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Aquatic Geochemistry (v.10, #1-2)

Dedication of this Special Issue of Aquatic Geochemistry in Celebration of Frank J. Millero's 65th Birthday by John W. Morse (pp. 1-2).

Coupling of the Perturbed C–N–P Cycles in Industrial Time by Abraham Lerman; Fred T. Mackenzie; L. May Ver (pp. 3-32).

Coupling of the C–N–P biogeochemical cycles is effected by the dependence of the land and aquatic primary producers on the availability of N and P. In general, the Redfield ratios C:P and N:P in the reservoirs supplying nutrients for primary production on land, in the oceanic coastal zone, and in the surface ocean differ from these ratios in the land phytomass and aquatic plankton. When N:P in the source is higher than in primary producers, this results in a potential accumulation of some excess nitrogen in soil water and coastal water, and increased denitrification flux to the atmosphere. The oceanic coastal zone plays an important role in the coupled C–N–P cycles and their dynamics because of its intermediate position between the land and oceanic reservoirs. These coupled cycles were analyzed for the last 300 years of exposure to four human-generated forcings (fossil fuel emissions, land use change, chemical fertilization of land, and sewage discharge to the coastal zone) and global temperature rise. In the period from 1700 to 2000, there has been a net loss of C, N, and P primarily from the land phytomass and soil humus, despite the rise in atmospheric CO₂, increased recycling of nutrients from humus, chemical fertilization, and re-growth of forests on previously disturbed land. The main mechanisms responsible for the net loss were increased riverine transport to the coastal zone of dissolved and particulate materials and, for N, increased denitrification on land. The oceanic coastal zone gained N and P, resulting in their accumulation in the organic pool of living biomass and dissolved and reactive particulates, as well as in their accumulation in coastal sediments from land-derived and in situ produced organic matter. Pronounced shifts in the rates and directions of change in some of the major land reservoirs occurred near the mid-1900s. Denitrification removes N from the pool available for primary production. It is the strongest on land, accounting for 73–83% of N removal from land by the combined mechanisms of denitrification and riverine export.

Keywords: C–N–P cycles; coupled cycles; C:N:P ratios; dentrification; human perturbations; land environment; ocean coastal zone; primary production; cycle dynamics

Acidification and Trace Metals of Lakes in Taiwan by Chen-Tung Arthur Chen; Jiunn-Tzong Wu; Bing-Jye Wang; Kuo-Ming Huang (pp. 33-57).

One hundred and eighty lakes, ponds and reservoirs in Taiwan, and the offshore islands have been investigated since 1985. Effects such as warming/cooling, mixing, photosynthesis and respiration on pH have been identified. These effects were found to be less profound than those resulting from the geology. Since there is little rain or lake water pH data available prior to 1970, it was necessary to use proxy data in order to ascertain the history of lake acidification. In this study, we present data based on diatoms in a sediment core collected from a lake in the remote subalpine region of southern Taiwan. The acidity of this lake water was found to have increased since 1900. We measured the concentrations of 55 chemical species in lake water samples. In addition, concentrations of 26 chemical species were obtained from sediments. Conductivity, alkalinity (HCO₃ ⁻), most other major and minor chemical species, as well as the non sea-salt SO₄/Na ratio in lake water clearly decrease with elevation. Distribution of pH is similar, although the trend is less clear. Distribution of trace metals, however, shows a different trend as anthropogenic pollution, diagenetic reduction and lake acidification are at play.

Keywords: acidification; alkalinity; heavy metals; lake water; pH; sediment; temporal variation

Comparative Scavenging of Yttrium and the Rare Earth Elements in Seawater: Competitive Influences of Solution and Surface Chemistry by Kelly A. Quinn; Robert H. Byrne; Johan Schijf (pp. 59-80).

