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Abiotic and secondary biotic reactions can contribute to the formation of cryptic biogeochemical cycles, resulting in an underestimation of carbon and nutrient budgets. This Texas coastal estuary sediment study provided a unique opportunity to use multidisciplinary RNA-based molecular and geochemical approaches to identify cryptic cycles associated with sulfate reduction, a commonly measured biogeochemical process considered to be the predominant anoxic terminal electron accepting process in shallow marine environments. Active sulfate reduction within an environment is typically determined by the detection of sulfides. However, a biologically driven cryptic cycle was determined by identifying metabolically active sulfate reducing and sulfur oxidizing lineages co-locating within the sediments, effectively masking sulfide production through re-oxidation back to sulfate. Similar co-location of sulfate and iron reducing lineages prevented the detection of sulfides through the formation of iron sulfide minerals, producing a geochemically driven cryptic cycle. We also showed that sulfate reduction rates determined by 35SO4 2− incubation analysis can be positively correlated with dsrA transcript abundance, and thus this molecular technique may be a proxy for the prohibitive radioactive method. Based on these results, support is given to a synergistic geochemical and molecular biological strategy to better provide an understanding of marine sulfur cycle.

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 102.8 to 103.8, 102.3 to 104.0, and 102.5 to 104.1 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 101.3 ± 0.12, 101.9 ± 0.06, and 102.6 ± 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).

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.

This paper presents the results of two cruises in the Northern Gulf of Mexico in 2008 that investigated local and short-term factors influencing the carbonate chemistry dynamics and saturation state with respect to aragonite (Ωaragonite) of surface seawater in this region. One cruise covered much of the northern half of the Gulf, and the other focused on the coastal zone west of the Atchafalaya Bay outlet of the Mississippi River—the region where the hypoxic “dead zone” occurs on the Louisiana shelf. Offshore waters (>100 m depth) exhibited only small variations in CO2 fugacity (fCO2), total alkalinity (TA) and Ωaragonite. Values were close to those typically observed in subtropical Atlantic Ocean and Caribbean Sea waters of similar salinity. However, inner shelf waters (

Experiments were conducted on CaCO 3 nucleation in seawater of estimated Cretaceous Period “calcite sea” (i.e., low-magnesium calcite being the primary inorganic marine calcium carbonate precipitate) composition to determine the influences of alkalinity and p CO 2 on calcite versus aragonite formation. The pH was continually monitored throughout the experiments and it was used, in combination with the initial alkalinity, to calculate the p CO 2 and saturation state of aragonite and calcite at the time of nucleation. The morphology and mineralogy of the precipitates were determined using scanning electron microscopy and X-ray diffraction analyses. It was observed that the initial alkalinity affected greatly the nucleation p CO 2 and the CaCO 3 polymorph that is precipitated. In seawater with Mg 2+ /Ca 2+ = 1.2, ∼10 mM alkalinity, and a p CO 2 p CO 2 is >2500 μatm. Seawater with Mg 2+ /Ca 2+ = 1.2 and a wide range of initial alkalinities (5–50 mM) produces variable results. Seawater with Mg 2+ /Ca 2+ = 1.7 produces only aragonite at lower alkalinities ( ∼18 mM. These results demonstrate the need to also consider the effects of alkalinity and p CO 2 in the critical Mg 2+ /Ca 2+ ratio region of ∼1–2 for calcite seas versus “aragonite seas” (in which the primary inorganic carbonate precipitates are aragonite and high-magnesium calcite) at various times throughout the Phanerozoic Eon and to use reasonable estimated values of alkalinity and p CO 2 in experimental studies. The results of this study indicate that it may have been possible for CaCO 3 to commonly nucleate directly from Cretaceous seawater due to elevated calcium and alkalinity concentrations, even though atmospheric p CO 2 was higher.

The effect of ionic strength (I), pCO2, and temperature on the dissolution rate of calcite was investigated in magnesium-free, phosphate-free, low calcium (mCa2+ ≈ 0.01 m) simple KCl and NaCl solutions over the undersaturation range of 0.4 ≤ Ωcalcite ≤ 0.8. First-order kinetics were found sufficient to describe the rate data where the rate constant (k) is dependent on the solution composition. Rates decreased with increasing I and were faster in KCl than NaCl solutions at the same I indicating that Na+ interacts more strongly with the calcite surface than K+ or that water is less available in NaCl solutions. Rates increased with increasing pCO2 and temperature, and their influences diminished at high I. Arrhenius plots yielded a relatively high activation energy (Ea ≈ 20 ± 2 kJ mol− 1) which indicated that dissolution was dominated by surface-controlled processes. The multiple regression model (MR) of Gledhill and Morse (2006a) was found to adequately describe the results at high I in NaCl solutions, but caution must be used when extrapolating to low I or pCO2 values. These results are consistent with the hypothesis that the mole fraction of “free” solvent (X“free”H2O) plays a significant role in the dissolution kinetics of calcite with a minimum value of ∼ 45–55% required for dissolution to proceed in undersaturated solutions at 25–55 °C and pCO2 = 0.1–1 atm. This hypothesis has been incorporated into a modified version of the MR model of Gledhill and Morse (2006a) where X“free”H2O has replaced I and the Ca2+ and Mg2+ terms have been dropped: k pred = β 0 + β 1 T + β 2 p C O 2 + β 3 X ”free”H 2 O .

Whitings (patches of suspended fine-grained calcium carbonate) on the Great Bahama Bank (GBB) have been studied since the 1940s. The source and cause of these whitings have been hotly debated for a number of years. Recent studies have shown that resuspension of underlying sediments act as seed crystals for the slow precipitation of CaCO3. Similar studies have not been conducted on the nearby more northerly Little Bahama Bank (LBB) where satellite photographs indicate the presence of extensive whitings north of the Grand Bahama Island. Research cruises were made to the LBB in July 2003 and May 2005. On board measurements were used to examine the distribution of the components of the carbonic acid system in LBB waters. The distribution of suspended calcium carbonate was also determined and samples of the suspended material and underlying sediments were collected for analyses, including 14C age determinations. The maximum residence time for waters on the LBB is a little over half (∼ 144 d) that on the GBB. A ∼ 20% decrease was found in the salinity-normalized titration alkalinity for inner bank waters. This yields a carbonate precipitation rate similar to waters on the GBB. However, unlike the waters in the whitings on the GBB where no changes in carbonic acid system parameters have been detected, large changes in the carbonic acid system parameters were found in the whiting waters of the LBB. These results clearly indicate that active precipitation was occurring in these waters. These observations are interpreted as indicating that there is a much slower mixing of waters associated with whitings and surrounding bank waters on the LBB than on the GBB. The more closed nature of whitings on the LBB makes them better targets for study of the processes taking place in whitings. Suspended carbonate mineralogy and geochemistry closely matched those of the fine-grained portion of the underlying sediments. These results support a close relationship between carbonates found in whitings and those in underlying sediments. This is further evidenced by a similar age for whiting carbonates (∼ 700 y) and easily resuspended sediment carbonates (∼ 1000 y) and is what was previously observed on the GBB. These observations indicate that the processes leading to whiting formation and associated calcium carbonate precipitation along with changes in water chemistry are quite similar in these two shallow carbonate bank environments in the Bahamas.

