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Archaeomagnetic dating is a technique that relies on changes in the intensity and direction of the Earth’s magnetic field over the last 10,000 years. When some natural materials like soil and clay are heated to high temperatures, the minerals lock in a distinct signature as they cool that reflects the strength and direction of the Earth’s magnetic field at that exact time and place. This can then be compared to models of known geomagnetic shifts in the relevant area in the past to work out when the artefacts were last heated.
However, a lack of published archaeomagnetic data from ancient Mesopotamia (modern-day Iraq and parts of Syria, Turkey, Iran, and Kuwait) between the 3rd and 1st millennia BC makes it difficult to develop local models of the Earth’s magnetic field for this region. This limits opportunities to use the technique for absolute dating of archaeological material from a time and place that are central to understanding the development of urbanism, social complexity, and early states on a global level. In particular, the absence of evidence for a phenomenon known as the Levantine Iron Age geomagnetic Anomaly (LIAA) in Mesopotamia has puzzled researchers for some time. The LIAA was a period between c.1050 and 550 BC when the strength of Earth’s magnetic field was especially high in the Levant. These geomagnetic spikes have been detected in many other regions, as far away as China and the Canary Islands, but until now there was no firm evidence that the LIAA extended to the area covered by ancient Mesopotamia.
A new study by an international team of researchers set out to remedy this gap in our knowledge of Earth’s magnetic field. The research, published in the journal PNAS (https://doi.org/10.1073/pnas.2313361120), analysed the geomagnetic signatures of 32 ancient Mesopotamian bricks. Mud bricks were used widely in construction across Mesopotamia; made of a combination of earth, chopped straw, and water, these were generally just left to dry in the sun. However, by the 5th millennium BC we also start to find kiln-fired bricks, which can be used for archaeomagnetic dating. By the end of the 3rd millennium BC, these ‘baked bricks’ had become increasingly common, although they remained expensive to produce, so were largely restricted to elite buildings like palaces or temples, and areas that required greater durability such as drains or floors. Some baked bricks from Mesopotamia have been subjected to archaeomagnetic analysis in the past, but studies have more commonly focused on other materials like ceramics and hearth fragments. Baked bricks represent an ideal resource for this technique, however, as many are stamped or inscribed with the name of the king within whose reign they were created, the dates of which are often known from historical records. It is therefore possible to tie the archaeomagnetic signature of the brick to a precise period, and thus build up a picture of fluctuations in the geomagnetic field at definite points in time.
The bricks analysed in this study were found at sites across Mesopotamia, including Ashur, Ur, Eshnunna, Isin, Tell al-Wilaya, Larsa, and Marad. The inscriptions reveal that they originate from the reigns of 12 Mesopotamian kings, who ruled between the late-3rd to mid-1st millennium BC. Tiny samples were taken from each brick and analysed for archaeomagnetic intensity. The results provide the first evidence that the high geomagnetic intensity of the Levantine Archaeomagnetic Curve seen in the Levant, Syria, and many other regions across the world during the LIAA also occurred in all parts of Mesopotamia. Additionally, the results identified several intense spikes in the magnetic field over a short span of time, adding further evidence to hypotheses suggesting that such rapid dramatic changes are indeed possible.
The 32 newly archaeomagnetically dated bricks represent a considerable addition to the known assemblage from Mesopotamia from the 3rd-1st millennium BC and include new geomagnetic data from several major sites for the first time. In addition to improving our knowledge of the LIAA, the study significantly enhances the reliability of geomagnetic intensity models for Mesopotamia in this period, which will refine dating of the region’s archaeology, as well as demonstrating the potential of kiln-fired bricks for future archaeomagnetic studies.
Text: Amy Brunskill / Images: Matthew D Howland
