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Aquatic Geochemistry (v.17, #4-5)

Preface to John W. Morse Special Issue of Aquatic Geochemistry by Fred T. Mackenzie; Alfonso Mucci; George W. Luther III (pp. 307-310).

Effect of Dissolved Organic Carbon and Alkalinity on the Density of Arctic Ocean Waters by Frank J. Millero; Fen Huang; Ryan J. Woosley; Robert T. Letscher; Dennis A. Hansell (pp. 311-326).

At constant temperature, the density of deep waters in the oceans is higher than that of surface waters due to the oxidation of plant material that adds NO₃, PO₄, and Si(OH)₄, and the dissolution of CaCO₃(s) that adds Ca²⁺ and HCO₃. These increases in the density have been used to estimate the absolute salinity of seawater that is needed to determine its thermodynamic properties. Density (ρ), total alkalinity (TA), and dissolved organic carbon (DOC) measurements were taken on waters collected in the eastern Arctic Ocean. The results were examined relative to the properties of North Atlantic Waters. The excess densities (Δρ = ρ_Meas − ρ_Calc) in the surface Arctic waters were higher than expected (maximum of 0.008 kg m⁻³) when compared to Standard Seawater. This excess is due to the higher values of the normalized total alkalinity (NTA = TA * 35/S) (up to ~2,650 μmol kg⁻¹) and DOC (up to ~130 μmol kg⁻¹) resulting from river water input. New measurements are needed to determine how the DOC in the river waters contributes to the TA of the surface waters. The values of Δρ in deep waters are slightly lower (−0.004 ± 0.002 kg m⁻³) than that in Standard Seawater. The deep waters in the Arctic Ocean, unlike the Atlantic, Pacific, Indian, and Southern Oceans, do not have significant concentrations of silicate (maximum ~15 μmol kg⁻¹) and that can affect the densities. Since the NTA of the deep Arctic waters (2,305 ± 6 μmol kg⁻¹) is the same as Standard Seawater (2,306 ± 3 μmol kg⁻¹), the decrease in the density may be caused by the lower concentrations of DOC in the deep waters (44–50 μmol kg⁻¹ compared to the Standard Seawater value of 57 ± 2 μmol kg⁻¹). The relative deficit of DOC (7–13 μmol kg⁻¹) in the deep Arctic waters appears to cause the lower densities (−0.004 kg m⁻³) and Absolute Salinities (S _A, −0.004 g kg⁻¹). The effect of increases or decreases in Δρ and δS _A due to DOC in other deep ocean waters may be hidden in the correlations of the changes with silicate. Further work is needed to separate the effects of SiO₂ and DOC on the density of deep waters of the world oceans.

Keywords: Dissolved organic carbon; DOC; Alkalinity; Density; Arctic Ocean

Balancing the Oceanic Calcium Carbonate Cycle: Consequences of Variable Water Column Ψ by Stephen V. Smith; Jean-Pierre Gattuso (pp. 327-337).

The paired chemical reactions, Ca²⁺ + 2HCO₃ ⁻ ↔ CaCO₃ + CO₂ + H₂O, overestimate the ratio of CO₂ flux to CaCO₃ flux during the precipitation or dissolution of CaCO₃ in seawater. This ratio, which has been termed ψ, is about 0.6 in surface seawater at 25°C and at equilibrium with contemporary atmospheric CO₂ and increases towards 1.0 as seawater cools and pCO₂ increases. These conclusions are based on field observations, laboratory experiments, and equilibrium calculations for the seawater carbonate system. Yet global geochemical modeling indicates that small departures of Ψ from 1.0 would cause dramatic, rapid, and unrealistic change in atmospheric CO₂. Ψ can be meaningfully calculated for a water sample whether or not it is in equilibrium with the atmosphere. The analysis presented here demonstrates that the atmospheric CO₂ balance can be maintained constant with respect to seawater CaCO₃ reactions if one considers the difference between CaCO₃ precipitation and burial and differing values for ψ (both <1.0) in regions of precipitation and dissolution within the ocean.

