Archeo- and paleomagnetic records from Saint Helena, South Atlantic

The South Atlantic Anomaly (SAA) is a dynamic feature of the geomagnetic field. Systematic observations have revealed an expansion of the SAA in terms of its geographical extent and a decrease in field strength over the last decades. Its evolution can be reconstructed further back in time by means of archeo- and paleomagnetic investigations conducted on suitable recorders of the ancient geomagnetic field. Historical geomagnetic field models indicate a westward movement of the SAA from Africa during the last centuries, while its emergence is still under debate, mainly due to the lack of ancient field records in the Southern hemisphere. However, even on much longer timescales, paleomagnetic studies on volcanic rocks from the South Atlantic have suggested a persistence or recurrence of the anomaly.
Here we present ancient geomagnetic field records from the island of Saint Helena from two different periods. Archeomagnetic directions and intensity were obtained from a historical lime kiln that operated until the middle of the 19th century. The reconstructed geomagnetic field components agree well with available instrumental measurements from the island. Geological timescales are covered by paleomagnetic examinations of lava flow profiles from Prosperous Bay and Munden’s Hill. A geomagnetic polarity transition was recorded by some flows at Properous Bay. Several other flows characterized by stable field directions are associated with weak intensities below 20 µT. These results suggest a prolonged anomalous field behavior in the South Atlantic region as indicated by previous studies.