Privileged Planet: life before plate tectonics (Introduction)

by David Turell , Saturday, June 17, 2023, 15:22 (315 days ago) @ David Turell

Dating plate tectonics dates it after life started:

https://www.universetoday.com/161952/did-life-need-plate-tectonics-to-emerge/

"It’s widely accepted that Earth’s plate tectonics are a key factor in life’s emergence. Plate tectonics allows heat to move from the mantle to the crust and plays a critical role in cycling nutrients. They’re also a key part of the carbon cycle that moderates Earth’s temperature.

"But new research suggests that there was no plate tectonic activity when life appeared sometime around 3.9 billion years ago. Does this have implications for our search for habitable worlds?

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"In new research, scientists examined zircons from between 3.9 to 3.4 billion years ago. The results are in a paper titled “Hadaean to Palaeoarchaean stagnant-lid tectonics revealed by zircon magnetism”.

“'We found there wasn’t plate tectonics when life is first thought to originate, and that there wasn’t plate tectonics for hundreds of millions of years after,” says Tarduno. “Our data suggests that when we’re looking for exoplanets that harbour life, the planets do not necessarily need to have plate tectonics.”

"Zircons contain magnetic particles, and as the zircons solidified, the particles are affected by Earth’s magnetic fields at the time, and the magnetic fields change in strength over time and over different parts of the Eart as the magnetic poles wander. So, the zircons that scientists study retain evidence of Earth’s ancient magnetic fields.

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"Scientists can date zircons through radiometric dating. The tiny rocks contain trace amounts of the radioactive elements uranium and thorium. Since scientists know the rate of decay for these elements and their decay chains, they can measure and compare the amounts of the elements and what they decay into to determine the zircons’ ages.

"Once they know the ages of the zircons, and the orientation of their magnetic particles, a picture of Earth’s ancient magnetic fields emerges.

"Earth’s geodynamo generates its magnetic field, but its strength and direction change with latitude and time. So if the geodynamo is consistent over time, then zircons that formed at different latitudes will have different magnetic properties, whereas zircons from the same latitude will have similar properties.

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These results pose another question: how did the early Earth shed heat?

"The answer might be what’s known as stagnant lid tectonics. In stagnant lid tectonics, Earth sheds heat through cracks in the planet’s solid cap.

"With no plate tectonics, the heat still has to escape somehow. In stagnant lid tectonics, there are no continents drifting around, with ocean ridges venting the planet’s internal heat. Instead, there’s a single, monolithic lid that remains in place. Magma plumes rise up at different locations under the lid, cracking it and releasing heat. Stagnant lid tectonics don’t release heat as efficiently as plate tectonics, but they can still form continents.

“'Early Earth was not a planet where everything was dead on the surface,” Tarduno says. “Things were still happening on Earth’s surface; our research indicates they just weren’t happening through plate tectonics. We had at least enough geochemical cycling provided by the stagnant lid processes to produce conditions suitable for the origin of life.”

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"Plate tectonics are also important for cycling nutrients and carbon. Carbon is released at mid-ocean ridges and sequestered through subduction when plates meet. Plate tectonics have helped Earth maintain its Goldilocks climate for billions of years. But it may not have always been this way.

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"This study shows that it’s not necessary, at least for the first billion years or so. Stagnant lid tectonics may have been enough. In fact, the similarity between the zircons also indicates that deep subduction was unlikely, meaning the poles didn’t wander, with all the rapid surface changes that can accompany that phenomenon. That could’ve helped provide the stability early life may have relied on. “The lack of large, rapid changes in environmental conditions induced by true polar wander likely fostered survival of nascent life on our planet,” the authors explain.

"This isn’t the first research showing that plate tectonics may not have been active in Earth’s earliest days. Earth likely had at least one magma ocean phase, and some research shows that the crust may have formed into one more-or-less complete crust before tectonics began. A 2020 paper showed that the end of the Archaean, about 2.5 billion years ago, “marks the period in which plate tectonics became the dominant tectonic regime on Earth.”

"There’s more. A 2011 study based on diamonds on the subcontinental mantle showed that modern plate tectonics started 3 billion years ago. And a 2005 research article based on ophiolites—chunks of oceanic plate thrust up at the edge of continental plates—suggested that modern subductive plate tectonics began in the Neoproterozoic, between 1 billion to 538.8 million years ago.

"It’s not surprising that there are different estimates for the appearance of plate tectonics. Earth’s history is deeply concealed, and in many cases, totally erased. Different studies have regarded different pieces of evidence as pivotal, and there’s no way to figure it all out except to keep studying it.

Comment: this shows early life did not need plate tectonics but it supported later more complex life.