Aquatic Geochemistry (AQUAT GEOCHEM)

Publications in this journal

• Article: Burial and Preservation of Carbonate Rocks Over Phanerozoic Time

  Robert A. Berner, Fred T. Mackenzie
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  ABSTRACT: Comparison of results for the original burial rate of carbonate sediments over Phanerozoic time, as calculated using the GEOCARBSULFvolc model, with their rate of preservation to the present (survival rate) shows a considerable loss of mass, partly by subduction of oceanic crust, during the past 250million years. Before that time, despite the evidence that preserved Paleozoic carbonates appear to have been deposited only in shallow water, we contend that there was also inorganic deposition of carbonates in the Paleozoic deep sea with subsequent loss by subduction. Inorganic carbonate deposition may have been abetted by the vastly different seawater and atmospheric composition for most of the Paleozoic than those of post-Cretaceous and end Paleozoic–early Mesozoic times. The hypothesis helps to explain the loss of mass greater than that predicted for shallow-water carbonates prior to 250Ma. KeywordsCarbonate rocks–Burial and preservation–Phanerozoic
  Aquatic Geochemistry 05/2012; 17(4):727-733.
• Article: Factors Controlling Sulfide Geochemistry in Sub-tropical Estuarine and Bay Sediments

  John W. Morse, Heather Thomson, David W. Finneran
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  ABSTRACT: The primary factors that control the concentration of total reduced (inorganic) sulfide in coastal sediments are believed to be the availability of reactive iron, dissolved sulfate and metabolizable organic carbon. We selected nine sites in shallow (<3m), close to sub-tropical, estuaries and bays along the central Texas coast that represented a range in sediment grain size (a proxy for reactive iron), salinity (a proxy for dissolved sulfate), and total organic carbon (a proxy for metabolizable organic carbon). Based on these parameters a prediction was made of which factor was likely to control total reduced sulfide at each site and what the relative total reduced sulfide concentration was likely to be. To test the prediction, the sediments were analyzed for total reduced sulfide, acid volatile sulfide, and citrate dithionate-extractable, HCl-extractable and total Fe in the solid phase. Using solid-state gold–mercury amalgam microelectrodes and voltammetry, we determined pore water depth profiles of Fe(II) and ΣH2S and presence or absence of FeS(aq). At five of the nine sites the calculated degree of sufildization of citrate dithionite-reactive-iron was close to or greater than 1 indicating that rapidly reactive iron was probably the limiting factor for iron sulfide mineral formation. At one site (salinity=0.9) dissolved Fe(II) was high, ΣH2S was undetectable and the total reduced sulfide concentration was low indicating sulfate limitation. At the last three sites a low degree of sulfidization and modest total reduced (inorganic) sulfide concentrations appeared to be the result of a limited supply of metabolizable organic carbon. Fe(II)–S(-II) clusters (FeS(aq)) were undetectable in 10 out of 12 bay sediment profiles where ΣH2S was close to or below detection limits, but was observed in all other porewater profiles. Acid volatile sulfide, but not total reduced sulfide, was well correlated with total organic carbon and ranged from being undetectable in some cores to representing a major portion of total reduced sulfide in other cores. Although predicted controls on total reduced sulfide were good for very low salinity water or sandy sediments, they were only right about half the time for the other sediments. The likely reasons for the wrong predictions are the poor correlation of total organic carbon with grain size and differing fractions of metabolizable organic carbon in different sedimentary environments. Differences in sediment accumulation rates may also play a role, but these are difficult to determine in this region where hurricanes often resuspend and move sediments. This study demonstrates the need to examine more complex and often difficult to determine parameters in anoxic “normal marine” sediments if we are to understand what controls the concentration and distribution of sulfides.
  Aquatic Geochemistry 05/2012; 13(2):143-156.
• Article: Calcium Carbonate Nucleation in an Alkaline Lake Surface Water, Pyramid Lake, Nevada, USA

