Distribution and significance of long-chain alkenones as salinity and temperature indicators in Spanish saline lake sediments

We investigated relationships between sedimentary solvent-extractable long-chain alkenone (LCA) concentration and composition and environmental factors in a suite of endorheic lakes from inland Spain. LCAs were found in 14 of the 54 lakes examined, with concentrations comparable with those from previously published lacustrine settings. The composition of LCAs in our sites, however, contrast from the majority of those previously reported from lake environments; in our study the tri-unsaturated component is the most abundant component at most sites where LCAs are detected, and C_38:3 is the most abundant LCA in the majority of sites. LCA occurrence appears to be restricted to brackish-hypersaline sites and C₃₇ LCAs are absent above a salinity of ∼40 g L⁻¹ suggesting a salinity control on LCA-producing organisms in these sites. Low concentrations of C₃₇ LCA components means and temperature indices are generally not applicable. Instead we find good relationships between C₃₈ components and (in particular mean autumn) temperature and the strongest LCA-temperature relationships are found when using a combination of all C₃₇ and C₃₈ compounds. We propose a new alkenone temperature index for lakes with elevated salinity and where the C₃₈ components dominate the LCA distributions. This is expressed as (r² = 0.80, n = 13). In this paper, we provide the first account of sedimentary LCA distributions from lakes in inland Spain, extending the range of environments within which these compounds have been found and highlighting their significance as indicators of both salinity and temperature in saline, endorheic lake environments. This has important implications for extending the potential role of LCAs as palaeoclimatic indicators in lacustrine environments.     

Climatic and environmental controls on the occurrence and distributions of long chain alkenones in lakes of the interior United States

Long chain alkenones (LCA) are temperature-sensitive lipids with great potential for quantitative reconstruction of past continental climate. We conducted the first survey for alkenone biomarkers from 55 different lakes in the Northern Great Plains and Nebraska Sand Hills of the United States. Among those surveyed, we found 13 lakes that contain LCAs in the surface sediments. The highest concentrations of alkenones in sediments are found in cold (mean annual air temperature ∼11 °C versus 17 °C in our warmest sites), brackish to mesosaline (salinity = 8.5-9.7 g/L), and alkaline (pH = 8.4-9.0) lakes with high concentrations of sodium and sulfate. The dynamics of stratification and nutrient availability also appear to play a role in LCA abundance, as early spring mixing promotes a bloom of alkenone-producing haptophytes. Four of the alkenone-containing sites contain the C_37:4 alkenone; however, we discovered an unprecedented lacustrine alkenone distribution in a cluster of lakes, with a total absence of C_37:4 alkenone. We attribute this unusual composition to a different haptophyte species and show that the sulfate:carbonate ratio may control the occurrence of these two distinct populations. We created a new in-situ temperature calibration for lacustrine sites that contain C_37:4 using a water-column calibration from Lake George, ND and show that is linearly correlated to lake water temperature (R² = 0.74), but is not. A number of lakes contain an unidentified compound series that elutes close to the LCAs, highlighting the importance of routine GC-MS examination prior to using lacustrine LCAs for paleotemperature reconstructions.     

Complexation of Cu in Hydrothermal NaCl Brines: Ab initio molecular dynamics and energetics

Chloride complexation of Cu⁺ controls the solubility of copper(I) oxide and sulfide ore minerals in hydrothermal and diagenetic fluids. Solubility measurements and optical spectra of high temperature CuCl solutions have been interpreted as indicating the formation of CuCl, , and complexes. However, no other monovalent cation forms tri- and tetrachloro complexes. EXAFS spectra of high temperature Cu-Cl solutions, moreover, appear to show only CuCl and complexes at T > 100 °C. To reconcile these results, I investigated the nature and stability of Cu-Cl complexes using ab initio cluster calculations and ab initio (Car-Parrinello) molecular dynamics simulations for CuCl-NaCl-H₂O systems at 25 to 450 °C. Ab initio molecular dynamic simulations of 1 m CuCl in a 4 m Cl solution give a stable complex at 25 °C over 4 ps but show that the third Cl⁻ is weakly bound. When the temperature is increased along the liquid-vapour saturation curve to 125 °C, the complex dissociates into and Cl⁻; only forms at 325 °C and 1 kbar. Even in a 15.6 m Cl brine at 450 °C, only the complex forms over a 4 ps simulation run.Cluster calculations with a static dielectric continuum solvation field (COSMO) were used in an attempt directly estimate free energies of complex formation in aqueous solution. Consistent with the MD simulations, the complex is slightly stable at 25 °C but decreases in stability with decreasing dielectric constant (ε). The complex is predicted to be unstable at 25 °C and becomes increasingly unstable with decreasing dielectric constant. In hydrothermal fluids (ε < 30) both the and complexes are unstable to dissociation into and Cl⁻.The results obtained here are at odds with recent equations of state that predict and complexes are the predominant species in hydrothermal brines. In contrast, I predict that only complexes will be significant at T > 125 °C, even in NaCl-saturated brines. The high-temperature (T > 125 °C) optical spectra of CuCl solutions and solubility measurements of Cu minerals in Cl-brines need to be reinterpreted in terms of only the CuCl and complexes.     

