Ancient earth’s weakened magnetic field may have driven mass extinction


A diorama depicting a typical seafloor ecosystem during Earth’s Ediacaran period.
PHOTO/Ryan Somma Flickr (CC BY-SA 2.0)

Some 565 million years ago, life on Earth dodged a bullet. The magnetosphere—the magnetic field that surrounds our planet like a protective shield—had degraded to its lowest intensity ever, according to a study published January 28 in Nature Geoscience. Stripped of this shielding, Earth could have been blasted by atmosphere-eroding outbursts from the sun, gradually losing most of its air and water until it became as dry and desolate as present-day Mars.

Instead, deep in the planet’s interior an event was taking place that would help the magnetosphere rebound, according to the study’s authors. Earth’s liquid-iron inner core crystallized, a process geophysicists call “nucleation.” Once solid, the rotating core acted as a whirling dynamo, strengthening the protective electromagnetic bubble that wrapped around Earth, staving off planet-wide devastation. That, in turn, could have set the stage for the Cambrian explosion, an event approximately 541 million years ago in which the biosphere suddenly experienced the greatest evolutionary expansion in the planet’s history.

To measure Earth’s magnetic field as it was more than a half billion years ago, University of Rochester geophysicist John Tarduno and colleagues looked at magnetic particles from ancient silicate crystals within a band of igneous rocks called the Sept-Îles Intrusive Suite in Quebec. The igneous band formed from upwelling magma that cooled before reaching the surface. As the magma cooled, evidence of the paleointensity, or strength of the Earth’s magnetic field at the time, was locked into the crystals.

The geophysicists were able to determine what that paleointensity was by heating single crystals to demagnetize them, and then reheating the samples in the presence of a magnetic field to impart magnetization. Averaging the results over the estimated 75,000-year period in which the crystals cooled, the researchers determined the paleointensity circa 565 million years ago was about 10 times weaker than Earth’s modern magnetosphere—a finding that comports with independent studies charting the magnetosphere’s slow, steady strengthening over geologic time.Tarduno and his colleagues surmise Earth’s growing core caused this upswing: iron and other heavy elements fell toward its center as the inner core crystallized, leaving a liquid layer of lighter elements in the core’s outer regions, sparking the long-lived convection that drives Earth’s dynamo.

According to scientists outside of the study, insights about Earth’s ancient magnetic field are as uncertain as they are rare. “Getting any paleomagnetic samples from earlier time periods is so important because we have so little data,” says Sabine Stanley, a geophysicist at Johns Hopkins University.“At the moment it’s one data point at a particular time interval.” More data points are needed, she says, although she also notes the magnetosphere’s apparent increase in strength across a half billion years does support the researchers’ analysis. Elisa Piispa, a geophysicist at Yachay Tech University in Ecuador, cautions the single-crystal method Tarduno’s group used is not yet universally accepted. “Some of the leading researchers in the paleomagnetic community are very skeptical on it,” she says. Then again, the team’s results are consistent with several other models of the core’s thermal evolution and a wealth of other paleomagnetic observations, says Krista Soderlund, a researcher at The University of Texas at Austin.

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