  Michael M. Reddy, Anthony Hoch
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  ABSTRACT: Calcium concentration and calcite supersaturation (Ω) needed for calcium carbonate nucleation and crystal growth in Pyramid Lake (PL) surface water were determined during August of 1997, 2000, and 2001. PL surface water has Ω values of 10–16. Notwithstanding high Ω, calcium carbonate growth did not occur on aragonite single crystals suspended PL surface water for several months. However, calcium solution addition to PL surface-water samples caused reproducible calcium carbonate mineral nucleation and crystal growth. Mean PL surface-water calcium concentration at nucleation was 2.33mM (n=10), a value about nine times higher than the ambient PL surface-water calcium concentration (0.26mM); mean Ω at nucleation (109 with a standard deviation of 8) is about eight times the PL surface-water Ω. Calcium concentration and Ω regulated the calcium carbonate formation in PL nucleation experiments and surface water. Unfiltered samples nucleated at lower Ω than filtered samples. Calcium concentration and Ω at nucleation for experiments in the presence of added particles were within one standard deviation of the mean for all samples. Calcium carbonate formation rates followed a simple rate expression of the form, rate (mM/min)=A (Ω)+B. The best fit rate equation “Rate (ΔmM/Δmin)=−0.0026 Ω+0.0175 (r=0.904, n=10)” was statistically significant at greater than the 0.01 confidence level and gives, after rearrangement, Ω at zero rate of 6.7. Nucleation in PL surface water and morphology of calcium carbonate particles formed in PL nucleation experiments and in PL surface-water samples suggest crystal growth inhibition by multiple substances present in PL surface water mediates PL calcium carbonate formation, but there is insufficient information to determine the chemical nature of all inhibitors. KeywordsPyramid Lake, Nevada, USA–Calcium carbonate–Nucleation–Calcium carbonate nucleation–Supersaturation–Mineral formation inhibition
  Aquatic Geochemistry 05/2012; 18(2):95-113.
• Article: Quantifying Sediment Nitrogen Releases Associated with Estuarine Dredging

  Jeffrey C. Cornwell, Michael S. Owens
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  ABSTRACT: Experimental studies of sediment pore water NH4 + chemistry, adsorbed NH4 + concentrations, sediment–water NH4 + exchange and N2–N flux were carried out to quantify the mass of labile N that can be released during large-scale dredging activities. Pore water NH4 + concentrations below 0.5-m sediment depth averaged 5±2mmolL−1 with average adsorbed NH4 + concentrations of 11μmolg−1. Elevated NH4 + concentrations found in rapidly accreting dredge channels are partly a result of the rapid advective burial of both reactive organic matter and pore water. Elutriate tests, a dilution of sediment with site water, yielded adsorbed NH4 + concentrations very similar to those using the more typical KCl extraction. Intact deep sediment sections exposed to overlying water, used to simulate postdredging conditions, showed high initial fluxes of ammonium and no development of coupled nitrification–denitrification under the cold incubation conditions. Despite high concentrations and effluxes of NH4 + during dredging, the amount of NH4 + release during dredging was <0.5% of northern Chesapeake Bay sediment fluxes. The likelihood of large environmental effects of nitrogen release during the dredging of navigational channels in the Chesapeake Bay is low. KeywordsDenitrification–Ammonium adsorption–Dredging–Pore water chemistry–Estuarine sediments
  Aquatic Geochemistry 05/2012; 17(4):499-517.
• Article: Dolomite Versus Calcite Weathering in Hydrogeochemically Diverse Watersheds Established on Bedded Carbonates (Sava and Soča Rivers, Slovenia)

