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.
| Identifer | oai:union.ndltd.org:bl.uk/oai:ethos.bl.uk:359381 |
| Date | January 1993 |
| Creators | Perkins, Andrew Mark |
| Publisher | University of East Anglia |
| Source Sets | Ethos UK |
| Detected Language | English |
| Type | Electronic Thesis or Dissertation |
Page generated in 0.1306 seconds