The history of the atmosphere rewritten by a sample of rock

Scientists have long debated when measurable levels of oxygen first appeared in Earth’s atmosphere.

In 2007, surprising geological discoveries suggested that transient “puffs” of oxygen appeared much earlier than had ever been thought possible. Our interpretation of the planet’s past has been completely rewritten accordingly.

Now new research published in Scientists progress brings us back to the drawing board.

The study indicates that chemical data that suggested atmospheric oxygen very early in Earth’s history may in fact have been introduced by events hundreds of millions of years later. This resurrects the superseded hypothesis that Earth’s atmosphere had extremely low oxygen levels before 2.3 billion years ago, when the atmosphere underwent a dramatic change known as the Great Event. oxygenation.

“Without the smell of oxygen reported by a series of previous studies, the scientific community must critically reassess its understanding of the first half of Earth’s history,” said Sarah slotznick, assistant professor of earth sciences in Dartmouth, USA, and first author of the study.

The idea of ​​the Great Oxygenation Event has developed over the past century. It is believed that this was when oxygen levels began to rise dramatically over 2 billion years ago, catalyzing the evolution of aerobic life and paving the way for the appearance of complex cells, animals and ultimately humans.

Most of the geological evidence indicates an almost complete absence of oxygen before the event.

When researchers in 2007 reported traces of oxygen based on samples from the 2.5 billion-year-old Mount McRae shale – part of a 2004 drill core collected in Western Australia by the program NASA’s astrobiology drill – the scientific community was stunned.

“When the results came out ten years ago, they were surprising,” says Joseph Kirschvink, professor of geobiology at the California Institute of Technology (Caltech) and co-author of the study. “The results appeared to contradict the abundant evidence of other geological indicators that militated against the presence of free oxygen prior to the large oxygenation event.”

The primitive oxygen observation was taken by some research groups to support earlier findings that microscopic cyanobacteria – the first innovators of photosynthesis – pumped oxygen into the ancient atmosphere, but others Earth processes kept oxygen levels low.

The 2007 study, including its implications for the origin of life and its evolution, has been widely accepted and has served as the basis for a series of other research papers over the past 14 years.

A rock sample used to reexamine Earth’s “breath of fresh air” before the GOE covers the Archean and Paleoproterozoic periods. This illustration shows what the Earth might have looked like billions of years ago. Credit: Ozark Natural History Museum.

But science never rests on its laurels.

Returning to breakthrough samples in 2009, a team led by Caltech began additional analysis. After more than a decade of work, they have now produced the first published study that directly refutes the finding of an early breath of fresh air.

These results seem to stem from a methodological error.

Since oxygen cannot be measured directly in rock, the 2007 study instead used chemical signals correlated with oxygen as indirect measures of the element’s abundance. Specifically, they used evidence of oxidation and reduction of molybdenum to infer oxygen concentrations in the atmosphere, estimating that oxidized molybdenum would come from the oxygen-based weathering of terrestrial rocks that subsequently collapsed. concentrated in the ocean.

Importantly, these studies used bulk analysis techniques including geochemical assessments of powdered samples from all over the Mount McRae shale – a method that inadvertently removed vital background information from the sample, skewing the results interpretation.

Rather than conducting chemical analysis on the powder, the new research inspected rock specimens using a suite of high-resolution techniques, including synchrotron-based X-ray fluorescence spectroscopy, providing insight on the geology and chemistry of the samples as well as on the relative timing of the processes that were identified.

“We used new tools to study the origins of trace oxygen signals”, explains the co-author Jena johnson, assistant professor of earth and environmental sciences at the University of Michigan. “We discovered that a series of changes after sediment deposition on the seabed were likely responsible for the chemical proof of oxygen.”

The new analysis shows that the molybdenum in the Mount McRae shale samples originated from volcanoes and then concentrated during what was previously characterized as the puff interval. As these sediments hardened into rock, they fractured, creating pathways for fluids to carry oxidation signals hundreds of millions of years after rock formed.

The results remove significant support for the original discovery of early atmospheric oxygen. Instead, the team confirmed that the level of atmospheric oxygen in the 150 million years before the great oxygenation event was negligible.

The conclusions are far-reaching. They question the early existence of cyanobacteria and instead support other hypotheses that oxygen-generating photosynthesis only evolved shortly before the great oxygenation event.

Importantly, the new methodology used in the study also calls into question other research that used the same style of mass-market techniques that led the 2007 research to stray.

“Our new high-resolution data clearly indicates that the sedimentary background of chemical signals must be carefully considered in all old recordings,” says Johnson.

“We expect our research to generate interest both for those who study Earth and for those who look beyond other planets,” Slotznick said. “We hope this will stimulate conversation and thought about how we analyze chemical signatures in rocks that are billions of years old.”

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