  Kathryn Szramek, Lynn M. Walter, Tjaša Kanduč, Nives Ogrinc
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  ABSTRACT: The relative contributions of dolomite to calcite weathering related to riverine fluxes are investigated on a highly resolved spatial scale in the diverse watersheds of Slovenia, which previous work has shown have some of the highest carbonate-weathering intensities in the world and suggests that dolomite weathering is favored over limestone weathering in mixed carbonate watersheds. The forested Sava and Soča River watersheds of Slovenia with their headwaters in the Julian Alps drain alpine regions with thin soils (<30cm) and dinaric karst regions with thicker soils (0 to greater than 70cm) all developed over bedded Mesozoic carbonates (limestone and dolomite), and siliclastic sediments is the ideal location for examining temperate zone carbonate weathering. This study extends previous work, presenting geochemical data on source springs and documenting downstream geochemical fluctuations within tributaries of the Sava and Soča Rivers. More refined sampling strategies of springs and discrete drainages permit directly linking the stream Mg2+/Ca2+ ratios to the local bedrock lithology and the HCO3 − concentrations to the relative soil depths of the tributary drainages. Due to differences in carbonate source lithologies of springs and tributary streams, calcite and dolomite weathering end members can be identified. The Mg2+/Ca2+ ratio of the main channel of the Sava River indicates that the HCO3 − concentration can be attributed to nearly equal proportions by mass of dolomite relative to calcite mineral weathering (e.g., Mg2+/Ca2+ mole ratio of 0.33). The HCO3 − concentration and pCO2 values increase as soil thickness and alluvium increase for discrete spring samples, which are near equilibrium with respect to calcite. Typically, this results in approximately 1.5meq/l increase in HCO3 − from the alpine to the dinaric karst regions. Streams in general do not change in HCO3 −, Mg2+/Ca2+, or Mg2+/HCO3 − concentrations down course, but warming and degassing of CO2 produce high degrees of supersaturation with respect to calcite. Carbonate-weathering intensity (mmol/km2-s) is highest within the alpine regions where stream discharge values range widely to extreme values during spring snowmelt. Overall, the elemental fluxes of HCO3 −, Ca2+, and Mg2+ from the tributary watersheds are proportional to the total water flux because carbonates dissolve rapidly to near equilibrium. Importantly, dolomite weathers preferentially over calcite except for pure limestone catchments. KeywordsDolomite–Carbonate–Weathering–Rivers–Slovenia–Watersheds–Fluxes
  Aquatic Geochemistry 05/2012; 17(4):357-396.
• Article: Generic Issues of Batch Dissolution Exemplified by Gypsum Rock

  Victor W. Truesdale
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  ABSTRACT: Recent work has emphasized that the empirical rate equation for batch dissolution of a solid consists of a forward term involving Recent work has emphasized that the empirical rate equation for batch dissolution of a solid consists of a forward term involving the surface area minus a back reaction term involving surface area and concentration of dissolved solid. Integrated forms the surface area minus a back reaction term involving surface area and concentration of dissolved solid. Integrated forms exist for use at extremes of high under-saturation and of very heavy solid loadings which lead to saturation. A middle conditiondings which lead to saturation. A middle condition allows for significant decrease in solid supply and simultaneous arrival at saturation. This study tests the three approaches allows for significant decrease in solid supply and simultaneous arrival at saturation. This study tests the three approaches simultaneously to the batch dissolution of gypsum, thereby demonstrating a consistent applicability of the afore-mentioned simultaneously to the batch dissolution of gypsum, thereby demonstrating a consistent applicability of the afore-mentioned rate equation. Previously, some mineral dissolutions have displayed so-called nonlinear kinetics and hence have not appeared rate equation. Previously, some mineral dissolutions have displayed so-called nonlinear kinetics and hence have not appeared to conform to this rate equation. This paper provides a template for future investigation of those situations; dissolution to conform to this rate equation. This paper provides a template for future investigation of those situations; dissolution experiments are not easy to perform, and instances of the so-called nonlinear kinetics may represent experimental artefact. experiments are not easy to perform, and instances of the so-called nonlinear kinetics may represent experimental artefact. The relationship between this empirical approach and that of Transition State Theory used in mineral dissolution is discussed, The relationship between this empirical approach and that of Transition State Theory used in mineral dissolution is discussed, and a new, linear proof for the applicability of the ‘middle ground’ equations is demonstrated. Stirring experiments highlight and a new, linear proof for the applicability of the ‘middle ground’ equations is demonstrated. Stirring experiments highlight the difference between the conditions in fluidized bed and laminar flow reactors. Gypsum dissolution is found to be transport the difference between the conditions in fluidized bed and laminar flow reactors. Gypsum dissolution is found to be transport limited at all but very vigorous laboratory stirring conditions, although the relationship between the rate of shrinkage of limited at all but very vigorous laboratory stirring conditions, although the relationship between the rate of shrinkage of gypsum particles and stirring seems to be relatively simple. A stirring factor is applied to the rate equation overall tofactor is applied to the rate equation overall to allow for differences in reactor design, and it is suggested that this should also be applicable to laminar flow reactors. allow for differences in reactor design, and it is suggested that this should also be applicable to laminar flow reactors. The link between batch and chemo-stat dissolutions is stressed, together with a need to contour dissolution data on a new The link between batch and chemo-stat dissolutions is stressed, together with a need to contour dissolution data on a new graph of particle size versus stirring rate. graph of particle size versus stirring rate. KeywordsBatch dissolution-Dissolution kinetics-Shrinking object-Shrinking sphere-Gypsum dissolution-Carbonate dissolution-Silica dissolution-Geo-engineering-Transition State Theory-Biogenic silica-Gypsum-Stirring rate-Common ion KeywordsBatch dissolution-Dissolution kinetics-Shrinking object-Shrinking sphere-Gypsum dissolution-Carbonate dissolution-Silica dissolution-Geo-engineering-Transition State Theory-Biogenic silica-Gypsum-Stirring rate-Common ion
  Aquatic Geochemistry 05/2012; 17(1):21-50.
• Article: Geochemistry and Behavior of Trace Elements During the Complete Evaporation of the Merouane Chott Ephemeral Lake: Southeast Algeria

