Continuous record of geomagnetic field intensity between 4.7 and 2.7 Ma from downhole measurements

A continuous record of the geomagnetic field intensity from 4.7 to 2.7 Ma has been obtained, together with a precise magnetostratigraphy, from downhole magnetic measurements at Site 884 of ODP Leg 145 in the North Pacific. The record confirms the saw-tooth pattern of geomagnetic field intensity proposed by Valet et Meynadier [10]. Reversals are characterized by a steep intensity decrease followed by a quick regeneration. Over each polarity interval, rapid variations are superimposed over a progressive decrease in the mean intensity. We find that the duration of each polarity interval is inversely proportional to the mean rate of decrease in the field intensity over this period, and thus this duration seems to be pre-determined.