Marine sediments, sulfidic

The case of bacterial reduction of sulfate to sulfide described by Berner (1984) provides a useful example. The dependence of sulfate reduction on sulfate concentration is shown in Fig. 5-4. Here we see that for [SO ] < 5 mM the rate is a linear function of sulfate concentration but for [SO4 ] > 10 itiM the rate is reasonably independent of sulfate concentration. The sulfate concentration in the ocean is about 28 mM and thus in shallow marine sediments the reduction rate does not depend on sulfate concentration. (The rate does depend on the concentration of organisms and the concentration of other necessary reactants - organic carbon in this case.) In freshwaters the sulfate concentration is... 

Copper concentrations in sediment interstitial pore waters correlate positively with concentrations of dissolved copper in the overlying water column and are now used to predict the toxicity of test sediments to freshwater amphipods (Ankley et al. 1993). Sediment-bound copper is available to deposit-feeding clams, especially from relatively uncontaminated anoxic sediments of low pH (Bryan and Langston 1992). The bioavailability of copper from marine sediments, as judged by increased copper in sediment interstitial waters, is altered by increased acid volatile sulfide (AYS)... 

Casas, A.M. and E.A. Crecelius. 1994. Relationship between acid volatile sulfide and toxicity of zinc, lead and copper in marine sediments. Environ. Toxicol. Chem. 13 529-536. 

Zheng Y, Anderson RF, van Geen A, Kuwabara J (2000a) Authigenic molybdenum formation in marine sediments A link to pore water sulfide in the Santa Barbara Basin. Geochim Cosmochim Acta 64 4165-4178... 

Each of these solid phases can be described in terms of their mineralogy. This classification scheme is based on crystal structure and chemical composition. The most common minerals found in marine sediments are listed in Table 13.2. Most are silicates in which Si and O form a repeating tetrahedral base unit. Other minerals common to marine sediments are carbonates, sulfates, and oxyhydroxides. Less common are the hydrogenous minerals as they form only in restricted settings. These include the evap-orite minerals (halides, borates, and sulfates), hydrothermal minerals (sulfides, oxides, and native elements, such as gold), and phosphorites. 

There may be a cycling of S compounds of different oxidation state between anaerobic and aerobic zones in the soil, such as at the soil—floodwater interface. In reduced lake and marine sediments this leads to accumulation of insoluble sulfides as S04 carried into the sediment from the water above is immobilized. Such deposits function as sinks for heavy metals. Plants absorb S through their roots as S04 H2S is toxic to them. Therefore HS must be oxidized to S04 in the rhizosphere before it is absorbed. 

Limnofix In situ Sediment Treatment (LIST) technology is offered by Limnofix, Inc., a Colder Associates Company. The technology allows for the in situ treatment of contaminated sediment in surface waters. LIST enhances bioremediation of organic contaminants oxidizes sediments to control odor, nutrient release, or sulfide toxicity and produces stable marine sediment surfaces via consolidation and flocculation. 

The sulfur-oxidizing bacteria. Anaerobic conditions prevail in marine sediments, in poorly stirred swamps, and around hydrothermal vents at the bottom of the sea. Sulfate-reducing bacteria form high concentrations (up to mM) of H2S (in equilibrium with HS and s2-)318-320 This provides the substrate for bacteria of the genus Thiobacillus, which are able to oxidize sulfide, elemental sulfur, thiosulfate, and sulfite to sulfate and live where the aerobic and anaerobic regions meet.311 321-323 Most of these small gram-negative... 

Iron frequently has been postulated to be an important electron acceptor for oxidation of sulfide (58, 84,119, 142, 152). Experimental and theoretical studies have demonstrated that Fe(III) will oxidize pyrite (153-157). Reductive dissolution of iron oxides by sulfide also is well documented. Progressive depletion of iron oxides often is coincident with increases in iron sulfides in marine sediments (94, 158, 159). Low concentrations of sulfide even in zones of rapid sulfide formation were attributed to reactions with iron oxides (94). Pyzik and Sommer (160) and Rickard (161) studied the kinetics of goethite reduction by sulfide thiosulfate and elemental S were the oxidized S species identified. Recent investigations of reductive dissolution of hematite and lepidocrocite found polysulfides, thiosulfate, sulfite, and sulfate as end products (162, 163). 

Stability of iron sulfides in lake sediments has not been thoroughly examined. Pyrite undergoes dynamic seasonal oxidation in salt marshes and coastal marine sediments (142, 184, 191-195). Pyrite oxidation is mediated... 