Sediment samples were selected from 28 sites across the deep (212–3527 m) northern Gulf of Mexico (GOM) as part of the Deep Gulf of Mexico Benthos (DGoMB) program, and analyzed for geochemical parameters related to organic carbon (OC) distribution and characteristics. The results of this study indicate that the OC content of sediments in the deep northern GOM is controlled by several factors; including water depth, overlying water productivity, sediment carbonate content, sediment oxygen exposure time, OC sources, and regional influences. The best correlation between sediment OC content, on a CaCO 3 -free basis, and other parameters examined was an inverse correlation of OC with water depth. The OC/SA ratio had a wide range of values and, along with variable sources of sedimentary OC, indicated that the organism-available concentration of metabolizable organic matter may not be simply related to sedimentary OC content. This was perhaps reflected in the observation that benthic macrofaunal and meiofaunal biomass abundances were well correlated with sedimentary OC, but the abundance of bacteria in sediments was not.

Bottom-water hypoxia effects on sediment–water interface nitrogen (N) transformations in Corpus Christi Bay (TX, USA) were examined using continuous-flow intact sediment core incubations. Sediment cores were collected from three sites in August 2002 (summer hypoxia) and April 2003 (normoxia). Oxygen (O2) and hydrogen sulfide (H2S) depth profiles were generated with microelectrodes. Membrane inlet mass spectrometry was used to measure sediment O2 demand and net N2 flux and combined with isotope pairing to determine potential denitrification and N fixation. Potential dissimilatory nitrate reduction to ammonium (DNRA) was measured using high-performance liquid chromatography. Sediment O2 penetration depths ranged from 5 to 10 mm. H2S ranged from being present in overlying water and throughout the sediment column in August to not detectable in overlying water or sediment in April. Sediment O2 demand was higher during bottom-water normoxia conditions versus hypoxia. Sediments were a significant source of \({\text{NH}}_{\text{4}}^{\text{ + }} \) to overlying water during hypoxia but not during normoxia. Net N2 fixation was observed at one station in August and all stations in April. Denitrification rates were significantly higher during hypoxia at two of three sites. Potential DNRA was observed during both oxic states, but rates were significantly higher during hypoxia, which may reflect sulfide enhancement and absence of cation exchange with \(^{{\text{14}}} {\text{NH}}_{\text{4}}^{\text{ + }} \). DNRA may contribute to formation and maintenance of bottom-water hypoxic events in this system. These results show that N transformation pathways and rates change when bottom-water O2 concentrations drop to hypoxic levels. Since south Texas is a semiarid region with few episodic runoff events, these results indicate that Corpus Christi Bay sediments are a N source most of the year, and denitrification may drive N limitation between episodic runoff events.

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 (

Carbonate-rich sediments at shoal to shelf depths ( 3 reservoir that can rapidly react to the decreasing saturation state of seawater with respect to carbonate minerals, produced by the increasing partial pressure of atmospheric carbon dioxide ( p CO 2 ) and “acidification” of ocean waters. Aragonite is usually the most abundant carbonate mineral in these sediments. However, the second most abundant (typically ∼24 wt%) carbonate mineral is high Mg-calcite (Mg-calcite) whose solubility can exceed that of aragonite making it the “first responder” to the decreasing saturation state of seawater. For the naturally occurring biogenic Mg-calcites, dissolution experiments have been used to predict their “stoichiometric solubilities” as a function of mol% MgCO 3 . The only valid relationship that one can provisionally use for the metastable stabilities for Mg-calcite based on composition is that for the synthetically produced phases where metastable equilibrium has been achieved from both under- and over-saturation. Biogenic Mg-calcites exhibit a large offset in solubility from that of abiotic Mg-calcite and can also exhibit a wide range of solubilities for biogenic Mg-calcites of similar Mg content. This indicates that factors other than the Mg content can influence the solubility of these mineral phases. Thus, it is necessary to turn to observations of natural sediments where changes in the saturation state of surrounding waters occur in order to determine their likely responses to the changing saturation state in upper oceanic waters brought on by increasing p CO 2 . In the present study, we investigate the responses of Mg-calcites to rising p CO 2 and “ocean acidification” by means of a simple numerical model based on the experimental range of biogenic Mg-calcite solubilities as a function of Mg content in order to bracket the behavior of the most abundant Mg-calcite phases in the natural environment. In addition, observational data from Bermuda and the Great Bahama Bank are also presented in order to project future responses of these minerals. The numerical simulations suggest that Mg-calcite minerals will respond to rising p CO 2 by sequential dissolution according to mineral stability, progressively leading to removal of the more soluble phases until the least soluble phases remain. These results are confirmed by laboratory experiments and observations from Bermuda. As a consequence of continuous increases in atmospheric CO 2 from burning of fossil fuels, the average composition of contemporary carbonate sediments could change, i.e., the average Mg content in the sediments may slowly decrease. Furthermore, evidence from the Great Bahama Bank indicates that the amount of abiotic carbonate production is likely to decline as p CO 2 continues to rise.

Sedimentary basins can contain close to 20% by volume of pore fluids commonly classified as brines. These fluids can become undersaturated with respect to calcite as a result of migration, dispersive mixing, or anthropogenic injection of CO2. This study measured calcite dissolution rates in geologically relevant Na–Ca–Mg–Cl synthetic brines (50–200 g L−1 TDS). The dissolution rate dependency on brine composition, pCO2 (0.1–1 bar), and temperature (25.0–82.5 °C) was modeled using the empirical rate equation R = k ( 1 - Ω ) n where R is the rate, k and n are empirical fitting terms and 1 − Ω the degree of disequilibrium with respect to calcite. When Ω is defined relative to an apparent steady-state solubility, n can be assumed first-order over the range of Ω investigated (Ω = 0.2–1.0). Rates increased with increasing pCO2 as did its sensitivity to increased brine total dissolved salt (TDS) concentration. At 0.1 bar, rates were nearly independent of the TDS (k = 13.0 ± 2.0 × 10−3 mol m−1 h−1). However, at higher CO2 partial pressures, rates became composition dependent and the rate constant, k, was shown to be a function of temperature, pCO2, ionic strength and calcium and magnesium activity. The rate constant (k) can be estimated from a multiple regression (MR) model of the form k = β 0 + β 1 ( t ) + β 2 ( pCO 2 ) + β 3 ( I ) + β 4 a Ca 2 + + β 5 a Mg 2 + . A relatively high activation energy (Ea = 20 kJ mol−1) was measured, along with a stirring rate independence suggesting that the dissolution is dominated by surface-controlled processes at Ω > 0.2 in these calcium-rich brines. The addition of 1 g L−1 SO 4 2 - resulted in a rate inhibition that was highly sensitive to increasing concentrations of calcium and magnesium. Consequently, even though sulfate concentrations in subsurface formation waters are generally less than that in seawater, at TDS concentrations greater than 200 g L−1, its effect on the rate may be of similar magnitude. These findings provide an opportunity to improve reaction-transport models in carbonate-bearing saline reservoirs, where pCO2 is >0.1 atm (pH

