Magnetic studies of speleothems.
The Natural Remanent Magnetizations (NRMs) of rocks and sediments relate to past
variations of the Earth's geomagnetic field (GMF). Studies of speleothems (cave
deposits such as stalagmites) have shown that they often possess measurable NRMs.
However, there have not been extensive studies of the magnetic minerals responsible
for the NRM, nor in determining the type and origin of the NRM.
A selection of speleothems has been studied by palaeomagnetic and rock magnetic
techniques to identify the magnetic minerals within them and the carriers of the
NRMs. Electron microscope studies of extracted magnetic phases provide
suggestions as to their origin. These studies have been combined with observations of
speleothem surfaces to address the question of how the NRM is acquired.
The NRMs and magnetic mineralogies of most speleothems are dominated by
magnetite, with hematite and goethite present as accessories. Some samples are
dominated by hematite. The bulk magnetic content of most speleothems does not
vary and consequently there is only a single primary component of magnetization.
However, there are exceptions. Rock magnetic data suggest that interaction between
magnetic phases may be occurring and thus these data cannot be interpreted
unambiguously in terms of magnetic grain size. Electron microscope studies have
shown that the techniques for extracting and preparing magnetic grains cannot be
used on a quantitative basis. On a qualitative basis, however, detrital grains (<0.01μm
to »1Oμm, composed of magnetite, hematite and titanomagnetite), hexagonal or cubic
grains (<0.1μm, composed of magnetite) and needle-like grains (<2μm, possibly
goethite) have been observed.
A detrital remanent magnetization contributes to the NRM of speleothems and is
probably more important than previously suggested. Detrital grains are introduced
into speleothems either via floodwaters or through feedwaters. It is suggested that the
NRM is acquired due to grains becoming trapped in depressions in the speleothem
surface. Experiments suggest that, in the near-absence of oxygen, inorganic
precipitation of magnetite could occur during speleothem growth. Iron-chelating
organic compounds could also introduce iron into caves. Further work is needed on
de-chelation mechanisms, the transport of detrital and organic material into caves, the
thermodynamic behaviour of iron at low temperatures and the oxygen content of
waters in and entering caves.
The recent introduction of mass spectrometry for dating speleothems suggests that the
reliability of speleothems as records of past behavioural features of the GMF could be
assessed to a greater degree.