Dawsonite synthesis and reevaluation of its thermodynamic properties from solubility measurements: Implications for mineral trapping of CO2

Over the last decade, a significant research effort has focused on determining the feasibility of sequestering large amounts of CO₂ in deep, permeable geologic formations to reduce carbon dioxide emissions to the atmosphere. Most models indicate that injection of CO₂ into deep sedimentary formations will lead to the formation of various carbonate minerals, including the common phases calcite (CaCO₃), dolomite (CaMg(CO₃)₂), magnesite (MgCO₃), siderite (FeCO₃), as well as the far less common mineral, dawsonite (NaAlCO₃(OH)₂). Nevertheless, the equilibrium and kinetics that control the precipitation of stable carbonate minerals are poorly understood and few experiments have been performed to validate computer codes that model CO₂ sequestration.In order to reduce this uncertainty we measured the solubility of synthetic dawsonite according to the equilibrium: , from under- and oversaturated solutions at 50-200 °C in basic media at 1.0 mol · kg⁻¹ NaCl. The solubility products (Q_s) obtained were extrapolated to infinite dilution to obtain the solubility constants (. Combining the fit of these values and fixing  at 25 °C, which was derived from the calorimetric data of Ferrante et al. [Ferrante, M.J., Stuve, J.M., and Richardson, D.W., 1976. Thermodynamic data for synthetic dawsonite. U.S. Bureau of Mines Report Investigation, 8129, Washington, D.C., 13p.], the following thermodynamic parameters for the dissolution of dawsonite were calculated at 25 °C: , and . Subsequently, we were able to derive values for the Gibbs energy of formation (, enthalpy of formation ( and entropy ( of dawsonite. These results are within the combined experimental uncertainties of the values reported by Ferrante et al. (1976). Predominance diagrams are presented for the dawsonite/boehmite and dawsonite/bayerite equilibria at 100 °C in the presence of a saline solution with and without silica-containing minerals.     

Comparative carbonate and hydroxide complexation of cations in seawater

Comparative concentrations of carbonate and hydroxide complexes in natural solutions can be expressed in terms of reactions with bicarbonate that have no explicit pH dependence (). Stability constants for this reaction with n = 1 were determined using conventional formation constant data expressed in terms of hydroxide and carbonate. Available data indicate that stability constants appropriate to seawater at 25 °C expressed in the form are on the order of 10^4.2 for a wide range of cations (M^z+) with z = +1, +2 and +3. Φ₁ is sufficiently large that species appear to substantially dominate MOH^z−1 species in seawater. Evaluations of comparative stepwise carbonate and hydroxide stability constant behavior leading to the formation of n = 2 and n = 3 complexes suggest that carbonate complexes generally dominate hydroxide complexes in seawater, even for cations whose inorganic speciation schemes in seawater are currently presumed to be strongly dominated by hydrolyzed forms (). Calculated stability constants, and , indicate that the importance of carbonate complexation is sufficiently large that carbonate and hydroxide complexes would be generally comparable even if calculated Φ₂ and Φ₃ values are overestimated by two or more orders of magnitude. Inclusion of mixed ligand species in carbonate-hydroxide speciation models allows cation complexation intensities (M_T/[M^z+]) to be expressed in the following form:

     

Zinc complexation in aqueous sulfide solutions: Determination of the stoichiometry and stability of complexes via ZnS(cr) solubility measurements at 100 °C and 150 bars