  Messaoud Hacini, Eric H. Oelkers
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  ABSTRACT: The Merouane Chott, located in arid southeastern Algeria, experiences annual cycles of filling from September through February followed by its complete evaporation from February through June. The concentration of 15 trace elements (Li, B, Ti; V, Cr, Mn, Co, Cu, Ni, Zn, As, Sr, Ba, Pb, Bi, and U) were measured in chott water samples collected from January through June 2003 during the complete evaporation of the lake. The corresponding concentrations of these trace elements in the major external inputs to this closed basin chott were also obtained. The trace metals show two distinct behaviors. Li, B, Cr, Co, and U tend to be conserved in the chott waters throughout its evaporation. Much of Cr, Co, and U originated from external sources. It is likely, therefore, that the concentration of these elements will increase in the chott waters in future years. In contrast, Ti, Sr, Ba, Zn, Ni, and Pb precipitate continuously during chott evaporation. Of these elements, most of the Sr, Ba, and Zn originated from outside the chott, and thus it is likely these elements will become increasingly concentrated in the chott bottom salts with time. V, As, and Cu exhibit intermediate behaviors. These contrasting behaviors are confirmed by analysis of chott bottom solids. KeywordsChott Merouane-Saline lakes-Trace elements-Water–rock interaction
  Aquatic Geochemistry 05/2012; 17(1):51-70.
• Article: A Brine Evolution Model and Mineralogy of Chemical Sediments in a Volcanic Crater, Lake Kitagata, Uganda

  Lichun Ma, Tim K. Lowenstein, James M. Russell
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  ABSTRACT: Lake Kitagata, Uganda, is a hypersaline crater lake with Na–SO4–Cl–HCO3–CO3 chemistry, high pH and relatively small amounts of SiO2. EQL/EVP, a brine evaporation equilibrium model (Risacher and Clement 2001), was used to model the major ion chemistry of the evolving brine and the order and masses of chemically precipitated sediments. Chemical sediments in a 1.6-m-long sediment core from Lake Kitagata occur as primary chemical mud (calcite, magadiite [NaSi7O13(OH)3·3H2O], burkeite [Na6(CO3)(SO4)2]) and as diagenetic intrasediment growths (mirabilite (Na2SO4·10H2O)). Predicted mineral assemblages formed by evaporative concentration were compared with those observed in salt crusts along the shoreline and in the core from the lake center. Most simulations match closely with observed natural assemblages. The dominant inflow water, groundwater, plays a significant role in driving the chemical evolution of Lake Kitagata water and mineral precipitation sequences. Simulated evaporation of Lake Kitagata waters cannot, however, explain the large masses of magadiite found in cores and the formation of burkeite earlier in the evaporation sequence than predicted. The masses and timing of formation of magadiite and burkeite may be explained by past groundwater inflow with higher alkalinity and SiO2 concentrations than exist today. KeywordsCrater lakes–Alkaline lakes–Brine evolution–Mineralogy–Magadiite–Lake Kitagata
  Aquatic Geochemistry 05/2012; 17(2):129-140.
• Article: Arsenic and Antimony in Groundwater Flow Systems: A Comparative Study

