GEOMAGNETIC REVERSAL BEHAVIOR SINCE 100 MA

The new magnetic polarity time scale of Cande and Kent (J. Geophys. Re s., 97: 13917-13951, 1992a) provides an opportunity to analyse further the random character of the geomagnetic polarity sequence since 100 M a. Large differences in the peak amplitudes of changes in magnetic rev ersal frequency are observed when the previous Harland et al. (A Geolo gical Time Scale, Cambridge University Press, 1990) and new Cande and Kent sequences are compared. These differences illustrate remaining un certainties in the significance of these changes. When the successive lengths of polarity intervals are displayed as a function of their ord er of occurrence in the sequence, a change in behaviour is observed mi dway through the series. Flat 'white noise'-like behaviour is observed for the last 100 intervals (from about 25 Ma to the present). For old er intervals, a quasi-periodic pattern in interval duration is observe d. This evolution is more apparent in the Cande and Kent time scale. T he quasi-periodicity of interval duration as a function of occurrence does not translate into periodicity of reversal frequency as a functio n of time (from 85 Ma to about 25 Ma). Our description of the sequence is reminiscent of the findings of Dubois and Pambrun (C.R. Acad. Sci. , 311: 643-650, 1990), who suggested that low-dimensional deterministi c chaos, between 85 and 25 Ma, preceded a random process occurring sin ce then. The details of a long-term control (or memory) of the geomagn etic field remain an open question. Addition of independent informatio n, such as palaeointensity data, will probably be necessary to uncover any determinism in reversal chronology.