Diagenetic imprints on magnetic mineral assemblages in marine ...

Chapter 2.1
from the experimentally calibrated band are related to the presence of a mixture of
magnetic-minerals as indicated by the IRM components. It could be that the remaining
magnetic minerals occur as inclusions in silicates. Electron microscopic analysis could be
used to verify this.
5 Conclusion and implications
30
The sequential extraction clearly separates different phases of iron that occur in oxic
and in anoxic sediment conditions. However, the separation between the ‘amorphous’ and
‘crystalline’ oxides seems to be grain-size selective. The dissolution of silicon during the
extraction of the ‘amorphous’ oxides implies the presence of a relatively easily soluble
ferro-silicate, which is most likely to be a by-product of bacterial metabolism.
IRM acquisition followed by IRM component analysis could identify three components,
e.g. different species of iron oxides. The magnetites found in the sapropelic sediments
can be subdivided into two categories: 1) ‘Detrital’ magnetite with a relatively low mean
coercivity, 2) Biogenic magnetite, formed by magnetotactic bacteria living in the lower part
of the active oxidation zone. The grains display single domain to super-paramagnetic
behaviour. The other component identified is a mixture of hematite and goethite.
The sequential extraction together with the magnetic parameters of the sediments
identified also two other iron phases of diagenetic origin: a) Coatings on mineral surfaces
mainly composed of ferro-silicates; b) ‘Amorphous’ iron oxides (hematite and/or goethite)
which are likely to have the same microbially induced origin as the coatings.
The hysteresis measurements and the IRM component analysis, verify earlier findings
that the coercivity peak observed in the sediments is of diagenetic origin. It is most likely
the result of the activity of magnetotactic bacteria forming in-situ magnetite (Tarduno et
al., 1998; Passier et al., 2001, Kruiver and Passier, 2001).
Acknowledgements
We thank captain J. Ellen and the crew and scientific party of the R.V. Pelagia
PASSAP cruise for their pleasant collaboration. Furthermore, H. de Waard, D. van de
Meent, and G. Nobbe are acknowledged for analytical assistance. The manuscript was
greatly improved through consideration of the comments of an anonymous reviewer. This
study was supported by the Netherlands Organisation of Scientific Research (NWO, in
particular the programs PASS2 and SAPS), and EU Mast project MAS3-CT97-0137
(SAP).