Earth’s magnetic field preserved in ancient bricks

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Archaeomagnetic dating is the term used to describe the study of the history of the Earth’s magnetic recorded in archaeological materials. Research published in the Proceedings of the National Academy of Sciences details how its authors used this method to gain insight into an anomaly in the Earth’s magnetic field 3,000 years ago though the analysis of ancient, Mesopotamian bricks inscribed with the names of their kings.

To do this, the team used 32 bricks sourced from around Mesopotamia, which overlaps with modern day Iraq, and analysed the latent magnetic signature in grains of iron oxide minerals present within them. This was possible because the Earth’s magnetic field strength was imprinted on these grains when the bricks were first fired by their millennia ago as these minerals, when hot, are sensitive to magnetic fields. The exact time the bricks were made could be ascertained because each contained an inscription of the reigning king meaning historians could match this with current knowledge of Mesopotamian rulers.

The Earth’s magnetic field strength at the time was imprinted on grains of iron oxide within the bricks that are sensitive to magnetic fields when hot

Co-author of this research, Professor Lisa Tauxe of the Scripps Institution of Oceanography, explained the importance of such archaeological materials as the “well-dated archaeological remains of the rich Mesopotamian cultures, especially bricks inscribed with names of specific kings, provide an unprecedented opportunity to study changes in the field in high time resolution, tracking changes that occurred over several decades or even less.”

By undertaking this project, the team were able to confirm a period in history between 1050 and 550 BCE where Earth had an unusually strong magnetic field. This is known as the ‘Levantine Iron Age geomagnetic Anomaly’ (LIAA) and evidence for its existence has been detected in China, Bulgaria, and the Azores.

Earth had an unusually strong magnetic field between 1050 and 550 BCE

This research is important because there is currently little data from the southern part of the Middle East. Some of the bricks analysed produced during the reign of Nebuchadnezzar II (604 to 562 BCE) even show drastic changes in the Earth’s magnetic field over a seemingly brief period of time. This research, however, does not help or claim to help explain the cause of such anomalies, with researchers still calling the reasons for such events as the LIAA “unclear”.

However, the process can also be used in the other direction. Co-author, Professor Mark Altaweel (UCL Institute of Archaeology) explained how they can use this information to find the age of cultural remains that have been heated like ceramics and bricks but that do not contain organic material and so cannot be dated using other methods such as radiocarbon dating.

Similarly, another benefit of archaeomagnetic dating is that it allows historians to create a more accurate picture of ancient kings who have ambiguous and debated reigns. Although the length and order of these reigns is usually known, there is debate over the precise years each king took the throne. Using this method, the researchers found their results matched with what archaeologists call the ‘Low Chronology’ of the reigns.

With both aspects of this research working hand in hand with each other, it will be interesting to see how it is further implemented in both fields of study in the future and whether it becomes a more common form of archaeological dating.

Image: Osama Shukir Muhammed Amin FRCP(Glasg) via Wikimedia Commons

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