A first archeomagnetic record from the South Atlantic: Investigation of a lime kiln on St. Helena

Abstract

The geomagnetic field is anomalously weak in the region of the South Atlantic causing a reduced shielding effect against energetic cosmic particles and malfunctions of satellite communication. The evolution of this so-called South Atlantic Anomaly can be reconstructed for past times by means of archeomagnetic investigations conducted on suitable recorders of the ancient geomagnetic field. Here we present a first archeomagnetic study from the South Atlantic region on a lime kiln operated until the middle of the 19th century on the island of St. Helena. The laboratory experiments comprised basic rock magnetic investigations as well as demagnetization and archeointensity experiments using the MT4 protocol along with cooling rate and anisotropy corrections. Reconstructed field directions show a rather large scatter mainly due to the limited number of samples. While the effects of anisotropy are found to be negligible, cooling rate effects account for ∼11% on average yielding a corrected intensity slightly lower than expected from instrumental measurements performed at St. Helena. Nevertheless, the comparison with historical records and field models yields a good agreement with respect to the obtained archeomagnetic field components and associated uncertainties. Furthermore, the new full-vector record represents an important complement for archeomagnetic databases and, therefore, will constrain archeomagnetic field modelling approaches in the South Atlantic more accurately.