Although the exact composition of the Earth’s earliest atmosphere is not known, geological evidence suggests that it was largely made of dinitrogen (N2) and carbon dioxide (CO2) with small amounts of water vapor and other gases such as methane. Such an atmosphere contained practically no free oxygen (O2).
Around 2 billion years ago, however, oxygen gradually became more common in the atmosphere, a process that culminated in what scientists term the Great Oxygenation Event. During this period (between 2.4 and 2.0 billion years ago), O2 began to accumulate in Earth’s atmosphere and levels rose to the extent that they could support life as we know it today. This coincided with the evolution of complex cells, followed by plants, animals and humans.
For decades scientists have debated when measurable levels of oxygen first appeared in Earth’s atmosphere, with some studies suggesting that there were chemical signals of the presence of O2 long before the Great Oxygenation Event commenced. The scientists refer to these earlier, transient traces of oxygen as “whiffs”.
In particular, two studies published in 2007 found evidence of such a whiff of oxygen based on samples of the 2.5-billion-year-old Mount McRae Shale, part of an intensively studied 2004 drill core collected in Western Australia by the NASA Astrobiology Drilling Program. The studies concluded that oxygen was present in Earth’s atmosphere long before the start of the Great Oxygenation Event.
“When the results came out a decade ago, they were startling,” said Joseph Kirschvink, professor of geobiology at Caltech, and member of the Earth-Life Science Institute at the Tokyo Institute of Technology. “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 2007 studies were based on evidence of oxidation and reduction of molybdenum and sulfur, two elements that are widely used to test for the presence of atmospheric oxygen since it cannot be measured directly in rock. The findings raised fundamental questions about the early evolution of life on Earth.
The observation of early oxygen was thought to support earlier findings that microscopic cyanobacteria were present on Earth at that early time and that photosynthesis by these organisms released oxygen into the ancient atmosphere. These 2007 studies, and their 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 last 14 years.
A new analysis of the same rock core samples has, however, cast some doubt on this approach. The new study dates back to 2009, when a Caltech-led team began to conduct additional analysis. The team, some of whom have since moved to other institutions, took over a decade to collect and analyze data, resulting now in the first published study that directly refutes the finding of a whiff of early oxygen.
“Rocks this old tell a complicated story that goes beyond what the world was like when the mud was deposited,” said Woodward Fischer, a professor of geobiology at Caltech and co-author of the study. “These samples also contain minerals that formed long after their deposition when ancient environmental signals were mixed with younger ones, confusing interpretations of the conditions on ancient Earth.”
For the new study, the research team recorded images of the 2004 drill core on a flatbed optical scanner. Based on those observations, they then collected thin samples for additional analyses. The suite of approaches used on the physical specimens, including synchrotron-based X-ray fluorescence spectroscopy, gave the team additional insight into the geology and chemistry of the samples as well as the relative timing of processes that were identified.
Their findings, published today in the journal Science Advances, suggested that the chemical signatures previously used as evidence of the presence of whiffs of oxygen were actually introduced into the rock at a later stage, long after the rock was formed. They explain that the Mount McRae Shale formed from organic carbon and volcanic dust and that fracturing processes long after the rock’s initial formation created cracks into which fluids (carrying molybdenum) could flow, thereby introducing signals of oxidation hundreds of millions of years after the rocks formed.
“Our observations of abundant pyroclastic glass shards and intercalated tuff beds, paired with the recent insight that volcanic glass is a major host of [molybdenum], offers a new explanation for the [molybdenum] enrichments in the ‘whiff’ interval,” the paper says.
If the molybdenum was not from oxygen-based weathering of rocks on land, its presence does not support the original finding of early atmospheric O2. The original 2007 studies used analysis of powdered samples sourced from throughout the Mount McRae Shale, but by using a different methodological approach and a series of high-resolution techniques, the new research challenges the original findings. In addition, it calls into question all the research that followed from those original studies.
“Our new, high-resolution data clearly indicates that the sedimentary context of chemical signals has to be carefully considered in all ancient records,” said Johnson.
In addition, the study reported “negligible” levels of atmospheric oxygen in the period 150 million years before the sudden change that marked the start of the Great Oxygenation Event. Furthermore, the findings challenge the proposed early existence of cyanobacteria, instead supporting the hypothesis that the process of photosynthesis only evolved shortly before 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,” said Sarah Slotznick, an assistant professor of earth sciences at Dartmouth and first author of the study.
“We expect that our research will generate interest both from those studying Earth and those looking beyond at other planets,” said Slotznick. “We hope that it stimulates further conversation and thought about how we analyze chemical signatures in rocks that are billions of years old.”