For the first time, researchers have recovered a magnetic field from ancient minerals from the Iron Age in southern Africa, and the information has helped them discover that the area of the Earth’s core beneath this region could play a key role in reversals of the planet’s magnetic poles.
The study, which appears in the latest edition of the journal Nature Communications, used the measurements collected by University of Rochester geophysicist John Tarduno and colleagues from Witwatersrand University and Kwa-Zulu Natal University in South Africa, along with the ongoing weakening of Earth’s magnetic field during an analysis of polar reversals.
Such reversals of the North and South Poles have taken place at irregular points throughout the planet’s history, with the last one occurring about 800,000 years ago. The researchers noted that once a reversal starts, it can take up to 15,000 years to complete, and the core region beneath southern Africa may have been the origin site of some of these events.
Tarduno’s team said that while it has long been believed that polar reversals started at random locations, the data they collected from five sites along the border separating South Africa from Botswana and Zimbabwe indicates that this might not actually be the case after all.
Collecting the data from burnt ancient villages
According to study co-author Rory D. Cottrell, a scientist in the UR Paleomagnetism Research Lab and the Department of Earth & Environmental Sciences, the agricultural populations living throughout southern Africa during the Iron Age lived in semi-permanent villages. They based their economy on herding cattle, sheep, and goats, and cultivating millet, beans, and peas.
These villages consisted of mud huts, grain bins, and animal kraals, and during times of sustained drought, poor growing seasons and unexpected cattle deaths, the villages would be burned as part of a ritualistic cleansing. These burnt features enable researchers “a snap-shot look at the Earth’s magnetic field throughout the past 2000 years,” Cottrell explained via email.
“The fires were hot enough to reset magnetic minerals (primarily magnetite) above their Curie Temperature,” or the temperature where a material’s magnetic characteristics change from being unable to retain a permanent magnetic field to “freezing” the magnetic field as the material cools down. “We sampled burnt features from localities in southern Africa to look at what magnetic fields were recorded for different time periods throughout the Iron Age of southern Africa.”
The Earth’s magnetic field has been decreasing in dipole intensity for the past 160 years. The study authors attributed this observed 16 percent decrease in field strength to the weakening field in the area known as the South Atlantic Anomaly, which stretches from the part of Africa where the research took place, all the way to South America and beyond.
Findings may reflect the South Atlantic Anomaly’s longevity
While Cottrell noted that these changes (which are also called secular variation of the magnetic field) “are not uncommon,” there has been speculation that the planet is in the beginning stages of a field reversal. In a statement, Tarduno pointed out that this is only speculative at this point, as weakening magnetic fields can recover without leading to a complete polar reversal.
The researchers’ analysis of burnt structures in southern Africa revealed that there was “more change in magnetic field direction and intensity than has been seen over the last 160 years. This may speak to the longevity of the South Atlantic Anomaly, how it has changed through time,” Cottrell said. Based on models of magnetic observatory measurements, they found that this area has fluctuated magnetically in terms of both size and intensity over the past century.
“Imagine a cup full of sharpened pencils,” he explained. “Each hemisphere has their pencils generally pointing in a single direction (into the cup in the northern hemisphere, out of the cup in the southern hemisphere). Each pencil represents the magnetic field vector at a particular locality at Earth’s surface. At the SAA, there [are a] number of pencils pointed in the wrong direction. So when you add all of the pencil vectors together, it is overall a smaller length (oppositely pointed pencils will effectively cancel each other out).”
“It is thought that the weak field at the SAA is related to core-flux expulsions, and these core-flux expulsions may be related to the large low shear velocity provinces (LLSVP) located at the core-mantle boundary,” Cottrell added. “The dipole field at the core-mantle boundary has lobes of lower field values (green blobs) near the edges of the LLSVP. Changing (growing) core-flux expulsions may be a focal point for magnetic dipole reversals.”
(Image credit: John Tarduno/University of Rochester)
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