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Earth’s magnetic field plays a key role in making our planet habitable. The protective bubble above the atmosphere protects the planet from solar radiation, winds, cosmic rays and unusual changes in temperature.
However, Earth’s magnetic field nearly collapsed 591 million years ago, and this change, paradoxically, may have played a key role in the rise of complex life, new research has found.
“In general, the field is defensive. “If we hadn’t had a field early in Earth’s history, water would have been removed from the planet by the solar wind (a stream of energized particles flowing from the sun to Earth),” said John Tarduno, professor of geophysics at the University of Rochester in New York. and senior author of the new study.
“But in the Ediacaran, we had a fascinating period in the development of the deep Earth when the processes creating the magnetic field … had become so ineffective after billions of years that the field almost completely collapsed.”
The study, published in the journal Communications Earth & Environment on May 2, found that Earth’s magnetic field, which is created by the movement of molten iron in Earth’s outer core, has been significantly weaker than its current strength for a period of at least -some 26 million years. The discovery of a sustained weakening of Earth’s magnetic field also helped solve an enduring geological mystery about when Earth’s solid inner core formed.
This time frame coincides with a period known as the Ediacaran, when the first complex animals appeared on the seafloor as the percentage of oxygen in the atmosphere and ocean increased.
These strange animals bore almost no resemblance to life today—fluffy fans, tubes and donuts and discs like Dickinsonia, which grew to 4.6 feet (1.4 meters) in size, and the slug-like Kimberella.
Before that time, life was mostly unicellular and microscopic. The researchers believe that the weak magnetic field may have led to an increase in oxygen in the atmosphere, allowing the development of early complex life.
Shuhai Xiao/Virginia Tech
Photo shows a cast of a 560 million year old Dickinsonia costata fossil found in South Australia. At over a meter long, the creature is the largest known animal from this period.
Earth’s magnetic field intensity is known to vary with time, and crystals preserved in rock contain tiny magnetic particles that record Earth’s magnetic field intensity.
The first evidence that Earth’s magnetic field weakened significantly during this period came in 2019 from a study of 565-million-year-old rocks in Quebec that suggested the field was 10 times weaker than today at that time.
The latest study gathered more geological evidence to show that the magnetic field weakened dramatically, with information contained in a 591-million-year-old rock from a site in southern Brazil suggesting the field was 30 times weaker than today.
The weak magnetic field wasn’t always this weak: The team studied similar rocks from South Africa dating back more than 2 billion years and found that at that time Earth’s magnetic field was as strong as it is today.
Unlike now, Tarduno explained, back then the innermost part of Earth was liquid, not solid, which affects how the magnetic field is generated.
“Over billions of years, this process becomes less and less efficient,” he said.
“And by the time we get to the Ediacaran, the terrain is on its last legs. It’s almost collapsing. But then, luckily for us, it got cool enough that the inner core started generating (strengthening the magnetic field).”
The appearance of the earliest complex life that would have spread across the sea floor at that time was associated with an increase in oxygen levels. Some animals can survive low oxygen levels, such as fungi and microscopic animals, but larger animals with more complex bodies that move around need more oxygen, Tarduno said.
Traditionally, the increase in oxygen during this time has been attributed to photosynthetic organisms such as cyanobacteria that produce oxygen, allowing it to accumulate in the water steadily over time, explained study co-author Shuhai Xiao, a professor of geobiology at Virginia Tech.
However, the new research suggests an alternative or complementary hypothesis involving increased loss of hydrogen to space when the geomagnetic field is weak.
“The magnetosphere shields the Earth from the solar wind, thereby keeping the atmosphere towards the Earth. Thus, a weaker magnetosphere means that lighter gases such as hydrogen will be lost from Earth’s atmosphere,” Xiao added via email.
Tarduno said multiple processes may have occurred at once.
“We do not dispute that one or more of these processes are occurring simultaneously. But the weak field may have allowed oxygenation to cross the threshold, aiding animal radiation (evolution),” Tarduno said.
Peter Driscoll, a scientist at the Earth and Planetary Laboratory at the Carnegie Institution for Science in Washington, D.C., said he agreed with the study’s findings about the weakness of Earth’s magnetic field, but the claim that a weak magnetic field may have affected the atmospheric oxygen and biological evolution was difficult to assess. He did not participate in the study.
“It is difficult for me to assess the truth of this claim because the influence that planetary magnetic fields can have on climate is not very well understood,” he said by email.
Tarduno said their hypothesis is “solid,” but proving a causal link could take decades of challenging work, given how little is known about the animals that lived at the time.
Shuhai Xiao/Virginia Tech
A 565-million-year-old fossil of an Ediacaran animal called Fractofusus misrai was found in the Mistaken Point Formation in Newfoundland, Canada.
Geological analysis has also revealed telling details about the innermost part of the Earth’s center.
Estimates of when the planet’s inner core may have solidified — when iron first crystallized in the planet’s center — have once ranged from 500 million to 2.5 billion years ago.
A study of Earth’s magnetic field intensity suggests that the age of Earth’s inner core is at the younger end of this time scale, solidifying after 565 million years ago and allowing Earth’s magnetic shield to bounce back.
“The observations seem to support the proposition that the inner core first nucleated soon after that time, pushing the geodynamo (the mechanism that creates the magnetic field) from a weak, unstable state into a strong, stable dipolar field,” Driscoll said.
Tarduno said the post-Ediacaran field strength recovery, with the growth of the inner core, is likely important in preventing the water-rich Earth from drying out.
As for the strange animals of the Ediacaran, they all disappeared during the following Cambrian period, when the diversity of life exploded and the branches of the tree of life known today formed in a relatively short time.
