Diagenetic imprints on magnetic mineral assemblages in marine ...

Chapter 2.1
stratified pore space of unconsolidated sedimentary deposits, just below the oxidation
front (Gorby et al., 1988Bleil, 2000). Biogeochemical processes at redox boundaries affect
remanent magnetisations. They may cause significant smoothing of paleomagnetic
records (e.g. Tarduno et al., 1998).
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Here, we discuss how diagenesis of iron phases influences magnetic hysteresis
parameters and magnetic coercivity components. To this end, we incorporate the results
of a recently evaluated nine-step sequential extraction procedure (Rutten and De Lange
2002a, 2002b) into the recentlynewly developed IRM component analysis (Kruiver et al.,
2001; Heslop et al., 2002). The merits of these two techniques may be enhanced by their
combination because dissolution puts constraints on the mineral-magnetic analysis, while
the sensitive magnetic techniques simultaneously allow validation of the sequential
extraction.
2 Material and Methods
Boxcore PSO36BC was recovered by the R/V Pelagia in May 2000 at 36°15.85’N;
21°48.38’E, in a water depth of 3286 meters. The 41 cm long boxcore contains the
partially oxidised sapropel S1, as found in large parts of the eastern Mediterranean (Fig.
1). In the overlying mud, a tephra layer occurs, tentatively interpreted as being originating
from the Minoan eruption of Santorini, 3.6 ka BP (de Rijk et al., 1999). At a sediment
depth of 26 centimetres (cmsbd) the sapropel starts, indicated by the darker sediments of
the markerbed (van Santvoort et al., 1996), continuing until the bottom of the boxcore.
The upper part of the sapropel has been altered by oxic diagenesis. This oxidised part
can be divided in two segments, the upper part being the oxidised sapropel and the lower
part the active oxidation zone, starting at a depth of 30 cmsbd. Below the active oxidation
zone the sapropel is still unchanged. In the present boxcore, sapropel S1 is interrupted
near the bottom of the core by a thin zone (3 cm thickness) of muddy sediment. The
position concurs with the interruption in S1 that was shown to be a distinct cooling event
around 7 ka BP by de Rijk et al. (1999).
Six subcores were taken from the boxcore. The sediments from the subcores, #2, #4,
#5 and #6 were subsampled, whereby the core was divided into 32 intervals according to
the colour of the sediment. An overview of the sampled intervals is given in table 1. The
sample intervals range from a few millimetres in the (oxidised) sapropel to two centimetres
in the oxic sediments overlying the sapropel. The samples were freeze-dried and gently
ground in an agate mortar. Coinciding intervals were mixed prior to analysis.