Seawater In Equilibrium Research Articles

Molecular speciation of Mo (VI) on goethite and its implications for molybdenum and its isotopic cycle in ocean

Sorption of Mo to goethite and related iron (hydr)oxides plays an important role in controlling the mobility of Mo in soil and aquatic environments. We first performed ab-initio molecular dynamics (MD) simulations of Mo complexation at the goethite {1 0 1}-water interface. We find that molybdenum adsorbed on goethite mainly exists as the inner-sphere tetrahedral bidentate and monodentate corner-sharing complex, instead of the previous argued bidentate edge-sharing or outer-sphere complex. Different from previous literature, we didn’t find any distorted octahedral structure of Mo on goethite. The predicted Mo-Fe distances of bidentate and monodentate corner-sharing complex based on ab-initio MD is 3.53 and 3.38 Å, respectively, which are consistent with the fitted Mo-Fe distances: 3.40 ± 0.15 Å and 3.41 ± 0.1 Å from EXAFS.We measured the adsorption of Mo (VI) on goethite (α-FeOOH) at different initial concentration and pH from 3 to 11 under N2 atmosphere and constructed a surface complexation model that is consistent with the molecular speciation based on ab-initio MD, XANES, EXAFS and reported in-situ ATR-IR analysis (Davantes and Lefevre, 2015, 2016). Application of SCM for Mo on goethite assuming iron oxides as the principal component of ferromanganese crust (containing average 276 mg/kg Mo) will have ∼0.3 μg/l Mo in seawater in equilibrium. The observed ∼10 μg/l Mo in seawater (c a. 400–800 yr) is between 0.3 (adsorption equilibrium by marine goethite) and 23 μg/l (adsorption equilibrium by marine birnessite, Kashiwabara et al., 2011), suggesting that molybdenum in seawater has reached equilibrium with ferromanganese crust, and goethite (57%) probably plays a more important role than birnessite (43%) in regulating molybdenum abundance in the ocean, which has not been fully recognized in past.Observed molybdenum isotopic fractionation between seawater and ferromanganese crust excesses the experimental molybdenum isotopic fractionation range during adsorption by birnessite and goethite (Goldberg et al., 2009). Based on reported experimental molybdenum isotopic fractionation values and our SCM prediction, the molybdenum isotopic fractionation δ98/95Mo value should vary between 1.6 and 2.3‰, which is lower than the observed range between seawater and ferromanganese crust: 2.7–3.2‰ about 1‰, suggesting the disequilibrium of molybdenum isotopic fractionation in ocean. To clarify the discrepancy, this study highlights the necessity of clarifying the molybdenum source in hydrogenic and hydrothermal ferromanganese crusts before understanding molybdenum isotopic fractionation between seawater and ferromanganese crust.

Fe(II)-carbonate precipitation kinetics and the chemistry of anoxic ferruginous seawater

The chemical evolution of anoxic seawater is chronicled in part by the abundance of sedimentary Fe minerals. Our view of the chemical state of the oceans throughout much of Earth's history, therefore, hinges on understanding the behaviour of Fe2+ in seawater and sedimentary pore water. However, the formation pathways and precipitation rates of a number of sedimentary Fe(II)-minerals are poorly understood. For example, because siderite is widely distributed in Precambrian rocks, it is frequently assumed that seawater Fe2+ concentrations would have been poised at siderite equilibrium. Yet a variety of studies indicate that siderite precipitation would have been strongly controlled by kinetic processes rather than equilibrium behaviour, prompting a reconsideration of the Precambrian Fe cycle and those element cycles to which it was connected. To address this issue, we experimentally investigated spontaneous (or homogeneous) precipitation of Fe(II)-carbonate across a range of pH and pCO2 conditions, and in the presence/absence of SiO2(aq). Our data show that the formation of siderite directly from seawater is initiated by the formation of precursor amorphous Fe carbonate (AFC). The precursor, once formed, rapidly transforms into either siderite or chukanovite (Fe2CO3(OH)2; metastable with respect to siderite), dependent on the availability of carbonate ions. Precipitation rates estimated from our experiments reveal a strong exponential dependence on the saturation state of the solution, consistent with theoretical predictions. This relationship, in turn, delineates a critical supersaturation threshold for homogeneous Fe(II)-carbonate precipitation from anoxic and ferruginous water. Incorporation of our experimental data with constraints on the ocean–atmosphere carbon system shows that significant accumulations of siderite in Precambrian sedimentary rocks require an origin from fluids that were distinct from seawater in equilibrium with the ocean–atmosphere inorganic carbon reservoir. In addition, these data show that the maximum Fe2+ concentration in Precambrian oceans was controlled by Fe(II)-silicate precipitation and not by Fe(II)-carbonate, consistent with the sedimentary record of Fe-rich Precambrian rocks. More broadly, these results contribute to a quantitative framework that underpins the utility of Fe(II)-carbonate as a sensitive palaeo-environmental proxy for ferruginous systems where CO2 may have been present.