Keywords: Calcium carbonate reactions; Seawater carbonate buffering; Geochemical cycles; CO₂ exchange

Aragonite Kinetics in Dilute Solutions by Christopher S. Romanek; John W. Morse; Ethan L. Grossman (pp. 339-356).

Aragonite was synthesized inorganically using a seeded-growth technique to characterize precipitation kinetics for the heterogeneous growth of solid from dilute solutions (ionic strength: 0.05–0.07 mol l⁻¹). The concentration of all aqueous constituents, including Ca (~5–15 mmol l⁻¹), Na (~10–35 mmol l⁻¹), Cl (~30–35 mmol l⁻¹), and carbon (as total alkalinity: ~10 to 17 meq l⁻¹), was held constant by the addition of titrants that contained excess solute concentrations to balance the growth of solid phase during the precipitation reaction, and a CO₂/N₂ gas mixture (0.009–0.178) was bubbled through each solution to facilitate mass exchange between gaseous and aqueous carbon species. Forty-three experiments were conducted at 10° (n = 13), 25° (n = 21), and 40°C (n = 9), over a range of average saturation states with respect to aragonite from 8.3 to 28.5, 2.9 to 19.6 and 2.0 to 12.2, and average precipitation rates from 10^2.8 to 10^3.8, 10^2.3 to 10^4.0, and 10^2.5 to 10^4.1 micromol m⁻² h⁻¹, respectively. Reaction orders averaged 1.7 ± 0.10 at 10°, 1.7 ± 0.07 at 25° and 1.5 ± 0.06 at 40°, and they were independent of temperature while rate constants averaged 10^1.3 ± 0.12, 10^1.9 ± 0.06, and 10^2.6 ± 0.04 micromol m⁻² h⁻¹, respectively, increasing one-half order of magnitude for each 15°C rise in temperature. From these data, an Arrhenius activation energy of 71.2 kJ mol⁻¹ is calculated for the heterogeneous precipitation of aragonite. This value is comparable to a sole independent measurement of 80.7 kJ mol⁻¹ reported for the solid-solution recrystallization of monohydrocalcite to aragonite (Munemoto and Fukushi in J Mineral Petrol Sci 103: 345–349, 2008).

Keywords: Aragonite; Precipitation rate; Activation energy; Kinetics

Dolomite Versus Calcite Weathering in Hydrogeochemically Diverse Watersheds Established on Bedded Carbonates (Sava and Soča Rivers, Slovenia) by Kathryn Szramek; Lynn M. Walter; Tjaša Kanduč; Nives Ogrinc (pp. 357-396).

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 (<30 cm) and dinaric karst regions with thicker soils (0 to greater than 70 cm) 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 1.5 meq/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.

Keywords: Dolomite; Carbonate; Weathering; Rivers; Slovenia; Watersheds; Fluxes

Biogeochemical Stratification and Carbonate Dissolution-Precipitation in Hypersaline Microbial Mats (Salt Pond, San Salvador, The Bahamas) by Mary K. Puckett; Karen S. McNeal; Brenda L. Kirkland; Margaret E. Corley; John E. Ezell (pp. 397-418).

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 8 mM. 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.

Keywords: Microbial Mats; Bahamas; Carbonates; Sulfides; Organic Carbon; Electrodes

Radiocarbon Ages Constraints on the Origin and Shedding of Bank-Top Sediment in the Bahamas during the Holocene by Niall C. Slowey; Gideon M. Henderson (pp. 419-429).

