Diagenetic imprints on magnetic mineral assemblages in marine ...

Chapter 1
1 The aim of this study
2
Magnetic minerals, mainly iron oxides, iron oxyhydroxides and some iron sulphides,
occur in minor or trace amounts in igneous and sedimentary rocks, soils, dusts and even
in living organisms. Depending on their grain size, magnetic minerals are able to carry a
stable natural remanent magnetisation (NRM) and act as recorders of the ancient Earth’s
magnetic (or geomagnetic) field. Paleomagnetic and geomagnetic studies investigate this
NRM history for plate tectonic reconstructions, magnetostratigraphy and short-term
variations in the Earth’s magnetic field. In the discipline of environmental magnetism,
properties of magnetic minerals are used as proxies for paleoclimatic and
paleoenvironmental change. Secondary magnetisations acquired in the period between
formation and the present time are generally considered as a bias on the primary
magnetisation, but can also in some cases be used when studying the nature and timing
of geological and geochemical events.
In considerably (chemically) altered sedimentary assemblages plausible magnetic
polarity information has often been retained, although the primary magnetic mineral
assemblage has been almost completely transformed. The mechanisms, which permit
oxic magnetic minerals to survive in strong reducing environments, are to be identified.
The NRM carried by the primary magnetic minerals can also be replaced by or passed
onto secondary minerals. By comparing the magnetic minerals present in distinct layers,
and under various sedimentary conditions, different preservation mechanisms may be
identified. An integrated geochemical and mineral magnetic approach is aimed at
establishing the primary or secondary nature of the magnetic signal. This will aid the
development of models that describe and explain reaction kinetics and past redox
conditions.
In the next sections a brief introduction is given of environmental magnetism and some
characteristics of magnetic minerals, encountered in this study, are described. In addition,
the main diagenetic processes occurring in marine sediments studied are introduced.
2 Environmental magnetism
Paleoclimatic and paleoenvironmental changes are recorded in sedimentary
sequences by magnetic minerals, the study thereof is called environmental magnetism.
The most important terrestrial record is arguably the wind-blown silt deposit of the
Chinese loess plateau, which spans approximately 2.6 Ma (e.g., Ding et al., 1998; Sun et
al., 2005). Water-laid lake sediments on the other hand, also have long been used for
obtaining climatic and environmental information (Thompson et al., 1975). However, the