This study investigated calcite solubility approached from undersaturation in geologically relevant Na–Ca–Mg–Cl synthetic brines (35 to 200 g l− 1 TDS) at CO2 partial pressures of ∼ 0.1, 0.5 and 1.0 bar at 25 °C. Total carbon dioxide content, total alkalinity and total calcium content were measured and the carbonic acid species distribution and ion activities calculated using the Pitzer equation-based program EQPITZER. Stoichiometric (pKsp⁎) equilibrium constants ranged from 6.4 to 4.8, decreasing with increasing brine TDS. In general, good agreement was found between measured and predicted solubility data. Evaluation of the Ca2+ and CO32− activity coefficients with EQPITZER in brines

Variability of dissolved Fe2+ and ∑H2S concentrations in porewaters were studied, using cathodic stripping voltammetry (CSV) microelectrodes, in sediments overlain by hypoxic waters in the summer from the southeastern region of Corpus Christi Bay, Texas (CCB) and the Mississippi River Bight (MRB), Louisiana. These measurements were complimented by sediment microcosm studies where oxygen concentrations in the overlying water were manipulated. Sulfate reduction rates, benthic oxygen demand, total reduced sulfide, porewater sulfate, and total organic carbon were also determined. Fe2+ and ∑H2S were the major dissolved redox-reactive dissolved species in these sediments. During hypoxic conditions, an upward migration of porewater Fe2+ and ∑H2S occurred, with Fe2+ reaching much higher maximum concentrations than ∑H2S. Statistically significant (p 0.05) difference in regard to ∑H2S between microcosm and field monitoring studies. Results indicated that, for Fe2+ and ∑H2S, relatively large and rapid changes occurred in both the concentrations and distributions of these important porewater constituents in response to relatively short-term changes in overlying water oxygen content. Model calculations indicated that conditions in the sediments can be responsible for the induction of hypoxic conditions in the formation of hypoxic overlying waters in

The sequestration of sulfur within sedimentary pyrite is a major sink in the global biogeochemical sulfur cycle and has consequent impacts on the related cycles of carbon and oxygen. In many sedimentary environments, a zone exists which releases H2S if treated with acid. The materials that produce this sulfide are described as acid volatile sulfides or AVS and the sulfide which is evolved and collected by this treatment is called acid volatile sulfide (AVS-S). In this research review we show that AVS represents part of a complex and dynamic biogeochemical system which is not defined simply by the analysis of AVS-S. During the 40 years since R.A. Berner first defined the concept of acid volatile sulfide, a number of scientific myths have developed regarding the nature and composition of AVS and its role in the global sulfur cycle. We show that: • AVS is not equivalent to FeS and solid FeS phases have rarely been identified in marine sediments. AVS is a complex and variable component which does not represent any singular or simple group of sediment components. • The commonly used extraction methods do not quantitatively extract AVS minerals (mackinawite and greigite) without also significantly releasing sulfide from pyrite. • AVS varies geographically and AVS is not the same component or set of components wherever it is found. • The solubilities of FeS and Fe3S4 in neutral to alkaline systems are presently poorly constrained and pore water saturation state calculations cannot be used to support the presence of specific AVS minerals. • The colored zone in sediments does not reflect the distribution of AVS. • AVS is not always a minor fraction of total reduced sulfide (TRS) which dominantly occurs near the sediment–water interface. AVS minerals are not required precursors for pyrite formation. A better understanding of the sedimentary sulfur system, including its nature, formation and transformations, is a key component in understanding the evolution of the Earth and its future development. However, some basic parameters, such as the solubility and composition of the metastable iron sulfide solids, are still not constrained. Even the presence of these minerals in most sedimentary systems remains equivocal. Aqueous FeS clusters appear to be ubiquitous and appear to have significant geochemical and biological roles. The roles of organisms, especially in modifying rates and products, remain a key area of research. Although AVS may have served its purpose in the earlier exploratory days of the study of sedimentary sulfides, the concept is now probably more of a hindrance than a help in advancing this field. It is reasonable to question if it is not time to simply do away with the AVS concept altogether and get on with the business of trying to deal more directly with the behavior of the more important components known to fall within the group of sulfide species that contribute to sulfide that is volatilized when acid is added to a sediment.

In light of recent studies that show oxygen isotope fractionation in carbonate minerals to be a function of HCO3− and CO32− concentrations, the oxygen isotope fractionation and exchange between water and components of the carbonic acid system (HCO3−, CO32−, and CO2(aq)) were investigated at 15°, 25°, and 40°C. To investigate oxygen isotope exchange between HCO3−, CO32−, and H2O, NaHCO3 solutions were prepared and the pH was adjusted over a range of 2 to 12 by the addition of small amounts of HCl or NaOH. After thermal, chemical, and isotopic equilibrium was attained, BaCl2 was added to the NaHCO3 solutions. This resulted in immediate BaCO3 precipitation; thus, recording the isotopic composition of the dissolved inorganic carbon (DIC). Data from experiments at 15°, 25°, and 40°C (1 atm) show that the oxygen isotope fractionation between HCO3− and H2O as a function of temperature is governed by the equation: 1000 ⁢ ln ⁡ α HCO 3 − − H 2 O = 2.59 ± 0.00 ⁢ ( 10 6 T − 2 ) + 1.89 ± 0.04 where α is the fractionation factor and T is in kelvin. The temperature dependence of oxygen isotope fractionation between CO32− and H2O is 1000 ⁢ ln α CO 3 2 − − H 2 O = 2.39 ± 0.04 ⁢ ( 10 6 T − 2 ) − 2.70 ± 0.46 The oxygen isotope fractionation between CO2(aq) and H2O was investigated by acid stripping CO2(aq) from low pH solutions; these data yield the following equation: 1000 ⁢ ln ⁡ α CO 2(aq) − H 2 O = 2.52 ± 0.03 ⁢ ( 10 6 T − 2 ) + 12.12 ± 0.33 These results show that pH can have a significant effect on the δ18O of the DIC, which can vary by as much as 17‰ at a given temperature.

Ammonium adsorption on sediments is an important process in aquatic environments where often about twice as much exchangeable NH 4 + is found adsorbed on sediments as that measured in associated porewaters. Adsorption of NH 4 + on fine-grained terrigenous sediments from the Louisiana Shelf west of the Mississippi River delta and from Lower Laguna Madre (LLM), Texas was studied under reducing and oxic conditions. Sediment from both sites yielded K * values of 2.3 and 4.0 ml g dw −1 under oxidizing and reducing conditions, respectively, even though the NH 4 + concentration was about 10 times greater in Laguna Madre sediments than sediments from the Louisiana Shelf. K * determined under reducing conditions was a better predictor of exchangeable NH 4 + in sediments from the two sites, based on NH 4 + porewater concentrations, than K * determined under oxidizing conditions (the mean square error of reducing K * was ∼1/4 that of oxidizing K *). However, when considering potential NH 4 + release from sediments if they are resuspended into overlying oxic waters, the oxic adsorption coefficient should be used.

