An international team of researchers has discovered evidence of a magnetic field anomaly in Southeast Asia roughly 800 years ago. The researchers conducted an archaeomagnetic study on an iron-smelting site from Cambodia, an area currently devoid of data. The results fill gaps in the present datasets and provide constraints for understanding specific geomagnetic features and their interior driving mechanism.
- The geomagnetic field contains information about the interior dynamics of the Earth and is closely related to human beings maintaining a habitable planet.
- Understanding variations of the field in the past, especially during the Holocene, is helpful for deciphering modern geomagnetic behaviors or even predicting future variations.
From the ~11th to the 14th century, smelters were producing iron objects at Tonle Bak at a time when the area was part of the Khmer Empire.
As part of the process, the laborers would dump residue periodically from their smelting operations onto a nearby site. Over time, 50 hills of the material built up, serving as a record of the magnetic field in the area for the years it was dumped.
The researchers extracted material from several of the hills then analyzed them to learn more about the magnetic field in that region from 1034 to 1391. The team found that over a century between 1200 and 1300, the magnetic field in Southeast Asia shifted direction by nearly .05 degrees per year.
The inclination dropped from about 30 degrees to just 5. They also discovered a change in intensity during the same time period as it dropped from 44 microteslas to 27.
It was found that there was a regional magnetic anomaly in Southeast Asia about 800 years ago. The team suggests the weakening they observed was possibly part of a wide anomaly that extended all the way to the equator– a phenomenon called a flux expulsion at low latitudes.
They were not able to find explanations for the anomaly, but suggest that it could have been due to interference from turbulence taking place at the Earth's core/mantle boundary.
It is also noted that many such anomalies have been found and studied, and one of them is happening today below the southern areas of the Atlantic Ocean.
"These results reveal a sharp directional change of the geomagnetic field between 1200 and 1300 CE, accompanied by an intensity dip between 1100 and 1300 CE. The fast geomagnetic variation recorded by our data provides evidence for the possible existence of low-latitude flux expulsion," the authors wrote.
"Related discussions in this paper will inspire a new focus on detailed geomagnetic research in low-latitude areas around the equator, and exploration of related dynamic processes."
"Archaeomagnetic results from Cambodia in Southeast Asia: Evidence for possible low-latitude flux expulsion" – Cai, S., et al. – Proceedings of the National Academy of Sciences – DOI: 10.1073/pnas.2022490118
Extensive spatial and temporal distribution of high-quality data are essential for understanding regional and global behaviors of the geomagnetic field. We carried out chronological and archaeomagnetic studies at the Angkor-era iron-smelting site of Tonle Bak in Cambodia in Southeast Asia, an area with no data available to date. We recovered high-fidelity full-vector geomagnetic information from the 11th to 14th century for this region, which fill gaps in the global distribution of data and will significantly improve the global models. These results reveal a sharp directional change of the geomagnetic field between 1200 and 1300 CE, accompanied by an intensity dip between 1100 and 1300 CE. The fast geomagnetic variation recorded by our data provides evidence for the possible existence of low-latitude flux expulsion. Related discussions in this paper will inspire a new focus on detailed geomagnetic research in low-latitude areas around the equator, and exploration of related dynamic processes.
Featured image credit: NASA/NOAA Suomi NPP/VIIRS
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