Complexation of cadmium to sulfur and oxygen functional groups in an organic soil

Cadmium (Cd) is a toxic trace element and due to human activities soils and waters are contaminated by Cd both on a local and global scale. It is widely accepted that chemical interactions with functional groups of natural organic matter (NOM) is vital for the bioavailability and mobility of trace elements. In this study the binding strength of cadmium (Cd) to soil organic matter (SOM) was determined in an organic (49% organic C) soil as a function of reaction time, pH and Cd concentration. In experiments conducted at native Cd concentrations in soil (0.23 μg g⁻¹ dry soil), halides (Cl, Br) were used as competing ligands to functional groups in SOM. The concentration of Cd in the aqueous phase was determined by isotope-dilution (ID) inductively-coupled-plasma-mass-spectrometry (ICP-MS), and the activity of Cd²⁺ was calculated from the well-established Cd-halide constants. At higher Cd loading (500-54,000 μg g⁻¹), the Cd²⁺ activity was directly determined by an ion-selective electrode (ISE). On the basis of results from extended X-ray absorption fine structure (EXAFS) spectroscopy, a model with one thiolate group (RS⁻) was used to describe the complexation (Cd²⁺ + RS⁻ ? CdSR⁺; log K_CdSR) at native Cd concentrations. The concentration of thiols (RSH; 0.047 mol kg⁻¹ C) was independently determined by X-ray absorption near-edge structure (XANES) spectroscopy. Log K_CdSR values of 11.2-11.6 (pK_a for RSH = 9.96), determined in the pH range 3.1-4.6, compare favorably with stability constants for the association between Cd and well-defined thiolates like glutathione. In the concentration range 500-54,000 μg Cd g⁻¹, a model consisting of one thiolate and one carboxylate (RCOO⁻) gave the best fit to data, indicating an increasing role for RCOOH groups as RSH groups become saturated. The determined log K_CdOOCR of 3.2 (Cd²⁺ +  RCOO⁻ ? CdOOCR⁺; log K_CdOOCR; pK_a for RCOOH = 4.5) is in accordance with stability constants determined for the association between Cd and well-defined carboxylates. Given a concentration of reduced sulfur groups of 0.2% or higher in NOM, we conclude that the complexation to organic RSH groups may control the speciation of Cd in soils, and most likely also in surface waters, with a total concentration less than 5 mg Cd g⁻¹ organic C.     

Mechanism of tremolite carbonation

The direct aqueous carbonation of fibrous tremolite was investigated by TEM, powder XRD, and FT-IR to elucidate the transformation mechanism of tremolite into calcite. The TEM study revealed that tremolite dissolves along the weak cleavage on (1 0 0) and (1 1 0) and the cleavage fragment fibers. The preferential release of Ca, relative to Si or Mg, into the solution resulted in the crystallization of compositionally homogeneous calcite and in the concurrent formation of proto-saponite. X-ray diffraction analysis revealed that the amount of product calcite increased rapidly within 15 min. Subsequently, after a 5 h carbonation period, it increased slowly to about 60%. The product calcite is euhedral. In contrast, the unreacted tremolite grains retain the original crystal structure even after prolonged carbonation. The morphological modification of tremolite from fibrous to round may have important implications for the elimination of its toxicity.     

Assessing the concentration,speciation, and toxicity of dissolved metals during mixing of acid-mine drainage and ambient river water downstream of the Elizabeth Copper Mine,Vermont, USA

The authors determine the composition of a river that is impacted by acid-mine drainage, evaluate dominant physical and geochemical processes controlling the composition, and assess dissolved metal speciation and toxicity using a combination of laboratory, field and modeling studies. Values of pH increase from 3.3 to 7.6 and the sum of dissolved base metal (Cd + Co + Cu + Ni + Pb + Zn) concentrations decreases from 6270 to 100 μg/L in the dynamic mixing and reaction zone that is downstream of the river’s confluence with acid-mine drainage. Mixing diagrams and PHREEQC calculations indicate that mixing and dilution affect the concentrations of all dissolved elements in the reach, and are the dominant processes controlling dissolved Ca, K, Li, Mn and SO₄ concentrations. Additionally, dissolved Al and Fe concentrations decrease due to mineral precipitation (gibbsite, schwertmannite and ferrihydrite), whereas dissolved concentrations of Cd, Co, Cu, Ni, Pb and Zn decrease due to adsorption onto newly formed Fe precipitates.     