Great quantities of fine-sized aragonite needles are produced in the shallow waters that cover the tops of the Bahama Banks and then exported to the bank margins where they accumulate with shells of pelagic organisms. To better understand these processes, we investigated Holocene-aged sediments in a core from the southwestern margin of Little Bahama Bank. The aragonite content of the sediments, δ¹⁸O of planktonic foraminifera shells, and radiocarbon ages of aragonite-rich <63 μm sediments and coexisting planktonic foraminifera shells were determined. Sediment deposition was rapid overall, and a significant increase in deposition rate occurred 3,500–4,000 years ago, shortly after rising sea level flooded the bank top with seawater and caused a dramatic increase in the shallow water area where aragonite production occurred. During the latest Holocene when high deposition rates minimize effects of bioturbation, aragonite-rich <63 μm sediments are 400–600 years older than coexisting foraminifera. This difference indicates the net age of aragonite when it was exported from the bank top. It is consistent with expectations of the “hip-hop’n” model (Morse et al. in Geochimica et Cosmochimica Acta 67: 2819–2826, 2003) whereby aragonite needles on the bank top, formed initially by biologic or other processes, continue to grow for hundreds of years via precipitation of epitaxial carbonate cement from seawater. Earlier in the Holocene, when sea level was lower and the top of Little Bahama Bank was subaerially exposed, the deposition rate and aragonite content of the sediments were less, and the aragonite-rich <63 μm sediments are about 1,000 years younger than coexisting foraminifera. This age difference can be explained by downward mixing of latest-Holocene <63 μm material into older early-Holocene sediments.

Keywords: Aragonite; Bahama Banks; Radiocarbon; Holocene; Marine; Carbonate sediment; Sea level

Sources of Terrestrial Organic Carbon in the Mississippi Plume Region: Evidence for the Importance of Coastal Marsh Inputs by Thomas S. Bianchi; Laura A. Wysocki; Kathryn M. Schreiner; Timothy R. Filley; D. Reide Corbett; Alexander S. Kolker (pp. 431-456).

High sedimentation rates along river-dominated margins make these systems important repositories for organic carbon derived from both allochthonous and autochthonous sources. Using elemental carbon/nitrogen ratios, molecular biomarker (lignin phenol), and stable carbon isotopic (bulk and compound-specific) analyses, this study examined the sources of organic carbon to the Louisiana shelf within one of the primary dispersive pathways of the Mississippi River. Surface sediment samples were collected from stations across the inner, mid, and outer Louisiana shelf, within the Mississippi River plume region, during two cruises in the spring and fall of 2000. Lignin biomarker data showed spatial patterns in terrestrial source plant materials within the river plume, such that sediments near the mouth of the Mississippi River were comparatively less degraded and richer in C₄ plant carbon than those found at mid-depth regions of the shelf. A molecular and stable isotope-based mixing model defining riverine, marsh, and marine organic carbon suggested that the highest organic carbon inputs to the shelf in spring were from marine sources (55–61% marine organic carbon), while riverine organic carbon was the highest (63%) in fall, likely due to lower inputs of marine organic carbon at this time compared with the spring season. This model also indicated that marsh inputs, ranging from 19 to 34% and 3–15% of the organic carbon in spring and fall, respectively, were significantly more important sources of organic carbon on the inner Louisiana shelf than previously suggested. Finally, we propose that the decomposition of terrestrial-derived organic carbon (from the river and local wetlands sources) in mobile muds may serve as a largely unexplored additional source of oxygen-consuming organic carbon in hypoxic bottom waters of the Louisiana shelf.

Keywords: Mobile muds; Lignin; Chemical biomarkers; Sediment carbon cycling; Louisiana shelf; Compound-specific isotopic analyses

An In Situ Multispectral Imaging System for Planar Optodes in Sediments: Examples of High-Resolution Seasonal Patterns of pH by Yanzhen Fan; Qingzhi Zhu; Robert C. Aller; Donald C. Rhoads (pp. 457-471).

A new sediment profile imaging (SPI) instrument, CHEM-SPI, was developed for in situ two-dimensional measurements of biogeochemical solutes using fluorosensor foils in sediments and overlying waters. The CHEM-SPI system was used to simultaneously measure vertical sections of pH, O₂, and pCO₂ distributions in subtidal, surface deposits of Long Island Sound, NY. Images are readily obtained in 5–15 min with inexpensive LED excitation and commercial grade digital cameras having typical pixel resolution of ~50–100 μm over areas >150 cm² sediment. Seasonal examples of in situ deployments of the instrument revealed extensive horizontal and vertical heterogeneity of pH distributions. pH dynamics were associated with complex biogenic structures in the upper few centimeters of marine sediment and the pulsed input of organic matter during the spring bloom period. The pH beneath the sediment–water interface was dramatically depressed by the bloom input of organic matter but macrofaunal activity otherwise dominated pH variations in the bioturbated zone. The CHEM-SPI system allows direct quantitative confirmation of biogeochemical patterns previously inferred qualitatively from color patterns in visible SPI images. The instrument is sufficiently adaptable in design to accommodate new optical sensor foils for other chemical variables.

