Relative geomagnetic field intensity and reversals from Upper Miocene sections in Crete

Two Late Miocene sections at Potamida and Kotsiana in western Crete have been resampled at about 10 cm intervals (288 and 130 horizons, respectively). The two sections record the same two reversals bounding a normal polarity zone just below the Tortonian-Messinian boundary. Mineral-magnetic analysis identifies pseudo single-domain magnetite as the carrier of the main fraction of magnetization in both sections. Normalizations with ARM, SIRM and chi, on different specimens of the same cores, yield very similar results. After checking that it is not correlated 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 appears to increase slightly then to decrease progressively to low values, prior to the R/N polarity transition. There is no evidence for a fast recovery of field intensity across the two transitions: the intensity remains very low after the R/N reversal and only progressively builds to a maximum in the middle of the following normal chron. It then progressively decreases until the N/R transition. No sudden recovery of the field intensity is observed across this reversal either. These results are thus inconsistent with the 'saw-tooth' record obtained by Valet and Meynadier for the last 4 million years. Either the behaviour of the field is different for different intervals of time or the hypothesis of sedimentary artefacts in the original saw-tooth record should be considered.