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, fr om downhole magnetic measurements at Site 884 of ODP Leg 145 in the No rth Pacific. The record confirms the saw-tooth pattern of geomagnetic field intensity proposed by Valet et Meynadier [10]. Reversals are cha racterized by a steep intensity decrease followed by a quick regenerat ion. Over each polarity interval, rapid variations are superimposed ov er a progressive decrease in the mean intensity. We find that the dura tion of each polarity interval is inversely proportional to the mean r ate of decrease in the field intensity over this period, and thus this duration seems to be pre-determined.