‘Sawtooth’ variation in magnetic intensity profiles and delayed acquisition of magnetization in deep sea cores

Recent records of normalised intensity of natural remanent magnetization (NRM) obtained from deep-sea sediments document an asymmetrical saw-tooth pattern, associated with geomagnetic reversals. This observation may reflect a fundamental aspect of the geodynamo time evolution or, alternatively, delay in acquisition of the NRM. It is demonstrated here that a simple model, in which a large fraction of the magnetic grains acquire NRM at deposition time while remaining grains reorientate or acquire magnetization after deposition, is able to produce a ‘saw-tooth’ profile strikingly similar to that observed. Interestingly, this mechanism of NRM acquisition does not damp out high frequency oscillations of the field, such as those observed in the deep-sea records exhibiting the saw-tooth profile. It provides an accurate record of field directions, except for periods of very low field intensity.