In situ Raman spectroscopic study of the pressure effect on the concentration of CO2 in water at hydrate-liquid water equilibrium up to 900 bar

CO2 hydrate may form during the injection of CO2 into the submarine environment for CO2 storage or enhanced recovery of CH4 hydrate. The growth of CO2 hydrate in seawater is dependent on the concentration of CO2 in seawater in equilibrium with hydrate. However, such data are currently only available at pressures below 250 bar, and the pressure effect on the concentration of CO2 in water at hydrate-liquid water equilibrium still remains controversial. In this study, Raman spectroscopy was applied to measure the concentration of CO2 in water in equilibrium with hydrate from 276.15 to 289.05 K and from 30 to 900 bar. Our results show that the concentration of CO2 in water at hydrate-liquid water equilibrium increases with increasing temperature (∼0.081 mol/(kg K) on average), but decreases slightly with increasing pressure (−0.016, −0.021 and −0.034 mol/(kg·100 bar) at 276.15, 280.15 and 285.15 K, respectively). An equation that can well reproduce the current and previous measurements of CO2 concentration in water at hydrate-liquid water equilibrium was proposed.

Chemical speciation of environmentally significant metals with inorganic ligands. Part 5: The Zn2+ + OH-, Cl-, CO3 2-, SO4 2-, and PO4 3- systems (IUPAC Technical Report)

Abstract The numerical modeling of ZnII speciation amongst the environmental inorganic ligands Cl–, OH–, CO3 2–, SO4 2–, and PO4 3– requires reliable values for the relevant stability (formation) constants. This paper compiles and provides a critical review of these constants and related thermodynamic data. It recommends values of log10 β p,q,r ° valid at I m = 0 mol·kg–1 and 25 °C (298.15 K), and reports the empirical reaction ion interaction coefficients, ∆ε, required to calculate log10 β p,q,r values at higher ionic strengths using the Brønsted–Guggenheim–Scatchard specific ion interaction theory (SIT). Values for the corresponding reaction enthalpies, ∆r H, are reported where available. There is scope for additional high-quality measurements for the Zn2+ + H+ + CO3 2– system and for the Zn2+ + OH– and Zn2+ + SO4 2– systems at I > 0. In acidic and weakly alkaline fresh water systems (pH < 8), in the absence of organic ligands (e.g., humic substances), ZnII speciation is dominated by Zn2+(aq). In this respect, ZnII contrasts with CuII and PbII (the subjects of earlier reviews in this series) for which carbonato- and hydroxido- complex formation become important at pH > 7. The speciation of ZnII is dominated by ZnCO3(aq) only at pH > 8.4. In seawater systems, the speciation at pH = 8.2 is dominated by Zn2+(aq) with ZnCl+, Zn(Cl)2(aq), ZnCO3(aq), and ZnSO4(aq) as minor species. This behaviour contrasts with that for CuII and PbII for which at the pH of seawater in equilibrium with the atmosphere at 25 °C (log10 {[H+]/c°} ≈ 8.2) the MCO3(aq) complex dominates over the MCl n (2–n)+ species. The lower stability of the different complexes of ZnII compared with those of CuII, PbII, and CdII is also illustrated by the percentage of uncomplexed M2+ in seawater, which is ca. 55, 3, 2, and 3.3 % of [MII]T, respectively.