Geochemical processes and mobilization of toxic metals and metalloids in an As-rich base metal waste pile in Zimapán,Central Mexico

The geochemistry and mineralogy of samples collected along depth profiles from an As-rich tailing deposit with abundant calcite was studied to determine the processes that influence the mobility of Fe, Zn, Cu, Ni, Cd, As, Sb, Cr and Tl. In spite of their near neutral pH, almost all of them are acid potential generators. Total concentrations decreased as: Fe > As > Zn > Pb > Cu > Sb > Cd > Cr > Ni > Tl. Soluble contents were lower and followed a slightly different order. Mobility decreased as: Tl > Cd, Zn, Cu, Sb, Ni, As > Fe, Pb > Cr. Higher soluble concentrations of Fe, Cu, Zn, As, Pb, and Ni were found in low-pH samples and of Sb and Tl in near-neutral samples. Sulfide oxidation processes are developing in the tailing’s dam. These processes do not have a trend with depth but occur mainly in acid layers. Near neutral layers formed by primary sulfides and calcite probably correspond to wastes produced from the processing of ore coming mainly from pods within the skarn, and acid layers with abundant secondary minerals from material mined from chimneys and mantos. The presence of calcite influences speciation, neutralizes acid mine drainage (AMD), and decreases the mobility of most toxic metals and metalloids (TMMs). However, a hard-pan layer was not observed in the studied profiles. Retention of TMM within tailings probably occurs through the formation of low solubility metal carbonates and from elevation of pH that promotes Fe hydroxides precipitation that may retain As, Sb and metals. Calcite occurrence promotes As, Cd, Cu, Fe, Zn, Pb, Cd and Cr retention, does not play a role on Tl and Ni mobilization, and increases Sb release.     

Response of the coccolithophores Emiliania huxleyi and Coccolithus braarudii to changing seawater Mg and Ca concentrations: Mg/Ca, Sr/Ca ratios and δCa, δMg of coccolith calcite

Calcium and magnesium concentrations in seawater have varied over geological time scales. On short time scales, variations in the major ion composition of seawater influences coccolithophorid physiology and the chemistry of biogenically produced coccoliths. Validation of those changes via controlled laboratory experiments is a crucial step in applying coccolithophorid based paleoproxies for the reconstruction of past environmental conditions. Therefore, we examined the response of two species of coccolithophores, Emiliania huxleyi and Coccolithus braarudii, to changes in the seawater Mg/Ca ratio (≈0.5 to 10 mol/mol) by either manipulating the magnesium or calcium concentration under controlled laboratory conditions. Concurrently, seawater Sr/Ca ratios were also modified (≈2 to 40 mmol/mol), while keeping salinity constant at 35. The physiological response was monitored by measurements of the cell growth rate as well as the production rates of particulate inorganic and organic carbon, and chlorophyll a. Additionally, coccolithophorid calcite was analyzed for its elemental composition (Sr/Ca and Mg/Ca) as well as isotope fractionation of calcium and magnesium (Δ^44/40Ca and Δ^26/24Mg). Our results reveal that physiological rates were substantially influenced by changes in seawater calcium rather than magnesium concentration within the range estimated to have occurred over the past 250 million years when coccolithophores appear in the fossil record. All physiological rates of E. huxleyi decreased at a calcium concentration above 25 mmol L⁻¹, whereas C. braarudii displayed a higher tolerance to increased seawater calcium concentrations. Partition coefficient of Sr was calculated as 0.36 ± 0.04 (±2σ) independent of species. Partition coefficient of Mg²⁺ increased with increasing seawater Ca²⁺ concentrations in both coccolithophore species. Calcium isotope fractionation was constant at 1.1 ± 0.1‰ (±2σ) and not altered by changes in seawater Mg/Ca ratio. There is a well-defined inverse linear relationship between calcium isotope fractionation and partition coefficient of Sr²⁺ in all experiments, suggesting similar controls on both proxies in the investigated species. Magnesium isotope ratios were relatively stable for seawater Mg/Ca ratios ranging from 1 to 5, with a higher degree of fractionation in Emiliania huxleyi (by ≈0.2‰ in Δ^26/24Mg). Although Mg/Ca ratios in the calcite of coccolithophores and foraminifera are similar, the former have considerably higher Δ^26/24Mg (by >+3‰), presumably due to differences in calcification mechanisms between the two taxa. These observations suggest, a physiological control over magnesium elemental and isotopic fractionation during the process of calcification in coccolithophores.     

