• Wed. May 25th, 2022

Atmospheric history rewritten by a rock sample

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

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

Now new research published in Scientists progress takes 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 This resurrects the superseded hypothesis that Earth’s atmosphere had extremely low oxygen levels prior to 2.3 billion years ago, when the atmosphere underwent a dramatic change known as the Great Earthquake Event. oxygenation.

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

The idea of ​​the great oxygenation event has developed over the last century. It is thought to be when oxygen levels began to rise dramatically more than 2 billion years ago, catalyzing the evolution of aerobic life and paving the way for the rise of complex cells, animals and possibly humans.

Most geological evidence points to an almost complete absence of oxygen prior to the event.

When researchers in 2007 reported evidence of trace 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 NASA’s astrobiology drill program – the scientific community was stunned.

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

The observation of early oxygen has been taken by some research groups to support earlier findings that microscopic cyanobacteria – the early innovators of photosynthesis – pumped oxygen into the ancient atmosphere, but that 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 re-examine Earth’s pre-GOE “breath of oxygen” spans the Archean and Paleoproterozoic periods. This illustration shows what Earth might have looked like billions of years ago. Credit: Ozark Museum of Natural History.

But science never rests on its laurels.

Returning to the 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 conclusions appear to be the result of a methodological error.

Since oxygen cannot be measured directly in rock, the 2007 study instead used chemical signals correlated to oxygen as indirect measures of the element’s abundance. Specifically, they used evidence of molybdenum oxidation and reduction to infer atmospheric oxygen concentrations, believing that oxidized molybdenum originated from oxygen-based weathering of Earth rocks which then concentrated in the atmosphere. ‘ocean.

Basically, these studies used bulk analysis techniques that included geochemical assessments of powdered samples from all of the Mount McRae Shale – a method that inadvertently removed vital contextual information from the sample, distorting the results interpretation.

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

“We used new tools to investigate the origins of trace oxygen signals,” says the co-author Jena Johnson, assistant professor of earth and environmental sciences at the University of Michigan. “We found that a series of changes after the sediments were deposited on the seafloor were likely responsible for the chemical evidence for oxygen.”

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

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

Discoveries go a long way. They question the early existence of cyanobacteria and instead support alternative hypotheses that oxygen-generating photosynthesis evolved only shortly before the great oxygenation event.

Importantly, the new methodology used in the study also challenges other research that used the same style of mass techniques that misled the 2007 research.

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

“We expect our research to pique the interest of both those studying Earth and those looking beyond to other planets,” Slotznick says. “We hope this stimulates conversation and thinking about how we analyze chemical signatures in rocks billions of years old.”