Earth’s magnetic field, a crucial shield against cosmic radiation, is not a permanent fixture. Generated by the dynamic movements of its liquid nickel-iron outer core, the field undergoes periodic geomagnetic reversals—events where the magnetic north and south poles swap places. These reversals, recorded in rocks and ocean floor sediments, typically unfold over several thousand years. However, a groundbreaking study led by a University of Utah geoscientist, in collaboration with researchers from France and Japan, suggests that some reversals took far longer, challenging previous assumptions about this planetary phenomenon.
Traditionally, it was believed that over the past 170 million years, Earth’s magnetic poles have reversed 540 times, with each reversal taking roughly 10,000 years. Yet, the new study, published in Nature Communications Earth & Environment, documents instances from 40 million years ago where the reversal process extended up to 70,000 years. This revelation offers fresh insights into the geomagnetic processes that envelop our planet and protect it from harmful solar radiation.
Implications of Extended Reversal Durations
Extended periods of reduced geomagnetic shielding have significant implications for Earth’s atmosphere and life. According to co-author Peter Lippert, an associate professor at the University of Utah’s Department of Geology & Geophysics, such prolonged reversals likely influenced atmospheric chemistry, climate processes, and the evolution of living organisms.
“The amazing thing about the magnetic field is that it provides the safety net against radiation from outer space,” Lippert explained. “If you are getting more solar radiation coming into the planet, it’ll change organisms’ ability to navigate. We are exposing higher latitudes in particular, but also the entire planet, to greater rates and durations of cosmic radiation, which could lead to higher rates of genetic mutation and atmospheric erosion.”
The study’s lead author, Yuhji Yamamoto from Japan’s Kochi University, expressed astonishment at the findings. “This finding unveiled an extraordinarily prolonged reversal process, challenging conventional understanding and leaving us genuinely astonished,” Yamamoto wrote in a summary published by Springer Nature.
Discovering the Unexpected
The research team, including Lippert and Yamamoto, embarked on a 2012 drilling expedition in the North Atlantic to investigate climate change during the Eocene Epoch, 56 to 34 million years ago. The two-month journey, part of the Integrated Ocean Drilling Program’s Expedition 342, involved drilling off the coast of Newfoundland to extract sediment cores, which serve as time capsules of Earth’s magnetic history.
As paleomagnetists, their task was to measure the direction and intensity of magnetization preserved in these cores. “We don’t know what triggers a reversal,” Lippert noted. “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.”
The sediments, locked with a reliable magnetic signal by tiny crystals of magnetite, revealed Earth’s polarity at the time of deposition. One particular 8-meter-thick layer surprised scientists by recording prolonged geomagnetic reversals in remarkable 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 recounted. “But the interval between them—of unstable polarity when it went to the other direction—was spread out over many, many centimeters.”
Realizing the significance, the team collected additional samples at extremely fine spacing to capture the sediments’ story in high resolution. Subsequent analysis confirmed these were indeed changes in the magnetic field, constructing high-precision timelines for two reversals, one lasting 18,000 years and another 70,000 years.
Revisiting Geomagnetic Models
While surprising, the study’s findings align with some computer models of Earth’s geodynamo, which suggest that reversal durations can vary significantly. Some transitions may last up to 130,000 years, indicating that Earth’s geomagnetism has always been unpredictable, though not previously captured in the geological record.
The study, titled “Extraordinarily Long Duration of Eocene Geomagnetic Polarity Reversals,” was published online on January 20 in Nature Communications Earth & Environment. The research was led by Yuhji Yamamoto of Kochi University, Slah Boulila of Sorbonne Université, and Futoshi Takahashi of Kyushu University, with funding from the Japan Agency for Marine-Earth Science and Technology, the Japan Society for the Promotion of Science, and Kochi University.
This new understanding of Earth’s magnetic field dynamics not only challenges existing theories but also opens avenues for further research into how these prolonged reversals might have influenced the planet’s history and its inhabitants.
