© Oded Lipschits
A stamped handle from Ramat Rahel. Such fragments are an archive of the strength of the Earth's magnetic field at the time they were made.
A collection of clay storage jar handles from ancient Judah - modern-day Jerusalem - is providing fresh insight into fluctuations in the Earth's magnetic field.

It's long been known that the magnetic field - extending from the centre of the planet out to about 65,000 kilometres above the surface - changes over time. The north and south poles shift to and fro over centuries, and every few hundred thousand years they even swap places.

Since the development of pottery, these movements have been inadvertently encoded in human artefacts. Clay contains small amounts of a mineral known as magnetite. When fired in a kiln, the magnetite transforms into a permanent record of the intensity and alignment of the magnetic field at that moment.

Today, movement in the magnetic fields are made accurately and easily, but recreating past shifts - a field known as archaeomagnetism - is confounded by, among other things, difficulties in accurately estimating the age of artefacts used in the dating process.

But now a research team led by Erez Ben-Yosef of Tel Aviv University in Israel has used samples from clay jars dating from between the eighth and second centuries BCE to produce extremely accurate measurements of field activity in the Levant.

The key to the team's precision lies in the welcome habit of the ancient Judeans of marking their jars with stamps specific to the ruling entities of the area. As rulers came and went, so too did the inscriptions.

© Oded Lipschits
A concentric incision on a jar handle from Ramat Rahel.
"The typology of the stamp impressions, which corresponds to changes in the political entities ruling this area, provides excellent age constraints for the firing event of these artefacts," write the researchers in the Proceedings of the National Academy of Sciences.

Because the stamps can be correlated against other records of Judean history, the samples used in the study could each be located within known periods ranging predominantly between 100 to 50 years.

The results, say the scientists, constitute an "unparalleled record of the geomagnetic field intensity".

The data showed an overall decline in field intensity during the 600-year period covered - but also revealed a short, sharp upward movement during the eighth century.

The evidence supports earlier studies that point towards two such upticks in the magnetic field, dubbed the Levantine Iron Age "geomagnetic spikes."

As well as confirming the second of these spikes, Ben-Yosef's team also found that the magnetic field, at least around the Middle East, went into equally sharp decline just after 732 BCE, losing 27% of its value in just three decades.

"Age constraints from archaeological contexts and stamped jar handles during the second half of the eighth century BCE southern Levant are exceptionally tight," the study reports. "The region was influenced by Assyrian interventions that resulted in excellent chronological markers in the archaeological record."