Diagenetic imprints on magnetic mineral assemblages in marine ...

Chapter 2.1
coercivity remains relatively constant, until the removal of the ‘crystalline’ oxides. The
coercivity then increases to approximately 10 logB½ of 3.2 (log mT) with a dispersion (DP)
of 0.12 (log mT). Apparently goethite is present and is more resistant to dissolution than
hematite.
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The third IRM component, biogenic magnetite, is only present in the oxidised sapropel
and the active oxidation zone. Like IRM component 1, it is harder in the active oxidation
zone than in the other zonations. Its contribution to the SIRM decreases after the removal
of the ‘amorphous’ oxides, indicating some dissolution of biogenic magnetite during this
step. The effect is more pronounced in the oxidised sapropel than in the zone of active
oxidation. A plausible explanation is that the magnetosome chains become disintegrated
upon cell lyses after the oxidation front has moved further down in the sediment.
Therefore they are more amenable to dissolution. Component 3 has disappeared
completely after the extraction of ‘crystalline’ oxides.
4.2.2 Hysteresis parameters
The coercivity pattern found in the present study (Fig. 4b) with peak values at the
lower boundary of the active oxidation zone was also observed in sapropel S1 by Passier
et al. (2001) and Kruiver and Passier (2001) and in other sediments by e.g. Tarduno et al.
(1998). They concluded that an unusually abundant population of bacterial magnetite
produces the peak at the iron-redox boundary. This coincides with the findings of the IRM
component analysis. The peak disappears after the removal of the ‘crystalline’ oxides,
after which ‘biogenic’ magnetite is absent in the IRM component analysis.
It is common to summarize hysteresis parameters by plotting Mrs/Ms versus Hcr/Hc
(i.e. Day plot), because different domain states plot in different areas (Day et al., 1977).
Fig. 5 shows the Day plot for the ten samples analysed after the different extraction steps.
The sample positions on the Day plot have to be viewed with some caution because the
domain state areas refer to monomineralic (titano-)magnetite assemblages in the absence
of superparamagnetic (SP) grains. SP grains tend to displace values for the hysteresis
ratios to the right of the experimentally calibrated band of Day et al. (1977) as a recent
study by Dunlop (2002) shows. The theoretical Day-plot curves as calculated by
Dunlop (2002) are added to Fig. 5. Compared to the calculated mixing curves of single
domain (SD) and multidomain (MD) grains and the experimentally calibrated band, all
data points are displaced to the right. The position of the samples conforms rather closely
to the trend line for unremagnetised limestones as found by Channell and McCabe (1994).
This indicates an assemblage of magnetic minerals dominated by ‘primary’ magnetite
concurring with results of the IRM component analysis. Surface oxidation and the