Evidence for short geomagnetic polarity intervals in the Early Cenozoic

Abstract

To test the validity of the theory that geomagnetic reversals are produced by a stochastic process in the earth's core, a search was made for heretofore undetected short polarity intervals in marine magnetic anomalies over segments of ocean floor with ages between 55 and 63 m.y. The original Heirtzler time scale shows almost no short events during this time, and it is thus indicated that reversals are not a Poisson-distributed stochastic process. In our analysis we reduced profiles to the pole to eliminate asymmetry, stretched them to bring them to a common spreading rate, and stacked them to attenuate incoherent noise. Six new short events were determined by modeling the profiles with a single crustal layer. To simulate the process by which new oceanic crust is created by dike injection and extrusion of lava flows, the magnetization of the layer was determined by convolving model time scales with a Gaussian function. A standard deviation of 3 km gave the best fit to the anomalies. All the new events were found to be <60,000 years. When the geomagnetic time scale is modified by the addition of these six short polarity intervals, it has a frequency distribution close to that predicted by the stochastic model.