Keywords: Sediment profile imaging; Planar optodes; Multispectral; In situ pH measurement; Fluorosensor; Sediment diagenesis

Nutrient Inputs, Phytoplankton Response, and CO₂ Variations in a Semi-Enclosed Subtropical Embayment, Kaneohe Bay, Hawaii by Patrick Drupp; Eric Heinen De Carlo; Fred T. Mackenzie; Paul Bienfang; Christopher L. Sabine (pp. 473-498).

The marine shelf areas in subtropical and tropical regions represent only 35% of the total shelf areas globally, but receive a disproportionately large amount of water (65%) and sediment (58%) discharges that enter such environments. Small rivers and/or streams that drain the mountainous areas in these climatic zones deliver the majority of the sediment and nutrient inputs to these narrow shelf environments; such inputs often occur as discrete, episodic introductions associated with storm events. To gain insight into the linked biogeochemical behavior of subtropical/tropical mountainous watershed-coastal ocean ecosystems, this work describes the use of a buoy system to monitor autonomously water quality responses to land-derived nutrient inputs and physical forcing associated with local storm events in the coastal ocean of southern Kaneohe Bay, Oahu, Hawaii, USA. The data represent 2.5 years of near-real time observations at a fixed station, collected concurrently with spatially distributed synoptic sampling over larger sections of Kaneohe Bay. Storm events cause most of the fluvial nutrient, particulate, and dissolved organic carbon inputs to Kaneohe Bay. Nutrient loadings from direct rainfall and/or terrestrial runoff produce an immediate increase in the N:P ratio of bay waters up to values of 48 and drive phytoplankton biomass growth. Rapid uptake of such nutrient subsidies by phytoplankton causes rapid declines of N levels, return to N-limited conditions, and subsequent decline of phytoplankton biomass over timescales ranging from a few days to several weeks, depending on conditions and proximity to the sources of runoff. The enhanced productivity may promote the drawing down of pCO₂ and lowering of surface water column carbonate saturation states, and in some events, a temporary shift from N to P limitation. The productivity-driven CO₂ drawdown may temporarily lead to air-to-sea transfer of atmospheric CO₂ in a system that is on an annual basis a source of CO₂ to the atmosphere due to calcification and perhaps heterotrophy. Storms may also strongly affect proximal coastal zone pCO₂ and hence carbonate saturation state due to river runoff flushing out high pCO₂ soil and ground waters. Mixing of the CO₂-charged water with seawater causes a salting out effect that releases CO₂ to the atmosphere. Many subtropical and tropical systems throughout the Pacific region are similar to Kaneohe Bay, and our work provides an important indication of the variability and range of CO₂ dynamics that are likely to exist elsewhere. Such variability must be taken into account in any analysis of the direction and magnitude of the air–sea CO₂ exchange for the integrated coastal ocean, proximal and distal. It cannot be overemphasized that this research illustrates several examples of how high frequency sampling by a moored autonomous system can provide details about ecosystem responses to stochastic atmospheric forcing that are commonly missed by traditional synoptic observational approaches. Finally, the work exemplifies the utility of combining synoptic sampling and real-time autonomous observations to elucidate the biogeochemical and physical responses of coastal subtropical/tropical coral reef ecosystems to climatic perturbations.

Keywords: Alkalinity; Carbon dioxide; Carbon dioxide flux; Chlorophyll; Coastal; Coral reef; Dissolved oxygen; Embayment; Fluvial inputs; Gas exchange between the ocean and atmosphere; Nitrate; Nutrients; Oligotrophic; Phosphate; Seawater; Silicate; Tropical; Turbidity

Quantifying Sediment Nitrogen Releases Associated with Estuarine Dredging by Jeffrey C. Cornwell; Michael S. Owens (pp. 499-517).

