Absolute paleointensity and reversal records from the Waianae sequence (Oahu, Hawaii, USA)

Paleointensity experiments were performed on samples from four parallel sections of the Waianae volcanics of Oahu (Hawaii) that document 0.4 Myr of geomagnetic field variations. The records of the three successive Gilbert-Gauss, Lower and Upper Mammoth reversals confirm that large oscillations of directions precede or follow the reversals, which are similar to waveforms generated by paleosecular variation with their amplitude being considerably amplified by the decrease of the dipole. Determinations of absolute paleointensity were attempted on 546 samples. We only selected data which were obtained from segments covering a large part of the remanent magnetization (more than 70% on the average) and without concave-up Arai diagrams. This procedure limited the success rate to 14% but provided consistent and reliable paleointensities. In addition to other time intervals, the results document the field variations surrounding the lower Mammoth transition. A weak field period dominated before the reversal, then the transition was initiated by a transit from normal to reverse polarity followed by a short restoration of the field intensity in reverse polarity. A second episode of a very weak field was accompanied by a return to positive inclinations before definitely reaching the reverse polarity. A strong and apparent rapid recovery of the dipole following the completion of the reversal culminated at a value of 16 × 1022 Am2 similar to field intensities reported for the other detailed volcanic records of reversals studied so far. The asymmetry between the pre- and the post-reversal phases indicates the importance of field regeneration to initiate a new stable polarity interval.