Sulfide-methane transition

Figure 8.35 shows the redox state and acidity of the main types of seawaters. The redox state of normal oceanic waters is almost neutral, but they are slightly alkaline in terms of pH. The redox state increases in aerated surface waters. Seawaters of euxinic basins and those rich in nutrients (eutrophic) often exhibit Eh-pH values below the sulfide-sulfate transition and below carbonate stability limits (zone of organic carbon and methane cf figure 8.21). We have already seen (section 8.10.1) that the pH of normal oceanic waters is buffered by carbonate equilibria. At the normal pH of seawater (pH = 8.2), carbonate alkalinity is 2.47 mEq per kg of solution. 

Sulfur reaches the sediment essentially only by means of diagenetic processes. Here, the fixation of sulfate-bound sulfur in the form of barite in the sulfate-methane transition (SMT) zone produces an interesting signal, but is quantitatively irrelevant on account of the low barium concentrations in the pore water. Sulfur precipitates in much greater amounts in shape of sulfide (Mackinawite FeS, pyrite FeS ) wherever sulfide resulting from the reduction of sulfate comes into contact with divalent iron by means of diffusion. In the core shown in Figure 3.29 this obviously proceeded at a depth of about 7 m below the sediment surface over a relatively long period of time. 

Fig. 14.22 Schematic illustration of gas hydrate deposits and biogeochemical reactions in near-surface sediments on southern Hydrate Ridge. High gradients in pore water sulfate and methane are typical of methane hydrate-rich environment close to sulfate-rich seawater. At the sulfate-methane interface (also named sulphate-methane transition in earlier chapters of the book) a microbial consortium of methanothrophic archaea and sulfate-reducing bacteria (Boetius et al. 2000) perform anaerobic oxidation of methane (AOM) leading to carbonate precipitation. AOM rates influence hydrogen sulfide fluxes and gradients, which are reflected on the seafloor by the distribution of vent communities around active gas seeps and gas hydrate exposures (Sahling et al. 2002).

In molecules containing atoms belonging to the second row of the periodic system the situation is in most cases similar to that found with molecules having the related precursor atom as the most characteristic atom. The lowest frequency bands of sulfides are usually of valence-shell or mixed type like those of oxides. In silane the first band is due to a transition to a relatively small 4s Rydberg orbital, in analogy to methane s 3s - ... 

Whereas the Mobil process starts with syn gas based methyl alcohol, Olah s studies were an extension of the previously discussed electrophilic functionalization of methane and does not involve any zeolite-type catalysts. It was found that bifunctional acidic-basic catalysts such as tungsten oxide on alumina or related supported transition metal oxides or oxyfluorides such as alumina or related supported transition metal oxides or oxyfluorides such as tantalum or zirconium oxyfluoride are capable of condensing methyl chloride, methyl alcohol (dimethyl ether), methyl mercaptan (dimethyl sulfide), primarily to ethylene (and propylene) (equation 65) . 

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