Thousands of Years Ago: Ancient Clay Tablets Reveal Mysterious Fluctuations in the Earth's Magnetic Field

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Clay tablet: Dated to the reign of Nebuchadnezzar II

Photo: Slemani Museum

At first glance, the clay tablets mainly contain written information, such as the names of kings who ruled Mesopotamia 3000 years ago, for example in the area of today's Iraq. But there are other clues in the material: a team of experts has used the burned writings to reconstruct a strange anomaly in the Earth's magnetic field. The study was published in the journal Proceedings of the National Academy of Sciences.

"We often rely on dating methods such as radiocarbon to trace the chronological sequence of events in ancient Mesopotamia," said study author Mark Altaweel of University College London. However, some of the most common remains from the period of fired clay and pottery could not be dated with it.

So the team of experts took a different approach: they analysed the magnetic signature of iron oxide-containing minerals from a total of 32 clay tablets from archaeological sites in Mesopotamia. Magnetic substances such as iron react to the Earth's magnetic field. Indications of its strength were thus burned into their works by the makers of the panels.

Reconciled with reigns of kings

At the time of production, the name of the reigning king was inscribed on each clay tablet. The experts combined this information with knowledge of the order and approximate reign of the rulers. From the engraved names and the information from the iron oxide grains, they created a kind of historical map of the changes in the Earth's magnetic field.

The results confirm the existence of the "geomagnetic anomaly of the Levantine Iron Age". This refers to a period in which the Earth's magnetic field in the area of present-day Iraq was unusually strong between about 1050 and 550 BC for unclear reasons. So far, experts have found evidence of the anomaly in China, Bulgaria and the Azores, but little data existed from the southern part of the Middle East itself.

The analysis also revealed that the Earth's magnetic field changed dramatically within a relatively short period of time almost 3000 years ago. The underlying samples are reportedly dated to the reign of Nebuchadnezzar II between 604 and 562 BC and support the hypothesis that rapid successive peaks in the intensity of the Earth's magnetic field are possible.

The Earth's magnetic field surrounds the globe like a protective mantle and is generated by currents of liquid iron in the Earth's outer core. It keeps harmful cosmic rays largely away from the Earth's surface. The magnetic field also allows orientation by compass. However, the Earth's magnetic field is not constant, the strength fluctuates permanently. However, the phenomenon still raises many questions today.

"The geomagnetic field is one of the most enigmatic phenomena in the geosciences," said co-author Lisa Tauxe of the Scripps Institution of Oceanography. "The well-dated archaeological remains provide an unprecedented opportunity to study changes in strength at high temporal resolution, tracing changes that have occurred over several decades or even less."

Conversely, experts can now also date archaeological finds more precisely on the basis of their magnetic signatures. "By comparing ancient artifacts with what we know about the conditions of the magnetic field at the time, we can estimate exactly when they were fired," said lead author Matthew Howland of Wichita State University.

Ani