RELATIVE GEOMAGNETIC-FIELD INTENSITY AND REVERSALS FROM UPPER MIOCENESECTIONS IN CRETE

Two Late Miocene sections at Potamida and Kotsiana in western Crete ha ve been resampled at about 10 cm intervals (288 and 130 horizons, resp ectively). The two sections record the same two reversals bounding a n ormal polarity zone just below the Tortonian-Messinian boundary. Miner al-magnetic analysis identifies pseudo single-domain magnetite as the carrier of the main fraction of magnetization in both sections. Normal izations with ARM, SIRM and chi, on different specimens of the same co res, yield very similar results. After checking that it is not correla ted to bulk magnetic parameters, the ARM-normalized record was used as a proxy for geomagnetic field intensity changes. In the lower part of the reverse polarity zone, sampled at Potamida, the field intensity a ppears to increase slightly then to decrease progressively to low valu es, prior to the R/N polarity transition. There is no evidence for a f ast recovery of field intensity across the two transitions: the intens ity remains very low after the R/N reversal and only progressively bui lds to a maximum in the middle of the following normal chron. It then progressively decreases until the N/R transition. No sudden recovery o f the field intensity is observed across this reversal either. These r esults are thus inconsistent with the 'saw-tooth' record obtained by V alet and Meynadier for the last 4 million years. Either the behaviour of the field is different for different intervals of time or the hypot hesis of sedimentary artefacts in the original saw-tooth record should be considered.