Winkler's method overestimates dissolved oxygen in seawater: Iodate interference and its oceanographic implications

Dissolved oxygen in seawater has been determined by using the Winkler's reaction scheme for decades. An interference in this reaction scheme that has been heretofore overlooked is the presence of naturally occurring iodate in seawater. Each mole of iodate can result in an apparent presence of 1.5 mol of dissolved oxygen. At the concentrations of iodate in the surface and deep open ocean, it can lead to an overestimation of 0.52 ± 0.15 and 0.63 ± 0.05 μmol kg − 1 of oxygen in these waters respectively. In coastal and inshore waters, the effect is less predictable as the concentration of iodate is more variable. The solubility of oxygen in seawater was likely overestimated in data sources that were based on the Winkler's reaction scheme for the determination of oxygen. The solubility equation of García and Gordon [Garcia H.E., Gordon, L.I., 1992. Oxygen solubility in seawater: Better fitting equations. Limnol. Oceanogr. 37, 1307–1312] derived from the results of Benson and Krause [Benson, F.B., Krause, D. Jr., 1984. The concentration and isotopic fractionation of oxygen dissolved in freshwater and seawater in equilibrium with the atmosphere. Limnol. Oceanogr. 29, 620–632] is free from this source of error and is recommended for general use. By neglecting the presence of iodate, the average global super-saturation of oxygen in the surface oceans and the corresponding efflux of oxygen to the atmosphere both have been overestimated by about 8%. Regionally, in areas where the degree of super-saturation or under-saturation of oxygen in the surface water is small, such as in the tropical oceans, the net air–sea exchange flux can be grossly under- or overestimated. Even the estimated direction of the exchange can be reversed. Furthermore, the presence of iodate can lead to an overestimation of the saturation anomaly of oxygen in the upper ocean attributed to biological production by 0.23 ± 0.07%. AOU may have been underestimated by 0.52 ± 0.15 and 0.63 ± 0.05 μmol kg − 1 in the surface mixed layer and deep water, while preformed phosphate and preformed nitrate may have been overestimated by 0.004 ± 0.001 and 0.06 ± 0.02 μmol kg − 1 in the surface mixed layer, and 0.005 ± 0.0004 and 0.073 ± 0.006 μmol kg − 1 in the deep water. These are small but not negligible corrections, especially in areas where the values of these parameters are small. At the increasing level of sophistication in the interpretation of oxygen data, this source of error should now be taken into account. Nevertheless, in order to avoid confusion, an internationally accepted standard needs to be adopted before these corrections can be applied.

Diagenetic Fe-carbonates in Paleoarchean felsic sedimentary rocks (Hooggenoeg Formation, Barberton greenstone belt, South Africa): Implications for CO2 sequestration and the chemical budget of seawater

