Rapid changes and near-stationarity of the geomagnetic field during a polarity reversal

Abstract

One of the most important suggestions from many of the new reports of sedimentary geomagnetic records is that the time dependence of the changes of the geomagnetic field during a reversal might be more complex than early records indicated. Indeed, alternating periods of rapid and slow field changes are apparent in some sedimentary records^1–4. In volcanic records, very fast directional) changes recorded by individual lava flows during their progressive cooling have been reported^5,6, but a major problem, when inferring the temporal behaviour of the transitional field, is that the extrusion rate of the successive lava flows is much more variable and more difficult to evaluate than the rate of accumulation of the sedimentary sequences. Here we report the results of a detailed study of a geomagnetic reversal recorded in Middle Miocene marine clays, with temporal resolution almost comparable with that of the volcanic records and, in our opinion, a more reliable chronological control. The record displays large fluctuations in the rate of directional change and different episodes of collapse and recovery of the field intensity. A very fast N–S directional change may be entirely controlled by a frozen–flux dominated reversal mechanism.