An Archeomagnetic Record From Southern Africa and its Bearing on the History of the South Atlantic Anomaly

Abstract:

The low intensity area in Earth’s recent magnetic field that spans the southern Atlantic Ocean, Africa and South America is commonly called the South Atlantic Anomaly (SAA). Many believe the SAA is linked to the dramatic decay of the dipole geomagnetic field intensity during the last 160 years, and the growth of a reversed core flux patch beneath South Africa. The dipole field decay has even motivated speculation that Earth is heading toward a geomagnetic field reversal. But understanding these phenomena within the context of longer-term geomagnetic history has been limited by a lack of Southern Hemisphere archeomagnetic data. We have recently presented the first archeomagnetic data from Iron Age sites of southern Africa (~1000-1650 AD) (Tarduno et al., 2014, 2015). Magnetic data show a sharp intensity drop at ~1300 AD, at a rate comparable to modern field changes in the SAA, (but to lower values). These changes motivated our conceptual model whereby the recurrence of low field values reflects magnetic flux expulsion from the core, promoted by the unusual core-mantle boundary composition and structure beneath southern Africa defined by seismology (specifically the African Large Low Velocity Seismic Province, or LLVSP). Because the African LLVSP is a longstanding structure, we expect this region to be a steady site of flux expulsion, and perhaps the triggering site for reversals, on time scales of millions of years. Here we discuss our ongoing efforts to extend the archeomagnetic record from southern Africa back in time, and further develop the flux expulsion- African LLVSP hypothesis, through new sampling and paleomagnetic analyses of Iron Age burnt huts, grain bins and kraals from South Africa, Zimbabwe and Botswana.