Y. Gallet et G. Hulot, STATIONARY AND NONSTATIONARY BEHAVIOR WITHIN THE GEOMAGNETIC POLARITYTIME-SCALE, Geophysical research letters, 24(15), 1997, pp. 1875-1878
We analyse the geomagnetic polarity time scale (GPTS) since the Upper
Jurassic by displaying the successive lengths of polarity intervals as
a function of their order of occurrence. The sequence consists of thr
ee segments. Between the Upper Jurassic and the Lower Cretaceous, segm
ent ''A'' comprises intervals of short duration, with a mean duration
of about 0.29 My, and no clear long-term evolution. Segment ''B'' begi
ns around 130 Ma, displays a sudden increase of the duration of the ma
gnetic intervals, an interval of maximum duration, the normal Cretaceo
us superchron, and a long and erratic sequence of intervals with decre
asing average duration between 85 Ma and about 25 Ma. From 25 Ma to th
e present, segment ''C'' consists of intervals of short duration with
a mean value of 0.23 My. This description suggests that the Earth's ma
gnetic field could have experienced a fairly stationary regime until s
lightly before the onset of the Cretaceous superchron, when the regime
has been rapidly and strongly perturbed before progressively returnin
g to another stationary regime about 25 Ma ago. A geophysical explanat
ion for this sequence of events could be that the geodynamo has been p
erturbed by the arrival of some cold material at the core mantle bound
ary. As this material would have heated up, the geodynamo would have b
een brought back to its stationary regime.