The solubility of ZnS_(cr) was measured at 100 °C, 150 bars in sulfide solutions as a function of sulfur concentration (m(S_total) = 0.02-0.15) and acidity (pH_t = 2-11). The experiments were conducted using a Ti flow-through hydrothermal reactor enabling the sampling of large volumes of solutions at experimental conditions, with the subsequent concentration and determination of trace quantities of Zn. Prior to the experiments, a long-term in situ conditioning of the solid phase was performed in order to attain the reproducible Zn concentrations (i.e. solubilities). The ZnS_(cr) solubility product was monitored in the course of the experiment. The following species were found to account for Zn speciation in solution: Zn²⁺ (pH_t < 3), (pH_t 3-4.5), (pH_t 5-8), and ZnS(HS)⁻ (pH_t > 8) (pH_t predominance regions are given for m(S_total) = 0.1). Solubility data collected in this study at pH_t > 3 were combined with the ZnS_(cr) solubility product determined at lower pH to yield the following equilibrium constants (t = 100 °C, P = 150 bars):

     

Dissociation constants and speciation in aqueous Li2SO4 and K2SO4 from measurements of electrical conductance to 673 K and 29 MPa

The electrical conductivities of aqueous solutions of Li₂SO₄ and K₂SO₄ have been measured at 523-673 K at 20-29 MPa in dilute solutions for molalities up to 2 × 10⁻² mol kg⁻¹. These conductivities have been fitted to the conductance equation of Turq, Blum, Bernard, and Kunz with a consensus mixing rule and mean spherical approximation activity coefficients. In the temperature interval 523-653 K, where the dielectric constant, ε, is greater than 14, the electrical conductance data can be fitted by a solution model which includes ion association to form , , and , where M is Li or K. The adjustable parameters of this model are the first and second dissociation constants of the M₂SO₄. For the 673 K and 300 kg m⁻³ state point where the Coulomb interactions are the strongest (dielectric constant, ε = 5), models with more extensive association give good fits to the data. In the case of the Li₂SO₄ model, including the multi-ion associate, , gave an extremely good fit to the conductance data.     

Synthesis and properties of ammoniojarosites prepared with iron-oxidizing acidophilic microorganisms at 22-65 °C

Ammoniojarosite [(NH₄,H₃O)Fe₃(OH)₆(SO₄)₂], a poorly soluble basic ferric sulfate, was produced by microbiological oxidation of ferrous sulfate at pH 2.0-3.0 over a range of concentrations (5.4-805 mM) and temperatures (22-65 °C). Ammoniojarosites were also produced by chemical (abiotic) procedures in parallel thermal (36-95 °C) experiments. At 36 °C, schwertmannite [ideally Fe₈O₈(OH)₆(SO₄)] was the only solid product formed at <10 mM concentrations. Between 11.5 and 85.4 mM , a mixed product of ammoniojarosite and schwertmannite precipitated, as identified by X-ray diffraction. In excess of 165 mM , ammoniojarosite was the only solid phase produced. An increase in the incubation temperature using thermoacidophiles at 45 and 65 °C accelerated the formation of ammoniojarosite in culture solutions containing 165 mM . Both the biogenic and chemical ammoniojarosites were yellow (2Y-4Y in Munsell hue), low surface area (<1 m²/g), well crystalline materials with average c_o and a_o unit cell parameters of 17.467 ± 0.048 Å and 7.330 ± 0.006 Å, respectively. Strong positive correlations were observed between unit cell axial ratios (c_o/a_o) and increasing synthesis temperature in both biotic and abiotic systems. All samples were N deficient compared to stoichiometric ammoniojarosite, and both chemical and X-ray data indicated partial replacement of by H₃O⁺ to form solid solutions with 0.14-0.24 mole H₃O⁺ per formula unit. The morphology of the biogenic jarosites included aggregated discs, pseudo-cubic crystals and botryoidal particles, whereas the chemical specimens prepared at 36-95 °C were composed of irregular crystals with angular edges. Morphological information may thus be useful to evaluate environmental parameters and mode of formation. The data may also have application in predicting phase boundary conditions for Fe(III) precipitation in biogeochemical processes and treatment systems involving acid sulfate waters.     

