The Early Diagenesis of Iron in Sediments

The fundamental work by Froelich et al. (1979) established a conceptual model for the organic matter respiration in marine sediments which has been modified, verified and extended in nnmerous aspects since then. Froelich and colleagnes found a snccession of electron acceptors nsed by dissimilatory bacteria according to their energy gain. Conseqnently, a biogeochemical zonation of the sediment resnlts where O, NO3, bioavailable Mn(IV) and Fe(III) and SO diminish sncces-sively with depth. Apart from the consnmption of electron acceptors the production of rednced species snch as Mn , Fe, MS and CH  

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Haese, R.R., Petermann, R, Dittert, L. and Schulz, H.D., 1998. The early diagenesis of iron in pelagic sediments - a multidisciplinary approach. Earth and Planetary Science Letters, 157 233-248. 

Mucci, A. and Edenborn, H.M. (1992) Influence of an organic-poor landslide deposit on the early diagenesis of iron and manganese in a coastal marine sediment. Geochimica et Cosmochimica Acta, 56, 3909-3921. 

In summary, it seems meaningful to consider both the formation of pyrite from the reaction of H2S with reactive ferric oxides and sulfate recycling as a result of this process in any discussion of the early diagenesis of sulfur and iron in sediments. 

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 ... 

Emerson, S. and Widmer, G. (1978). Early diagenesis in anaerobic lake sediments II. Equilibrium and kinetic factors controlling the formation of iron phosphate. Geochim. Cosmochim. Acta 42,1307-1316. 

Schematic diagram of the coupled iron and phosphate cycles during early diagenesis in marine sediments. Light gray ovals and circles represent solid phases black arrows are solid-phase fluxes. White outlined black arrows Indicate reactions white arrows are diffusion pathways. Source From Ruttenberg, K. C. (2003). Treatise on Geochemistry, Elsevier Ltd. pp. 585-643.

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 leaching with HF/H SO as described above and the subsequent polarographic determination of ferrous and ferric iron is based on work by Beyer et al. (1975) and Stucki (1981) in order to quantify the silicate bound ferrous and ferric iron. This extraction has hardly been applied with respect to questions of early diagenesis so far, yet, the silicate bound iron fraction is quantitatively very important in marine sediments and even a small reactive fraction of this pool may be of overall significance for the iron reactivity. As a complementary method to the commonly applied extractions (Table 7.4) it renders the calculation of the total iron speciation in the sediment which may then be compared to Mdssbauer-spectroscopic results (Haese et al. 1997 Haese et al. 2000). 

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