonate and water. Geochim. Cosmochim. Acta 33, 987-996). Nevertheless, extrapolation of our relationship to the pure calcite end member yielded a value of 27.9 ± 0.02, which is similar in magnitude to published values for the calcite-water system. Although no kinetic effect was observed on 10³lnα_Mg-cl-H2O for precipitation rates that ranged from 10^3.21 to 10^4.60 μmol · m⁻² · h⁻¹, it was impossible to disentangle the potential effect(s) of precipitation rate and Mg-content on 10³lnα_Mg-cl-H2O due to the heterogeneous nature of the solid.The results of this study suggest that paleotemperatures inferred from the δ¹⁸O values of high magnesian calcite (>10 mol% MgCO₃) may be significantly underestimated. Also, the results underscore the need for additional experiments to accurately characterize the effect of Mg coprecipitation on the isotope systematics of calcite from a chemically homogeneous precipitate or a heterogeneous material that is analyzed at the scale of chemical and isotopic zonation.     

Molecular and isotopic stratigraphy in an ombrotrophic mire for paleoclimate reconstruction

A 40 cm deep Sphagnum-dominated peat monolith from Bolton Fell Moss in Northern England was systematically investigated by lipid molecular stratigraphy and compound-specific δ¹³C and δD analysis using gas chromatography (GC), GC-mass spectrometry (GC-MS), GC-combustion-isotope ratio-MS (GC-C-IRMS) and GC-thermal conversion-IRMS (GC-TC-IRMS) techniques. ²¹⁰Pb dating showed the monolith accumulated during the last ca. 220 yr, a period encompassing the second part of Little Ice Age. While the distributions of lipids, including n-alkan-1-ols, n-alkan-2-ones, wax esters, sterols, n-alkanoic acids, α,ω-alkandioic acids and ω-hydroxy acids, display relatively minor changes with depth, the cooler climate event was recorded in the concentrations of n-alkanes and organic carbon, CPI values of n-alkanes and n-alkanoic acids, and the ratio of 5-n-alkylresorcinols/sterols. Superimposed on the fossil fuel effect, the relatively cooler climate event was also recorded by δ¹³C values of individual hydrocarbons, especially the C₂₃n-alkane, a major compound in certain Sphagnum spp. The δD values of the C₂₉ and C₃₃n-alkanes correlated mainly with plant composition and were relatively insensitive to climatic change. In contrast the C₂₃n-alkane displayed variation that correlated strongly with recorded temperature for the period represented by the monolith, agreeing with previously reported deuterium records in tree ring cellulose spanning the same period in Scotland, Germany and the USA, with more negative values occurring during the second part of Little Ice Age. These biomarker characteristics, including the compound-specific δ¹³C and δD records, provide a new set of proxies of climatic change, potentially independent of preserved macrofossils which will be of value in deeper sections of the bog where the documentary records of climate are unavailable and humification is well advanced.     

Strontium incorporation into calcite generated by bacterial ureolysis

Strontium incorporation into calcite generated by bacterial ureolysis was investigated as part of an assessment of a proposed remediation approach for ⁹⁰Sr contamination in groundwater. Urea hydrolysis produces ammonium and carbonate and elevates pH, resulting in the promotion of calcium carbonate precipitation. Urea hydrolysis by the bacterium Bacillus pasteurii in a medium designed to mimic the chemistry of the Snake River Plain Aquifer in Idaho resulted in a pH rise from 7.5 to 9.1. Measured average distribution coefficients (D_EX) for Sr in the calcite produced by ureolysis (0.5) were up to an order of magnitude higher than values reported in the literature for natural and synthetic calcites (0.02-0.4). They were also higher than values for calcite produced abiotically by ammonium carbonate addition (0.3). The precipitation of calcite in these experiments was verified by X-ray diffraction. Time-of-flight secondary ion mass spectrometry (ToF SIMS) depth profiling (up to 350 nm) suggested that the Sr was not merely sorbed on the surface, but was present at depth within the particles. X-ray absorption near edge spectra showed that Sr was present in the calcite samples as a solid solution. The extent of Sr incorporation appeared to be driven primarily by the overall rate of calcite precipitation, where faster precipitation was associated with greater Sr uptake into the solid. The presence of bacterial surfaces as potential nucleation sites in the ammonium carbonate precipitation treatment did not enhance overall precipitation or the Sr distribution coefficient. Because bacterial ureolysis can generate high rates of calcite precipitation, the application of this approach is promising for remediation of ⁹⁰Sr contamination in environments where calcite is stable over the long term.     

