Early diagenetic reactions

Figure 1). The solute exchanges resulting from reactions in surface sediments can be important for the chemistry of oceanic deep water, and to the overall cycles of several elements in the oceans. In addition, the alteration of particles by these reactions must be taken into account when down-core records of the accumulation of sedimentary components are interpreted in terms of past oceanic and atmospheric chemistry. Understanding early diagenetic reactions in the upper few centimeters of the marine sediment column is important both to the study of geochemical cycles in the contemporary ocean and to the reconstruction of past oceanic conditions. 

In fully marine systems siderite formation is probable to occur below the sulfate reduction zone where dissolved sulfide is absent, if reactive iron is still present and the Fe/Ca-ratio of pore water is high enough to stabilize siderite over calcite (Berner 1971). The coexistence of siderite and pyrite in anoxic marine sediments was shown by Ellwood et al. (1988) and Haese et al. (1997). Both studies attribute this observation to the presence of microenvironments resulting in different characteristic early diagenetic reactions next to each other within the same sediment depth. It appears that in one microenvironment sulfate reduction and the formation of pyrite is predominant, whereas at another site dissimilatory iron reduction and local supersaturation with respect to siderite occurs. Similarly, the importance of microenvironments has been pointed out for various other processes (Jorgensen 1977 Bell et al. 1987 Canfield 1989 Gingele 1992). 

The first two pathways of formation will not be discussed here as they were found to be only of local/regional importance and are not eonsidered to be of major importance for early diagenetic reactions. Iron bearing clay mineral formation under high-temperatnre conditions near a hydro-thermal system of the Red Sea was studied by Bischoff (1972). A direct precipitation of an iron-rich smectite (nontronite) within the metalliferons sediments was found. This pathway of clay mineral formation was shown to occur at temperatures... 

Schinzel, U., 1993. Laboratory experiments on early diagenetic reactions of iron(III) oxyhydroxides in marine sediments (in German). Berichte, 36, Fachbereich Geowissenschaften, Universitat Bremen, 189 pp. 

Analytical Solutions for Diffusion and Early Diagenetic Reactions... 

The model CoTReM (Column Transport and Reaction Model) (Adler et al. 2000b) is primarily designed for its application to complex early diagenetic reactions in marine sediments. In the one-dimensional transport part of the model (as a continuance of the model DISPER by Fliihler and Jury 1983) the model s territory is divided up into a variable number of REVs (representative elementary volumes). These REVs may differ in... 

Both models are not easy to operate and require at least some background knowledge in modeling procedures and early diagenetic reactions. 

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