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

A flip-flop of the Earth's magnetic poles between 42,000 and 41,000 years ago recently, but has dramatically reduced the strength of the magnetic field and may trigger a cascade of environmental crises on Earth, a new study suggests.

With the help of accurate new carbon dating obtained from fossils of ancient trees, researchers correlated changes in climate patterns, mammal extinctions, and even changes in human behavior just before and during the Laschamps excursion, a brief reversal of by the magnetic ones that lasted less than a thousand years. It is the first study to directly link magnetic pole reversal to large-scale environmental changes, the team reported in the February 19 issue of the journal Science.

During an inversion, the Earth’s protective magnetic field, which protects the planet from a bunch of charged particles coming out of the sun, can lose strength (SN: 28/01/19). Thus, some researchers have suggested that these flip-flops may be linked to extinction events (SN: 19/11/20).

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

Enter the kauri tree of New Zealand, one of the oldest in the world. The swampy swamps of the country preserve the relics of the kauri trees that already date from the Laschamps excursion. Cooper and colleagues obtained cross-sections of four ancient trees recovered from a swamp in Ngāwhā Springs in northern New Zealand and analyzed them to determine carbon-14, a radioactive form of carbon. (This is the first newspaper Cooper has run since he was fired from the University of Adelaide in December 2019 for misconduct, allegations he denied).

Kauri trees (one of them) have grown in New Zealand for thousands of years. By analyzing tree rings preserved in the Ngawha swamp, scientists have identified evidence to suggest that a magnetic pole turned about 41,000 years ago.Mark Meredith / Moment / Getty Images

In particular, a huge conserved trunk dating back about 41,000 years ago offered a carbon 14 record of 1,700 years ago. That record revealed significant changes in carbon-14 during the period that lasted until the Laschamps excursion, included, the team reports. That makes sense: increasing incoming cosmic rays, as with a weakened magnetic field, also produces more carbon-14 in the atmosphere, a carbon signature that would then be incorporated into tree tissues.

The team simulated how a weakened magnetic field could alter atmospheric patterns. Computer analysis suggested that the increase in charged particles entering the atmosphere would also increase the production of atmospheric oxides and hydrogen, molecules that tend to consume ozone. That would reduce the ability of stratospheric ozone to protect Earth’s inhabitants from ultraviolet radiation. Atmospheric changes would also affect the amount of sunlight absorbed in different layers of the atmosphere, which would cause large-scale changes in the weather patterns that would have cooled the planet.

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Those effects may in fact occur at that time. Using carbon-14 dates from kauri trees, the team examined sediment, pollen and other data from before and during the Laschamps excursion that point to a sudden cooling in places from Australia to the Andes.

Surprisingly, the most intense effects did not occur during the actual reversal of the pole, the team found, but in the several hundred years that preceded it, spanning about 42,300 to 41,600 years ago. During the real investment, the field was only about 28 percent as strong as it is today. But during that transition period, the intensity of the field was reduced to about 6 percent of the current intensity. Researchers have dubbed this "Adams' Transition Geomagnetic Event" – for Douglas Adams, author of The Hitchhiker's Guide to the Galaxy. Adams is often associated with the number 42, which is said in his books as the answer to the "last question about life, the universe, and everything."

Scientists have long debated whether climate change or human hunters were to blame for the extinction events that wiped out giant mammals such as woolly mammoths and Diprotodon, a species of large Australian wombat. “It was actually one of the reasons for this study,” says study co-author Chris Turney, a paleoclimatologist at the University of New South Wales in Sydney.

In a 2015 study by Cooper, Turney, and colleagues, “we noticed that some of the megafaunal extinctions appear to be grouping together and we started asking why,” says Turney (SN: 23/07/15). One of these groups of Australian megafauna extinctions, including the extinction of Diprotodon and the giant kangaroo Procoptodon goliah, occurred about 42,000 years ago.

The team compared the dates of the magnetic event with previous records of ice cores that may reflect changes in solar activity. These data suggested that solar activity was at least at that time. The combination of a weak magnetic field and this decrease in sun production at the same time “created the perfect storm” of climate and broader environmental changes, which posed significant stress on megafauna populations, Turney says. Those factors can also lead to increased competition between megafauna and human populations, as well as with Neanderthals, he said.

Another possible line of evidence for a depletion of the ozone layer: a growing abundance of red ocher handprints made by humans in cave paintings, the researchers note. It is believed that red ocher was used as sunscreen (SN: 7/3/20). The researchers reported that there may also be an increasing use of the caves between about 42,000 and 40,000 years ago, possibly as a refuge from the more intense sun.

This is the first study to consider a wide range of environmental consequences of extreme changes in the magnetic field, says Monika Korte, geomagnetist at the German Geosciences Research Center GFZ in Potsdam. “The suggested links seem conceivable to me,” Korte says. But, he adds, "the biggest value of the newspaper is that it is publishing several ideas that should be further investigated."

It is still unknown whether other magnetic inversions have caused similar disorders in the past, but “we expect the community to see sets of biological and archaeological data through this different lens,” Turney says. Improving the accuracy of isotopic citations for these events will be key, and radiocarbon dates collected from New Zealand kauri trees may help with other recent reversals, such as a brief reversal event called Mono Lake Excursion that occurred about 34,000 years ago. .

An even more tempting issue is that long-term environmental damage may have been caused by much longer reversal events recorded in ancient rocks, such as the 20,000-year-old Brunhes-Matuyama inversion, which began 781,000 years ago. "The impacts can be huge."

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