Geomagnetic field intensity during the last 5 Myr: LTD-DHT Shaw palaeointensities from volcanic rocks of the Society Islands, French Polynesia

Abstract

SUMMARY On the basis of the Thellier palaeointensities reported so far it has been thought that the time-averaged virtual dipole moment (VDM) and virtual axial dipole moment (VADM) for the last few million years are almost the same as the present geomagnetic dipole moment (∼8 × 1022 A m2). This estimate has been called into question, however, because recent studies have revealed that the Thellier method occasionally overestimates palaeointensities by as much as twice the true values. In contrast, a recently developed palaeointensity technique, namely the double heating technique of the Shaw method combined with low-temperature demagnetization (the LTD-DHT Shaw method), can yield expected field intensities from samples which give unreliable palaeointensities using the Thellier method. Therefore, we have measured absolute palaeointensities from 0.5–4.6 Ma volcanic rocks from the Society Islands, French Polynesia, mainly using the LTD-DHT Shaw method. As a result, 195 out of 361 samples passed the selection criteria, some of which are compared with additional results obtained with Coe's version of the Thellier method. In the Thellier experiments, 18 out of 40 samples passed the criteria, giving palaeointensities both consistent and inconsistent with the LTD-DHT Shaw results. These samples are characterized mostly by two-segmented Arai diagrams. If we take the LTD-DHT Shaw palaeointensities, 24 reliable site means are available and give a mean VADM of (3.64 ± 2.10) × 1022 A m2. This is nearly half of the mean of the 0–5 Ma Thellier data selected from the latest palaeointensity database [(7.46 ± 3.10) × 1022 A m2, N= 458] as well as the present dipole moment. The LTD-DHT Shaw palaeointensities newly determined in this study suggest that the present-day field is so strong that it may not be typical of the past geomagnetic field.