Barite deposition resulting from phototrophic sulfide-oxidizing bacterial activity

Barite (BaSO₄) deposits generally arise from mixing of soluble barium-containing fluids with sulfate-rich fluids. While the role of biological processes in modulating barium solubility has been shown, no studies have shown that the biological oxidation of sulfide to sulfate leads to barite deposition. Here we present an example of microbially mediated barite deposition in a continental setting. A spring in the Anadarko Basin of southwestern Oklahoma produces water containing abundant barium and sulfide. As emergent water travels down a stream to a nearby creek, sulfate concentration increases from 0.06 mM to 2.2 mM while Ba²⁺ concentration drops from 0.4 mM to less than 7 μM. Stable isotope analysis, microbial activity studies, and in situ experiments provide evidence that as sulfide-rich water flows down the stream, anaerobic, anoxygenic, phototrophic bacteria play a dominant role in oxidizing sulfide to sulfate. Sulfate then precipitates with Ba²⁺ producing barite as travertine, cements, crusts, and accumulations on microbial mats. Our studies suggest that phototrophic sulfide oxidation and concomitant sulfur cycling could prove to be important processes regulating the cycling of barium in continental sulfur-containing systems.     

Long-range vs. short-range ordering in synthetic Cr-substituted goethites

A series of Cr-substituted goethites with (Cr:Fe molar ratio up to 0.12) were prepared. Thermal analysis of the solids indicates the formation of cation-deficient compounds that are more stable towards the transformation to hematite as the Cr content increases. Powder X-ray diffraction (PXRD) and extended X-ray absorption fine structure (EXAFS) techniques were used to assess the structural characteristics of the whole series of the substituted solids. XRD patterns demonstrate that the order around Fe remains typical of a goethite-like structure. Rietveld refinement of X-ray diffraction data indicates that the incorporation of Cr causes a slight decrease in the cell volume with the c-cell parameter following the Vegard’s law. This decrease is accompanied by changes in opposite directions of the various Me-Me distances. EXAFS spectra at the Fe K-edge indicate that the local order around the Fe atom changes slightly upon Cr substitution: Measurements in the Cr K-edge show that the Cr environment remains unchanged in the whole series. All the observed trends in both average Rietveld and local EXAFS distances can be traced back to the differences in the coordination polyhedra around Cr and Fe. The polyhedron around Cr is more symmetric and can be described as Cr(OH_0.5)₆ as opposed to the polyhedron around Fe that contains two distinct sets of ligands, FeO₃(OH)₃. The effects caused by substitution are governed by this difference, rather than by the smaller size of Cr(III) as compared to Fe(III). Simultaneous use of XAS and Rietveld refinement of XRD data permits tracing the trends in the average long range ordering (Me-Me distances) to local changes in distances and angles when Cr³⁺ substitutes Fe³⁺ in goethite. Complex changes in the various interatomic distances and angles may result in deceivingly simple long-range trends. These trends are therefore of limited value as probes for the atomic scale changes. On the other hand, XAS provide direct information on the fundamental, atomic-scale changes.     

Reversibility of radiocaesium sorption on illite

Adsorption of trace amounts of radiocaesium on NH₄-, K-, and Na-saturated Fithian illite and subsequent desorption by 1 M NH₄ showed that a substantial amount of radiocaesium (44%, 46%, and 91% for NH₄-, K-, and Na-illite, respectively) cannot be desorbed after only 5 min of adsorption. Our results suggest that this instantaneous fixation is caused by the collapse of the frayed edges of the clay mineral and the relatively high concentration of radiocaesium building up in solution in the batch desorption experiments. Consequently, commonly applied high-NH₄ extractions underestimate truly exchangeable amounts of radiocaesium in soils and sediments containing illitic clay minerals. The rate of desorption of trace amounts of radiocaesium from the solids using high NH₄ or Cs concentrations has a half-life of about 2 yr, reflecting radiocaesium desorption from (partially) collapsed interlayers. Extraction of radiocaesium from illite after 5 min of contact time with a Cs-selective adsorbent or a 1 × 10^-6 M CsCl solution shows that 100% of the bound radiocaesium is readily available. The desorption rate in the presence of a Cs-selective adsorbent has a half-life of about 0.2 yr. Desorption of radiocaesium from illite using different ammonium concentrations shows that radiocaesium partitioning follows reversible ion-exchange theory if the NH₄ concentration is below 1 × 10^-4 M, and sufficient time (weeks) is allowed for the reaction to proceed. Thus, radiocaesium sorption reversibility in the natural environment is much higher than generally assumed, and equilibrium solid/liquid partitioning may be assumed for the long-term modelling of radiocaesium mobility in the natural environment. In the particular case of anoxic freshwater sediments with very high NH₄ concentrations in the pore waters (up to several mmol.L^-1), collapse of the frayed edges of illite may occur, influencing radiocaesium partitioning. If collapse occurs before radiocaesium adsorbs to illite, high caesium sorption reversibility as measured by high-NH₄ extraction can be expected because further collapse of the frayed edges during the extraction procedure will be limited. This effect has indeed been observed earlier in the extraction of radiocaesium from anoxic freshwater sediments with high-NH₄ solutions and was as yet unexplained.     

