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Clay tablets with the names of ancient Mesopotamian kings have revealed valuable information about the mysterious anomaly in Earth's magnetic field about 3,000 years ago. Traces of large changes in the magnetic field have been preserved, particularly in the iron oxide grains embedded in the plates. This discovery could lead to a new reference for geophysical and archaeological dating that is much more precise than radiocarbon dating.
Over the billions of years of its existence, the Earth's magnetic field has undergone major dynamic changes, ranging from intensification to weakening and back again. Among the most important changes during the Holocene is the so-called “Levantine Iron Age geomagnetic anomaly,” which occurred between 1050 and 550 BC. took place in ancient Mesopotamia. Between these dates, the intensity of the magnetic field in this region increased significantly, and the exact reasons for this event remain unknown to this day. However, while evidence of this anomaly has been traced to China, Bulgaria and the Azores, data from the southern part of the Middle East is relatively rare.
The study of the earth's magnetic field generally consists of evaluating its intensity and direction according to the declination and inclination angles. Its evaluation in the Holocene also requires the analysis of archaeological materials. This allows us to track both its intensity and direction. This area of research, called “archaeomagnetism,” would reduce reliance on radiocarbon dating. The latter is actually limited to remains containing organic material, which excludes important cultural artifacts such as clay and ceramic objects.
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However, archaeomagnetic samples are still subject to a temporal uncertainty on the order of several hundred years. Only 30% of exhumed samples have an uncertainty of less than or equal to 50 years. To fill these gaps, researchers in the new study relied on high-quality samples from ancient Mesopotamia, including inscriptions of monarchs' names and whose geographic locations are precisely specified. This makes it possible to pre-determine dating based on historical archives and carry out comparative analyses.
“The Earth's magnetic field is one of the most puzzling phenomena in geosciences,” explains study co-author Lisa Tauxe from the Scripps Institution of Oceanography (USA) in a press release. “The well-dated archaeological remains of rich Mesopotamian cultures, particularly bricks bearing the names of specific kings, provide an unprecedented opportunity to study changes in field intensity at high temporal resolution,” she suggests. The technique of archaeomagnetism covered in the study would allow both archaeological artifacts and changes in the Earth's magnetic field to be more accurately dated.
Regional map of the Levant and Mesopotamia illustrating the location of samples with high-quality magnetic signatures from previous studies and the present study. © Matthew D. Howland et al.
Magnetic signatures embedded in minerals
Changes in the Earth's magnetic field give certain minerals, such as iron oxide, a distinct signature when heated. As part of their study, the researchers selected a set of 32 Mesopotamian clay tablets inscribed with the names of ancient kings whose reigns had previously been estimated based on historical records. Iron oxide grains were extracted from the latter to analyze the magnetic signatures that were imprinted there when the bricks were first fired thousands of years ago.
Together, the estimated dating of the monarch's reign and the intensity of the magnetic signature measured in iron oxide grains (using a magnetometer) provide a historical map of changes in the Earth's magnetic field. On the other hand, “by comparing ancient objects with what we know about ancient magnetic field conditions, we can estimate the dates of all objects that were heated in ancient times,” suggests the study's lead author, Matthew Howland from Wichita State University (USA). and Tel Aviv (Israel).
Since the reigns of kings lasted from several years to several decades, this archaeomagnetism technique provides much better resolution than radiocarbon dating. The latter generally date the artifacts to only a few hundred years, without taking into account the need for the presence of organic matter. During the analyses, the research team found that in five of the samples taken from the plates from the reign of Nebuchadnezzar II (604 to 562 BC), the Earth's magnetic field changes dramatically over a relatively short period of time. This finding is consistent with data previously collected in China and Bulgaria.
Furthermore, archaeomagnetic dating of these artifacts would potentially put an end to the debate about the reigns of specific Mesopotamian kings. Although the duration and chronology of their reign are well known, there is disagreement within the archaeological community as to the exact years in which they ascended the throne due to incomplete historical records. The study's experts found that the estimates of monarch reigns obtained using their dating technique were consistent with those of “low chronology.” “This research establishes a basis for the use of archaeomagnetic analysis as an absolute dating technique for archaeological materials from Mesopotamia,” the researchers concluded in their paper published in the journal PNAS.
Source: PNAS