When Earth’s magnetic field took its time flipping

(Press-News.org) Earth’s magnetic field is generated by the churn of its liquid nickel-iron outer core, but it is not a constant feature.

Every so often, the magnetic north and south poles swap places in what are called geomagnetic reversals, and the record of these flips is preserved in rocks and sediments, including those from the ocean floor. These reversals don’t happen suddenly, but over several thousand years, where the magnetic field fades and wobbles while the two poles wander and finally settle in the opposite positions of the globe.

Over the past 170 million years, the magnetic poles have reversed 540 times, with the reversal process typically taking around 10,000 years to complete each time, according to years of research. Now, a new study by a University of Utah geoscientist and colleagues from France and Japan has upended this scenario after documenting instances 40 million years ago where the process took far longer to complete, upwards of 70,000 years. These findings offer a new perspective on the geomagnetic phenomenon that envelops our planet and shields it from solar radiation and harmful particles from space.

Extended periods of reduced geomagnetic shielding likely influenced atmospheric chemistry, climate processes and the evolution of living organisms, according to co-author Peter Lippert, an associate professor in the U Department of Geology & Geophysics.

“The amazing thing about the magnetic field is that it provides the safety net against radiation from outer space, and that radiation is observed and hypothesized to do all sorts of things. If you are getting more solar radiation coming into the planet, it’ll change organisms’ ability to navigate,” said Lippert, who heads the Utah Paleomagnetic Center. “It’s basically saying we are exposing higher latitudes in particular, but also the entire planet, to greater rates and greater durations of this cosmic radiation and therefore it’s logical to expect that there would be higher rates of genetic mutation. There could be atmospheric erosion.”

The results appear in Nature Communications Earth & Environment. The lead author is Yuhji Yamamoto of Japan’s Kochi University.

“This finding unveiled an extraordinarily prolonged reversal process, challenging conventional understanding and leaving us genuinely astonished,” Yamamoto wrote in a summary posted by Springer Nature.

Yamamoto and Lippert worked together on a 2012 drilling expedition in the North Atlantic that was investigating climate change during the Eocene Epoch, 56 to 34 million years ago. The two-month trip was facilitated by the Integrated Ocean Drilling Program’s Expedition 342. The team drilled off the coast of Newfoundland in the North Atlantic, extracting sediment cores, layered time capsules built grain by grain over millions of years, from up to 300 meters below the sea floor.

As paleomagnetists, Yamamoto and Lipperts’ job was to “measure the direction and the intensity of the magnetization that’s preserved in those cores,” Lippert said. “We don’t know what triggers a reversal. Individual reversals don’t last the same amount of time, so that creates this unique barcode. We can use the magnetic directions preserved in the sediments and correlate them to the geologic timescale.”

These sediments carry a reliable magnetic signal locked in by tiny crystals of magnetite produced by ancient microorganisms and from dust and erosion from the continents. Like a compass, the direction they point reveals Earth’s polarity at the time the sediments were deposited.

One 8-meter-thick layer took the scientists by surprise, appearing to record prolonged geomagnetic reversals in incredible detail.

“Yuhji noticed, while looking at some of the data when he was on shift, this one part of the Eocene had really stable polarity in one direction and really stable polarity in another direction,” Lippert said. “But the interval between them—of unstable polarity when it went to the other direction—was spread out over many, many centimeters.”

They realized this was no ordinary flip and collected extra samples at extremely fine spacing, just a few centimeters apart, to capture the sediments’ story in high resolution.

resolution and to test if the strange magnetic behavior was due to changes in the magnetic field or the sediments. In subsequent analysis of these cores over several years, Lippert and his colleagues confirmed this was recording changes in the magnetic field and constructed high-precision timelines for two reversals, one lasting 18,000 years and another for 70,000 years.

While the finding was a surprise, it may not have been unexpected, according to the study. Computer models of Earth’s geodynamo—in the swirling outer core that generates the electrical currents supporting the magnetic field—had indicated reversals’ durations vary, with many short ones, but also occasional long, drawn-out transitions, some lasting up to 130,000 years.

In other words, Earth’s geomagnetism may have always had this unpredictable streak, but scientists hadn’t caught it in the rocks until now.

The study, titled “Extraordinarily long duration of Eocene geomagnetic polarity reversals,” was posted online Jan. 20 in Nature Communications Earth & Environment. Lead author is Yuhji Yamamoto of Kochi University, Slah Boulila of Sorbonne Universite and Futoshi Takahashi of Kyushu University. Funding came from the Japan Agency for Marine-Earth Science and Technology, the Japan Society for the Promotion of Science and Kochi University.

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