Seawater nutrient and carbonate ion concentrations recorded as P/Ca, Ba/Ca, and U/Ca in the deep-sea coral Desmophyllum dianthus

As paleoceanographic archives, deep sea coral skeletons offer the potential for high temporal resolution and precise absolute dating, but have not been fully investigated for geochemical reconstructions of past ocean conditions. Here we assess the utility of skeletal P/Ca, Ba/Ca and U/Ca in the deep sea coral D. dianthus as proxies of dissolved phosphate (remineralized at shallow depths), dissolved barium (trace element with silicate-type distribution) and carbonate ion concentrations, respectively. Measurements of these proxies in globally distributed D. dianthus specimens show clear dependence on corresponding seawater properties. Linear regression fits of mean coral Element/Ca ratios against seawater properties yield the equations: P/Ca_coral (μmol/mol) = (0.6 ± 0.1) P/Ca_sw(μmol/mol) - (23 ± 18), R² = 0.6, n = 16 and Ba/Ca_coral(μmol/mol) = (1.4 ± 0.3) Ba/Ca_sw(μmol/mol) + (0 ± 2), R² = 0.6, n = 17; no significant relationship is observed between the residuals of each regression and seawater temperature, salinity, pressure, pH or carbonate ion concentrations, suggesting that these variables were not significant secondary dependencies of these proxies. Four D. dianthus specimens growing at locations with Ω_arag ? 0.6 displayed markedly depleted P/Ca compared to the regression based on the remaining samples, a behavior attributed to an undersaturation effect. These corals were excluded from the calibration. Coral U/Ca correlates with seawater carbonate ion: U/Ca_coral(μmol/mol) = (−0.016 ± 0.003) (μmol/kg) + (3.2 ± 0.3), R² = 0.6, n = 17. The residuals of the U/Ca calibration are not significantly related to temperature, salinity, or pressure. Scatter about the linear calibration lines is attributed to imperfect spatial-temporal matches between the selected globally distributed specimens and available water column chemical data, and potentially to unresolved additional effects. The uncertainties of these initial proxy calibration regressions predict that dissolved phosphate could be reconstructed to ±0.4 μmol/kg (for 1.3-1.9 μmol/kg phosphate), and dissolved Ba to ±19 nmol/kg (for 41-82 nmol/kg Ba_sw). Carbonate ion concentration derived from U/Ca has an uncertainty of ±31μmol/kg (for ). The effect of microskeletal variability on P/Ca, Ba/Ca, and U/Ca was also assessed, with emphasis on centers of calcification, Fe-Mn phases, and external contaminants. Overall, the results show strong potential for reconstructing aspects of water mass mixing and biogeochemical processes in intermediate and deep waters using fossil deep-sea corals.     