Precipitation experiments were carried out using chemo-stat technique in order to determine the effect of humic acid on the precipitation rate and composition of magnesian calcite from seawater under laboratory conditions (25°C, 1 atm). Results of our study clearly show that the presence of humic acid in seawater reduces the precipitation rate of magnesian calcite. However, MgCO3 composition in magnesian calcite overgrowths is not influenced by the presence of up to 9.2 mg C/l of humic acid.

In hydrocarbon seep sediments associated with chemosynthetic communities along the continental slope of the northern Gulf of Mexico, the distribution of sulfate reduction rates is very heterogeneous, with rates ranging from values typical of off-seep Gulf environments to very high rates (up to ∼14.8 μmol cm −3 day −1 ). These extreme rates deplete porewater sulfate within 20 cm and are associated with increases in dissolved sulfide and DIC concentrations. Large depletions of porewater calcium and high concentrations of solid phase CaCO 3 indicate that carbonate precipitation is driven by sulfate reduction as well. Although total organic carbon concentrations (reflecting an unresolved mix of ambient biomass, crude oil and other seep hydrocarbons) are much higher than elsewhere in the Gulf, sulfate reduction rates are not related in any simple fashion to POC. In addition, apparent diffusion coefficients ( D S *) computed from sulfate gradients and reduction rates typically exceed molecular values. In tubeworm- Beggiatoa mat associations these excursions approach two orders of magnitude. We suggest that these elevated D S * values may indicate specific organism-sediment interactions that replenish sulfate, either through sediment irrigation or in situ sulfur recycling. Interactions between Beggiatoa and tubeworms may lead to large enhancements of the apparent flux of sulfate to the zone of SR, and give rise to very high SR rates that are otherwise probably limited in duration and lateral extent.

The simultaneously extracted metal-acid volatile sulfide (SEM-AVS) method is widely used to estimate wether or not metals in sediments are toxic. Although considerable attention has been given to metal-sulfide interactions and related toxicological research, the complexities of the sedimentary sulfide system and how they may influence the SEM-AVS relationship have received scant consideration. These measurements have implications for metal toxicity but lack consistent definitions.

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.

The source of whitings on the Great Bahama Bank and their relationship to major changes in the chemistry of Bank waters have been among the longest and most hotly debated topics in carbonate geochemistry. In this paper, we demonstrate that the reaction kinetics of calcite with Bank waters for a given saturation state are similar to, but somewhat slower (2 to 3 times) than with Gulf Stream water. The interpretation of the reaction kinetics of suspended Bank sediment with Bank water requires that the precipitating phase be about twice as soluble as aragonite. Good agreement at equivalent saturation states was found between experimental precipitation rates and those calculated for the rate of change of Bank water chemistry in the region of whitings. These results indicate that the dominant mode of carbonate removal is via precipitation on resuspended sediments rather than the rapid pseudo-homogeneous precipitation of calcium carbonate in the water column resulting in the formation of a whiting. Estimates indicate that single aragonite needles may be resuspended many times over a period of decades during which they experience repeated overgrowth. A major portion (>98%) of suspended calcium carbonate is outside the visually dramatic whitings. Thus, as visually spectacular as they are, whitings do not represent a short-term locally massive precipitation of carbonate on the Great Bahama Bank, nor are they even likely to be the dominant sites of carbonate removal in this region. Although future refinements are needed that include seafloor processes, we have at this point arrived at a mechanistic kinetic model that provides a reasonably quantitative explanation for the hydrochemistry of the carbonate system on the northern Great Bahama Bank.

Net fluxes of respiratory metabolites (O2, dissolved inorganic carbon (DIC), NH4+, NO3−, and NO2−) across the sediment-water interface were measured using in-situ benthic incubation chambers in the area of intermittent seasonal hypoxia associated with the Mississippi River plume. Sulfate reduction was measured in sediments incubated with trace levels of35S-labeled sulfate. Heterotrophic remineralization, measured as nutrient regeneration, sediment community oxygen consumption (SOC), sulfate reduction, or DIC production, varied positively as a function of temperature. SOC was inversely related to oxygen concentration of the bottom water. The DIC fluxes were more than 2 times higher than SOC alone, under hypoxic conditions, suggesting that oxygen uptake alone cannot be used to estimate total community remineralization under conditions of low oxygen concentration in the water column. A carbon budget is constructed that compares sources, stocks, transformations, and sinks of carbon in the top meter of sediment. A comparison of remineralization processes within the sediments implicates sulfate reduction as most important, followed by aerobic respiration and denitrification. Bacteria accounted for more than 90% of the total community biomass, compared to the metazoan invertebrates, due presumably to hypoxic stress.

Among the most important set of chemical reactions occurring under near Earth surface conditions are those involved in the dissolution of sedimentary carbonate minerals. These minerals comprise about 20% of Phanerozoic sedimentary rocks. Calcite and, to a significantly lesser extent, dolomite are the major carbonate minerals in sedimentary rocks. In modern sediments, aragonite and high-magnesian calcites dominate in shallow water environments. However, calcite is by far the most abundant carbonate mineral in deep sea sediments. An understanding of the factors that control their dissolution rates is important for modeling of geochemical cycles and the impact of fossil fuel CO 2 on climate, diagenesis of sediments and sedimentary rocks. It also has practical application for areas such as the behavior of carbonates in petroleum and natural gas reservoirs, and the preservation of buildings and monuments constructed from limestone and marble. In this paper, we summarize important findings from the hundreds of papers constituting the large literature on this topic that has steadily evolved over the last half century. Our primary focus is the chemical kinetics controlling the rates of reaction between sedimentary carbonate minerals and solutions. We will not attempt to address the many applications of these results to such topics as mass transport of carbonate components in the subsurface or the accumulation of calcium carbonate in deep sea sediments. Such complex topics are clearly worthy of review papers on their own merits. Calcite has been by far the most studied mineral over a wide range of conditions and solution compositions. In recent years, there has been a substantial shift in emphasis from measuring changes in solution composition, to determine “batch” reaction rates, to the direct observation of processes occurring on mineral surfaces using techniques such as atomic force microscopy (AFM). However, there remain major challenges in integrating these two very different approaches. A general theory of surface dissolution mechanisms, currently lacking (although see Lasaga and Luttge [Science 291 (2001) 2400]), is required to satisfactorily relate observations of mineral surfaces and the concentration of dissolved components. Studies of aragonite, high-magnesian calcites, magnesite, and dolomite dissolution kinetics are much more limited in number and scope than those for calcite, and provide, at best, a rather rudimentary understanding of how these minerals are likely to behave in natural systems. Although the influences of a limited number of reaction inhibitors have been studied, probably the greatest weakness in application of experimental results to natural systems is understanding the often profound influences of “foreign” ions and organic matter on the near-equilibrium dissolution kinetics of carbonate minerals.