In order to evaluate the potential of felsic sediments as a CO2-sink in the Archean, we studied felsic volcaniclastic/epiclastic sedimentary rocks of the 3.45Ga Hooggenoeg Formation, Barberton greenstone belt, which were affected by metasomatic processes during seafloor alteration and diagenesis. Water–rock interactions leading to K-, Si- and CO2-metasomatism were quantified. The precursor rock, a K-metasomatized dacite, was leached of Ca, Mg, Fe, Na, Sr, and Ba and enriched in K, Rb and Si prior to erosion and deposition. The formation of K-mica and quartz (and minor K-feldspar) in the dacites suggests low pH metasomatic conditions, likely produced by hydrothermally induced circulation of hot, acidic and reduced Archean seawater in equilibrium with a high PCO2 atmosphere. Erosion and transport of K-metasomatized dacitic detritus away from the felsic volcanic centers resulted in the deposition of conglomerate, sandstone and shale by mass flow processes. Early diagenetic silicification affected mainly the fine-grained sediments with higher silica sorption capacity, forming impermeable layers, while sand-rich sediments were partly silicified and remained permeable. Trapped fluids precipitated two generations of Fe-rich dolomites and finally calcite. Up to 30vol.% of siliciclastic coarse-grained sediment was replaced by carbonates in a shallow-burial, high heat-flow diagenetic regime (depth: ∼750m, temperature: 80–160°C), and likely throughout deposition of overlying volcano–sedimentary units. The carbon isotopic composition of Fe-rich dolomites (δ13CPDB=+1.9 to +2.4‰) and the strong Fe–Ca–Mg leaching of the Paleoarchean volcanic formations support the influence of seawater-derived fluids throughout CO2-metasomatism. For each gram of eroded dacite, the overall chemical exchange involved by K–Si–CO2-metasomatism was characterized by a mass transfer of Fe (1.2mmol/g), Na (2.1mmol/g) and O2− (2.0mmol/g) to seawater. In contrast, seawater was depleted in Si (10mmol/g), Ca (0.51mmol/g), Mg (0.43mmol/g), K (1.5mmol/g) and H (0.93mmol/g) during incorporation of these elements in the volcanic and sedimentary rocks. The average CO2 uptake by the sedimentary rocks studied here is estimated to be 1.8mmol/g, in the same order of magnitude as previous estimates for the Paleoarchean basaltic crust. Although mafic-ultramafic rocks are the most abundant rocks in Paleoarchean greenstones belts, and represent the most important atmospheric CO2-sink upon seafloor alteration in the Paleoarchean, coarse felsic sedimentary rocks provide a non-negligible contribution to the build-up of the continental CO2 reservoir.

Carbon isotopic characterization for the origin of excess methane in subsurface seawater

We collected samples of seawater, zooplankton, and sinking particles in the northwestern North Pacific to determine the source of excess CH4 over the saturation value in equilibrium with the atmospheric CH4 in the oxygenated open ocean, using stable carbon isotope as a tracer. We found that subsurface (∼100 m depth) seawater is supersaturated (up to 12%) with 13C‐enriched CH4 (up to −33.1‰) relative to surface seawater in equilibrium with the atmosphere (−47‰), suggesting that in situ addition of 13C‐enriched CH4 must be responsible for CH4 enrichment at depth. The δ13C of CH4 emitted from sinking particles (from −36.7 ± 1.2‰ to +5.9 ± 7.5‰) is within the range of that of excess CH4 in seawater, suggesting that the major source of subsurface excess CH4 is sinking particles. The unusually 13C‐enriched δ13C composition of CH4 emitted from sinking particles suggests that active microbial CH4 oxidation occurs within the oxic/anoxic boundary of these particles. On the basis of the Rayleigh equation, we estimated that at least 62% of CH4 produced within the anoxic center of sinking particles is oxidized within 100 m of the surface.

Sorption of Ni by birnessite: Equilibrium controls on Ni in seawater

Synthetic hexagonal birnessite (Hx-birnessite) is a close analogue to natural poorly crystalline phyllomanganate phases found in soils and marine ferromanganese deposits. These phases are often highly enriched in trace metals such as Ni and Co. We measured the sorption of Ni(II) onto synthetic hexagonal birnessite (Hx-birnessite) from pH 1 to 7. EXAFS spectra show that, at pH 3.7, Ni is adsorbed to the Hx-birnessite surface above vacancy sites on {001} as a tridentate corner-sharing complex. We developed a surface complexation model for Ni adsorption based on the equilibria 3(≡Mn 2O − 2/3 ) + Ni + 2 = (≡Mn 2O) 3Ni 0 3(≡Mn 2O − 2/3 ) + Ni + 2 + H 2O = (≡Mn 2O) 3Ni(OH) − + H + Using this surface complexation model, we predict the concentration of Ni in seawater in equilibrium with Ni-bearing birnessite found in hydrogenetic FeMn crusts and nodules. Our predicted results are in good agreement with observed Ni concentrations in seawater and suggest that the concentration of dissolved Ni in seawater is buffered by sorption to birnessite or a related MnO 2 phase. However, in addition to the surface complex, Ni also sorbs by structural incorporation into the vacancy site. In our synthetic samples at pH 7, EXAFS shows ∼ 10% of Ni is structurally incorporated into Hx-birnessite. In natural birnessites found in marine ferromanganese crusts and nodules, EXAFS shows that all of the sorbed Ni is structurally incorporated. Structural incorporation suggests that Ni sorption may be irreversible.

