Do changes in geomagnetic secular variation, dipole moment and polarity reversal frequency correlate over the past 155 Myr?

Changes in palaeosecular variation, dipole moment and polarity reversal frequency are salient features of the Earth's magnetic field over the geological past, yet how these changes are linked by the geodynamo remains controversial. To further understand this issue, we provide new absolute (API) and relative (RPI) palaeointensities from the similar to 1-km-thick basaltic sequence of Waja (North Ethiopia) emplaced around 31 Ma, yielding an instantaneous virtual dipole moment of 57 +/- 9 ZAm(2) (1 sigma, N = 18) and a relative variability in intensity epsilon(F) = 0.39 +/- 0.07 (1 sigma, N = 19). Our analysis of the API database with strict selection criteria (inclusion of Thellier-style determinations with pTRM checks only, at least five determinations per cooling unit, and within-unit relative standard error lower than 10 per cent) fails to identify any robust correlation between changes in dipole moment and reversal frequency over the past 155 Myr. More convincingly, the available RPI results are consistent with an increase of the palaeosecular-variation proxy epsilon(F) with reversal rate, as predicted by numerical dynamo simulations. We also find that the API-based estimate epsilon(F) = 0.40 +/- 0.03 (1 sigma, N = 104), computed from the filtered version of the World Palaeointensity Database for the 0.77-31 Ma interval, is consistent with the scaling rule, suggesting that the API record has been sufficiently sampled over the past 31 Ma. We thus speculate that the absence of negative correlation between changes in dipole moment and reversal frequency in the API database over the past 155 Myr may be the result of insufficient sampling prior to 31 Ma rather than the signature of an intrinsic geomagnetic feature.