Diagenetic imprints on magnetic mineral assemblages in marine ...

Chapter 2.1
of the oxidised sapropel (Fig.s 1 and 3). In the peak of the active oxidation zone the iron is
allocated over the same species but in different proportions, 45% ‘amorphous’ iron, 10%
‘crystalline’ iron, and 45% ‘other’ iron. In the reduced part of the sapropel the proportions
are: approximately 20% iron incorporated into oxides, 25% pyrite-related iron, and 55%
‘other’ iron. Manganese is incorporated into carbonates and ‘amorphous’ oxides. In the
markerbed the oxides are the dominant species, whereas in the oxidised part of the
sapropel and the active oxidation zone and approximately equal amount of carbonate
related manganese is present. In the reduced part of the sapropel mainly
carbonate-related manganese is present.
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In this study, in the dissolution step with ascorbate (step 4, Table 2), a significant
amount of silicon was dissolved together with aluminium and potassium, whereas in the
dithionite step (step 6, Table 2), little dissolution was observed of the latter two elements.
The extraction of silicon and related elements occurs in the oxidised sapropel and in the
zone of active oxidation. A plausible explanation could be given by the reaction of ferrous
iron with dissolved oxygen to initially form abiological ferrihydrite, that with time transforms
into goethite or hematite depending on the pH conditions (cf. Konhauser, 1998 and
references cited herein). Ferrihydrite can sorb a wide variety of elements and anionic
groups including silicon as Si(OH)4 groups and phosphate (Carlson and Schwertmann,
1981; Schwertmann, 1988). Although both refer to pedogenic conditions, the underlying
principles may be extended toward a marine setting: electrical double layers are smaller in
the marine environment but the pH in the pore waters is often slightly acidic as well,
i.e. ~6–7. Sorption of silicate, that is a bidendate inner-sphere complex, distinctly reduces
crystallinity. It may favour a ferrihydrite aging mechanism toward more stable iron oxides
via dissolution and precipitation from solution. This would yield goethite and would explain
the extreme magnetic hardness in the active oxidation zone that was found by Passier et
al. (2001) and Kruiver and Passier (2001). Note that a yellowish brown colour occurs
within the active oxidation zone (cf. Table 1). This would point to goethite rather than to
hematite. It is unclear, however, why it would ‘disappear’ higher up in the oxidised
sapropel. A reason could be that the disappearance is only apparent: one could speculate
that in the oxidised sapropel more hematite-like ferric oxide is added so that the colour
that is ascribed to a goethite phase would no longer be traceable because hematite is
known to be a strong pigment. In addition to inorganically precipitated ferric oxides also
bacterially induced precipitation occurs often yielding ferrihydrite. This ferrihydrite is also a
precursor of more stable iron oxides, as goethite and hematite. These iron-phases are
by-products of bacterial metabolism, and may be formed in large quantities
(cf. Konhauser, 1998 and references cited herein). With time and depth the downward
progression of the oxidation front retards, due to continuing (oxic) sedimentation, which