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, wind, cosmic radiation and wild temperature fluctuations.
However, Earth's magnetic field nearly collapsed 591 million years ago, and this change may, paradoxically, have played a crucial role in the flourishing of complex life, new research shows.
"Generally the field is protective. If we had not had a field early in Earth's history, the planet would have been stripped of water by the solar wind (a stream of high-energy particles flowing from the Sun to Earth)," said John Tarduno, professor of geophysics at the University of California. 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 processes that created the magnetic field... became so inefficient 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, was significantly weaker than its current strength for a period of at least 26 million years. years. The discovery of the persistent weakening of Earth's magnetic field also helped solve an enduring geological mystery about the origins of Earth's solid core.
This time frame corresponds to a period known as the Ediacaran, when the very first complex animals appeared on the seafloor as the percentage of oxygen in the atmosphere and ocean increased.
These strange animals bore little resemblance to today's life: swampy fans, tubes and doughnuts, and disks like Dickinsonia, which grew up to 1.4 meters, and the snail-like Kimberella.
Before then, life was largely single-celled and microscopic. The researchers believe that a weak magnetic field may have led to an increase in oxygen in the atmosphere, allowing early complex life to develop.
Exposing the near collapse of the magnetic field
The intensity of the Earth's magnetic field is known to fluctuate over time, and crystals preserved in rocks contain tiny magnetic particles that record the intensity of the Earth's magnetic field.
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, which found that the field was ten times weaker at the time than it is today.
The latest study gathered more geological evidence indicating that the magnetic field was weakening dramatically, with information in 591-million-year-old rocks from a site in southern Brazil indicating that the field was 30 times weaker than today.
The weak magnetic field had not always been this way: the team examined similar rocks from South Africa that were more than 2 billion years old and found that Earth's magnetic field was just as strong at that time as it is today.
Unlike today, Tarduno explained, the interior of the Earth at the time was liquid and not solid, which affected how the magnetic field was generated.
"Over billions of years, that process becomes less and less efficient," he said.
"And by the time we get to the Ediacaran, the field is on its last legs. It's almost collapsing. But luckily for us it got cool enough that the inner core started generating (strengthening the magnetic field).
The emergence of the earliest complex life that would have spread across the seabed at that time is accompanied by a rise in oxygen levels. Some animals can survive on low oxygen levels, such as sponges and microscopic animals, but larger animals with more complex bodies that move require more oxygen, Tarduno said.
Traditionally, the increase in oxygen levels during this period has been attributed to photosynthetic organisms such as cyanobacteria, which produced oxygen, allowing it to accumulate steadily in the water over time, explained co-author Shuhai Xiao, professor of geobiology at Virginia Tech, out.
However, the new research suggested an alternative or complementary hypothesis that implies greater loss of hydrogen to space when the geomagnetic field is weak.
"The magnetosphere protects the Earth from solar wind and thus holds the atmosphere against the Earth. So a weaker magnetosphere means that lighter gases such as hydrogen are lost from Earth's atmosphere," Xiao added via email.
Tarduno said multiple processes could take place at the same time.
"We do not dispute that one or more of these processes took place simultaneously. But the weak field may have caused oxygenation to cross a threshold, promoting animal radiation (evolution)," Tarduno said.
Peter Driscoll, a staff scientist at the Earth and Planets 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 with the assertion that it weak magnetic field could have affected the oxygen in the atmosphere. and biological evolution was difficult to assess. He was not involved in the investigation.
"It is difficult for me to assess the veracity of this claim because the influence that planetary magnetic fields may have on climate is not very well understood," he said via email.
Tarduno said their hypothesis was "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.
Inner core mystery
The geological analysis also revealed telling details about the inner part of the Earth's center.
Estimates about when the planet's inner core might have solidified - when iron first crystallized in the planet's center - once ranged from 500 million to 2.5 billion years ago.
The research into the intensity of Earth's magnetic field suggests that the age of Earth's inner core is on the younger end of that time scale, solidifying after 565 million years ago and allowing Earth's magnetic shield to return to bounce.
"The observations appear to support the claim that the inner core first nucleated shortly afterwards, pushing the geodynamo (the mechanism that creates the magnetic field) from a weak, unstable state to a strong, stable dipolar field," Driscoll said.
Tarduno said the post-Ediacaran recovery of field strength, with the growth of the inner core, was likely important to prevent the drying out of the water-rich Earth.
As for the bizarre animals of the Ediacaran, they were all gone by the subsequent Cambrian period, when the diversity of life exploded and the branches of the tree of life we know today formed in a relatively short time.
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