Uptake of Zn,Cu, Pb,and Cd by water hyacinth in the initial stage of water system remediation

The removal efficiency of water hyacinth for Zn, Cu, Pb and Cd after their entry into an undisturbed fresh water body was studied using minicosms placed within a reservoir. Variable parameters were water pH (6 or 8), single or multi-metal additions, and the plant biomass. The initial concentrations of Zn, Cu, Pb and Cd in water (500, 250, 250 and 50 μg/L, respectively) quickly decreased in the order Pb ≈ Cu ? Cd ≈ Zn in the first days. Metal removal was more efficient at pH 8 than at pH 6, and it was only slightly higher for single metals compared to multi-metal additions. After 8 days the remaining amounts of metals relative to their initial concentrations for multi-metal pollution treatments were 8% and 24% (Cu), 11% and 26% (Pb), 24% and 50% (Cd), and 18% and 57% (Zn) at pH 8 and pH 6, respectively. Increasing plant biomass promoted faster metal removal. The bioconcentration factor (the ratio of the metal concentration in whole plants to the initial metal concentration in water) exceeds 2000 for all metals (with the exception of Zn and Cd at pH 6). It was concluded that the water hyacinth can be successfully used for fast removal of metals in the initial stage of water body remediation.     

Effects of diagenesis on paleoclimate reconstructions from modern and young fossil corals

The integrity of coral-based reconstructions of past climate variability depends on a comprehensive knowledge of the effects of post-depositional alteration on coral skeletal geochemistry. Here we combine millimeter-scale and micro-scale coral Sr/Ca data, scanning electron microscopy (SEM) images, and X-ray diffraction with previously published δ¹⁸O records to investigate the effects of submarine and subaerial diagenesis on paleoclimate reconstructions in modern and young sub-fossil corals from the central tropical Pacific. In a 40-year-old modern coral, we find secondary aragonite is associated with relatively high coral δ¹⁸O and Sr/Ca, equivalent to sea-surface temperature (SST) artifacts as large as −3 and −5 °C, respectively. Secondary aragonite observed in a 350-year-old fossil coral is associated with relatively high δ¹⁸O and Sr/Ca, resulting in apparent paleo-SST offsets of up to −2 and −4 °C, respectively. Secondary Ion Mass Spectrometry (SIMS) analyses of secondary aragonite yield Sr/Ca ratios ranging from 10.78 to 12.39 mmol/mol, significantly higher compared to 9.15 ± 0.37 mmol/mol measured in more pristine sections of the same fossil coral. Widespread dissolution and secondary calcite observed in a 750-year-old fossil coral is associated with relatively low δ¹⁸O and Sr/Ca. SIMS Sr/Ca measurements of the secondary calcite (1.96-9.74 mmol/mol) are significantly lower and more variable than Sr/Ca values from more pristine portions of the same fossil coral (8.22 ± 0.13 mmol/mol). Our results indicate that while diagenesis has a much larger impact on Sr/Ca-based paleoclimate reconstructions than δ¹⁸O-based reconstructions at our site, SIMS analyses of relatively pristine skeletal elements in an altered coral may provide robust estimates of Sr/Ca which can be used to derive paleo-SSTs.     

Calcium isotope fractionation in calcite and aragonite

Calcium isotope fractionation was measured on skeletal aragonite and calcite from different marine biota and on inorganic calcite. Precipitation temperatures ranged from 0 to 28°C. Calcium isotope fractionation shows a temperature dependence in accordance with previous observations: 1000 · ln(α_cc) = −1.4 + 0.021 · T (°C) for calcite and 1000 · ln(α_ar) = −1.9 + 0.017 · T (°C) for aragonite. Within uncertainty the temperature slopes are identical for the two polymorphs. However, at all temperatures calcium isotopes are more fractionated in aragonite than in calcite. The offset in δ^44/40Ca is about 0.6‰. The underlying mechanism for this offset may be related to the different coordination numbers and bond strengths of the calcium ions in calcite and aragonite crystals, or to different Ca reaction behavior at the solid-liquid interface. Recently, the observed temperature dependence of the Ca isotope fractionation was explained quantitatively by the temperature control on precipitation rates of calcium carbonates in an experimental setting (Lemarchand et al., 2004). We show that this mechanism can in principle also be applied to CaCO₃ precipitation in natural environments in normal marine settings. Following this model, Ca isotope fractionation in marine Ca carbonates is primarily controlled by precipitation rates. On the other hand the larger Ca isotope fractionation of aragonite compared to calcite can not be explained by different precipitation rates. The rate control model of Ca isotope fractionation predicts a strong dependence of the Ca isotopic composition of carbonates on ambient CO₃²⁻ concentration. While this model is in general accordance with our observations in marine carbonates, cultured specimens of the planktic foraminifer Orbulina universa show no dependence of Ca-isotope fractionation on the ambient CO₃²⁻ concentration. The latter observation implies that the carbonate chemistry in the calcifying vesicles of the foraminifer is independent from the ambient carbonate ion concentration of the surrounding water.     