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 ± 2 mmol L⁻¹ with average adsorbed NH₄ ⁺ concentrations of 11 μmol g⁻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.

Keywords: Denitrification; Ammonium adsorption; Dredging; Pore water chemistry; Estuarine sediments

Going West: Nutrient Limitation of Primary Production in the Northern Gulf of Mexico and the Importance of the Atchafalaya River by Antonietta Quigg; Jason B. Sylvan; Anne B. Gustafson; Thomas R. Fisher; Rod L. Oliver; Sasha Tozzi; James W. Ammerman (pp. 519-544).

To investigate controls on phytoplankton production along the Louisiana coastal shelf, we mapped salinity, nutrient concentrations (dissolved inorganic nitrogen (DIN) and phosphorus (P_i), silicate (Si)), nutrient ratios (DIN/P_i), alkaline phosphatase activity, chlorophyll and ¹⁴C primary productivity on fine spatial scales during cruises in March, May, and July 2004. Additionally, resource limitation assays were undertaken in a range of salinity and nutrient regimes reflecting gradients typical of this region. Of these, seven showed P_i limitation, five revealed nitrogen (N) limitation, three exhibited light (L) limitation, and one bioassay had no growth. We found the phytoplankton community to shift from being predominately N limited in the early spring (March) to P limited in late spring and summer (May and July). Light limitation of phytoplankton production was recorded in several bioassays in July in water samples collected after peak annual flows from both the Mississippi and Atchafalaya Rivers. We also found that organic phosphorus, as glucose-6-phosphate, alleviated P limitation while phosphono-acetic acid had no effect. Whereas DIN/P_i and DIN/Si ratios in the initial water samples were good predictors of the outcome of phytoplankton production in response to inorganic nutrients, alkaline phosphatase activity was the best predictor when examining organic forms of phosphorus. We measured the rates of integrated primary production (0.33–7.01 g C m⁻² d⁻¹), finding the highest rates within the Mississippi River delta and across Atchafalaya Bay at intermediate salinities. The lowest rates were measured along the outer shelf at the highest salinities and lowest nutrient concentrations (<0.1 μM DIN and Pi). The results of this study indicate that P_i limitation of phytoplankton delays the assimilation of riverine DIN in the summer as the plume spreads across the shelf, pushing primary production over a larger region. Findings from water samples, taken adjacent the Atchafalaya River discharge, highlighted the importance of this riverine system to the overall production along the Louisiana coast.

Keywords: Primary production; Nutrient limitation; Mississippi River; Atchafalaya River

FeS-Induced Radical Formation and Its Effect on Plasmid DNA by D. Rickard; B. Hatton; D. M. Murphy; I. B. Butler; A. Oldroyd; A. Hann (pp. 545-566).

Plasmid DNA was incubated at 25°C with aqueous solutions of dissolved Fe(II), S(-II), and nanoparticulate FeS with a mackinawite structure, FeS_m. At ≥0.1 mM total dissolved Fe(II) and S(-II), an increase in the proportion of the relaxed plasmid DNA occurs, through scission of the DNA backbone. In solutions where FeS_m was precipitated, nanoparticulate FeS_m binds to the DNA molecules. In solutions with concentrations below the FeS_m solubility product, nicking of supercoiled pDNA occurs. Plasmid DNA appears to be a sensitive proxy for radical reactions. The reactant is proposed to be a sulfur-based radical produced from the iron-catalyzed decomposition of bisulfide, in a manner analogous to the Fenton reaction. This is further supported by experiments that suggest that sulfide free radicals are produced during the photolysis of aqueous solutions of polysulfides. Supercoiling of DNA affects nearly all DNA–protein transactions so the observation of relaxation of supercoiled forms through reaction with FeS solutions has direct implications to biochemistry. The results of this experimentation suggest that genotoxicity in FeS-rich systems is a further contributory factor to the limited survival of organisms in sulfidic environments. Mutations resulting from the interactions of organisms and mobile elements, such as plasmids, in sediments will also be affected in sulfide-rich environments.

