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A magnetic field reversal 42,000 years ago may have contributed to mass extinctions


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A flip-flop of Earth’s magnetic poles between 42,000 and 41,000 years ago briefly but dramatically shrank the magnetic field’s strength — and may have triggered a cascade of environmental crises on Earth, a new study suggests.

With the help of new, precise carbon dating obtained from ancient tree fossils, the researchers correlated shifts in climate patterns, large mammal extinctions and even changes in human behavior just before and during the Laschamps excursion, a brief reversal of the magnetic poles that lasted less than a thousand years. It’s the first study to directly link a magnetic pole reversal to large-scale environmental changes, the team reports in the Feb. 19 Science.

During a reversal, Earth’s protective magnetic field, which shields the planet from a barrage of charged particles streaming from the sun, can lose strength (SN: 1/28/19). So some researchers have suggested that these flip-flops may be linked to extinction events (SN: 11/19/20).

But evidence for this has proven elusive. In fact, “the general belief had been that geomagnetic changes had no impact on climate or anything else,” says Alan Cooper, an evolutionary biologist at BlueSky Genetics in Adelaide. One reason for that belief is a dearth of precise dates for the timing and duration of the geomagnetic event to correlate with environmental, ice core and magnetic rock records.

Enter New Zealand’s kauri tree, among the most ancient in the world. The country’s swampy bogs preserve the relics of kauri trees dating as far back as the Laschamps excursion. Cooper and his colleagues obtained cross-sections from four ancient trees recovered from a swamp at Ng­āwhā Springs in northern New Zealand, and analyzed them for carbon-14, a radioactive form of carbon. (This is the first paper Cooper has led since he was fired from the University of Adelaide in December 2019 for misconduct, allegations which he has denied.)

In particular, one massive preserved log dating to about 41,000 years ago offered up a 1,700-year-long carbon-14 record. That record revealed major changes in carbon-14 during the time period running up to and including the Laschamps excursion, the team reports. That makes sense: Increasing incoming cosmic rays — as would occur with a weakened magnetic field — also produce more carbon-14 in the atmosphere, a carbon signature which would then become incorporated into the tree’s tissues.

The team simulated how a weakened magnetic field might alter atmospheric weather patterns. The computer analysis suggested that the increase of charged particles entering the atmosphere would also increase the production of atmospheric hydrogen and nitrogen oxides — molecules that tend to consume ozone. That would reduce the ability of stratospheric ozone to shield Earth’s denizens from ultraviolet radiation. The atmospheric changes would also affect how much sunlight is absorbed at different layers in the atmosphere, leading to large-scale changes in weather patterns that would have cooled the planet.




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