MISSING REVERSALS IN THE GEOMAGNETIC POLARITY TIMESCALE - THEIR INFLUENCE ON THE ANALYSIS AND IN CONSTRAINING THE PROCESS THAT GENERATES GEOMAGNETIC REVERSALS

A major problem in defining the chronology of geomagnetic reversals is linked to the detection of short (<30 kyr) time intervals between rev ersals (TIBR). Published polarity timescales do not usually include th e shortest TIBR; therefore the timescales are inherently incomplete. T he purpose of this paper is to investigate the effects of this incompl eteness on the analysis of the geomagnetic polarity timescale; the ult imate goal is to provide constraints on the Earth's core processes. Th e effects of inclusion/exclusion of the shortest TIBR are verified by comparing the results obtained by statistical analysis of real and syn thetic series of events; for the real sequence, two basic cases are co nsidered in which the ''tiny wiggles'' are attributed either to short TIBR or to paleointensity fluctuations. Particular attention is paid t o the influence of measurement errors estimated for the most recently published Cenozoic timescale. By following the minimalist philosophy o f Occam's razor, which is particularly suitable for studying poorly kn own processes, the reliability of the simplest model, i.e., the Poisso n process which is symmetric in polarity, can be checked. The results indicate the plausibility of a generalized renewal process; the only r egularity is relative to the long-term trend, which is probably linked to core-mantle coupling. In detail, a uniform exponential trend in th e last 80 Myr is found for the timescale; it is not presently possible to estimate the influence of the inclusion of tiny wiggles because th ey are well-resolved only in the last 30 Myr, a period in which both s eries are stationary. The sequences, with and without tiny wiggles, ar e symmetric in polarity, with no evidence of low-dimensional chaos and memory of past configurations. The empirical statistical distribution of the TIBR departs slightly from a theoretical exponential distribut ion, i.e., from a Poisson process, which can be explained by a lack of short anomalies, and/or by a generating process with wear-out propert ies (a more general renewal process). A real exponential distribution is sustainable only if the number of missing short TIBR in the last 30 Myr is larger than the number of tiny wiggles observed in the same pe riod.