In sediments from the continental slope of the Northern Gulf of Mexico, generally,the degree of iron pyritization (DOP) is low (

The objective of this modeling effort was to better understand the dynamic relationship between seagrass beds and their sedimentary environment using a diagenetic model. The model was developed and optimized for sediments in the Laguna Madre, Texas, which is one of the world's largest (∼140 km long) negative estuaries with close to 85% of the basin floor covered with seagrass beds. Although high levels of organic matter decomposition occur in the near-surface sediments, the model was unable to produce enough metabolism to satisfy dissolved inorganic carbon (DIC) profiles from organic matter oxidation alone. Carbon isotope analyses of DIC verified that carbonate mineral dissolution contributes more than 50% of DIC added to porewaters during early diagenesis and is especially important below ∼5 cm. In comparison to unvegetated areas, a common characteristic of seagrass bed sediments was their low sulfide concentrations in the seagrass rootzone. Model simulations indicate that rootzone fluxes of O2 are essential to maintaining non-toxic levels of sulfide and consequently promote healthy conditions for seagrass growth. Further, the model simulations suggest that the position of maximum organic matter metabolism relative to the position of the seagrass rootzone can be used to predict several properties of seagrass sediment geochemistry. These predictions include the comparative role of anaerobic and aerobic metabolism, the sulfide to ammonium ratio, and the presence or absences of sulfides in the rootzone. In summary, the results of this model clearly demonstrate a dynamic interaction between seagrasses and diagenetic processes in the underlying sediments. The primary impact of these interactions is to lower sedimentary sulfide concentrations below toxic levels for seagrasses. Such interactions not only modify the sedimentary record but also play an important role influencing the health and productivity of seagrasses.

Interactions of trace metals with sulfide in anoxic environments are important in determining their chemical form and potential toxicity to organisms. In recent years, a considerable body of observational data has accumulated that indicates very different behavior for various trace metals in sulfidic sediments. These differences in behavior cannot be entirely attributed to thermodynamic relationships, but also reflect differences in ligand exchange reaction kinetics, and redox reaction pathways. Pb, Zn, and Cd, which are generally pyritized to only a few percent of the “reactive” fraction, have faster water exchange reaction kinetics than Fe 2+ , resulting in MeS phases precipitating prior to FeS formation and subsequent pyrite formation, whereas, Co and Ni, which have slower H 2 O exchange kinetics than Fe 2+ , are incorporated into pyrite. Although Hg and Cu have faster reaction kinetics than Fe 2+ , both are incorporated into pyrite or leached from the pyrite fraction with nitric acid. Hg undergoes significant chloride complexation, which can retard reaction with sulfide, but can also replace Fe in FeS to form HgS, which can only be dissolved in the pyrite fraction. Cu 2+ is reduced by sulfide and forms a variety of sulfides with and without Fe that can only be dissolved with nitric acid. Mn 2+ does not form a MnS phase easily and is incorporated into pyrite at high iron degrees of pyritization (DOP). Oxyanions of Mo and As are first reduced by sulfide. These reduced forms may then react with sulfides resulting in incorporation into pyrite. However, the oxyanion of Cr is reduced to Cr 3+ , which is kinetically inert to reaction with sulfide and, therefore, not incorporated into pyrite.

A benthic chamber has been developed that utilizes an artificial `gill' to maintain close to ambient dissolved oxygen concentrations within the chamber. The `gill' consists of approximately 12.8 m of silicone tubing through which chamber water is pumped by a controlled submersible peristaltic pump. Via turning this pump on and off, benthic oxygen demand can be determined at various times after deployment; ambient oxygen levels over the sediment can be manipulated so that the influences of hypoxic and anoxic conditions can be determined. Because the tubing is impermeable to charged ions, long-term deployments, at close to ambient internal oxygen levels, also make possible the measurement of fluxes of dissolved components whose fractional rate of change is much less than that of oxygen.

Although calcite is a common vein-filling mineral in rocks, there has been little understanding of the kinetics of vein-filling that gives a quantitative estimation of the fluid volume and time needed for calcite vein formation in subsurface fracture systems. A new experimental system capable of simulating calcite precipitation in veins was used to determine the influences of flow velocity and initial solution saturation state on the quantitative precipitation pattern of calcite in synthetic veins. Results are in relatively good agreement with the model predictions. Use of a Mn2+ tracer indicated that even at low concentrations manganese can have a substantial effect on the morphology of the calcite precipitate. Our model predicts that an extremely large volume of fluid is required to produce calcite veins in nature; typically 105 to 106 for a fluid/calcite volume ratio for formation of thin short veins. The model also predicts the time necessary for vein filling is thousands to millions of years, and is highly variable depending on factors such as fluid saturation state, flow velocity, vein dimension, and the precipitation kinetics that can differ under subsurface conditions.

Experimental evidence suggests that formation of metal sulfides in anoxic sediments limits the bioavailability of several toxic elements. Our ability to quantify the processes by which these metal sulfides form is dependent upon our ability to determine the speciation of solid phase metals in sediments. Our work indicates that an entire suite of Cu-Fe and Ni-Fe sulfide minerals can form upon the exposure of mackinawite to aqueous Cu and Ni. Furthermore, we have demonstrated that the solubility of pure metal sulfide minerals and their iron-metal derivatives in HCl directly correlates with the observed trends in the ''Degree of Trace Metal Pyritization'' in natural sediments. Current extraction schemes cannot distinguish discrete trace metal sulfides from trace metals associated with pyrite.

The solid phase chemical speciation of Fe, Cu, Ni, and Zn was investigated in two different biogeochemical environ ments in Chesapeake Bay to quantify the seasonal variability in the concentration of sulfide minerals and associated trace metals in the top 10 cm of sediment. When acid volatile sulfides (AVS) constitute less than 20% of HCl-soluble Fe (“reactive” Fe), the system is dynamic with respect to metals, and the formation and destruction of AVS minerals can potentially result in statistically significant seasonal shifts in both the flux of Cu across the sediment−water interface and the sedimentary concentration of Cu and Zn. When greater than 20% of HCl-soluble Fe is bound within AVS minerals, there is enough sulfide to effectively trap metals within the sediments, and temporal variations in AVS and pyrite do not necessarily result in temporal variations in the concentration of diagenetically available trace metals. Analysis of the ratio of reactive Fe to AVS can potentially increase the reliabilit...