Distribution coefficients were obtained for yttrium and the rare earth elements (YREEs) in aqueous solutions containing freshly precipitated hydroxides of trivalent cations (Fe³⁺, Al³⁺, Ga³⁺, and In³⁺). Observed patterns of log i K _S–, where i K _S = [MS i][M³⁺]⁻¹[S i]⁻¹, [MS i] is the concentration of a sorbed YREE, [M³⁺] is the concentration of a free hydrated YREE ion, and [S _i] is the concentration of a sorptive solid substrate (Fe(III), Al, Ga, In)– exhibited similarities to patterns of YREE solution complexation constants with hydroxide (_OH β ₁) and fluoride (_F β ₁), but also distinct differences. The log i K _S pattern for YREE sorption on Al hydroxide precipitates is very similar to the pattern of YREE hydroxide stability constants (log_OH β ₁) in solution. Linear free-energy relationships between log i K _S and log_OH β ₁ showed excellent correlation for YREE sorption on Al hydroxide precipitates, good correlation for YREE sorption on Ga or In hydroxide precipitates, yet poor correlation for YREE sorption on Fe(III) hydroxide precipitates. Whereas the correlation between log i K _S and log_F β ₁ was generally poor, patterns of log( i K _S/_F β ₁) displayed substantially increased smoothness compared to patterns of log i K _S. This indicates that the conspicuous sequence of inflections along the YREE series in the patterns of log i K _S and log_F β ₁ is very similar, particularly for In and Fe(III) hydroxide precipitates. While the log i K _S patterns obtained with Fe(III) hydroxide precipitates in this work are quite distinct from those obtained with Al, Ga, and In hydroxide precipitates, they are in good agreement with patterns of YREE sorption on ferric oxyhydroxide precipitates reported by others. Furthermore, our log i K _S patterns for Fe(III) hydroxide precipitates bear a striking resemblance to predicted log i K _S patterns for natural surfaces that are based on YREE solution chemistry and shale-normalized YREE concentrations in seawater. Yttrium exhibits an itinerant behavior among the REEs: sorption of Y on Fe(III) hydroxide precipitates is intermediate to that of La and Ce, while for Al hydroxide precipitates Y sorption is similar to that of Eu. This behavior of Y can be rationalized from the propensities of different YREEs for covalent vs. ionic interactions. The relatively high shale-normalized concentration of Y in seawater can be explained in terms of primarily covalent YREE interactions with scavenging particulate matter, whereby Y behaves as a light REE, and primarily ionic interactions with solution ligands, whereby Y behaves as a heavy REE.

Keywords: yttrium; rare earth elements; ICP-MS; sorption; scavenging; iron hydroxide; seawater; linear free-energy relationships

Kinetics of the Reactions of Water, Hydroxide Ion and Sulfide Species with CO₂, OCS and CS₂: Frontier Molecular Orbital Considerations by George W. Luther III (pp. 81-97).

The kinetics of the reactions of water, hydroxide ion and sulfide species with CO₂, OCS and CS₂ are investigated using the molecular orbital approach and available kinetic data. Although these reactions are symmetry allowed, the lowest unoccupied molecular orbital (LUMO) for CO₂ is a poor electron accepting orbital as it has a positive potential energy. At low pH, hydration of CO₂ requires that the waters interact with CO₂ via hydrogen bonding for subsequent formation of H₂CO₃ in an effort to overcome the high energy of activation. These factors are significant for the slow kinetics of hydration and the persistence of CO₂ in water. The reaction of hydroxide ion with CO₂ has a much smaller energy of activation. For the isoelectronic species OCS and CS₂, their LUMO orbitals are good electron acceptor orbitals, and the energy of activation is less than that for the corresponding CO₂ reactions. The LUMO orbitals for OCS and CS₂ have less carbon character whereas the LUMO for CO₂ has more carbon character. The relative rates of these reactions (CO₂ > OCS > CS₂) reflect the increased carbon character of the π^* LUMO orbital for CO₂ over CS₂ and the fact that the LUMO for OCS is σ^*, which when filled can readily break the C—S bond leading to sulfide (even though the C character of the LUMO is less than those for CO₂ and CS₂). Also, the higher hydrogen bonding interactions with nearest water molecules is in the order CO₂ > OCS > CS₂ indicating that hydrolysis via water catalysis is retarded as the number of S atoms increases. Solid phase FeS has a highest occupied molecular orbital (HOMO) with a potential energy similar to that of CO₂ and can activate (or bond with) the carbon atom in CO₂ so that organic compounds can be produced under hydrothermal vent conditions.

Keywords: carbon dioxide; carbonyl sulfide; carbon disulfide; hydrolysis; hydroxide ion; molecular orbital theory

Impact of Carbonate Precipitation on Riverine Inorganic Carbon Mass Transport from a Mid-continent, Forested Watershed by Kathryn Szramek; Lynn M. Walter (pp. 99-137).

Physiochemical controls on the carbonate geochemistry of large river systems are important regulators of carbon exchange between terrestrial and marine reservoirs on human time scales. Although many studies have focused on large-scale river carbon fluxes, there are few investigations of mechanistic aspects of carbonate mass balance and transport at the catchment scale. We determined elemental and carbonate geochemistry and mass balances for net carbonate dissolution fluxes from the forested, mid-latitude Huron River watershed, established on carbonate-rich unconfined glacial drift aquifers. Shallow groundwaters are near equilibrium with respect to calcite at pCO₂ values up to 25 times atmospheric values. Surface waters are largely groundwater fed and exhibit chemical evolution due to CO₂ degassing, carbonate precipitation in lakes and wetlands, and anthropogenic introduction of road salts (NaCl and CaCl₂). Because the source groundwater Mg²⁺/HCO₃ ⁻ ratio is fairly constant, this parameter permits mass balances to be made between carbonate dissolution and back precipitation after groundwater discharge. Typically, precipitation does not occur until IAP/K calcite values exceed 10 times supersaturation. Stream chemistry changes little thereafter even though streams remain highly supersaturated for calcite. Our data taken together with historical United States Geological Survey (USGS) data show that alkalinity losses to carbonate precipitation are most significant during periods of lowest discharge. Thus, on an annual basis, the large carbon flux from carbonate dissolution in soil zones is only decreased by a relatively small amount by the back precipitation of calcium carbonate.