A potentiometric study of the stability of aqueous yttrium-acetate complexes from 25 to 175 °C and 1-1000 bar

The stability of yttrium-acetate (Y-Ac) complexes in aqueous solution was determined potentiometrically at temperatures 25-175 °C (at P_s) and pressures 1-1000 bar (at 25 and 75 °C). Measurements were performed using glass H⁺-selective electrodes in potentiometric cells with a liquid junction. The species YAc²⁺ and were found to dominate yttrium aqueous speciation in experimental solutions at 25-100 °C (log [Ac⁻] < −1.5, pH < 5.2), whereas at 125, 150 and 175 °C introduction of into the Y-Ac speciation model was necessary. The overall stability constants β_n were determined for the reaction

     

Olivine/melt transition metal partitioning, melt composition, and melt structure—Influence of Al for Si substitution in the tetrahedral network of silicate melts

The influence on olivine/melt transition metal (Mn, Co, Ni) partitioning of substitution in the tetrahedral network of silicate melt structure has been examined at ambient pressure in the 1450-1550 °C temperature range. Experiments were conducted in the systems NaAlSiO₄-Mg₂SiO₄- SiO₂ and CaAl₂Si₂O₈-Mg₂SiO₄-SiO₂ with about 1 wt% each of MnO, CoO, and NiO added. These compositions were used to evaluate how, in silicate melts, substitution and ionization potential of charge-balancing cations affect activity-composition relations in silicate melts and mineral/melt partitioning.The exchange equilibrium coefficient, , is a positive and linear function of melt Al/(Al + Si) at constant degree of melt polymerization, NBO/T. The is negatively correlated with the ionic radius, r, of the M-cation and also with the ionization potential (Z/r², Z = electrical charge) of the cation that serves to charge-balance Al³⁺ in tetrahedral coordination in the melts. The activity coefficient ratio, (γ_M/γ_Mg)^melt, is therefore similarly correlated.These melt composition relationships are governed by the distribution of Al³⁺ among coexisting Q-species in the peralkaline (depolymerized) melts coexisting with olivine. This distribution controls Q-speciation abundance, which, in turn, controls (γ_M/γ_Mg)^melt and . The relations between melt structure and olivine/melt partitioning behavior lead to the suggestion that in natural magmatic systems mineral/melt partition coefficients are more dependent on melt composition and, therefore, melt structure the more alkali-rich and the more felsic the melt. Moreover, mineral/melt partition coefficients are more sensitive to melt composition the more highly charged or the smaller the ionic radius of the cation of interest.     

An XAS study of the structure and thermodynamics of Cu(I) chloride complexes in brines up to high temperature (400 °C, 600 bar)

The transport and deposition of copper in saline hydrothermal fluids are controlled by the stability of copper(I) complexes with ligands such as chloride. Despite their role in the formation of most hydrothermal copper deposits, the nature and stability of Cu(I) chloride complexes in highly saline brines remains controversial. We present new X-ray absorption data (P = 600 bar, T = 25-400 °C, salinity up to 17.2 m Cl), which indicate that the linear (x = 1, 2) complexes are stable up to supercritical conditions. Distorted trigonal planar complexes predominate at room temperature and at high salinity (>3 m LiCl): subtle changes in the XANES spectrum with increasing salinity may reflect geometric distortions of this complex. Similar changes were observed in UV-Vis data [Liu, W., Brugger, J., McPhail, D.C., Spiccia, L., 2002. A spectrophotometric study of aqueous copper(I) chloride complexes in LiCl solutions between 100 °C and 250 °C. Geochim. Cosmochim. Acta66, 3615-3633], and were erroneously interpreted as a new species, . Our XAS data and ab-initio XANES calculations show that this tetrahedral species is not present to any significant degree in our solutions. The stability of the complexe decreases with increasing temperature; under supercritical conditions and in brines under magmatic-hydrothermal conditions (e.g., 15.58 m Cl, 400 °C, 600 bar), only the linear Cu(I) chloride complexes were observed. This result and the instability of the complex are also consistent with the recent ab-initio molecular dynamic calculations of Sherman [Sherman D. M.(2007) Complexation of Cu⁺ in hydrothermal NaCl brines: ab-initio molecular dynamics and energetics. Geochim. Cosmochim. Acta71, 714-722]. This study illustrates the power of the quantitative nature of XANES and EXAFS measurements for deciphering the speciation of weak transition metal complexes up to magmatic-hydrothermal conditions.The systematic XANES data are used to retrieve the formation constant for at 150 °C, which is in good agreement with the reinterpretation of the UV-Vis data of Liu et al. (Liu et al., 2002). At high temperatures (?400 °C), the solubility of chalcopyrite in equilibrium with hematite-magnetite-pyrite and K-feldspar-muscovite-quartz calculated with the new properties is lower than that calculated using the previous model, and the calculated solubilities are at the lower end of the range of values measured in brine inclusions from porphyry copper systems.