8.09 Magnetic Polarity Reversals in the Core

(Matuyama–Brunhes), which occurred 0.78 Ma, the dating error of the most favorable basalt flows is comparable to the transition duration (Singer et al., 2005). For this reason transition duration must be obtained from sedimentary records, using the stratigraphic thicknesses of the transition zones and estimates of the deposition rate. From 30 selected records of the last four reversals, Clement (2004) found that the average time for the field to change direction from one polarity to the other was 7000 years (Figure 1). These reversals, so defined, tended to occur more quickly at low latitudes than at mid-to-high latitudes, with individual durations ranging from 2000 to 12 000 years. This regionally varying behavior implies that the nondipole field played a large role. In detail, reversal records display a wide variety of field behavior (Coe and Glen, 2004). Almost all exhibit large intensity drops during transition, but their directional behavior varies greatly. The most detailed, high-deposition-rate lava-flow records and sedimentary records show that at least some reversals are complex, with episodes of oscillatory and rapid field change (Figures 2 and 3). Often the field reverses briefly, but relapses to intermediate directions one or more times before finally attaining stable opposite polarity. This behavior complicates estimation of duration. For instance, if one includes an early unsuccessful swing to normal polarity in the most recent reversal transition, its duration is about 18 000 years (Figure 4), three times longer than if one considers that swing to be an unrelated precursor (Singer et al., 2005). Inclusion of the precursory swing in direction seems reasonable, in light of the complex transition paths of Figures 2 and 3. Even excluding the precursor, high-resolution sediment records show that this transition is complex, with five or six large swings of the field direction (Figure 5) (Channell and Lehman, 1997). Some dynamo simulations have also produced comparably complex and long reversal transitions – for example, the second reversal during 8.09.1 Observations