High-resolution magnetic analysis of sediment cores: strengths, limitations and strategies for maximizing the value of long-core magnetic data
Physics of the Earth and Planetary Interiors, 156, (3-4), . (doi:10.1016/j.pepi.2005.03.021).
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Narrow-access long-core cryogenic magnetometers enable measurement of a range of magnetic parameters at a speed and resolution that cannot be matched by other techniques. Despite the revolutionary impact that these instruments have had on paleomagnetic and environmental magnetic investigations, some fundamental constraints limit their usefulness. First, the pick-up coils have different response functions for the transverse and axial measurement axes. Transverse coils typically have regions of negative response on either side of the main response peak, whereas the axial coil usually lacks the negative response lobes. Zones of negative response affect the measured remanence intensity, for which corrections can be made by normalizing the measured magnetic moment by the area under each respective response curve. This correction works adequately for homogeneously magnetized cores. Second, in cores with significant changes in remanence intensity, the ratio of axial to transverse moment varies with intensity change, which can introduce spurious artefacts into the paleomagnetic directional record. Deconvolution is required to remove such effects. Third, measurements of non-centred samples with irregular cross-section (e.g., split core measurements), cause geometric effects that can introduce small but paleomagnetically important artefacts. Corrections for such effects are only possible if spatial variability of the magnetometer response is known throughout the entire measurement volume rather than solely along the centre-line of the magnetometer. Fourth, analysis of cores deposited at rates >10 cm/ky is desirable to minimize the effects of measurement smoothing. Finally, measurements of magnetic susceptibility should be conducted using loop sensors with a similar response function as a u-channel magnetometer to ensure comparability of data. Routine adoption of these five strategies should help to maximize the value of long-core magnetic measurements.
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