  Stephanie S. Willis, Shama E. Haque, Karen H. Johannesson
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  ABSTRACT: Arsenic (As) and antimony (Sb) concentrations and speciation were determined along flow paths in three groundwater flow systems, the Carrizo Sand aquifer in southeastern Texas, the Upper Floridan aquifer in south-central Florida, and the Aquia aquifer of coastal Maryland, and subsequently compared and contrasted. Previously reported hydrogeochemical parameters for all three aquifer were used to demonstrate how changes in oxidation–reduction conditions and solution chemistry along the flow paths in each of the aquifers affected the concentrations of As and Sb. Total Sb concentrations (SbT) of groundwaters from the Carrizo Sand aquifer range from 16 to 198pmolkg−1; in the Upper Floridan aquifer, SbT concentrations range from 8.1 to 1,462pmolkg−1; and for the Aquia aquifer, SbT concentrations range between 23 and 512pmolkg−1. In each aquifer, As and Sb (except for the Carrizo Sand aquifer) concentrations are highest in the regions where Fe(III) reduction predominates and lower where SO4 reduction buffers redox conditions. Groundwater data and sequential analysis of the aquifer sediments indicate that reductive dissolution of Fe(III) oxides/oxyhydroxides and subsequent release of sorbed As and Sb are the principal mechanism by which these metalloids are mobilized. Increases in pH along the flow path in the Carrizo Sand and Aquia aquifer also likely promote desorption of As and Sb from mineral surfaces, whereas pyrite oxidation mobilizes As and Sb within oxic groundwaters from the recharge zone of the Upper Floridan aquifer. Both metalloids are subsequently removed from solution by readsorption and/or coprecipitation onto Fe(III) oxides/oxyhydroxides and mixed Fe(II)/Fe(III) oxides, clay minerals, and pyrite. Speciation modeling using measured and computed Eh values predicts that Sb(III) predominate in Carrizo Sand and Upper Floridan aquifer groundwaters, occurring as the Sb(OH)30 species in solution. In oxic groundwaters from the recharge zones of these aquifers, the speciation model suggests that Sb(V) occurs as the negatively charged Sb(OH)6− species, whereas in sufidic groundwaters from both aquifers, the thioantimonite species, HSb2S4 − and Sb2S4 2−, are predicted to be important dissolved forms of Sb. The measured As and Sb speciation in the Aquia aquifer indicates that As(III) and Sb(III) predominate. Comparison of the speciation model results based on measured Eh values, and those computed with the Fe(II)/Fe(III), S(-II)/SO4, As(III)/As(V), and Sb(III)/Sb(V) couples, to the analytically determined As and Sb speciation suggests that the Fe(II)/Fe(III), S(-II)/SO4 couples exert more control on the in situ redox condition of these groundwaters than either metalloid redox couple. KeywordsArsenic–Antimony–Groundwater–Carrizo Sand–Upper Floridan–Aquia
  Aquatic Geochemistry 05/2012; 17(6):775-807.
• Article: Partition of Elements Between Solid and Liquid Phases in Aquatic Environments

  Yuan-Hui Li
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  ABSTRACT: The average composition of natural waters such as rivers, lakes, ocean, and hydrothermal vents and corresponding solids in equilibrium (e.g., river-suspended particles or shale; lake sediments; oceanic pelagic clay, organisms, and manganese nodules; and the mid-ocean ridge basalts) do not change randomly. The observed positive correlation between the electron binding energy (I z [*I z ]) and logarithms of bulk distribution coefficient (log K d ) for cations with charge of 1–4, and the negative correlation between I z [*I z ] and log K d for anions in various aquatic systems are consistent with the prediction from the surface complexation model. In other words, the bond strength between the adsorbed cation and the surface oxygen of hydrated metal oxides, and between the oxygen of adsorbed oxyanion and the surface metal of hydrated metal oxides control the partition of elements between solid and associated liquid in natural aquatic systems. For Mn, Co, Ce, Pb, and Tl, the oxidative uptake at the solid–water interface in the ocean is an additional important process. For alkali and alkaline-earth cations with large ionic radius (such as Cs, Rb, K, and Ba), their relatively small secondary solvation energy further enhances their adsorption onto solid particles. For living and non-living organic matter, the adsorbed B-type cations form extra strong bindings with hydrophilic functional groups such as –SH and –NH2 on organic matter surface. KeywordsDistribution coefficient–Surface complexation model–Electron binding energy–Chemical compositions–Natural aquatic systems–Mean residence time–Biophilic/biophobic elements
  Aquatic Geochemistry 05/2012; 17(4):697-725.
• Article: Dolomite Controls on Phanerozoic Seawater Chemistry