Relatively soon (∼0.2 Ga) after the Earthformed, it is likely that major oceans appeared in ahot (∼100°C) reducing environment where carbondioxide was probably the dominant atmospheric gas,with PCO2, values reaching perhaps in excess of 10atm. During the Hadean Eon between 4.3 and 3.8 Ga BP,major changes in the concentration of atmosphericCO2 and associated temperature changes had aprofound influence on the carbonate geochemistry ofthe Hadean Ocean. Although no rocks are known to havesurvived prior to the Archean Eon, it is stillpossible to calculate approximate values for importantseawater parameters during the Hadean Eon based onother sources of information and reasonableassumptions about processes such as weatheringreactions. Our calculations are based on a linear temperaturechange from 100°C to 70°C and logPCO2 change from 1 to -1.5 over the Hadean Eon. Over this range in temperature and P CO2, theinfluence of T is relatively small, but changes inP CO2 result in large compositional variations inthe carbonate chemistry of Hadean seawater. In theearly Hadean, seawater pH was probably about5.8 ± ∼ 0.2, DIC may have reached close to 130 mM,and alkalinity was perhaps close to 30 mM. By thelate Hadean, seawater pH probably had changed to closeto neutral (∼6.8), and DIC and alkalinity were closerto present-day values. Even large uncertainties inNa+ + Cl-, K+ and Mg2+concentrations produce relatively small uncertaintiesin our calculated values for the carbonic acid system. However, larger uncertainties result from reasonableranges for Ca2+ concentrations and the saturationstate of Hadean seawater with respect to calcite. Our calculations support the hypothesis that acarbonate chemistry of seawater roughly similar tothat of modern oceans could have been acquired veryearly in Earth history. If seawater composition werebuffered by reactions involving carbonates andsilicates, then the composition of late Hadean-earlyArchean seawater was not vastly different from that oftoday. Thus, by the conclusion of the Hadean Eon, ifnot before, environmental conditions at the Earth's surface, including temperature and seawatercomposition, were sufficiently equable for theevolution of life, including the Archaebacteria: theextreme halophiles and thermophiles and methanogens.Contrary to the hypothesis of an early Na-bicarbonateocean, our calculations suggest the possibility thatthe early oceans of Earth were a NaCl-dominatedaqueous solution, with somewhat higher DIC andalkalinity concentrations, higher saturation state,and the possibility of lower calcium concentrations.The time course of approach of Hadean seawater to acarbonate composition closer to that of today isdifficult to predict. It is distinctly possible thatthe concentration of calcium in seawater did not reachlevels like that of modern seawater until the latePrecambrian and thus constrained the timing of the"Big Bang" of organic evolution, the emergence of theshelled invertebrates at the beginning of thePhanerozoic.

The first edition of this chapter provided a brief review of the geochemistry of carbonate minerals, summarized general aspects of their thermodynamics and the kinetics of their precipitation and dissolution, and then used the natural division between deep-sea marine carbonates (chalks and pelagic muds) versus shoal-water carbonates (skeletal and nonskeletal materials) to organize details of the formation, distribution, and subsequent diagenetic alteration of these phases. A substantial focus in this treatment was thus the description of the behavior and fate of carbonate minerals in natural settings. In this chapter, this latter material appears largely unchanged. The aim in this revision is not simply to trudge through calcite, dolomite, and magnesite rate equations, detailed reviews of which are available elsewhere; instead, the intent is to provide background for the selective discussion of recent work on carbonate mineral surfaces (mostly calcite).

To determine the opening and precipitation history and characteristics of vein-forming fluids, analyses of oxygen and carbon isotopes and trace elements were carried out on multilayered crack-seal calcite veins in the Austin Chalk Formation near San Antonio, Texas. The veins developed within the normal fault zones possessing unique chemical and textural characteristics which indicate sequential vein development. They are composed of alternating millimeter- to submillimeter-thick calcite veinlets and host lithons, occasionally crosscut by coarse, equant-grained secondary calcite veins. Regular changes in δ18O (e.g., −2.6 to −5.6‰, Pee Dee belemnite (PDB)) of the calcite veinlets along the length of veins suggest that the individual calcite veinlets were sequentially developed. A systematic δ18O decrease in the vein opening direction primarily resulted from a continuous increase in temperature of the ascending fluids delivered to the Austin Chalk. Relatively constant δ13C (approximately +1.4±0.4‰, PDB) for the multilayered veins and most secondary veins indicates that the composition of fluids from which the calcite veins precipitated was initially buffered by the bulk chalk. There is no spatial variation in trace element composition of the calcite veinlets along the length of veins. Low Sr concentrations in both calcite veinlets and secondary veins relative to those of the host chalk reflect a low partition coefficient of Sr in calcite during vein formation. Normal faults in the Cretaceous Austin Chalk were conduits to upwardly mobile vein-forming fluids.

In the paper (Wang and Morse, 1996) that preceded this study, we presented results of experiments performed using a silica gel crystal growth technique to produce pyrite under conditions approximating those commonly occurring in anoxic marine sediments. The primary focus of that study was on the chemical pathways that pyrite formation follows and how differing conditions influenced reaction kinetics and morphology of pyrite crystals. In this paper, we present results of further long-term (up to 1 y) studies of pyrite formation, using the silica gel experimental technique, in which we investigated the role that different precursor iron (hydr)oxide minerals and marine organic matter play in pyrite formation. The minerals studied were akaganeite (β-FeOOH), ferrihydrite (Fe5HO8 · 4H2O), goethite (α-FeOOH), hematite (α-Fe2O3), lepidocrocite (γ-FeOOH), and magnetite (Fe3O4). Marine organic matter used in this study was freeze-dried plankton collected from near-surface water in the Gulf of Mexico. The influence of precursor iron (hydr)oxide mineralogy, although important for initial iron sulfidization rates, was relatively minor compared to other variables, such as solution pH and sulfide concentration, in controlling the rate of pyrite formation. Consequently, major variations in the observed rate of pyritization of different iron (hydr)oxide minerals in sediments (e.g., Canfield and Berner, 1987) may reflect large differences in surface areas of the minerals rather than their intrinsic reactivity and is a confirmation of the estimates of Canfield et al. (1992) that most iron oxides have similar reactivity. The presence of marine organic matter (freeze-dried plankton) caused an increase in the sulfidization rate of goethite and a major (about 20 ×) decrease in the rate of pyrite formation. This can be interpreted as indicating that organic matter-iron interactions are important in both iron (hydr)oxide dissolution, and pyrite nucleation and growth. A possible explanation for this behavior is that dissolved organic matter produced during the long experiments (up to 1 year) increased the rate of goethite dissolution while inhibiting pyrite nucleation and growth by complexing iron. The lessons learned in the study of other mineral reaction kinetics (e.g., calcite and aragonite), that rates determined in pure inorganic systems, may not always be reliably applied to natural systems where organic matter can significantly influence mineral dissolution and growth rates, are, alas, repeated here for pyrite.