Keywords: Iron sulfide; DNA; Sulfide; Free radicals

Examination and Refinement of the Determination of Aqueous Hydrogen Sulfide by the Methylene Blue Method by Brandi Kiel Reese; David W. Finneran; Heath J. Mills; Mao-Xu Zhu; John W. Morse (pp. 567-582).

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.

Keywords: Sulfide; Methylene blue; Cline method; Diamine

Sulfide Oxidation across Diffuse Flow Zones of Hydrothermal Vents by Amy Gartman; Mustafa Yücel; Andrew S. Madison; David W. Chu; Shufen Ma; Christopher P. Janzen; Erin L. Becker; Roxanne A. Beinart; Peter R. Girguis; George W. Luther III (pp. 583-601).

The sulfide (H₂S/HS⁻) that is emitted from hydrothermal vents begins to oxidize abiotically with oxygen upon contact with ambient bottom water, but the reaction kinetics are slow. Here, using in situ voltammetry, we report detection of the intermediate sulfur oxidation products polysulfides [ $$ { ext{S}}_{ ext{x}}^{2 - } $$ ] and thiosulfate [ $$ { ext{S}}_{ 2} { ext{O}}_{ 3}^{ 2- } $$ ], along with contextual data on sulfide, oxygen, and temperature. At Lau Basin in 2006, thiosulfate was identified in less than one percent of approximately 10,500 scans and no polysulfides were detected. Only five percent of 11,000 voltammetric scans taken at four vent sites at Lau Basin in May 2009 show either thiosulfate or polysulfides. These in situ data indicate that abiotic sulfide oxidation does not readily occur as H₂S contacts oxic bottom waters. Calculated abiotic potential sulfide oxidation rates are <10⁻³ μM/min and are consistent with slow oxidation and the observed lack of sulfur oxidation intermediates. It is known that the thermodynamics for the first electron transfer step for sulfide and oxygen during sulfide oxidation in these systems are unfavorable, and that the kinetics for two electron transfers are not rapid. Here, we suggest that different metal catalyzed and/or biotic reaction pathways can readily produce sulfur oxidation intermediates. Via shipboard high-pressure incubation experiments, we show that snails with chemosynthetic endosymbionts do release polysulfides and may be responsible for our field observations of polysulfides.

Keywords: Sulfide oxidation; Kinetics; Hydrothermal vents; Diffuse flow; Lau Basin; In situ chemistry

Iron and Trace Metals in Microbial Mats and Underlying Sediments: Results From Guerrero Negro Saltern, Baja California Sur, Mexico by Miguel Angel Huerta-Diaz; Francisco Delgadillo-Hinojosa; X. L. Otero; José Antonio Segovia-Zavala; J. Martin Hernandez-Ayon; Manuel Salvador Galindo-Bect; Enrique Amaro-Franco (pp. 603-628).

Total trace metals (Cd, Co, Cu, Fe, Mn, Ni, Pb, Zn), Al, and pyrite- and reactive-associated metals were measured for the first time in a microbial mat and its underlying anoxic-sulfidic sediment collected in the saltern of Guerrero Negro (GN), Baja California Sur, Mexico. It is postulated that the formation of acid volatile sulfide (AVS) and pyrite in the area of GN could be limited by the availability of reactive Fe, as suggested by its limited abundance (mat and sediment combined average value of only 19 ± 10 μmol g⁻¹; n = 22) as well as the low pyrite (0.89–7.9 μmol g⁻¹) and AVS (0.19–21 μmol g⁻¹) concentrations (for anoxic-sulfidic sediments), intermediate degrees of pyritization (12–50%), high degrees of sulfidization (14–100%), generally low degrees of trace metal pyritization, and slight impoverishment in total Fe. This is a surprising result considering the large potential reservoir of available Fe in the surrounding desert. Our findings suggest that pyrite formation in the cycling of trace metals in the saltern of GN is not very important and that other sedimentary phases (e.g., organic matter, carbonates) may be more important reservoirs of trace elements. Enrichment factors [EF_Me = (Me/Al)_sample/(Me/Al)_background] of Co, Pb, and Cd were high in the mat (EF_Me = 2.2 ± 0.4, 2.8 ± 1.6 and 34.5 ± 9.8, respectively) and even higher in the underlying sediment (EF_Me = 4.7 ± 1.5, 14.5 ± 6.2 and 89 ± 27, respectively), but Fe was slightly impoverished (average EF_Fe of 0.49 ± 0.13 and 0.50 ± 0.27 in both mat and sediment). Organic carbon to pyrite-sulfur (C/S) molar ratios measured in the mat (2.9 × 10²–27 × 10²) and sediment (0.81 × 10²–6.6 × 10²) were, on average, approximately 77 times higher than those typically found in marine sediments (7.5 ± 2.1). These results may indicate that ancient evaporation basins or hypersaline sedimentary environments could be identified on the basis of extremely high C/S ratios (e.g., >100) and low reactive Fe.