Controls on calcium isotope fractionation in cultured planktic foraminifera, Globigerinoides ruber and Globigerinella siphonifera

Specimens of two species of planktic foraminifera, Globigerinoides ruber and Globigerinella siphonifera, were grown under controlled laboratory conditions at a range of temperatures (18-31 °C), salinities (32-44 psu) and pH levels (7.9-8.4). The shells were examined for their calcium isotope compositions (δ^44/40Ca) and strontium to calcium ratios (Sr/Ca) using Thermal Ionization Mass Spectrometry and Inductively Coupled Plasma Mass Spectrometry. Although the total variation in δ^44/40Ca (∼0.3‰) in the studied species is on the same order as the external reproducibility, the data set reveals some apparent trends that are controlled by more than one environmental parameter. There is a well-defined inverse linear relationship between δ^44/40Ca and Sr/Ca in all experiments, suggesting similar controls on these proxies in foraminiferal calcite independent of species. Analogous to recent results from inorganically precipitated calcite, we suggest that Ca isotope fractionation and Sr partitioning in planktic foraminifera are mainly controlled by precipitation kinetics. This postulation provides us with a unique tool to calculate precipitation rates and draws support from the observation that Sr/Ca ratios are positively correlated with average growth rates. At 25 °C water temperature, precipitation rates in G. siphonifera and G. ruber are calculated to be on the order of 2000 and 3000 μmol/m²/h, respectively. The lower δ^44/40Ca observed at ?29 °C in both species is consistent with increased precipitation rates at high water temperatures. Salinity response of δ^44/40Ca (and Sr/Ca) in G. siphonifera implies that this species has the highest precipitation rates at the salinity of its natural habitat, whereas increasing salinities appear to trigger higher precipitation rates in G. ruber. Isotope effects that cannot be explained by precipitation rate in planktic foraminifera can be explained by a biological control, related to a vacuolar pathway for supply of ions during biomineralization and a pH regulation mechanism in these vacuoles. In case of an additional pathway via cross-membrane transport, supplying light Ca for calcification, the δ^44/40Ca of the reservoir is constrained as −0.2‰ relative to seawater. Using a Rayleigh distillation model, we calculate that calcification occurs in a semi-open system, where less than half of the Ca supplied by vacuolization is utilized for calcite precipitation. Our findings are relevant for interpreting paleo-proxy data on δ^44/40Ca and Sr/Ca in foraminifera as well as understanding their biomineralization processes.     

Incorporation of lead into calcium carbonate granules secreted by earthworms living in lead contaminated soils

The influence of soil organisms on metal mobility and bioavailability in soils is not currently fully understood. We conducted experiments to determine whether calcium carbonate granules secreted by the earthworm Lumbricus terrestris could incorporate and immobilise lead in lead- and calcium-amended artificial soils. Soil lead concentrations were up to 2000 mg kg⁻¹ and lead:calcium ratios by mass were 0.5-8. Average granule production rates of 0.39 ± 0.04 mg_calcite earthworm⁻¹ day⁻¹ did not vary with soil lead concentration. The lead:calcium ratio in granules increased significantly with that of the soil (r² = 0.81, p = 0.015) with lead concentrations in granules reaching 1577 mg kg⁻¹. X-ray diffraction detected calcite and aragonite in the granules with indications that lead was incorporated into the calcite at the surface of the granules. In addition to the presence of calcite and aragonite X-ray absorption spectroscopy indicated that lead was present in the granules mainly as complexes sorbed to the surface but with traces of lead-bearing calcite and cerussite. The impact that lead-incorporation into earthworm calcite granules has on lead mobility at lead-contaminated sites will depend on the fraction of total soil lead that would be otherwise mobile.     

