Rock sample re-writes Earth’s early atmospheric history – again

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

In 2007, startling geological findings suggested that transient ‘whiffs’ of oxygen appeared much earlier than had ever been thought possible. Our interpretation of the planet’s past was entirely rewritten as a result.

Now, new research published in Science Advances is taking us back to the drawing board.

The study indicates that the chemical data that suggested atmospheric oxygen very early in Earth’s history may actually have been introduced by events hundreds of millions of years later. This resurrects the supplanted hypothesis that Earth’s atmosphere had exceedingly low oxygen levels prior to 2.3 billion years ago, when the atmosphere underwent a dramatic shift known as the Great Oxygenation Event.

“Without the whiff of oxygen reported by a series of earlier studies, the scientific community needs to critically re-evaluate its understanding of the first half of Earth’s history,” says Sarah Slotznick, an assistant professor of earth sciences at Dartmouth, US, and first author of the study.

The idea of the Great Oxygenation Event developed over the last century. It’s thought to be when oxygen levels began to increase dramatically more than 2 billion years ago, catalysing the evolution of aerobic life and paving the way for the rise of complex cells, animals, and eventually humans.

Most geological evidence indicates a near-complete absence of oxygen prior to the event.

When researchers in 2007 reported evidence of trace amounts of oxygen based on samples of the 2.5-billion-year-old Mount McRae Shale – part of a 2004 drill core collected in Western Australia by the NASA Astrobiology Drilling Program – the scientific community was stunned.

“When the results came out a decade ago, they were startling,” says Joseph Kirschvink, professor of geobiology at the California Institute of Technology (Caltech), and a study co-author. “The findings seemed to contradict abundant evidence from other geological indicators that argued against the presence of free oxygen before the Great Oxygenation Event.”

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

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

But science never rests on its laurels.

Returning to the ground-breaking samples in 2009, a Caltech-led team began additional analysis. After more than a decade of work, they have now produced the first published study that directly refutes the finding of a whiff of early oxygen.

Those findings seem to have arisen from a methodological error.

Because oxygen can’t be measured directly in rock, the 2007 study instead used chemical signals correlated with oxygen as proxy measures for the abundance of the element. Specifically, they used evidence of oxidation and reduction of molybdenum to infer atmospheric oxygen concentrations, reasoning that oxidised molybdenum would have originated from oxygen-based weathering of rocks on land that subsequently concentrated in the ocean.

Crucially, these studies used bulk analysis techniques featuring geochemical assessments of powdered samples sourced from throughout the Mount McRae Shale – a method that inadvertently removed vital contextual information from the sample, skewing the interpretation of results.

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

“We used new tools to investigate the origins of the signals of trace oxygen,” says co-author Jena Johnson, an 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 of oxygen.”

The new analysis shows that the molybdenum in the Mount McRae Shale samples came from volcanoes and subsequently concentrated during what has been previously characterised as the whiff interval. As these sediments hardened into rock, they fractured – creating pathways for fluids to carry in signals of oxidation hundreds of millions of years after the rocks formed.

The results remove significant support for the original finding of early atmospheric oxygen. Instead, the team confirmed that the level of atmospheric oxygen in the 150-million-year lead up to the Great Oxygenation Event was negligible.

The findings are far-reaching. They call into question the early existence of cyanobacteria, and instead support other hypotheses that oxygen-generating photosynthesis evolved only shortly before the Great Oxygenation Event.

But importantly, the new methodology used in the study also calls into question other research that has used the same style of bulk techniques that led the 2007 research astray.

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

“We expect that our research will generate interest both from those studying Earth and those looking beyond at other planets,” says Slotznick. “We hope that it stimulates further conversation and thought about how we analyse chemical signatures in rocks that are billions of years old.”