We present 45 new determinations of the intensity of the geomagnetic field from basaltic rocks, less than 5 Ma in age, at sites mainly in the southern hemisphere. For 43 samples both Thelliers' (modified by Coe) and Shaw's (modified by Kono) methods were used and in some cases repeat measurements were made on several specimens. The results using the two methods are in excellent agreement and show no systematic difference. The within-specimen and between-sample measurements at each site exhibit internal consistency that can be associated with standard errors in the palaeointensities of 10%.
Very importantly the specimens for the determinations were individually selected using a new simple method based on low temperature susceptibility variation down to liquid nitrogen temperatures. Better than 90% success rate was achieved with such specimens in providing good palaeointensities using either method. We attribute the high success rate and the agreement between methods to the method of specimen selection.

View full abstract

Virtual Dipole Moments (VDMs) are analysed to determine the distribution of True Dipole Moments (TDMs) from which they were derived. The data set used is all the available VDMs for the past 5 million years which have associated Virtual Geomagnetic Poles (VGPs) with latitudes greater than 45°. These data do not support a model which predicts a cyclic variation of the dipole moment during stable polarity periods. Neither do they support a model in which the field strength of the non-dipole components is a constant ratio of the mean dipole field strength. Instead they support a model in which the TDMs have a truncated Gaussian distribution and the field strength of the non-dipole components is linearly proportional to the TDM. These components introduce a Gaussian distributed scatter to the observed VDMs with standard deviation about 18.6% of the dipole moment, and the palaeointensity determinations themselves introduce a further Gaussian distributed scatter with standard deviation about 10%. The data give no reason to reject the hypothesis of a common mean and variance for the (untruncated) Gaussian distribution of the normal and reverse polarity TDMs but the hypothesis of a common truncation point can be rejected. This indicates a difference in the overall properties of the two polarity states.
The term Palaeomagnetic Dipole Moment (PDM) is introduced for the value at which the truncated Gaussian distribution of TDMs peaks. As determined from the present data the PDM for the past 5 million years is 8.67±0.65×10²²Am² with 95% confidence. The estimated standard deviation of the untruncated distribution is 3.63±0.75×10²²Am² with 95% confidence, this variation (41.9%) relating solely to variation in the dipole moment itself. This reinforces the view that fluctuations in the dipole moment are much greater than are observed in data for the past 10⁴ years, suggesting that they must occur over time scales of 10⁵ or 10⁶ years.

View full abstract