Evolution of the geomagnetic field prior to the Matuyama-Brunhes transition: radiometric dating of a 820 ka excursion at La Palma

We present Cassignol technique K-Ar dating of lava flows from La Palma (Canary Islands, Spain) that bracket the Matuyama-Brunhes transition. An age of 821 +/- 13 ka obtained for a transitionally magnetized flow (LS118) provides the first volcanic evidence for a geomagnetic excursion occurring about 40 kyr prior to the transition. This interval has been successfully correlated with intensity minima present in high-resolution deep-sea records before the Matuyama-Brunhes transition. This study, along with the growing number of well-dated excursions reported for the Brunhes and Matuyama Chrons, shows that the occurrence of excursions is a common feature of the evolution of the geomagnetic field during stable polarity intervals. However, the presence of two successive excursional states within 40 kyr prior to the actual reversal, conjugated with moderate averaged palaeointensity in this interval as deduced from deep-sea records, could suggest that favorable conditions for the reversal to occur existed since about 820 ka. The present study highlights the importance of detailed chronological constraints for our understanding of the transition of the geomagnetic field from a stable polarity to a reversal state.