The planet's wandering magnetic poles

Most centuries during the past 4 millennia are represented by at least 10 data points. However, few archaeological sites have been dated to the centuries between A.D. 600 and 800, which historians often refer to as the Dark Ages. Data for the centuries before 1000 B.C. are similarly sparse.

During the past 4 millennia, magnetic declinations in Britain have varied through an angle of 70° and their magnetic dips have ranged about 25°, the researchers report.

To test their model, Batt and her colleagues analyzed an archaeological site that was exposed during construction in downtown Exeter, England. The city has been continuously populated since the Roman period, so sites there often include a jumble of artifacts from different periods. One sample the team analyzed probably came from a fireplace in a home or other structure. Another sample was, most likely, just a spot of burned soil.

The combination of declination and dip found in the fireplace sample suggest the material could have been last heated during any of three intervals during the past 4 millennia, says Batt. However, because two of those intervals long predate known occupation in the area, the researchers dismiss those possibilities. Therefore, the fireplace probably last hosted a fire in the 11th century, during Europe's early-medieval period. The burnt spot of soil is a century or so older than that, the magnetic data suggest.

Using paleomagnetic data offers archaeologists "a good tool to figure out who occupied a particular area at a particular time, and what they were doing," says Batt.