Thermal Sulfate Reduction

Two types of deposits, where the internal S-isotope variations fit the expected scheme of bacterial reduction, but where the biogenic nature was already known from other geological observations, are the sandstone-type uranium mineralization in the Colorado Plateau (Warren 1972) and the Kupferschiefer in Central Europe (Marowsky 1969), although thermal sulfate reduction may have occurred at the base of the Kupferschiefer (Bechtel et al. 2001). 

Reduction of sulfate by hydrocarbons (either biotic or thermal) occurs because their association in the aqueous diagenetic environment is thermodynamically unstable (25). Depending upon the exact reaction (Table II) and the presence or absence of geocatalysts, abiotic or thermal sulfate reduction/hydrocarbon oxidation can occur at temperatures as low as 100-140oC (25). 

In the presence of alkali metals, thermal sulfate reduction will result in precipitation of carbonate cements (mainly calcite and dolomite) or carbonate replacement of dissolving sulfates (gypsum/anhydrite) (2. The reaction of polysulfides with bicarbonate has been suggested as a cause of calcite precipitation. Further, transition and base metals present in formation waters during thermal sulfate reduction could lead to the deposition of disseminated or stratiform base... 

Decarboxylation of organic acid anions (i.e., alkalinity dominated by organic acid anions with elevated PC02) Decarboxylation and/or abiotic thermal sulfate reduction... 

The extent of H2S release by plants as the result of sulfate reduction is another unknown flux in the sulfur cycle. Relatively rapid reduction of sulfate and thiosulfate to HjS by a thermophilic blue-green alga Synechococ-cus lividus isolated from a thermal spring in Yellowstone has been reported (Sheridan, 1966 Sheridan and Castenholz, 1968). Wilson et al. (1977) described light-dependent emission of H2S from leaves of a variety of plants at a maximum rate of 8 n mol min g (fresh-wei t) which they judged to be comparable to the activity associated with sulfate uptake. However, the emission was not a steady phenomenon and increased markedly with stresses of root injury, increases in light intensity and increased bisulfite or sulfate ion concentrations. Emissions with bisulfite solutions were higher than with sulfate solutions. 

Reactions other than those of the nucleophilic reactivity of alkyl sulfates iavolve reactions with hydrocarbons, thermal degradation, sulfonation, halogenation of the alkyl groups, and reduction of the sulfate groups. Aromatic hydrocarbons, eg, benzene and naphthalene, react with alkyl sulfates when cataly2ed by aluminum chloride to give Fhedel-Crafts-type alkylation product mixtures (59). Isobutane is readily alkylated by a dipropyl sulfate mixture from the reaction of propylene ia propane with sulfuric acid (60). 

For the extraction of sulfates and total sulfur a suitable acid and reducing agent, such as tin(II)-phosphoric acid (the Kiba solution of Sasaki et al. 1979) is needed. The direct thermal reduction of sulfate to SO2 has been described by Holt and Engelkemeier (1970) and Coleman and Moore (1978). Ueda and Sakai (1984) described a method in which sulfate and sulfide disseminated in rocks are converted to SO2 and H2S simultaneously, but analyzed separately. With the introduction of on-line combustion methods (Giesemann et al. 1994), multistep off-line preparations can be reduced to one single preparation step, namely the combustion in an elemental analyzer. Sample preparations have become less dependent on possibly fractionating wet-chemical extraction steps and less time-consuming. 

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