Calcite is commonly found as a vein-filling mineral in rocks. However, the factors controlling its deposition are complex and not well understood in quantitative terms. In order to advance our understanding of the processes involved, we have refined the model for calcium carbonate mass transport in subsurface carbonate rocks of Morse and Mackenzie (1993) and developed a new experimental technique to test it. This technique uses a flow-through reactor that simulates a vein opening. Agreement was observed between model predictions and experimental observations for the deposition of calcite in synthetic veins. The influences of surface area to solution volume ratio, solution saturation state with respect to calcite, and flow velocity were well predicted by the model. The model predicts that in order to have a fairly uniform deposition of calcite within a vein, solution flow must be quite rapid (tens to thousands of cm h−1) or the solution must be only slightly supersaturated with respect to calcite. A low degree of solution supersaturation demands what may be unreasonably large volumes of solution flow to achieve vein filling for the vein configuration we have studied.

Experiments were performed utilizing the silica gel technique to synthesize pyrite crystals in four different chemical systems, at room temperature (ca. 23 °C), for periods of up to 24 months. These systems were: FeS(1 − x) (mackinawite) + S(s) ° + H2S(aq), Fe3S4 (greigite) + H2S(aq). FeOOH(goethite) + H2S(aq) and Fe(aq)2 + + S4(aq)2 −. Results indicate that the rate of pyrite formation strongly depends on the nature of solid reactant and solution chemistry (pH and total sulfide concentration). The formation of pyrite is most rapid in the presence of greigite. The pyritization of metastable iron sulfide minerals follows a dissolution-precipitation pathway. Two mechanisms are proposed for the nucleation behavior of pyrite on metastable iron sulfide minerals. The first mechanism is heterogeneous nucleation. This is a result of epitaxial overgrowth of pyrite nuclei at defects on a precursor mineral's surface. A second possible mechanism is that pyrite clusters, smaller than the critical nucleus, are concentrated on the surface of precursor iron sulfide minerals under the influence of crystal molecular field. This forms a transition layer between the precursor sulfide mineral and the bulk solution where nucleation of pyrite is accelerated. SEM observations indicate that the morphology of pyrite crystals formed at room temperature is primarily controlled by the degree of supersaturation in the solution from which pyrite is precipitated. With the increasing supersaturation, pyrite morphology changes in the following order: cube → octahedron → spherulite. The influence of other parameters, such as relative abundance of Fe2 + and dissolved zero-valance sulfur, were not observed to be of major significance for morphology.

The weight ratio of organic carbon to pyrite sulfur (C/S) has been observed to fall within a relatively narrow range (2.8 ± 0.8) in most fine-grained normal marine siliciclastic sediments. Although an increasing number of exceptions to this typical C/S ratio have been observed, it is nonetheless remarkable that so many modern and ancient marine sediments fall within a range of about a factor of two in C/S ratios. Here we demonstrate that this demands that the three major factors that control this ratio must be quite closely coupled to each other. These factors are (1) the fraction of organic carbon that is metabolized, (2) the fraction of metabolized organic carbon that is metabolized via sulfate reduction, and (3) the fraction of reduced sulfide that is not oxidized and is buried in pyrite. A simple model is presented that has utility in demonstrating the relationships among these parameters that are necessary to produce typical normal marine sediment C/S ratios.

Metal complexes of trimethylamine carboxyborane successfully suppressed calcium flux from both paired pup calvaria bones and rat UMR-106 osteosarcoma cultured cells over a 48 h period. These agents increased uptake of calcium into the cell cultures and accelerated [ 3 H]proline incorporation into collagen. Copper and iron complexes of the trimethylamine carboxyborane were more potent compared with the cobalt and chromium complexes. The agents effectively reduced lysosomal enzyme activity and also proteolytic enzyme activities of macrophages. Since macrophages invade the bone surface and assist in the demineralization and digestion of collagen, those agents may be potentially useful to retard diseases involving bone reconstruction. Influx of white blood cells and macrophages to sites of degradation most probably would be inhibited by the agents, based on sponge test observations in mice. Osteoporosis induced by ovariectomy was minimized by injections of tetrakis[μ-(trimethylamine-borane-carboxylato)-bis(trimethylamine-carboxyborane)-dicopper(II) into rats at 3.5 mg kg -1 day -1 for 14 days. Bone volume, density, weight and calcium content returned to normal baseline control values. In addition, the copper complex returned serum calcium, serum parathyroid hormone (PTH) and vitamin D 3 values to normal levels. One possible mode of action of these derivatives is the regulation of the production and release of chemical mediators initiating bone loss, e.g. tumor necrosis factor, TNFα and interleukins I1 or I1-2

Acyclic amine-carboxyboranes were effective anti-inflammatory agents in mice at 8 mg/kg x 2. These amine-carboxyboranes were more effective than the standard indomethacin at 8 mg/kg x 2, pentoxifylline at 50 mg/kg x 2, and phenylbutazone at 50 mg/kg x 2. The heterocyclic amine derivatives as well as amine-carbamoylboranes, carboalkoxyboranes, and cyanoboranes were generally less active. However, selected aminomethyl-phosphonate-N-cyanoboranes demonstrated greater than 60% reduction of induced inflammation. The boron compounds were also active in the rat induced edema, chronic arthritis, and pleurisy screens, demonstrating activity similar to the standard indomethacin. The compounds were effecive in reducing local pain and decreased the tail flick reflex to pain. The derivatives which demonstrated good anti-inflammatory activity were effective inhibitors of hydrolytic lysosomal, and proteolytic enzyme activities with IC50 values equal to 10-6M in mouse macrophages, human leukocytes, and Be Sal osteofibrolytic cells. In these same cell lines, the agents blocked prostaglandin cyclooxygenase activity with IC50 values of 10-6M. In mouse macrophage and human leukocytes, 5′ lipoxygenase activity was also inhibited by the boron derivatives with IC50 values of 10-6M. These IC50 values for inhibition of these enzyme activities are consistent with published values of known anti-inflammatory agents which target these enzymes.

The metal complexes and salts of amine carboxyboranes were shown to be potent anti-inflammatory agents in rodents at 8 mg kg−1. They were effective in blocking induced edema, pleurisy and endotoxic shock while blocking both local and central pain processes. The ability of the agents to function as anti-inflammatory agents is multi-fold. First, lysosomal enzymes of specific cells, e.g. macrophages, were inhibited with IC50 values in the range of 10−6M. Collagenase I and II activities were inhibited with IC50 values approximately equal to 10−4M. The anti-inflammatory activity of these agents at the molecular levels appears to be due to inhibition of the release of TNFα and Il-1 from macrophages which indirectly control chemotaxic migration of white blood cells as indicated by the suppression of PMN and macrophage invasion into sponges implanted subcutaneously (SC) in mice. Furthermore, in these invading cells, the agents' blockage of TNFα and Il-1 or Il-2 release down-regulates prostaglandin and leukotriene enzymatic synthetic rates and, consequently, their release, resulting in a reduction of the inflammation process.