Keywords: calcite saturation; carbon; Michigan; rivers; alkalinity

The Behavior of Mixed Ca–Mn Carbonates in Water and Seawater: Controls of Manganese Concentrations in Marine Porewaters by Alfonso Mucci (pp. 139-169).

The dissolution behavior of natural, ordered kutnahorite (Mn_1.14Ca_0.82Mg_0.04Fe_0.012(CO₃)₂) and a disordered, calcian rhodochrosite (Mn_1.16Ca_0.78Mg_0.06(CO₃)₂) precipitated in the laboratory was investigated in deionized distilled water and artificial seawater in both open and closed systems at 25 °C, one atmosphere total pressure, and various pCO₂s. Both solids dissolved congruently in distilled water in an open system and yielded identical long-term equilibration or extrapolated ion activity products, IAP_pkt = a_Ca ²⁺a_Mn ²⁺(a_CO ₃ ²⁻)² = 1.7 (±0.12)× 10⁻²¹ or pIAP_pkt = 20.77 (±0.03). This value is believed to be the thermodynamic solubility product of pseudokutnahorite. In contrast, the steady state ion concentration products, ICP_pkt = [Ca²⁺][Mn²⁺][CO₃ ²⁻]², measured following the dissolution of both minerals in artificial seawater increase as the CO₂ partial pressure decreases and the [Mn²⁺]:[Ca²⁺] ratio increases. These observations are interpreted as resulting from the formation of phases of different stoichiometry in response to large variations of the [Mn²⁺]:[Ca²⁺] ratio in solution. These data and results of calcite-seawater equilibration experiments in the presence of various dissolved Mn(II) concentrations define the fields of stability of manganoan calcites and calcian rhodochrosites in seawater within Lippmann phase diagrams for the CaCO₃–MnCO₃–H₂O system. Results of this study reveal that the nature (i.e., mineralogy) and composition of manganese-rich carbonate phases that may form under suboxic/anoxic conditions in marine sediments are dictated by the porewater [Mn²⁺]:[Ca²⁺] ratio, the abundance of calcite surfaces and reaction kinetics.

Keywords: solubility; seawater; solid solutions; pseudokutnahorite; manganoan calcite; calcian rhodochrosite

Dissolution Kinetics of Calcite in NaCl–CaCl₂–MgCl₂ Brines at 25 °C and 1 bar pCO₂ by Dwight K. Gledhill; John W. Morse (pp. 171-190).

Calcite dissolution rates were measured as a function of saturation state in NaCl–CaCl₂–MgCl₂ solutions at 1 bar (0.1 MPa) pCO₂ and 25 °C. Rates measured in phosphate- and sulfate-free pseudo-seawater (Ca²⁺:Mg²⁺= 0.2, I= 0.7) were compared with those in synthetic brines. The brines were prepared by co-varying calcium and magnesium (Ca²⁺:Mg²⁺= 0.9; 2.0; 2.8; 3.1; 4.8; 5.8) along with ionic strength (I= 0.9; 1.1; 1.6; 2.1; 3.0; 3.7; 4.4 m) to yield solutions approximating those of subsurface formation waters. The rate data were modeled using the equation, R = k(1 − Omega;) n, where k is the empirical rate constant, n describes the order of the reaction and ω is saturation state. For rates measured in the pseudo-seawater, n= 1.5 and k= 4.7 × 10⁻² mol m⁻² hr⁻¹. In general, rates were not significantly faster in the synthetic brines (n= 1.4 ± 0.2 and k= 5.0 ± 7 × 10⁻² mol m⁻² hr⁻¹). The rate coefficients agree within experimental error indicating that they are independent of ionic strength and Ca²⁺:Mg²⁺ over a broad range of brine compositions. These findings have important application to reaction-transport modeling because carbonate bearing saline reservoirs have been identified as potential repositories for CO₂ sequestration.

Keywords: calcite; dissolution; kinetic; brine; CO₂ sequestration

Instructions for Authors (pp. 191-195).

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Aquatic Geochemistry (v.10, #1-2)