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Earth’s magnetic field plays a key role in making our planet habitable. The protective bubble over the atmosphere protects the planet from solar radiation, winds, cosmic rays and large temperature fluctuations.
However, Earth’s magnetic field nearly collapsed 591 million years ago, and this change, paradoxically, may have played a key role in the flourishing of complex life, new research has found.
“In general, the field is protective. If we hadn’t had a field at the beginning of Earth’s history, water would have been removed from the planet by solar wind (a stream of energized particles flowing from the Sun toward 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 that create the magnetic field… became so inefficient after billions of years, that the field almost completely collapsed.”
The study, published in the journal Earth and Environment Communications on May 2, discovered that Earth’s magnetic field, which is created by movement of molten iron in the Earth’s outer core, was significantly weaker than its current strength over a period of at least 26 million years. The discovery of the sustained weakening of Earth’s magnetic field also helped solve a long-standing geological mystery about when Earth’s solid inner core formed.
This period coincides with a period known as the Ediacaran, when the first complex animals emerged on the sea floor as the percentage of oxygen in the atmosphere and ocean increased.
These strange animals barely resembled current life – fluffy fans, tubes and donuts, and discs like Dickinsoniawhich grew to 4.6 feet (1.4 meters) in size, and the Slug-like Kimberella.
Before this time, life was largely single-celled and microscopic. Researchers believe that a weak magnetic field may have led to an increase in oxygen in the atmosphere, allowing the evolution of early complex life.
Discovering the near collapse of the magnetic field
The strength of Earth’s magnetic field is known to fluctuate over time, and crystals preserved in rocks contain tiny magnetic particles that record the strength of Earth’s magnetic field.
The first evidence that Earth’s magnetic field weakened significantly during this period came in 2019 from a study of rocks 565 million years old in Quebec, which suggested the field was 10 times weaker than it is today at that time.
The latest study gathered further geological evidence that indicated the magnetic field had weakened dramatically, with information contained in 591-million-year-old rocks from a site in southern Brazil suggesting the field was 30 times weaker than it is today.
The weak magnetic field wasn’t always like this: the team examined similar rocks from South Africa dating back more than 2 billion years and found that Earth’s magnetic field was as strong then as it is today.
Unlike now, Tarduno explained, at that time the innermost part of the Earth was liquid, not solid, influencing the way the magnetic field was generated.
“Over billions of years, this process is becoming less and less efficient,” he said.
“And when we get to Ediacaran, the field is at its limit. It’s almost falling apart. But then, fortunately for us, it cooled down enough for the inner core to start generating (strengthening the magnetic field).”
The emergence of the first complex life that would have floated along the sea floor at this time is associated with an increase in oxygen levels. Some animals can survive with low oxygen levels, such as sponges and microscopic animals, but larger animals with more complex, moving bodies need more oxygen, Tarduno said.
Traditionally, the increase in oxygen during this period has been attributed to photosynthetic organisms such as cyanobacteria, which produce oxygen, allowing it to accumulate in the water steadily over time, explained study co-author Shuhai Xiao, professor in geobiology at Virginia Tech.
However, the new research suggested an alternative, or complementary, hypothesis involving an increased loss of hydrogen to space when the geomagnetic field was weak.
“The magnetosphere protects the Earth from the solar wind, thus trapping the atmosphere to the Earth. Thus, a weaker magnetosphere means that lighter gases such as hydrogen would be lost from Earth’s atmosphere,” Xiao added in an email.
Tarduno said that several processes could be occurring at the same time.
“We do not dispute that one or more of these processes were happening simultaneously. But the weak field may have allowed oxygenation to exceed a limit, aiding animal radiation (evolution),” said Tarduno.
Peter Driscoll, a scientist at the Carnegie Institution for Science’s Earth and Planetary Laboratory in Washington, D.C., said he agreed with the study’s findings about the weakness of Earth’s magnetic field, but the claim that the weak magnetic field could have affected oxygen Atmospheric and biological evolution was difficult to assess. He was not involved in the study.
“It is difficult for me to assess the veracity of this statement because the influence that planetary magnetic fields can have on climate is not very well understood,” he said in an email.
Tarduno said his 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 this time.
Internal central mystery
The geological analysis also revealed revealing details about the innermost part of the Earth’s center.
Estimates for when the planet’s inner core may have solidified – when iron first crystallized in the planet’s center – have ranged between 500 million and 2.5 billion years ago.
O research into the intensity of the Earth’s magnetic field suggests that the age of Earth’s inner core it is at the youngest end of that time scale, solidifying after 565 million years ago and allowing Earth’s magnetic shield to recover.
“The observations appear to support the claim that the inner core first nucleated shortly after this period, pushing the geodynamo (the mechanism that creates the magnetic field) from a weak, unstable state to a strong, stable dipole field,” Driscoll said. .
Tarduno said the recovery of the field’s strength after the Ediacaran, with the growth of the inner core, was probably important in preventing the water-rich Earth from drying out.
As for the bizarre animals of the Ediacaran, they all disappeared in the following Cambrian Period, when the diversity of life exploded and the branches of the tree of life now familiar to us were formed in a relatively short time.
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