Secular variation of the geomagnetic field in Sub-Saharan Africa over the last millennia: new archaeomagnetic data from West Africa and Ethiopia.

Instrumental observations highlight that intertropical Africa is one of the areas with the most rapid changes in the modern geomagnetic field. The secular variation beyond the last centuries is poorly known due to lack of data, this large region representing only 0.7% of the GEOMAGIA50 database. The resulting large uncertainties in global geomagnetic models in this area make in particular the history of the present South Atlantic anomaly unclear. Here, we present archaeomagnetic studies in Southern Benin, Northern Togo, Eastern Senegal and Northeastern Ethiopia. In order to study the full geomagnetic vector, most fieldworks focus on in-situ archaeological structures sampled using the plaster cap technique, one oven in Ethiopia (mid 2nd mill. BCE), 11 iron furnaces in Senegal (500 BCE – 800 CE), 5 iron furnaces in Benin (13-15th c. CE) and 7 iron furnaces in Togo (18-20th c. CE). In Benin, we also sampled displaced baked clays associated to iron metallurgy (kiln walls, tuyeres, potteries).Archaeodirections were determined after thermal demagnetization. We obtained 13 average directions, one in Ethiopia, 5 in Benin and 7 in Togo). The Senegalese structures provided scattered directions, probably because of tilting of the structure walls after the last heating. Archaeointensities were acquired using the classical Thellier-Thellier protocol with anisotropy and cooling rate corrections. The relatively high success rate (75% on average) allowed us to obtain 21 new average data, one on the Ethiopian oven, ten on the Beninese furnaces, 7 on the Togolese ones and 3 preliminary data on the Senegalese structures. The most recent 19-20th c. CE data from Togo are in very good agreement with the local predictions of the geomagnetic global models. At earlier periods, the new data are in agreement with, if any, already published data as in the 13-15th c. CE in West Africa. They highlight faster changes than the models, highlighting their importance to improve our knowledge of the secular variation of the geomagnetic field over the last 4 millennia.