Trace metals that are mobilized during early diagenesis can be coprecipitated with and adsorbed on authigenic sulfide minerals in anoxic sediments. A major portion (generally > 60%) of the non-silicate bound fraction of potentially toxic metals, such as As, Cu and Hg, is commonly coprecipitated with pyrite near the sediment-water interface. Traditional methods (e.g. 1 N HCl or citrate dithionite extractions) for determining the concentration of potentially bioavailable metals in sediments do not include the pyrite bound fraction. However, pyritized metals can be potentially bioavailable if the pyrite is oxidized. Pyrite oxidation commonly occurs during sediment resuspension, seasonal migration of the redoxocline in sediments and when dredge spoils are dumped on land. Experimental measurements of metal release from pyrite, in initially anoxic sediments exposed to oxic seawater, indicate that a major portion (20% to over 90%) of the pyrite bound metals can be released in a day or less. The As, Cu and Hg released from pyrite usually exceeds the concentration of their potentially bioavailable fraction determined by traditional methods. Consequently, the pyritization-depyritization of trace metals is probably an important process in controlling the bioavailability of many important trace metals.

The metal complexes of amine-carboxyborane including copper, chromium, zinc, calcium amd cobalt were effective hypolipidemic agents lowering both serum cholesterol and triglyceride levels significantly in mice at 8 mg/kg/day, I.P. after 16 days. The agents reduced acetyl CoA synthetase, ATP-dependent citrate lyase, acyl CoA cholesterol acyl transferase, sn¯-glycerol-3-phosphate acyl transferase activities of rat liver and small intestinal mucosa after 14 days treatment. The neutral cholesterol ester hydrolase activity was elevated by the agents in both tissues. The metal complexes altered lipid levels in the bile of rats after treatment as well as the bile acid composition after 14 days administration, orally. The agents blocked enterohepatic absorption of cholesterol from rat isolated intestinal loops.

The interactions of divalent metal ions with the common metastable sedimentary iron monosulfide mineral mackinawite were investigated in dilute solutions and seawater. The surface affinities of metals whose sulfide phases are more soluble than mackinawite are similar, whereas those metals whose sulfide phases are less soluble than mackinawite exhibit an increasing surface affinity with decreasing solubility. Adsorption of metals from dilute solutions and seawater was the same, within our experimental uncertainty, and not strongly sensitive to temperature. The partition coefficients of metals in mackinawite strongly increase with decreasing metal solubility and are very temperature sensitive. These results are interpreted as indicating that mackinawite is potentially a major temporary sink for many trace metals during early diagenesis in anoxic sediments.

CO 2 solubility, and the first and second stoichiometric dissociation constants of carbonic acid ( K 1 ∗ and K 2 ∗ ), have been determined in aqueous solutions containing Cl and SO 4 salts of Na, K, Ca and Mg over a wide range of ionic strength from 0 to 90°C at 0.1032 MPa (1 atm) pressure. Activity coefficients for CO 2(aq) , HCO 3 − and CO 3 2 − ions were calculated from m co 2 , K 1 ∗ and K 2 ∗ . These activity coefficients were then used to derive Pitzer parameters for the interaction between these carbonate species and other ions between 0 and 90 °C. The resulting Pitzer model for the carbonic acid system was verified by its ability to accurately predict experimentally determined calcite solubility in brines from 0 to 90°C. However, calcite solubility measurements in complex brines initially supersaturated with respect to calcite indicate that Na + , Mg 2+ and SO 4 2− ions are probably incorporated into calcite lattice and significantly increase calcite solubility in brines.

Simple models for carbonate transport in subsurface sedimentary environments that do not carefully consider solution chemistry are unable to produce even order of magnitude predictions of the volume of water or time needed to transport calcium carbonate into or out of a carbonate rock unit. We feel that the time intervals necessary for creation or destruction of porosity cannot usually be well established, because the solution chemistry (e.g., PCO2) and flow velocities of ancient waters are generally not determinable. Our calculations also indicate that major changes in porosity of medium- to large-sized carbonate rock units cannot take place by transporting a significant fraction of the calcium carbonate into or out of the rock unit. This conclusion is based on both the time needed for sufficient volumes of water to flow through the unit, and the probable reaction kinetics between the solution and rock that will usually result in relatively sharp reaction fronts. Based on these considerations, we suggest that most changes in porosity in carbonate rocks are the result of the redistribution of material within the rocks, rather than massive importation or exportation of calcium carbonate. However, in some sequences, allochthonous cementation of limestones may occur, because of mobilization of CaCO3 brought about by high effective tectonic stresses and high pore-water fluid pressures.

The influences of temperature, salinity and PCO2 on the pseudo-homogeneous nucleation of calcium carbonate from natural seawater were investigated to determine if whitings could be produced by this mechanism under ‘reasonable’ natural conditions. Increasing temperature and salinity substantially raises the PCO2 at which calcium carbonate nucleation can occur. The saturation state of the water, at which precipitation takes place, is essentially independent (range 20–25 times with respect to calcite) of temperature and salinity, from 25 to 40°C and for S = 35–70 psu. These findings indicate that formation of whitings on the Great Bahama Bank are unlikely to be due to the nucleation of calcium carbonate. However, whitings observed near the Trucial Coast of the Persian Gulf could possibly be the result of direct nucleation of calcium carbonate in waters of lowered PCO2 associated with phytoplankton blooms.

The metastable iron sulfide mineral mackinawite commonly forms in anoxic sediments in the region where elevated concentrations of dissolved Mn2+ are also observed. There is, consequently, a potential for significant interactions between dissolved Mn and mackinawite. We have experimentally investigated adsorption of Mn2+ on mackinawite and its coprecipitation with mackinawite using chemostat techniques. The very fine-grained nature of mackinawite results in adsorption being quantitatively (generally > 90%) much more important than coprecipitation. Adsorption of Mn2+ follows a Langmuir-type isotherm and decreases substantially with increasing salinity. The partition coefficient for Mn2+ with mackinawite, in low ionic strength solutions at 25°C extrapolated to zero reaction rate, is 4.8 (±0.5) × 103. It approximately doubles for every 20°C increase in temperature, and, over the range of conditions investigated, was found to be close to independent of pH and the mole fraction of MnS. The partition coefficient is 1.8 (±0.35) × 10−3 in seawater of S = 35 at 25°C at a slow reaction rate (0.5 μmol L−1 min.−1). It also decreases by about a factor of 3 at high precipitation rates, possibly because of the formation of a greater proportion of amorphous-FeS rather than mackinawite or an inability of the sorption reaction to keep up with the rate of formation of new surfaces.

Prediction of the solubility of the evaporite minerals halite, gypsum, and anhydrite in brines has numerous scientific and practical applications. This paper presents a Pitzer equation-based model for predicting the solubility of these minerals in Na + -K + -H + -Ca 2+ -Mg 2+ -Cl − -OH − -SO 4 2− -H 2 O solutions of varying composition as a function of temperature and pressure. Model predictions compare well with experimental observations. As an example of the utility of this program, the volume of CaSO 4 precipitated as a brine flows up a pipe from 6000 m depth is predicted. This is done as a function of flow distance for different cooling rates of the brine. Results indicate that if the brine can be cooled to near the geothermal gradient no precipitation will take place.