Magnetic field and mass extinction

Source: By the New York Times

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 recently 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 the 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 crystallised, 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 of 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 demagnetise them, and then reheating the samples in the presence of a magnetic field to impart magnetisation. 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 towards its centre as the inner core crystallised, 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 that the single-crystal method Tarduno’s group used is not yet universally accepted. “Some of the leading researchers in the paleomagnetic community are very sceptical 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.

Shields down

The weakened magnetic field Tarduno and his colleagues discovered roughly coincided with the end-Ediacaran extinction around 542 million years ago — a mass die-off of primitive, sessile, sea-dwelling organisms that preceded the Cambrian explosion. In 2016, Carlo Doglioni, a geologist at the Sapienza University of Rome, proposed the Cambrian’s profusion of new life-forms took place in part because of the magnetosphere’s growing strength. “The magnetic dipole was increasing after the Ediacaran,” Doglioni says. “We have a good, thick atmosphere that is protecting us from ionising radiation because we have a good, strong magnetic field.”

Fossil evidence suggests the organisms that endured the end-Ediacaran extinction survived by burrowing into the seafloor — a trait not shared by the immobile Ediacaran period biota that died out. As for the actual culprits in the killings, a 2016 study from Joseph Meert, a geologist at the University of Florida, blames harmful ultraviolet light and cosmic radiation that bathed the surface after passing through ancient Earth’s weakened magnetic field and thinning atmosphere. “When the shields went down, the Ediacaran organisms went extinct, clearing the ecological space for the later Cambrian explosion,” he says.

Tarduno urges caution. “The problem with this (hypothesis) is that the evidence of it in the geological record is pretty scarce,” he says. “If we look at other times of profound magnetic weakness, that would be at the very depth of a magnetic reversal. So that’s a very short time period, maybe a few hundred to a few thousands of years.”

Meert acknowledges other periods of magnetic instability are not obviously tied to extinction events. “But it’s the fact that the Earth’s magnetic field was weaker overall for a long period of time which drove that extinction,” he says. “The way I look at it is, we have this weak magnetic field from the Ediacaran into the early Cambrian, so it was an extended period of time of a weak magnetic field.”

Tarduno says despite the loss of magnetic protection, Earth’s atmosphere and the fact that Ediacaran creatures lived in the sea provided sufficient shielding from harmful radiation. But Meert notes the Ediacaran predates the existence of land-based plants that cloud modern-day waters with organic material; it may be the waters of the Ediacaran oceans were exceptionally clear, allowing ultraviolet radiation to reach greater depths. “Water does attenuate UV rays, but it’s not a cure-all,” he says. “UV rays can penetrate to significant depths, on the order of 10 metres or so. A lot of these Ediacarans were probably in even shallower waters than that.”

Courtney Sprain, a geoscientist at the University of Liverpool in England who was not involved in either study, says more data are needed to pin down the drivers of the Ediacaran extinction. “I do think there are avenues forward into understanding this at a higher level in the future,” she says. One avenue is determining whether the magnetic field was diminished everywhere in the world at this time or if the phenomenon was localised around the Sept-Îles Intrusive Suite, she notes. Another location would be to better constrain the timing of the magnetosphere’s vicissitudes.

Ultimately, Sprain says, determining the cause of the Ediacaran extinction is essential to understanding the evolution of life since then. “This has important implications for what’s going on with Earth today, for the modern changes that we’re seeing in Earth’s climate and for helping us understand what processes humans are potentially contributing to (that) may lead to these large-scale ecological collapses,” she adds. “It helps us infer something about our own future.”