Sulfide diagenesis

In the context of diagenesis in recent anoxic sediments, reduced carotenoids, steroids, and hopanoids have been identified, and it has been suggested that reduction by sulhde, produced for example, by the reduction of sulfate could play an important part (Hebting et al. 2006). The partial reduction of carotenoids by sulfide has been observed as a result of the addition of sulfide to selected allylic double bonds, followed by reductive desulfurization. This is supported by the finding that the thiol in allylic thiols could be reductively removed by sulhde to produce unsaturated products from free-radical reactions (Hebting et al. 2003). 

Nodules formed via suboxic diagenesis tend to have a low iron content because most is precipitated as FeS below the redox boimdary. This is true for other trace metals, such as Cu and Ni, which form highly insoluble sulfides. Because biogenic grains tend to be incorporated in these nodules, all metal concentrations tend to be low, making these nodules economically unattractive. 

The redox processes and processes linked to them in the basin sediments can be written in the following or some analogous way. At least some of these processes take place during the early diagenesis, as indicated by the presence of sulfides and iron (II) carbonate in many buried sediments ... 

As shown in the discussion accompanying Figure 7.4, sulfate is reduced before methane is generated in the diagenesis process. At the subsediment SMI, sulfate from the seawater above, and methane from the dissolved (or free) gas below, are consumed to produce carbonate and hydrogen sulfide. The anaerobic reaction for... 

Detrital iron (oxy)(hydr)oxides, organic matter, and other arsenic-bearing materials in sediments may be transported by water or wind into wetlands and contribute arsenic to peats. Once buried, reductive dissolution releases sorbed arsenic from iron (oxy)(hydr)oxides. Under sulfate-reducing conditions, the arsenic coprecipitates in sulfide minerals or organic matter. During diagenesis, additional arsenic may be released from the organic matter and coprecipitate in sulfide minerals (Eskenazy, 1995), 253. 

Chernoff, C.B. and Barton, M.D. (2001) Trace elements in black shale Fe-sulfides during diagenesis and metamorphism. Abstracts with Programs. The Geological Society of America, 33(6), 129. 

Lein, A. Y. Formation of carbonate and sulfide minerals during diagenesis of reduced sediments, in Environmental biogeochemistry and geomicrobiology 1 (ed. Krumbein, W. E.) p. 339, Ann Arbor Mich. Ann Arbor Publishers Sci. 1978... 

These examples convincingly demonstrate that specific OSC are formed during the early stages of diagenesis by reactions of reduced sulfur species with specific biogenic substrates. The reactive substrates are proposed to contain either carbon-carbon double bonds or other reactive functional groups that react with either hydrogen sulfide or polysulfides to form the OSC (88). These views are consistent with evidence from sulfur isotopes that H2S produced by microbial sulfate reduction is the major source of reduced sulfur in sediments... 

In marine and lacustrine muds, the initial sulfide phase precipitated during early diagenesis is mackinawite (FeS09) which is subsequently converted to greigite (Fe3S4) and pyrite (FeS2) (85-89). This reaction path leads to the formation of framboidal pyrite (88.90). However, in salt marsh sediments under low pH and low sulfide ion activity conditions, direct precipitation of pyrite by reaction of ferrous iron with elemental sulfur without the formation of iron monosulfides as intermediates has been reported (85-87.89.91.92). This reaction is one possible pathway for the precipitation of pyrite as single crystals (89). 

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... 

The resin fractions of organic sulfur-rich bitumens are for a substantial part composed of monomers with linear, isoprenoid, steroid, hopanoid and carotenoid carbon skeletons connected to each other by (poly)sulfide linkages. These structural units may contain additional intramolecular sulfur linkages. This sulfur-rich geopolymer is also formed by sulfur incorporation into functionalised lipids in an intermolecular fashion during early diagenesis. 

Machel H.G. (1989) Relationships between sulfate reduction and oxidation of organic compounds to carbonate diagenesis, hydrocarbon accumulations, salt domes, and metal sulfide deposits. Carbonates and Evaporites 4,137-151. 

Table System of equations for comprehensive model of sulfide diagenesis ...

However because the rate law expressions for iron sulfide formation (equations 8 and lO) are non-linear the differential equations for H2S and the iron sulfides are not amenable to explicit solution Thus it is important to develop an equation for that can be incorporated in a numerical solution technique such as that of Runge-Kutta (15) Fortunately an appiropriate differential equation for can be developed firom charge balance considerations Here it is assumed that dissolved substances other than those listed in Table I are not affected by diagenesis If this is true, then a charge balance difference equation can be written (16) ... 

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