Availability of reactive iron in sediments also has been postulated to control S retention. Reactive iron may limit fluxes of S recycled from sediments by rendering sulfide immobile and less amenable to oxidation by bacteria or chemical agents. Availability of iron strongly influences total S content and isotopic signature of marine sediments (198). Canfield (94) ob-... 

Iron is a common constituent of marine sediments. Magnetite is found in beach sands and iron is coituuoti in glauconitic manne silicates. Iron oxides and sulfides occur where anaerobic conditions and elevated temperatures arc found, as in the hot. salty brines found near rifts. Iron is a major constituent of the ferromanganese nodules. 

Table 3.8 lists the arsenic concentrations of different types of marine and estuary sediments from various locations. Overall, low organic-carbon carbonate muds, oxidizing sands, and coarser-grained sediments have relatively little arsenic. In contrast, reducing marine sediments may contain as much as 3000 mg kg-1 of arsenic (Mandal and Suzuki, 2002), 202. Arsenic also tends to be enriched in fine-grained silicate-rich sediments, such as deep-sea clays and marine muds. In most cases, arsenic-rich sediments contain abundant arsenic-accumulating (oxy)(hydr)oxides, organic matter, or sulfides. 

Like estuary and marine sediments, arsenic may also migrate into deeper sediments where bacteria reduce sulfate to sulfide (Figure 3.4). In such highly reducing environments, arsenic might be incorporated into relatively stable sulfide minerals (Ford, Wilkin and Hernandez, 2006). 

The anaerobic metabolism of acrylate and 3-mercaptopropionate (3-MPA) was studied in slurries of coastal marine sediments. The rate of these compounds is important because they are derived from the algal osmolyte dimethylsulfoniopropionate (DMSP), which is a major organic sulfur compound in marine environments. Micromolar levels of acrylate were fermented rapidly in the slurries to a mixture of acetate and propionate (1 2 molar ratio). Sulfate-reducing bacteria subsequently removed the acetate and propionate. 3-MPA has only recently been detected in natural environments. In our experiments 3-MPA was formed by chemical addition of sulfide to aciylate and was then consumed by biological processes. 3-MPA is a known inhibitor of fatty acid oxidation in mammalian systems. In accord with this fact, high concentrations of 3-MPA caused acetate to accumulate in sediment slurries. At lower concentrations, however, 3-MPA was metabolized by anaerobic bacteria. We conclude that the degradation of DMSP may ultimately lead to the production of substrates which are readily metabolized by microbes in the sediments. 

The Michael addition mechanism, whereby sulfur nucleophiles react with organic molecules containing activated unsaturated bonds, is probably a major pathway for organosulfur formation in marine sediments. In reducing sediments, where environmental factors can result in incomplete oxidation of sulfide (e.g. intertidal sediments), bisulfide (HS ) as well as polysulfide ions (S 2 ) are probably the major sulnir nucleophiles. Kinetic studies of reactions of these nucleophiles with simple molecules containing activated unsaturated bonds (acrylic acid, acrylonitrile) indicate that polysulfide ions are more reactive than bisulfide. These results are in agreement with some previous studies (30) as well as frontier molecular orbital considerations. Studies on pH variation indicate that the speciation of reactants influences reaction rates. In seawater medium, which resembles pore water constitution, acrylic acid reacts with HS at a lower rate relative to acrylonitrile because of the reduced electrophilicity of the acrylate ion at seawater pH. 

Formation of Pyrite. Iron is carried to the peat swamp, before seawater transgression, as ferric oxide and hydroxides adsorbed on fluvial clays (123). During early diagenesis in a reducing environment, ferric iron is reduced to ferrous, which reacts with hydrogen sulfide to form iron monosulfide. If the basic mechanism of pyrite formation is similar to that in marine sediments... 

Depositional and Diagenetic Behavior of Sulfur. The key controls on sulfur behavior in freshwater-lake, saline-lake, and marine sediments include the concentration of reactants for bacteriogenic H2S formation and sulfide-mineral formation. Table IV illustrates the relative importance of the independent reactants (organic matter, dissolved sulfate, and iron) in each of the lacustrine environments studied. We have shown that in saline... 

Aller R.C. and Rude P.D. (1988) Complete oxidation of solid phase sulfides by manganese and bacteria in anoxic marine sediments. Geochim. Cosmochim. Acta 52, 751-765. 

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