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A variety of redeposition experiments have been conducted in order to investigate the alignment efficiency of magnetic grains in low field intensities. The field direction is reproduced by the magnetic remanence with good fidelity and there is low directional dispersion for samples obtained with field intensities larger than 10 μT. However, there is an increase in dispersion for lower field strengths and a critical threshold is reached at 5 μT (i.e., similar to the geomagnetic intensity during reversals), below which no coherent signal is observed. We notice also that there is a good linear relationship (within about 10%) between the intensity of magnetization (corrected for magnetic concentration) and the value of the applied field. A column of sediment, deposited in the presence of two successive polarities of the field, shows downcore changes in direction and intensity which confirm the occurrence of post-depositional reorientations. About 50% of the magnetization was locked in within 2 cm of sediment and 60% within about 10 cm. An analysis of the magnetization components shows that reorientation did not occur exactly along the direction of the post-reversal field, which suggests that grain mobility is rapidly affected at small depths within the sediment. |
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