Porewater Sulfide Research Articles

Geochemistry of inorganic and organic sulphur in organic-rich sediments from the Peru Margin

We have determined (1) the abundance and isotopic composition of pyrite, monosulphide, elemental sulphur, organically bound sulphur, and dissolved sulphide; (2) the partition of ferric and ferrous iron; (3) the organic carbon contents of sediments recovered at two sites drilled on the Peru Margin during Leg 112 of the Ocean Drilling Program. Sediments at both sites are characterised by high levels of organically bound sulphur (OBS). OBS comprises up to 50% of total sedimentary sulphur and up to 1% of bulk sediment. The weight ratio of S to C in organic matter varies from 0.03 to 0.15 (mean = 0.10). Such ratios are like those measured in lithologically similar, but more deeply buried petroleum source rocks of the Monterey and Sisquoc formations in California. The sulphur content of organic matter is not limited by the availability of porewater sulphide. Isotopic data suggest that sulphur is incorporated into organic matter within a metre of the sediment surface, at least partly by reaction with polysulphides.Most inorganic Sulphur occurs as pyrite. Pyrite formation occurred within surface sediments and was limited by the availability of reactive iron. But despite highly reducing sulphidic conditions, only 35–65% of the total iron was converted to sulphide; 10–30% of the total iron still occurs as Fe (III). In surface sediments, the isotopic composition of pyrite is similar to that of both iron monosulphide and dissolved sulphide. Either pyrite, like monosulphide, formed by direct reaction between dissolved sulphide and detrital iron, and/or the Sulphur species responsible for converting FeS to FeS2 is isotopically similar to dissolved sulphide. Likely stoichiometries for the reaction between ferric iron and excess sulphide imply a maximum resulting FeS2 :FeS ratio of 1:1. Where pyrite dominates the pool of iron sulphides, at least some pyrite must have formed by reaction between monosulphide and elemental sulphur and/or polysulphide. Elemental sulphur (S0) is most abundant in surface sediments and probably formed by oxidation of sulphide diffusing across the sediment-water interface. In surface sediments, S0 is isotopically heavier than dissolved sulphide, FeS and FeS2 and is unlikely to have been involved in the conversion of FeS to FeS2. Polysulphides are thus implicated as the link between FeS and FeS2.

Reexamination of Pore Water Sulfide Concentrations and Redox Potentials Near the Aerial Roots of Rhizophora mangle and Avicennia germinans

Soil redox potentials and pore water sulfide concentrations on a mangrove island in the Belizean barrier reef system were significantly correlated with the presence of the aerial roots of mangrove trees. Sulfide concentrations were three to five times lower near the prop roots of Rhizophora mangle (red mangrove) and the pneumatophores of Avicennia germinans (black mangrove) than in adjacent (≤ 1 meter away) unvegetated sediment. Soil redox potentials were also significantly higher near the aerial roots. A comparison of the two species revealed that sulfide concentrations in the rhizosphere of R. mangle were as low as that of A. germinans. However, sulfide concentrations in areas occupied by the black mangrove were variable and a function of pneumatophore density. The occurrence of an oxidized rhizosphere around the roots of both species suggests that the adult trees are equally capable of exploiting reduced sediments as long as their respective pathways for root aeration are functional.

REEXAMINATION OF PORE WATER SULFIDE CONCENTRATIONS AND REDOX POTENTIALS NEAR THE AERIAL ROOTS OF RHIZOPHORA MANGLE AND AVICENNIA GERMINANS

Soil redox potentials and pore water sulfide concentrations on a mangrove island in the Belizean barrier reef system were significantly correlated with the presence of the aerial roots of mangrove trees. Sulfide concentrations were three to five times lower near the prop roots of Rhizophora mangle (red mangrove) and the pneumatophores of Avicennia germinans (black mangrove) than in adjacent (≤ 1 meter away) unvegetated sediment. Soil redox potentials were also significantly higher near the aerial roots. A comparison of the two species revealed that sulfide concentrations in the rhizosphere of R. mangle were as low as that of A. germinans. However, sulfide concentrations in areas occupied by the black mangrove were variable and a function of pneumatophore density. The occurrence of an oxidized rhizosphere around the roots of both species suggests that the adult trees are equally capable of exploiting reduced sediments as long as their respective pathways for root aeration are functional.

Dissolution and pyritization of magnetite in anoxie marine sediments

Concentrations of magnetite were determined, with depth, in sediments of varying H 2S content taken from Long Island Sound (FOAM, NWC, and Sachem sites) and from a site in the rapidly depositing subaqueous portion of the Mississippi Delta. For the three Long Island Sound sites, dissolution of magnetite during burial is clearly demonstrated, and for all four sites the rate of magnetite dissolution is proportional to the concentration of dissolved pore water sulfide. By combining magnetite depth distributions with sedimentation rate data and measurements of dissolved sulfide, we find that magnetite dissolution is well described by the following rate law: dC mag dt = -1.1× 10 −5C 0.5 BC magA mag where C mag is the concentration of magnetite (grams per gram of sediment), C s is the concentration of dissolved sulfide (mM), and A mag is the specific surface area of magnetite in the sediment (cm 2 per gram of magnetite.) This equation means that, for a typical range of magnetite surface areas and dissolved sulfide concentrations, the “half-life” of magnetite in anoxic marine sediments ranges from about 50 to 1000 years. SEM observations of magnetite grains from the different sediment sites reveal that magnetite dissolution may be accompanied by extensive replacement by pyrite. This only occurs if magnetite is in contact with high concentrations of H 2S (>1 mM) for relatively long periods of time (several hundred years). With low concentrations of H 2S (<1 mM) magnetite dissolution occurs without identifiable pyrite replacement. We suggest that this feature of magnetite diagenesis may prove to be a useful paleosulfide indicator.

Association between pore water sulfide concentrations and the distribution of mangroves

At Humingbird Cay, Exuma, Bahamas, distributions of bothRhizophora mangle (red mangrove) andAvicennia germinans (black mangrove) are closely correlated with amounts of hydrogen sulfide (H2S) in the soil.R. mangle is primarily distributed within large areas of low to moderate H2S concentrations (mean= 40 mg/l). H2S levels underA. germinans are lower (mean = 22 mg/l), but the area immediately beyond their root zone often has extremely high sulfide concentrations (mean= 120 mg/l). These results suggest that past attempts to explain mangrove distribution in terms of monotonic soil gradients, the dispersal characteristics of propagules, and interspecific competition are incomplete, and that it will be necessary to examine the link between soil sulfur chemistry and mangrove distribution more fully.

Uptake of Dissolved Sulfide by Spartina alterniflora : Evidence from Natural Sulfur Isotope Abundance Ratios

The difference in the stable sulfur isotope ratios of sulfate and sulfide in marsh pore water was used to verify the uptake of hydrogen sulfide by the salt marsh cordgrass Spartina alterniflora in a North Carolina salt marsh. Most of the plant sulfur derived from pore-water sulfide was recovered as sulfate, an indication that the sulfide had been oxidized within the plant. The analysis of the sulfur isotope ratios of other coastal halophytes may be a useful technique for determining whether sulfide is taken up by plants in saline wetlands.