Early diagenesis sulfate reduction

Sediment deposition on the seafloor traps interstitial water. After deposition, complex reactions take place in the sediment, most of them fueled by the decay of organic matter, such as sulfate reduction, denitrification. Because of fast diffusion rates of most cations in seawater, the presence of interstitial water makes exchange between overlying sedimentary layers a much easier process than if sediment deposition was dry. The book by Berner (1980) is entirely dedicated to these processes and only a short example is given here. 

Let us consider sulfate reduction by bacterial activity at the expense of decaying solid organic matter. Berner suggests the simplified equation 

Neglecting the movement of water relative to the surrounding sediment, we write the steady-state transport equation in one dimension with burial, e.g., in a medium 

In this equation, C is the concentration of element i in pore water at depth z below the seafloor and A is a reaction (sink and source) term. For reactions involving the oxidation of organic matter, A can be evaluated independently. For constant porosity / , the sulfate transport equation becomes 

Inserting this expression into the transport equation gives 

---

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... 

Birnbaum S.J. and Wireman J.W. (1984) Bacterial sulfate reduction and pH Implications for early diagenesis. Chem. Geol. 43, 143-149. 

Methanogenesis occurs after sulfate reduction, but it is important in early diagenesis only in rare circumstances, and it will not be discussed any further in this chapter. 

Many studies of the impact of chemical diagenesis on the carbonate chemistry of anoxic sediments have focused primarily on the fact that sulfate reduction results in the production of alkalinity, which can cause precipitation of carbonate minerals (see previous discussion). However, during the early stages of sulfate reduction (—2-35%), this reaction may not cause precipitation, but dissolution of carbonate minerals, because the impact of a lower pH is greater than that of increased alkalinity (Figure 4). Carbonate ion activity decreases rapidly as it is titrated by CO2 from organic matter decomposition leading to a decrease in pore-water saturation state. This process is evident in data for the Fe-poor, shallow-water carbonate sediments of Morse et al. (1985) from the Bahamas and has been confirmed in studies by Walter and Burton (1990), Walter et al. (1993), and Ku et al. (1999) for Florida Bay, Tribble (1990) in Checker Reef, Oahu, and Wollast and Mackenzie (unpublished data) for Bermuda sediments. 

Dissolved sulfides (H2S and HS ), formed from sulfate reduction, can react with Fe(ll) and precipitate as iron sulfides (Berner, 1970). Under conditions of early diagenesis of amorphous iron sulfides (FeS), mackinawite (FeSg 9) is formed. One of the first minerals formed is hydrotrolite (FeS H20), which is labile under acidic conditions. During diagenesis the hydrotrolite converts to pyrite, which is a more stable mineral. The reaction can be simply represented as... 

---