Lower mantle convection and geomagnetism

Abstract

The statistical properties of recent marine magnetic anomaly time scales, covering the period from 165 Ma to the present, are analyzed via the methods presented in the companion paper (McFadden, this issue). The data no longer support the hypothesis that during the last 86 Ma there were two periods, separated by a discontinuity at about 45 Ma, when the properties were stationary. Instead there is strong evidence that the rate of reversals decreased in an approximately linear manner with time from 165 Ma until it reached zero, resulting in the Cretaceous Normal Polarity Interval. At about 86 Ma ago the process of reversals began again, and the rate of reversals increased linearly with time. There is a slight indication that the rate of reversals reached a maximum about 10 Ma ago and is again decreasing. Analysis of the reversal sequences shows there is no good evidence for the concept of differences in the relative stabilities of the two polarity states, nor need such a concept be invoked to explain long intervals without reversals, such as the Cretaceous Normal Polarity Interval. Quite clearly there has been some form of thermal perturbation at the base of the mantle with a characteristic time of the order of 108 years. Our interpretations require convection deep within the mantle and suggest that the temperature at the base of the mantle attains a maximum during long magnetic quiet intervals.