Arsenate uptake by calcite: Macroscopic and spectroscopic characterization of adsorption and incorporation mechanisms

Batch uptake experiments and X-ray element mapping and spectroscopic techniques were used to investigate As(V) (arsenate) uptake mechanisms by calcite, including adsorption and coprecipitation. Batch sorption experiments in calcite-equilibrated suspensions (pH 8.3; PCO₂ = 10^−3.5 atm) reveal rapid initial sorption to calcite, with sorption rate gradually decreasing with time as available sorption sites decrease. An As(V)-calcite sorption isotherm determined after 24 h equilibration exhibits Langmuir-like behavior up to As concentrations of 300 μM. Maximum distribution coefficient values (K_d), derived from a best fit to a Langmuir model, are ∼190 L kg⁻¹.Calcite single crystals grown in the presence of As(V) show well-developed rhombohedral morphology with characteristic growth hillocks on surfaces at low As(V) concentrations (?5 μM), but habit modification is evident at As(V) concentrations ?30 μM in the form of macrostep development preferentially on the − vicinal surfaces of growth hillocks. Micro-X-ray fluorescence element mapping of surfaces shows preferential incorporation of As in the − vicinal faces relative to + vicinals. EXAFS fit results for both adsorption and coprecipitation samples confirm that As occurs in the 5+ oxidation state in tetrahedral coordination with oxygen, i.e., as arsenate. For adsorption samples, As(V) forms inner-sphere surface complexes via corner-sharing with Ca octahedra. As(V) coprecipitated with calcite substitutes in carbonate sites but with As off-centered, as indicated by two Ca shells, and with likely disruption of local structure. The results indicate that As(V) interacts strongly with the calcite surface, similar to often-cited analog phosphate, and uptake can occur via both adsorption and coprecipitation reactions. Therefore, calcite may be effective for partial removal of dissolved arsenate from aquatic and soil systems.     

Kinetics of Cd sorption, desorption and fixation by calcite: A long-term radiotracer study

Time-dependent sorption and desorption of Cd on calcite was studied over 210 days utilizing ¹⁰⁹Cd as a tracer to distinguish between ‘labile’ and ‘non-labile’ forms of sorbed Cd. Stabilizing the calcite suspensions for 12 months under atmospheric P_CO2 and controlled temperature was necessary to reliably follow Cd dynamics following initial sorption. Results revealed time-dependant Cd sorption and marked desorption hysteresis by calcite under environmentally relevant conditions. Data obtained were fitted to a first-order kinetic model and a concentric shell diffusion model. Both models described the progressive transfer of Cd²⁺ to a less reactive form within calcite and subsequent desorption of Cd subject to different initial contact times. The kinetic model provided a better fit to the combined sorption and desorption data (R² = 0.992). It differentiates between two ‘pools’ of sorbed Cd²⁺ on calcite, ‘labile’ and ‘non-labile’, in which labile sorbed Cd is in immediate equilibrium with the free Cd²⁺ ion activity in solution whereas non-labile Cd is kinetically restricted. For the diffusion model (R² = 0.959), the rate constants describing Cd dynamics in calcite produced a half-life for Cd desorption of ∼175 d, for release to a ‘zero-sink’ solution. Results from this study allow comment on the likely mechanisms occurring at the calcite surface following long-term Cd sorption.     

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     

Incorporation of trivalent actinides into calcite: A time resolved laser fluorescence spectroscopy (TRLFS) study

