10.05.2024

Decreased protection from cosmic radiation could have boosted oxygen levels in the atmosphere and oceans

Earth’s magnetic field buffers our planet against barrages of cosmic particles. It fluctuates in strength on timescales of thousands to millions of years. A weaker field in the past could have led to an explosion in the size of marine animals, a new study proposes.

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Earth’s magnetic field protects life from harmful cosmic radiation. But sometime between about 590 million and 565 million years ago, that security blanket seems to have been much thinner — with far-reaching effects for the development of life on Earth, researchers suggest.

A weaker magnetic field could account for the higher levels of oxygen recorded in the Earth’s atmosphere and oceans around that time — and for the ensuing proliferation of macroscopic marine animals, the team reports in the May 2 Communications Earth & Environment.

Earth’s magnetic field is the result of churning molten iron in the planet’s interior. With a strength of just 0.00005 tesla, it’s about one ten-thousandth the strength of the field in a magnetic resonance imaging machine. That might not seem like much, but it’s strong enough for several animals to perceive and use to navigate (SN: 3/29/24).

The strength of Earth’s magnetic field fluctuates, typically over timescales of thousands to millions of years. For instance, 565-million-year-old rocks found in Canada contain magnetic minerals that suggest Earth’s magnetic field then was only a tenth as strong as it is today (SN: 1/28/19). Our planet’s field periodically becomes weaker during events known as magnetic field reversals, but it’s unlikely that these rocks are reflecting those relatively short-term events, says John Tarduno, a geophysicist at the University of Rochester in New York who was on the team that studied the Canadian rocks (SN: 2/18/21).

Now, the same team has examined rocks from Brazil dating to about 590 million years ago. Earth’s magnetic field was even weaker back then, the researchers found — just one-thirtieth the modern-day value. That’s the lowest magnetic field strength ever measured for our planet, Tarduno says. “The field almost completely collapsed.”

If Earth’s magnetic field remained low during the roughly 25-million-year interval bracketed by those samples — and less-precise data from other teams suggest that it did — that’s a remarkable coincidence, Tarduno says. Earth’s magnetic field was dramatically weaker right around the time of the Ediacaran Period, when oxygen levels increased in both the atmosphere and oceans; rock records show higher-than-normal levels of oxygen around that time. It’s also a period when macroscopic animals began to proliferate in the world’s oceans.

Larger marine animals, like this fossilized, 41-centimeter Dickinsonia costata, evolved around the time that our planet’s magnetic field was weaker and oxygen levels were higher. SHUHAI XIAO

Perhaps there’s a link there, Tarduno and his colleagues propose in the new paper. A weaker magnetic field would have meant less protection from energetic cosmic particles. “Our shield was down,” Tarduno says. Those particles would have broken apart water molecules in the early Earth’s atmosphere. Hydrogen, being extremely light, would have readily escaped into space, while oxygen would have remained behind. Over time, that imbalance would have tipped the scales in favor of a more oxygen-rich atmosphere and oxygen-enriched oceans, the researchers suggest.

The larger, more mobile animals that the fossil record shows developed during the Ediacaran Period would have needed all that oxygen, Tarduno and his collaborators suggest. It’s no secret that bigger animals require more oxygen than their microscopic brethren, Tarduno says. “This oxygenation set the stage for large life.”

There are a lot of conceptual leaps to this research, but the measurements are all solid, says Joe Meert, a geoscientist at the University of Florida in Gainesville who was not involved in the research. Studies like this one are useful for getting big ideas out there, he says. “I like papers like this. It’s good to lay it all out.”

Quelle: Science News