Absolute paleointensities recorded during the Brunhes chron at La Guadeloupe Island

Ten absolute paleointensity determinations within the Brunhes chron have been obtained on andesitic lava flows from La Guadeloupe Island, French West Indies (F.W.I.). The Thellier and Thellier method performed on 124 specimens under either ambient or argon atmosphere allows reliable determinations from high temperature steps. A satisfactory within flow reproducibility has been observed and at least two samples for each flow have been used for calculation of the weighted-mean paleointensity. The average virtual axial dipole moment (VADM=7.1±1.8×10²² A m²) for normal polarity flows from the Brunhes chron recorded at La Guadeloupe is compatible with the historical field. The 10 VADM values obtained show a relatively good agreement with the deep-sea composite record (SINT800) of relative paleointensity. Focusing on the 100–75 ka interval, a 20-ka oscillation observed from a compilation of available volcanic data and present in high-resolution sedimentary records has been tentatively related to the axial dipole moment variations. Finally, a paleointensity decrease of a factor of 10 is observed for flows emitted during the Matuyama–Brunhes (M–B) transition, as already observed in other records of this reversal. Because a large area of the Globe around the Caribbean Islands was previously devoid of data, the present dataset fills a gap towards construction of a global paleointensity database for the Brunhes chron.     

Over how much time should the geomagnetic field be averaged to obtain the mean-palaeomagnetic field?

Average geomagnetic field models are now available over six orders of magnitude in time. Models based on Present and historical, archaeomagnetic and palaeomagnetic data illustrate how the amplitudes of secular variation of nonaxial dipole terms strongly decrease as a function of averaging window, mostly when jumping from the 1-yr to the 1700-yr average. The mean archaeomagnetic field bridges the gap between the complex historical field and the far simpler palaeomagnetic field, and appears to be much closer to the latter. Confidences in average field coefficients based on statistical field models give a quantitative basis to the practice of averaging data over a few millenia in order to 'suppress secular variation'. In particular, the axial quadrupole emerges from noise when the available time series exceed about 6 kyr. This happens to be close to the timescales of excursions and reversals. It is now a priority to obtain detailed data spanning the last 50 kyr.