Diagenetic imprints on magnetic mineral assemblages in marine ...

Chapter 1
In the last stage of reduction the OM is fermented producing methane and carbon dioxide.
The extent of theses reactions greatly depends on the amount of OM added to the
sediments, the sedimentation rate and the bio-availability of the reactants (Evans and
Heller, 2003).
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A new geochemical classification model was presented by Berner in (1981). It is
based on the presence or absence of oxygen and dissolved hydrogen sulphide (i.e. HS -
and H2S), at the time of authigenic mineral formation in marine sediments. The
environments identified occur in the following order: oxic, post-oxic, sulphidic and
methanic. In Fig. 2 a schematic representation is given, together with some characteristic
pore water profiles and authigenic minerals.
Fig. 2. Classification of the different early diagenetic environments and some stable authigenic iron
and manganese minerals characteristic for the corresponding environment (Froelich et al., 1979;
Berner, 1981).
Knowledge about the chemical stability of the (authigenically formed) magnetic
minerals is essential when trying to understand the sediment geochemistry or to
reconstruct the paleoenvironment.