Imaging and mechanical property measurements of kerogen via nanoindentation

Most analyses of kerogens rely on samples that have been isolated by dissolving the rock matrix. The properties of the kerogen before and after such isolation may be different and all sample orientation information is lost. We report a method of measuring kerogen mechanical properties in the rock matrix without isolation. An atomic force microscope (AFM) based nanoindenter is used to measure the hardness and reduced modulus of the kerogen within Woodford shale. The same instrument also provides useful images of polished rock sections on a submicrometer scale. Measurements were carried out both parallel and perpendicular to the bedding plane.     

Rates of weathering rind formation on Costa Rican basalt

Weathering rind thicknesses were measured on ∼ 200 basaltic clasts collected from three regionally extensive alluvial fill terraces (Qt 1, Qt 2, and Qt 3) preserved along the Pacific coast of Costa Rica. Mass balance calculations suggest that conversion of unweathered basaltic core minerals (plagioclase and augite) to authigenic minerals in the porous rind (kaolinite, allophane, gibbsite, Fe oxyhydroxides) is iso-volumetric and Ti and Zr are relatively immobile. The hierarchy of cation mobility (Ca ≈ Na > K ≈ Mg > Si > Al > Fe ≈ P) is similar to other tropical weathering profiles and is indicative of differential rates of mineral weathering (anorthite > albite ≈ hypersthene > orthoclase ? apatite). Alteration profiles across the cm-thick rinds document dissolution of plagioclase and augite and the growth of kaolinite, with subsequent dissolution of kaolinite and precipitation of gibbsite as weathering rinds age. The rate of weathering rind advance is evaluated using a diffusion-limited model which predicts a parabolic rate law for weathering rind thickness, r_r, as a function of time, t(r_r =), and an interface-limited model which predicts a linear rate law for weathering rind thickness as a function of time (r_r = k_appt). In these rate laws, κ is a diffusion parameter and k_app is an apparent rate constant. The rate of advance is best fit by the interface model.Terrace exposures are confined to the lower reaches of streams draining the Pacific slope near the coast where the stream gradient is less than ∼3 m/km, and terrace deposition is influenced by eustatic sea level fluctuations. Geomorphological evidence is consistent with terrace deposition coincident with sea level maxima when the stream gradient would be lowest. Assigning the most weathered regionally extensive terrace Qt 1 (mean rind thickness 6.9 ± 0. 6cm) to oxygen isotope stage (OIS) 7 (ca. 240 ka), and assuming that at time = 0 rind thickness = 0, it is inferred that terrace Qt 2 (r_r = 2.9 ± 0.1 cm) is coincident with stage 5e (ca. 125 ka) and that Qt 3 (r_r = 0.9 ± 0.1 cm) is consistent with OIS 3 (ca. 37 ka). These assignments yield a value of k_app of 8.6 × 10⁻¹³ cm s⁻¹ (R² = 0.99). Only this value satisfies both the existing age controls and yields ages coincident with sea level maxima. Using this value, elemental weathering release fluxes across a weathering rind from Qt 2 range from 6.0 × 10⁻⁹ mol Si m⁻² s⁻¹ to 2.5 × 10⁻¹¹ mol K m⁻² s⁻¹. The rate of rind advance for the Costa Rican terraces is 2.8 × 10⁻⁷ m yr⁻¹. Basalt rind formation rates in lower temperature settings described in the literature are also consistent with interface-controlled weathering with an apparent activation energy of about 50 kJ mol⁻¹. Rates of rind formation in Costa Rica are an order of magnitude slower than reported for global averages of soil formation rates.