  Rolf S. Arvidson, Michael W. Guidry, Fred T. Mackenzie
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  ABSTRACT: We investigate the potential role of dolomite as a long-term buffer on Phanerozoic seawater composition. Using a comprehensive model of Phanerozoic geochemical cycling, we show how variations in the formation rate of sedimentary marine dolomite have buffered seawater saturation state. The total inventory of inorganic carbon reflects the sum of fluxes derived from continental weathering, basalt-seawater exchange, alumino-silicate diagenesis (reverse weathering), and global deposition of calcium carbonate. Although these fluxes are approximately balanced, model results indicate that seawater saturation state is sensitive to the marine dolomite depositional flux. This conclusion is consistent with and constrained by independent proxy data for seawater ion ratios, paleo-atmospheric CO2 concentrations, and paleo-pH data, and dolomite mass-age distribution through Phanerozoic time. Abundant research indicates that dolomite’s occurrence in marine sediments is sensitive to many factors: temperature, seawater composition, paleogeographic setting, continental organization, etc. Although the complexity of the process of dolomite formation prevents a complete understanding of the relative role of these factors, our model results clearly underscore the importance of this mineral in the chemical history of Phanerozoic seawater. KeywordsDolomite–Carbonates–Phanerozoic–Geochemical cycling–Carbon cycle–Seawater saturation state
  Aquatic Geochemistry 05/2012; 17(4):735-747.
• Article: The Hydrolysis of Al(III) in NaCl solutions: A Model for M(II), M(III), and M(IV) Ions

  Ryan J. Woosley, Frank J. Millero
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  ABSTRACT: Understanding the identity and stability of the hydrolysis products of metals is required in order to predict their behavior in natural aquatic systems. Despite this need, the hydrolysis constants of many metals are only known over a limited range of temperature and ionic strengths. In this paper, we show that the hydrolysis constants of 31 metals [i.e. Mn(II), Cr(III), U(IV), Pu(IV)] are nearly linearly related to the values for Al(III) over a wide range of temperatures and ionic strengths. These linear correlations allow one to make reasonable estimates for the hydrolysis constants of +2, +3, and +4 metals from 0 to 300°C in dilute solutions and 0 to 100°C to 5m in NaCl solutions. These correlations in pure water are related to the differences between the free energies of the free ion and complexes being almost equal \Updelta \textG° ( \textAl3 + ) - \Updelta \textG° ( \textAl( \textOH )j( 3 - j ) ) @ \Updelta \textG° ( \textMn + ) - \Updelta \textG° ( \textM( \textOH )j( n - j ) ) \Updelta {\text{G}}^\circ \left( {{\text{Al}}^{3 + } } \right) - \Updelta {\text{G}}^\circ \left( {{\text{Al}}\left( {\text{OH}} \right)_{j}^{{\left( {3 - j} \right)}} } \right) \cong \Updelta {\text{G}}^\circ \left( {{\text{M}}^{n + } } \right) - \Updelta {\text{G}}^\circ \left( {{\text{M}}\left( {\text{OH}} \right)_{j}^{{\left( {n - j} \right)}} } \right) The correlation at higher temperatures is a result of a similar relationship between the enthalpies of the free ions and complexes \Updelta \textH° ( \textAl3 + ) - \Updelta \textH° ( \textAl( \textOH )j3 - j ) @ \Updelta \textH° ( \textMn + ) - \Updelta \textH° ( \textM( \textOH )jn - j ) \Updelta {\text{H}}^\circ \left( {{\text{Al}}^{3 + } } \right) - \Updelta {\text{H}}^\circ \left( {{\text{Al}}\left( {\text{OH}} \right)_{j}^{3 - j} } \right) \cong \Updelta {\text{H}}^\circ \left( {{\text{M}}^{n + } } \right) - \Updelta {\text{H}}^\circ \left( {{\text{M}}\left( {\text{OH}} \right)_{j}^{n - j} } \right) The correlations at higher ionic strengths are the result of the ratio of the activity coefficients for Al(III) being almost equal to that of the metal. g( \textMn + )/g( \textM( \textOH )jn - j ) @ g( \textAl3 + )/g( \textAl( \textOH )j3 - j ) \gamma \left( {{\text{M}}^{n + } } \right)/\gamma \left( {{\text{M}}\left( {\text{OH}} \right)_{j}^{n - j} } \right) \cong \gamma \left( {{\text{Al}}^{3 + } } \right)/\gamma \left( {{\text{Al}}\left( {\text{OH}} \right)_{j}^{3 - j} } \right) The results of this study should be useful in examining the speciation of metals as a function of pH in natural waters (e.g. hydrothermal fresh waters and NaCl brines). KeywordsHydrolysis constants-Divalent, trivalent, and quadrivalent metals
  Aquatic Geochemistry 05/2012; 16(3):317-324.
• Article: Biogeochemical Stratification and Carbonate Dissolution-Precipitation in Hypersaline Microbial Mats (Salt Pond, San Salvador, The Bahamas)

