A first regional model of the past Earth's magnetic field from Africa for the last 4000 years

Abstract

The understanding of the Earth's magnetic field variations over time on the African continent is fundamental for several reasons. For instance, the most important feature of the present geomagnetic field, the South Atlantic Anomaly (SAA) characterized by weaker geomagnetic strength values than those expected for their latitudes, may have emerged in South Africa at the beginning of the second millennium CE. Here, we first selected the available volcanic and archaeomagnetic data following a set of three criteria inspired by the FAIR principles. We then built a first regional geomagnetic model for Africa covering the last 4000 years, using a revised version of the spherical cap harmonic (SCH) analysis in 2 dimensions. The new regional model shows, at the Earth's surface, the westward migration of the SAA from the Indian Ocean over Africa since 1100 CE. In addition, the regional model is tested as a paleomagnetic dating tool by re-dating previous archaeomagnetic data from Africa and thus can be used to date other African archeological sites and the numerous active and dormant volcanoes of the East African System. Plain language summaryThe spatial and temporal evolution of the Earth's magnetic (geomagnetic) field is based on direct historical or instrumental data and the indirect information extracted from rocks and archeological artifacts (palaeo- and archaeomagnetic data). While the present field is well sampled at both spatial and temporal domains, this is not the case for the past evolution of the geomagnetic field, where the paleomagnetic data are not globally distributed. There are large areas in the Southern Hemisphere yet to be investigated. However, recent data from Africa enables us the building of the first regional geomagnetic model for the last 4000 years. The new regional model sheds light on the past evolution of the most remarkable feature of the present geomagnetic field, the South Atlantic Anomaly (SAA), showing details of the westward migration of the SAA from the Indian Ocean over Africa since 1100 CE. Moreover, the new model refines the use of paleomagnetism as a dating tool for volcanic units from East African Rift System and for the abundant African archeological sites improving the knowledge of the history of human civilizations.