Diagenetic imprints on magnetic mineral assemblages in marine ...

Chapter 2.1
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A large peak is present in the remanent coercivity (Hcr, Fig. 4b), located at the lower
boundary of the active oxidation zone. After the removal of the carbonates, the
‘amorphous’ oxides and the remaining carbonates, the down-core shape of the coercivity
profile remains unchanged. The peak in coercivity disappears after the removal of the
‘crystalline’ oxides: a constant remanent coercivity of approximately 30*10 3 A/m is
observed.
3.4 IRM component analysis
The results of the IRM component analysis are summarised in table 3. In the untreated
sediments, three components could be identified in the oxidised sapropel and the active
oxidation zone. These are a relatively strong low-coercivity component (component 1) and
a relatively weak high coercivity component (component 2), identified in the studies of
Passier et al. (2001), and Kruiver and Passier (2001), as respectively ‘detrital’ magnetite
and hematite. Component 3 was interpreted along similar lines as Kruiver and Passier
(2001) as biogenic magnetite. It displays a characteristic low dispersion (DP) in the
coercivity, indicating a narrow grain-size range distribution. In the sediments of the
reduced sapropel component 3 could not be identified.
The removal of the ‘amorphous’ oxides changes the distribution of the components in
the sapropel. A relative increase to the SIRM contribution of component 1, ‘detrital’
magnetite is observed. Consequently, the other components, biogenic magnetite and
hematite, display a decrease in their magnetic contribution. The contribution of component
2 (hematite) decreases strongly in the reduced part of the sapropel. After the removal of
the ‘crystalline’ oxides, component 3 (biogenic magnetite) disappears from the sediments.
The components 1 and 2, respectively ‘detrital’ magnetite and hematite remain detectable.
4 Discussion
4.1 <strong>Diagenetic</strong> interpretation
The identification of the diagenetic zones in the sediments of boxcore PS036BC is
mainly based on the colour transitions observed in the sediments. The geochemical
analyses confirmed the presence and location of the zones (Fig. 1 and 3). The markerbed,
indicated by darker sediments and a manganese peak, is located at 26 cmsbd. It also
marks the original upper boundary of sapropel S1 (van Santvoort et al., 1996). The
transition from pale brown to slightly yellow/orange sediments occurs at a depth of
approximately 30 cm, and marks the upper boundary of the active oxidation zone, in