Paleomagnetic data from recent volcanics of Piton de La Fournaise, Réunion Island: constraints on the evolution of the South Atlantic Anomaly.

The South Atlantic Anomaly (SAA) is an ongoing regional minimum of the Earth’s magnetic field, located beneath South America. The field intensity there is substantially weaker than is expected from the current geocentric axial dipole moment. Currently, the SAA is observed to be expanding and gradually moving westwards. Some geomagnetic field models suggest that the SAA developed underneath the Indian Ocean and has been moving westwards since, while others propose that the anomaly originated under Southern Africa. The current field models that track the onset and evolution of the anomaly, however, are hampered by a lack of paleomagnetic data from the Southern Hemisphere.

Here we present new paleomagnetic directions and paleointensity estimates from Piton de la Fournaise, an active shield volcano on Réunion Island. Réunion Island is located in the Western Indian Ocean (21°S, 55°E) and therefore ideally located to assess whether the SAA once moved from the Indian Ocean westwards, or whether it originated underneath the African continent. A recent study reporting data from Réunion Island revealed a low field intensity of around 30µT for a lava flow dated at ~1600CE. This suggests that the SAA has had an influence in the region at that time. Unfortunately, there are very few datapoints between 1400-1600CE. In our study, we sampled an additional 18 lava flows with ages between 573 and 2019CE, of which seven were deposited between 1400-1600CE. Samples are subjected to rock-magnetic analyses, thermal demagnetization and AF demagnetization methods. Obtained paleointensities with the IZZI-Thellier technique illustrate a high geomagnetic field around 1400CE, after which the field rapidly decreased until 1600CE. This behavior deviates significantly from current geomagnetic models. We discuss the implications these preliminary results have for the evolution of the SAA in the South African region.