In order to characterize and quantify the substitution of Ca(II) by Cm(III) (coordination, charge compensation), homogeneous Cm(III) coprecipitated calcite was synthesized in a mixed-flow-through experiment. Two sets of experiments were conducted at pH 8.1 and at pH 12.5.At pH 8.1 two calcites, a calcite with a low Cm³⁺ concentration (LCMpH8.1) and a calcite with a high M³⁺ (Gd³⁺ and Cm³⁺) concentration (HCMpH8.1) were grown and investigated by time resolved laser fluorescence spectroscopy. The Cm(III) emission spectra of LCMpH8.1 and HCMpH8.1 show the same Cm(III) fluorescence signals for two Cm(III) species; Cm(III) species (1) with a peak maximum at 606.2 nm and Cm(III) species (2) with a peak maximum at 620.3 nm. Cm(III) species (1) has a mean lifetime of τ = 386 ± 40 μs and Cm(III) species (2) has a mean lifetime of τ = 1874 ± 200 μs. A lifetime of 386 μs correlates with 1.3 water molecule in the first coordination sphere of the Cm ion whereas a lifetime of 1874 μs indicates the total loss of the Cm(III) hydration sphere. According to the fluorescence emission peak position and the fluorescence emission lifetime, Cm(III) species (1) is identified as a surface sorbed species whereas Cm(III) species (2) is identified as a Cm(III) incorporated into the calcite lattice.Cm(III) fluorescence emission spectra of Cm(III) doped calcite grown at pH 12.5 (LCMpH12.5) show the same peak maxima which are found for LCMpH8.1 and HCMpH8.1 grown at pH 8.1 but an additional emission band at 608.2 nm (3) is found, which can be assigned to a further Cm(III) species. Fluorescence emission lifetime measurements show that this Cm(III) species (3) has a lifetime of τ = 477 ± 25 μs, which correlates with 0.9 water molecules in the first coordination sphere. Cm(III) species (3) is suggested to be a CmOH²⁺ incorporated species.     

Application of calcite Mg partitioning functions to the reconstruction of paleocean Mg/Ca

Calcite Mg/Ca is usually assumed to vary linearly with solution Mg/Ca, that a constant partition coefficient describes the relationship between these two ratios. Numerous published empirical datasets suggests that this relationship is better described by a power function. We provide a compilation of these literature data for biotic and abiotic calcite in the form of Calcite Mg/Ca = F(Solution Mg/Ca)^H, where F and H are empirically determined fitting parameters describing the slope and deviation from linearity, respectively, of the function. This is equivalent to Freundlich sorption behavior controlling Mg incorporation in calcite. Using a power function, instead of a partition coefficient, lowers Phanerozoic seawater Mg/Ca estimates based on echinoderm skeletal material by, on average, 0.5 mol/mol from previous estimates.These functions can also be used to model the primary skeletal calcite Mg/Ca of numerous calcite phases through geologic time. Such modeling suggests that the Mg/Ca of all calcite precipitated from seawater has varied through the Phanerozoic in response to changing seawater Mg/Ca and that the overall range in Mg/Ca measured among various calcite phases would be greatest when seawater Mg/Ca was also high (e.g., “aragonite seas”) and lowest when seawater Mg/Ca was low (e.g., “calcite seas”). It follows that, during times of “calcite seas” when the seawater Mg/Ca is presumed to have been lower, deposition of calcite with low Mg contents would have resulted in a depressed drive for diagenetic stabilization of shelfal carbonate and, in turn, lead to greater preservation of crystal and skeletal microfabrics and primary chemistries in biotic and abiotic calcites.     

Precise determination of cadmium isotope fractionation in seawater by double spike MC-ICPMS

