Tracing the history of the South Atlantic Anomaly on an Archeomagnetic Timescale

The dramatic decay of the dipole geomagnetic field during the last 160 years has been driven by growth of a low intensity region in the South Atlantic that we refer to as the South Atlantic Anomaly (SAA). The rapid decay relative to ohmic resistance has motivated models of core flux expulsion. A prominent core patch of reversed polarity flux beneath South Africa has been called for on the basis of analyses of modern field data recorded by satellites. The rapid decay and similarity of the SAA to patterns seen in some numerical simulations of reversals has driven speculation that Earth is heading toward a field reversal. Putting the SAA in a longer temporal context, however, has been difficult because of the lack of archeomagnetic data from this key region of the Southern Hemisphere. To address this data deficiency, we report the first archeomagnetic curve from Early, Middle and Late Iron Age sites from South Africa, dated between ca. 700 and 1650 AD. The floors of burnt structures (huts, grain bins and kraals) resulting from ritualistic village cleansing provide opportunities to obtain directional data. Directions reveal times of modest change (ca. 0.04 degrees/yr, ca. 730 to 1225 AD), followed by a time of rapid change (ca. 0.11 to 0.15 degrees/yr, 1225 to 1580 AD). Preliminary paleointensity data from the burnt structures and coeval pottery show field strengths that are comparable to the lowest values seen in the SAA today during the period of rapid field directional change. Implications of these data for core processes will be discussed.