Oxygen solubility in seawater: Better fitting equations

We examined uncertainties associated with the routine computation of O2 solubility (Co*) at 1 atm total pressure in pure water and seawater in equilibrium with air as a function of temperature and salinity. We propose formulae expressing C*(at STP, real gas) in cm3 dm−3 and µmol kg−1 in the range (tF ≥ t 40°C; 0 ≥ S 42‰) based on a fit to precise data selected from the literature.

Solvent extraction of copper acetylacetonate in studies of copper(II) speciation in seawater

A liquid-liquid partition, ligand exchange procedure involving the formation of copper(II) complexes with acetylacetone is presented for the determination of stability constants and concentrations of copper chelators in seawater. Acetylacetone competes with natural ligands for copper, and the equilibrium concentration of the copper acetylacetonate complex is used in speciation calculations. The concentration of the complex is calculated by partitioning a fraction of it into an organic phase and determining the total Cu concentration in that phase by back extracting with acid, and analyzing by flameless atomic absorption spectroscopy. The concentration of Cu acetylacetonate in seawater in equilibrium with the organic phase is calculated from the partition coefficient. The simple, thermodynamically well characterized procedure offers several advantages over previous techniques. Studies using organic free seawater and model ligands show good agreement between experimental and calculated conditional stability constants. Studies from seawater in Biscayne Bay, Florida, indicate two ligand types are present; type 1, K 1 = 1.2 × 10 12, C L1 = 5.1 × 10 −9 M; type 2, K 2 = 2.8 × 10 10, C L2 = 1.1 × 10 −7 M. Speciation is dominated by ligand type 1. Depth profiles of [ Cu( II)] free /[ Cu( II)] total measured with the procedure at ambient copper concentrations show an increase from < 5 × 10 −5 at 50–60 m to > 1 × 10 −3 at the surface at two stations off the Florida coast.

The concentration and isotopic fractionation of oxygen dissolved in freshwater and seawater in equilibrium with the atmosphere1

Previous work on the solubility of oxygen in freshwater has been extended to seawater. Measurements of the Henry coefficient in the ranges 0° &lt; t &lt; 45°C and 0&lt; S &lt; 50 fit the Setschenow relationship for the variation with salinity. The temperature dependence of the Setschenow coefficient for oxygen is found to be K = 0.0225034 ‒ 13.6083/T + 2,565.68/T2. The equation for the Henry coefficient as a function of temperature and salinity is used to calculate values for unit standard atmospheric concentrations (USAC) in freshwater and seawater in equilibrium with air at a total pressure of 1 atmosphere. It is estimated that the possible error in the new USAC values is no greater than ±0.1% and probably less. Tables and equations are presented for obtaining accurate USAC values in the ranges 0° &lt; t &lt; 40°C and 0 &lt; S &lt; 40. Simple procedures are given for calculating standard atmospheric concentrations at pressures different from 1 atm. The presence of sea salt has a negligible effect on the fractionation of the oxygen isotopes during solution.

