A 6000 years speleothem paleomagnetic record of the South Atlantic Anomaly in central South America

The decay of the Earths magnetic dipole over the past few centuries has been accompanied by the increase of the South Atlantic Anomaly (SAA). Yet, the origin and persistence of this feature at millennial scales remains a matter of debate. Here, we report results from two caves from central South America located near the present-day SAA minimum spanning the last 6000 years. Speleothems from Pau d'Alho cave track the geomagnetic record from the present to 1500 BP and show rapid directional variations (>0.1 /yr) between ~1090-990 BP and ~500-200 BP. A similar pattern of directional variation is observed in data from South Africa, but precedes the South American record by 224 50 years. These results suggest that such fast geomagnetic field variations are a recurrent feature associated with the SAA, and the lag between records may reflect the westward migration of the anomaly. This geomagnetic behavior qualitatively resembles behavior observed in synthetic models of reversed magnetic flux patches at the core-mantle boundary and their expression at the Earths surface over 400 year timespans. A stalagmite from Dona Benedita cave provides a record from 3147 BP to 5306 BP, and reveals angular variations less than 0.1/yr with relatively steady paleointensity variations. These different regimes suggest that when the ratio of non-dipole to dipole components is high (> 0.02), as in the past 1500 BP, the observed field behavior is strongly influenced by the mid-latitude reversed flux-patches. Conversely, when the non-dipole components are weak, as in the 3000-5000 BP period, the field at the surface remains more stable and features like the South Atlantic Anomaly are not expressed. These results document a quiescent period of geomagnetic field behavior during the mid-to-late Holocene in an area now influenced by the SAA, and suggest that this kind of anomaly has an intermittent or absent expression at the Earths surface at multimillennial timescales.