Discussion The Importance of Fe-and Mn-Reactivity in Various Environments

The above sections of this chapter have shown the high variability of iron-inpnt modes, flnxes and reactivity towards oxidized and reduced species in marine sediments. Within this section the importance of iron and manganese reactivity with respect to the mineralization of organic matter as well as to the chemical oxidation (reduction) of reduced (oxidized) species within different depositional enviromnents will be discussed and hopefully inspire further considerations. 

In order to investigate the importance of iron and manganese reduction and oxidation processes one needs to determine their rates and compare 

Location Fe/Mn Turn-over time [d] Times cycied before buriai 

A conceptual model describing the importance of iron and manganese reactivity in different environments is only sketchy as results are scarce and manifold aspects deserve further investigations. A general complication concerns the differentiation of dissimilatory and chemical reduction of Mn-oxide when concurrent iron reduction is apparent and further investigations need to discriminate each reaction pathway. The reactivity of smectite-bound iron has only been shown qualitatively (Kostka et al. 1996 Konig et al. 1997), yet quantifications with respect to dissimilatory and/or chemical reactions are missing. An important role of adsorbed Fe + / 

Mn is indicated (Sorensen and Thorling 1991 Roden and Zachara 1996) but not proved by sediment studies. Investigations focusing on the importance of metal cycling and its influence on pathways of organic matter decomposition are scarce (Canfield et al. 1993a, Wang and Van Capellen 1996), yet necessary to understand an important link of the carbon cycle. 

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