f Geomagnetic Field Strength Recorded in Iron Age Ceramics from Southern Africa

The Earth’s dipole magnetic field has decreased in intensity by 5 to 10 percent over the last 150 years. To understand more about the nature of this decline, it is desirable to have a longer record, extending back in time beyond the data available from modern magnetic observatories. Firing of archeologic objects (e.g. ceramics and hut floors) raises temperatures above the Curie temperatures of their constituent magnetic minerals. Paleointensity analyses of fired artefacts-- commonly known as archeomagnetism—can provide longer term geomagnetic records. Such studies are well developed at North Hemisphere sites, but there are no extensive records from the Southern Hemisphere for the millennium prior to the era of observatory recording. There is an extensive record of Iron Age ceramics from southern Africa, with associated burnt structures that has the potential to provide a Southern Hemisphere geomagnetic field record. We discuss our efforts to test the potential of this record using rock magnetic and paleointensity analyses of Iron age test samples. Rock magnetic results to date suggest that Iron age Iron age ceramics contain magnetic minerals of appropriate composition and size to retain a high resolution record of past field intensity. Data from preliminary paleointensity experiments further support this conclusion.