Reconstructing the behavior of Earth’s magnetic field during archaeological periods is crucial for both achieving a better understanding of the field and related natural phenomena and for providing a basis for absolute dating archaeological materials. In new research, University College London’s Professor Mark Altaweel and his colleagues analyzed 32 inscribed baked bricks from Mesopotamia from the 3rd-1st millennia BCE (the reigns of 12 Mesopotamian kings).
This ancient brick from the Slemani Museum dates to the reign of Nebuchadnezzar II (ca. 604 to 562 BCE) based on the interpretation of the inscription; this object was looted from its original context before being acquired by the Slemani Museum and stored in that museum with agreement from the central government. Image credit: Howland et al., doi: 10.1073/pnas.2313361120.
“We often depend on dating methods such as radiocarbon dates to get a sense of chronology in ancient Mesopotamia,” said Professor Altaweel, co-author of a paper published in the Proceedings of the National Academy of Sciences.
“However, some of the most common cultural remains, such as bricks and ceramics, cannot typically be easily dated because they don’t contain organic material.”
“This work now helps create an important dating baseline that allows others to benefit from absolute dating using archaeomagnetism.”
The magnetic field of the Earth weakens and strengthens over time, changes which imprint a distinct signature on hot minerals that are sensitive to the magnetic field.
The study authors analyzed the latent magnetic signature in grains of iron oxide minerals embedded in 32 clay bricks originating from archaeological sites throughout Mesopotamia, which now overlaps with modern day Iraq.
The strength of the magnetic field was imprinted upon the minerals when they were first fired by the brickmakers thousands of years ago.
At the time they were made, each brick was inscribed with the name of the reigning king which archaeologists have dated to a range of likely timespans.
Together, the imprinted name and the measured magnetic strength of the iron oxide grains offered a historical map of the changes to the strength of the Earth’s magnetic field.
The researchers were able to confirm the existence of the Levantine Iron Age geomagnetic Anomaly, a period when Earth’s magnetic field was unusually strong around modern Iraq between about 1050 to 550 BCE for unclear reasons.
Evidence of the anomaly has been detected as far away as China, Bulgaria and the Azores, but data from within the southern part of the Middle East itself had been sparse.
“By comparing ancient artifacts to what we know about ancient conditions of the magnetic field, we can estimate the dates of any artifacts that were heated up in ancient times,” said Wichita State University’s Professor Matthew Howland, lead author of the study.
To measure the iron oxide grains, the scientists carefully chipped tiny fragments from broken faces of the bricks and used a magnetometer to precisely measure the fragments.
By mapping out the changes in Earth’s magnetic field over time, these data also offer archaeologists a new tool to help date some ancient artifacts.
The magnetic strength of iron oxide grains embedded within fired items can be measured and then matched up to the known strengths of Earth’s historic magnetic field.
The reigns of kings lasted from years to decades, which offers better resolution than radiocarbon dating which only pinpoints an artifact’s date to within a few hundred years.
The team also found that in five of their samples, taken during the reign of Nebuchadnezzar II from 604 to 562 BCE, the Earth’s magnetic field seemed to change dramatically over a relatively short period of time, adding evidence to the hypothesis that rapid spikes in intensity are possible.
“The geomagnetic field is one of the most enigmatic phenomena in earth sciences,” said co-author Professor Lisa Tauxe, a researher at the Scripps Institution of Oceanography.
“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 strength in high time resolution, tracking changes that occurred over several decades or even less.”
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Matthew D. Howland et al. 2023. Exploring geomagnetic variations in ancient Mesopotamia: Archaeomagnetic study of inscribed bricks from the 3rd-1st millennia BCE. PNAS 120 (52): e2313361120; doi: 10.1073/pnas.2313361120
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