Keywords: Pyrite; AVS; Trace metals; C/S ratios; Microbial mats; Reactive iron

Reduction Rates of Sedimentary Mn and Fe Oxides: An Incubation Experiment with Arctic Ocean Sediments by Cédric Magen; Alfonso Mucci; Bjorn Sundby (pp. 629-643).

To test the hypothesis that manganese- and iron-reducing bacteria in marine sediments respond rapidly to seasonal pulses of fresh organic carbon settling to the sea floor, we amended wet metal oxide–rich and metal oxide–poor sediments from the Beaufort Sea, Canadian Arctic, with organic carbon in the form of shrimp powder and incubated them at room temperature. Neither Mn nor Fe was released to the aqueous phase from unamended metal oxide–rich sediment during a 41-day incubation, but both elements were released from sediment aliquots amended with organic carbon. Dissolved Mn appeared in the aqueous phase after a lag period of 2 days or less and reached levels as high as 600 μmol l⁻¹ before levelling out. The release of dissolved Mn was accompanied by a decrease in the concentration of solid-phase reducible Mn. Dissolved Fe did not appear until 2 weeks into the incubation and only after the concentration of dissolved Mn had levelled out. For low concentrations of amended organic carbon (0.3%), the kinetics of Mn reduction fit a second-order rate law with a rate constant k = 2 × 10⁻³ g μmol⁻¹ day⁻¹, but at intermediate and high organic carbon concentrations (0.7 and 1.3%), the reduction kinetics was better described by a pseudo-first-order rate law with a rate constant k′ = 1.6 × 10⁻¹ day⁻¹. A pulse of organic carbon settling to the sea floor can trigger reduction of Mn and Fe oxides within a few days in strongly seasonal sedimentary environments, such as in the Arctic.

Keywords: Manganese; Iron; Kinetics; Reduction rates; Organic carbon; Arctic Ocean

Natural and Post-Urbanization Signatures of Hypoxia in Two Basins of Puget Sound: Historical Reconstruction of Redox Sensitive Metals and Organic Matter Inputs by Jill M. Brandenberger; Patrick Louchouarn; Eric A. Crecelius (pp. 645-670).