  Mary K. Puckett, Karen S. McNeal, Brenda L. Kirkland, Margaret E. Corley, John E. Ezell
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  ABSTRACT: Microbial mat communities host complex biogeochemical processes and play a role in the formation of most carbonate rocks by influencing both carbonate precipitation and dissolution. In this study, the biogeochemistry of microbial mats from the hypersaline Salt Pond, San Salvador, Bahamas are described using scanning electron microscopy, X-ray diffraction, microelectrode profiling, fatty acid methyl esters, and carbon and nitrogen analyses. These microbial mats are distinctly layered both chemically and with regard to composition of microbial community, where significant (ρ<0.05) differences are noted between layers and cores. Furthermore, an oxic upper zone and an H2S-rich lower zone dominate the Salt Pond microbial mats, where H2S concentrations were measured approaching 8mM. The high H2S concentrations along with the lacking evidence of mineral precipitation in SEM images point to the prevalence of carbonate dissolution. Moreover, the high concentrations of organics (3–9%) reveal that the mats are self-sourcing and can provide ample fuel to sustain the highly active heterotrophic (both aerobic and anaerobic) metabolism. Seasonal differences in sulfide and oxygen concentrations in Salt Pond mats indicate that the carbonate dissolution and precipitation reactions are dynamic in this hypersaline lake. KeywordsMicrobial Mats–Bahamas–Carbonates–Sulfides–Organic Carbon–Electrodes
  Aquatic Geochemistry 04/2012; 17(4):397-418.
• Article: Diel Aquatic CO2 System Dynamics of a Bermudian Mangrove Environment

  John A. Zablocki, Andreas J. Andersson, Nicholas R. Bates
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  ABSTRACT: Mangrove ecosystems play an important, but understudied, role in the cycling of carbon in tropical and subtropical coastal ocean environments. In the present study, we examined the diel dynamics of seawater carbon dioxide (CO2) and dissolved oxygen (DO) for a mangrove-dominated marine ecosystem (Mangrove Bay) and an adjacent intracoastal waterway (Ferry Reach) on the island of Bermuda. Spatial and temporal trends in seawater carbonate chemistry and associated variables were assessed from direct measurements of dissolved inorganic carbon, total alkalinity, dissolved oxygen (DO), temperature, and salinity. Diel pCO2 variability was interpolated across hourly wind speed measurements to determine variability in daily CO2 fluxes for the month of October 2007 in Bermuda. From these observations, we estimated rates of net sea to air CO2 exchange for these two coastal ecosystems at 59.8±17.3 in Mangrove Bay and 5.5±1.3mmolm−2d−1 in Ferry Reach. These results highlight the potential for large differences in carbonate system functioning and sea-air CO2 flux in adjacent coastal environments. In addition, observation of large diel variability in CO2 system parameters (e.g., mean pCO2: 390–2,841μatm; mean pHT: 8.05–7.34) underscores the need for careful consideration of diel cycles in long-term sampling regimes and flux estimates. KeywordsMangrove–CO2 –Diel–Gas flux–Coastal ocean
  Aquatic Geochemistry 04/2012; 17(6):841-859.
• Article: Coastal Ocean Last Glacial Maximum to 2100 CO2-Carbonic Acid-Carbonate System: A Modeling Approach