A new technique has been developed for the accurate and precise determination of the stable Cd isotope composition of seawater. The method utilizes a ¹¹⁰Cd-¹¹¹Cd double spike, and it involves separation of Cd from seawater by column chromatography and isotopic analyses by multiple collector inductively coupled plasma mass spectrometry. As a by-product, it also generates precise Cd concentration data. Repeated analyses of three pure Cd reference materials and three seawater samples yielded reproducibilities of about ±1.0 to ±1.6 ε^114/110Cd (2 SD), based on measurements that each consumed about ∼8 ng of natural Cd (ε^114/110Cd is the deviation of the ¹¹⁴Cd/¹¹⁰Cd isotope ratio of a sample from the standard in parts per 10,000). This demonstrates that the new double spike technique is superior to published methods of Cd isotope analyses, with regard to the acquisition of precise data for samples of limited size. Additional experiments showed that as little as 1-5 ng of seawater Cd could be analyzed with a precision of about ±2 to ±6 ε^114/110Cd (2 SD). The accuracy of the seawater isotope data was ascertained by experiments in which a Cd-free seawater matrix was doped with small quantities of isotopically well-characterized Cd. Repeated mass scans that were carried out on purified Cd fractions of several samples furthermore demonstrated the absence of significant spectral interferences. The isotope data that were acquired for the three seawater samples reveal, for the first time, small but resolvable Cd isotope fractionations in the marine environment. Cadmium-rich intermediate water from the North Pacific was found to have an isotope composition of ε^114/110Cd = 3.2 ± 1.0. In contrast, Cd-depleted seawater from the upper water column of the Atlantic and Arctic Oceans displayed isotope compositions of ε^114/110Cd = 6.4 ± 1.1 and 6.6 ± 1.6, respectively. These observations are in accord with the interpretation that the isotope effects are due to the biological fractionation that occurs during the uptake of dissolved seawater Cd by phytoplankton.     

Trace/minor element:calcium ratios in cultured benthic foraminifera. Part II: Ontogenetic variation

Ontogenetic (developmental stage) measurements of Mg/Ca and Sr/Ca were made on the benthic foraminifer Bulimina aculeata, which were cultured under controlled physicochemical conditions of temperature, pH, alkalinity, salinity, and trace- and minor-element concentrations. We utilized two methods of ontogenetic sampling—whole specimens progressively increasing in length and laser microdissection of a single specimen with subsequent analysis of dissected portions. A novel high-resolution laser-microdissection (HRLM) method allowed for precise (10 μm) cuts of the foraminiferal tests (shells) along the geometrically complex sutures distinguishing individual chambers. This new microdissection method limited sample loss and cross-contamination between foraminiferal chambers. Little or no variation in D_Sr was observed at different foraminiferal developmental stages. Conversely, D_Mg was enriched during a mid-developmental stage of whole-specimen samples (150-225 μm D_Mg = 1.6 × 10⁻³) compared to earlier and later stages (<150 μm, >225 μm D_Mg = 8.3 × 10⁻⁴). Further analysis of HRLM ontogenetic samples showed a larger, age-dependent D_Mg signature variation. This increase in shell Mg/Ca may contribute substantially to the measured inter-individual variability in Mg/Ca temperature prediction for cultured B. aculeata. Due to relatively large Mg/Ca inter- and intra-individual variability, measuring similar-size foraminiferal samples may improve the precision of paleotemperature prediction. Additionally, partial dissolution of the highest ontogenetically Mg-enriched calcite (D_Mg = 1.3 × 10⁻²-1.6 × 10⁻²) may occur in undersaturated bottom-water environments or during reductive cleaning procedures. Thus, the calcite phases remaining after partial dissolution by either natural or laboratory cleaning processes may not accurately represent the calcification environment.     

Modeling the adsorption of Cd (II), Cu (II), Ni (II), Pb (II) and Zn (II) onto montmorillonite

The adsorption of five toxic metallic cations, Cd(II), Cu(II), Ni(II), Pb(II) and Zn(II), onto montmorillonite was investigated as a function of pH and ionic strength and a two-site surface complexation model was used to predict the adsorption data. The results showed that in the lower pH range, 3∼6 for Cd, Cu, Ni and Zn, and 3∼4.5 for Pb, the adsorption was greatly affected by ionic strength, while in the higher pH range, the adsorption was not. In the lower pH range, the metallic cations were mainly bound through the formation of outer-sphere surface on the permanently charged basal surface sites (≡X⁻), while in the higher pH range the adsorption occurred mainly on the variably charged edge sites (≡SOH) through the formation of inner-sphere surface complexes. Acid-base surface constants and metal binding constants for the two sites were optimized using FITEQL. The adsorption affinity of the five metallic cations to the permanently charged sites of montmorillonite was Pb > Cu > Ni ≈ Zn ≈ Cd, while that to the variable charged sites was Pb ? Cu > Zn > Cd > Ni.     

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

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