Hypoxia has been observed in Hood Canal, Puget Sound, WA, USA since the 1970s. Four long sediment cores were collected in 2005 and age-dated to resolve natural and post-urbanization signatures of hypoxia and organic matter (OM) sources in two contrasting basins of Puget Sound: Main Basin and Hood Canal. Paleoecological indicators used for sediment reconstructions included pollen, stable carbon and nitrogen isotopes (δ¹³C and δ¹⁵N), biomarkers of terrestrial OM (TOM), biogenic silica (BSi), and redox-sensitive metals (RSM). The sedimentary reconstructions illustrated a gradient in RSM enrichment factors as Hood Canal > Main Basin, southern > northern cores, and pre-1900s > 1900–2005. The urbanization of Puget Sound watersheds during the 1900s was reflected as shifts in all the paleoecological signatures. Pollen distributions shifted from predominantly old growth conifer to successional alder, dominant OM signatures recorded a decrease in the proportion of marine OM (MOM) concomitant with an increase in the proportion of TOM, and the weight % of BSi decreased. However, these shifts were not coincidental with an overall increase in the enrichment of RSM or δ¹⁵N signatures indicative of cultural eutrophication. The increased percentage of TOM was independently verified by both the elemental ratios and lignin yields. In addition, isotopic signatures, BSi, and RSMs all suggest that OM shifts may be due to a reduction in primary productivity rather than an increase in OM regeneration in the water column or at the sediment/water interface. Therefore, the reconstructions suggested the Hood Canal has been under a more oxygenated “stance” during the twentieth century compared to prior periods. However, these 2005 cores and their resolutions do not encompass the period of high resolution water column measurements that showed short-lived hypoxia events and fish kills in Hood Canal during the early twenty-first century. The decoupling between the increased watershed-scale anthropogenic alterations recorded in the OM signatures and the relatively depleted RSM during the twentieth century suggests that physical processes, such as deep-water ventilation, may be responsible for the historical variation in oxygen levels. Specifically, climate oscillations may influence the ventilation and/or productivity of deep water in Puget Sound and particularly their least mixed regions.

Keywords: Sediment cores; Redox sensitive metals; Organic matter; Hypoxia; Paleoecological indicators; Climatic cycles

Preliminary Evidence for Iodate Reduction in Bottom Waters of the Gulf of Mexico During an Hypoxic Event by Piers Chapman; Victor W. Truesdale (pp. 671-695).

The distributions of iodate and total inorganic iodine concentrations in the waters on the Texas–Louisiana shelf in April, June, and August 2004 are described. Iodine–salinity graphs show three-end-member mixing involving onshore and offshore surface waters and deep offshore water. The April survey showed simple mixing on the surface, but in the later surveys, iodate concentrations were often much lower than predicted by the mixing curve while those for total inorganic iodine were higher. This demonstrated both iodate reduction in the water and iodide addition, although individual samples did not show equivalent speciation changes. Hydrographically, the system consists of the estuaries of the Mississippi and Atchafalaya rivers as they spill onto the shelf. The waters are stratified seasonally by a robust halocline, leading to hypoxia in the bottom waters from the combined effect of restricted downward diffusion of oxygen and the sinking of the luxuriant growth of phytoplankton induced by riverine nutrient supply. The distributions of iodate and total inorganic iodine are, therefore, interpreted in terms of water–sediment interaction as the shelf shoals to the north.

Keywords: Gulf of Mexico; Texas; Anoxia; Hypoxia; Iodine; Total inorganic iodine; Iodate

Partition of Elements Between Solid and Liquid Phases in Aquatic Environments by Yuan-Hui Li (pp. 697-725).

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.

Keywords: Distribution coefficient; Surface complexation model; Electron binding energy; Chemical compositions; Natural aquatic systems; Mean residence time; Biophilic/biophobic elements

Burial and Preservation of Carbonate Rocks Over Phanerozoic Time by Robert A. Berner; Fred T. Mackenzie (pp. 727-733).

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 250 million 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 250 Ma.

Keywords: Carbonate rocks; Burial and preservation; Phanerozoic

Dolomite Controls on Phanerozoic Seawater Chemistry by Rolf S. Arvidson; Michael W. Guidry; Fred T. Mackenzie (pp. 735-747).

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.

Keywords: Dolomite; Carbonates; Phanerozoic; Geochemical cycling; Carbon cycle; Seawater saturation state

Coastal Ocean Last Glacial Maximum to 2100 CO₂-Carbonic Acid-Carbonate System: A Modeling Approach by Abraham Lerman; Michael Guidry; Andreas J. Andersson; Fred T. Mackenzie (pp. 749-773).

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: 18,000 year 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 24 ppmv of the 100 ppmv 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 pH_T (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 0.028 μmol CO₃ ²⁻ per decade. In comparison, the decrease in coastal water pH_T 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 200 years, the carbonate ion concentration will fall by ~120 μmol kg⁻¹ or 6 μmol kg⁻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”.

Keywords: Glacial; Interglacial; Last Glacial Maximum; Carbon dioxide; Coastal ocean acidification; Coral reef hypothesis

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