The concentration and isotopic fractionation of oxygen dissolved in freshwater and seawater in equilibrium with the atmosphere : Benson, B.B. and Daniel Krause Jr., 1984. Limnol. Oceanogr., 29(3):620–632

The concentration and isotopic fractionation of oxygen dissolved in freshwater and seawater in equilibrium with the atmosphere : Benson, B.B. and Daniel Krause Jr., 1984. Limnol. Oceanogr., 29(3):620–632

Recent Carbonate Cementation from Seawater in Some Weathered Dolostones, Northumberland, England

ABSTRACT Inorganic precipitation of calcium carbonate from modern seawater is currently taking place inside pore spaces of some recently weathered and dedolomitized ferroan dolostones of Carboniferous age exposed in the littoral zone on the North Sea coast of northeast England. The precipitating carbonates fill partly or completely microcavities, solution pores and microfractures and occur in four texturally different forms, namely: 1) brown finely crystalline fringes; 2) clear bladed-equant crusts; 3) granular mosaic; and 4) blocky and massive cements. The only precipitating carbonate phase is magnesian calcite whilst aragonite is entirely absent. The ferroan dolostones are currently weathered by modern seawater and are gradually dedolomitized and altered into rusty-looking yellowish brown rocks containing iron hydroxides, dedolomite calcite with or without some dolomite relics. During the process of dedolomitization and alteration, apparently, some of the ferroan dolomites undergo dissolution releasing some Ca2+, Mg2+ and HCO3- into seawater remaining within pore spaces during low tides. Aided by evaporation, pore water may become sufficiently supersaturated with respect to CaCO3 to allow the precipitation of intergranular calcite cement to take place. Presumably, this level of supersaturation as well as the temperature of the waters are never high enough to permit the pre ipitation of aragonite. The MgCO3 contents of the intergranular calcite cements are below 10 mole percent. This is lower than expected in calcites precipitating from seawater in equilibrium with the atmosphere. Temperature and dissolution of altered ferroan dolomites appear to be the critical factors which determine the MgCO3 content of these cements.

Significance of Strontium Distribution in Some Carbonate Rocks in the Carboniferous of Northumberland, England

ABSTRACT Data obtained on the distribution of strontium in limestones and dolostones of the Middle Limestone Group (Carboniferous) in England show that most of the strontium in the limestones (Sr = 618 ± 267 ppm, mSr2+/mCa2+ = 0.7 X 10-3) was removed by dolomitization and only a small fraction was retained within the newly formed dolostones (Sr = 132 ± 46 ppm, mSr2+/mCa2+ = 0.225 X 10-3). The strontium of the resulting dolostone appears to be located in the dolomite lattice. Little of it could be related to the insoluble residue of these rocks, suggesting that none or little of the strontium liberated during dolomitization was adsorbed onto clay minerals. Data obtained on the strontium distribution in some associated recently dedolomitized dolostones show that the strontium content of dolostones suffered a considerable loss during dedolomitization (Sr = 95 ± 26 ppm, mSr2+/mCa2+ = 0.157 X 10-3). However, in view of the fact that the dedolomitization of the dolostones was affected by modern seawater and not by meteoric water (Al-Hashimi and Hemingway, 1973), this loss should not have taken place: the strontium content and the mSr2+/mCa2+ ratio of the dedolomites would be expected to be similar to the strontium content and to the mSr2+/mCa2+ ratio of a calcite precipitated from seawater in equilibrium with the atmosphere, i.e., Sr = 1,110 ppm, mSr2+ UP>/mCa2+= 1.26 X 10-3. The present strontium content and mSr2+/mCa2+ ratio of the dedolomitized rocks suggests that the mSr2+/mCa2+ ratio of the dedolomitizing solution, i.e., seawater, must have suffered a considerable dilution while dedolomitizing the dolostones. This dilution could be achieved only through the dissolution of altered dolomite crystals.

Interstitial Waters of Recent Marine Muds Off Cape Cod

Interstitial waters of Recent muds off Cape Cod have salinities higher than the present ocean water in the same location and pH values lower than normal sea water in equilibrium with the atmosphere. The salinities may be either "paleosalinities" or an effect produced by behavior of the clay-water system as a semipermeable membrane. The low pH values are due to higher pressures of CO(2) in the sediment, probably coming from bacterial decomposition of organic matter.