  Abraham Lerman, Michael Guidry, Andreas J. Andersson, Fred T. Mackenzie
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  ABSTRACT: Using coupled terrestrial and coastal zone models, we investigated the impacts of deglaciation and anthropogenic inputs on the CO2–H2O–CaCO3 system in global coastal ocean waters from the Last Glacial Maximum (LGM: 18,000year BP) to the year 2100. With rising sea level and atmospheric CO2, the carbonate system of coastal ocean water changed significantly. We find that 6×1012 metric tons of carbon were emitted from the coastal ocean, growing due to the sea level rise, from the LGM to late preindustrial time (1700 AD) because of net heterotrophy and calcification processes. This carbon came to reside in the atmosphere and in the growing vegetation on land and in uptake of atmospheric CO2 through the weathering of rocks on land. It appears that carbonate accumulation, mainly, but not exclusively, in coral reefs from the LGM to late preindustrial time could account for about 24ppmv of the 100ppmv rise in atmospheric CO2, lending some support to the “coral reef hypothesis”. In addition, the global coastal ocean is now, or soon will be, a sink of atmospheric CO2. The temperature rise of 4–5°C since the LGM led to increased weathering rates of inorganic and organic materials on land and enhanced riverine fluxes of total C, N, and P to the coastal ocean of 68%, 108%, and 97%, respectively, from the LGM to late preindustrial time. During the Anthropocene, these trends have been exacerbated owing to rising atmospheric CO2, due to fossil fuel combustion and land-use practices, other human activities, and rising global temperatures. River fluxes of total reactive C, N, and P are projected to increase from late preindustrial time to the year 2100 by 150%, 380%, and 257%, respectively, modifying significantly the behavior of these element cycles in the coastal ocean, particularly in proximal environments. Despite the fact that the global shoal water carbonate mass has grown extensively since the LGM, the pHT (pH values on the total proton scale) of global coastal waters has decreased from ~8.35 to ~8.18 and the carbonate ion concentration declined by ~19% from the LGM to late preindustrial time. The latter represents a rate of decline of about 0.028μmol CO3 2− per decade. In comparison, the decrease in coastal water pHT from the year 1900 to 2000 was about 8.18–8.08 and is projected to decrease further from about 8.08 to 7.85 between 2000 and 2100, according to the IS92a business-as-usual scenario of CO2 emissions. Over these 200years, the carbonate ion concentration will fall by ~120μmolkg−1 or 6μmolkg−1 per decade. This decadal rate of decline of the carbonate ion concentration in the Anthropocene is 214 times the average rate of decline for the entire Holocene. Hence, when viewed against the millennial to several millennial timescale of geologic change in the coastal ocean marine carbon system, one can easily appreciate why ocean acidification is the “other CO2 problem”. KeywordsGlacial–Interglacial–Last Glacial Maximum–Carbon dioxide–Coastal ocean acidification–Coral reef hypothesis
  Aquatic Geochemistry 04/2012; 17(4):749-773.
• Article: Examination and Refinement of the Determination of Aqueous Hydrogen Sulfide by the Methylene Blue Method

  Brandi Kiel Reese, David W. Finneran, Heath J. Mills, Mao-Xu Zhu, John W. Morse
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  ABSTRACT: The accurate and precise measurement of total sulfide has been of major interest for well over a century. The most commonly used method involves the formation of a methylene blue–sulfide complex and spectrophotometric measurement of its concentration. The study presented herein compares the two most commonly used methods as outlined in Standard Methods for the Examination of Water and Wastewater (in APHA, Standard methods for the examination of water and wastewater, Washington, 1960) and by Cline (Limnol Oceanogr 14:454–458, 1969). In addition, this study clarifies the existing confusion of Cline’s reagent preparation procedure, as it is apparent that various interpretations exist among research groups regarding reagent preparation. After evaluating both methods with respect to precision and accuracy, detection limit, sample storage time, and ease of use, the method outlined in Cline was determined to be superior. Furthermore, we suggest that the reagent concentration has to be optimized depending on the range of sulfide concentrations to increase the accuracy and precision of the method. KeywordsSulfide–Methylene blue–Cline method–Diamine
  Aquatic Geochemistry 04/2012; 17(4):567-582.