A rock sample used to reexamine Earth's pre-GOE "whiff of oxygen" spans the Archean and Paleoproterozoic time periods. The Earth might have looked like this billions of years ago. The Museum of Natural History is in the city.
The evidence for a "whiff of oxygen" before the Great Oxygenation Event 2.3 billion years ago is chemical signatures that were probably introduced later.
The result changes previous research findings that atmospheric oxygen existed prior to the Great Oxygenation Event, and has the potential to rewrite what is known of the planet's past.
Sarah Slotznick, an assistant professor of earth sciences at Dartmouth and the first author of the study, said that without the whiff of oxygen reported by a series of earlier studies, the scientific community needs to critically reexamine its understanding of the first half of Earth's history.
The study shows that the chemical data may have been introduced hundreds of millions of years later than originally thought.
The analysis shows that Earth's atmosphere had low oxygen levels prior to 2.3 billion years ago.
"We used new tools to investigate the origins of the signals of trace oxygen," said Jena Johnson, an assistant professor of earth and environmental sciences at the University of Michigan and co-author of the study. The chemical evidence of oxygen was likely caused by a series of changes after the seafloor was deposited.
Oxygenation is initiated.
Scientists have debated when the first measurable levels of oxygen appeared in the atmosphere. Over the last century, the idea of the Great Oxygenation event has developed and is thought to be when oxygen levels began to increase over 2 billion years ago, paving the way for the rise of complex cells, animals, and eventually humans.
Transient appearances of oxygen, known as "whiffs," have been suggested by research on chemical signals correlated to oxygen.
Two studies in 2007, one in Western Australia and the other in the US, found evidence of a whiff of oxygen in samples of the 2.5 billion-year-old Mount McRae Shale.
"When the results came out a decade ago, they were quite shocking," said Joseph Kirschvink, a professor at Caltech and a co-author of the new study. The findings seemed to contradict the evidence from other geological indicators that argued against the presence of free oxygen before the Great Oxygenation event.
A research origin story.
The evidence of oxidation and reduction of Molybdenum and sulfur was used in the studies to determine the presence of atmospheric oxygen. Questions about the early evolution of life on Earth were raised by the findings.
Some research groups took the observation of early oxygen to support earlier findings that the ancient atmosphere was filled with oxygen but that other Earth processes kept it low.
The implications of the 2007 studies about the origin of life and its evolution have been accepted and used as the basis for a number of other research papers over the last 14 years.
The Mount McRae Shale is made of volcanic glass shards, which could be a source of the molybdenum concentrated in the "whiff" interval during later fluid flow events that have previously been taken to indicate early atmospheric oxygen. The iron-sulfur mineral within the dark grey shale of the "whiff" interval is where these events are recorded. Slotznick et al., from Science Advances, reexamined the Oxygen Interval Points to Anoxic Ocean before GOE. The work is licensed under the CC BY-NC.
The Caltech-led team began efforts to conduct additional analysis in 2009. The team, some of whom have since moved to other institutions, took over a decade to collect and analyze data, resulting in the first published study that directly refutes the finding of a whiff of early oxygen.
"Rocks tell a complicated story that goes beyond what the world was like when the mud was deposited," said Woodward Fischer, a professor at Caltech and co-author of the study. "These samples 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."
There is a matter of approach.
The bulk analysis techniques used in the research papers that found the whiff of oxygen prior to Earth's full oxygenation used powdered samples from throughout the Mount. The new research used high-resolution techniques to inspect the specimen of the rock rather than using a chemical analysis.
The research team recorded the images of the drill core on a flatbed. They collected thin samples for more analysis. The team was given additional insight into the geology and chemistry of the samples as well as the relative timing of processes that were identified by using the suite of approaches.
According to the research paper, the bulk chemical datasets point to a 'whiff' of oxygen developed during post-depositional events.
The analysis shows that the Mount McRae Shale was formed from volcanic dust and organic carbon. The research shows that Molybdenum came from volcanoes and then concentrated during the whiff interval. During a series of chemical and physical changes that turned these sediments into rock, fracturing created pathways for several distinct fluids to carry in signals of oxidation hundreds of millions of years after the rocks formed.
The recent insight that volcanic glass is a major host of [molybdenum] enrichment offers a new explanation for our observations of abundant pyroclastic glass shards and intercalated tuff beds.
Looking back to point a way forward.
The original finding of early atmospheric oxygen is not supported by the presence of molybdenum if it was not from weathering of rocks on land and concentration in the ocean. The new research uses a completely different methodology than the first studies used, and it also calls into question the methods used in the other studies.
"Our new data clearly shows that chemical signals have to be considered in ancient records," said Johnson.
In addition to providing an alternate explanation for oxygen proxies that were found in the Mount McRae Shale, the team confirmed that the level of atmospheric oxygen at the time before the Great Oxygenation Event was very low.
The findings question the early existence of the cyanobacteria, instead supporting other hypotheses that oxygen-generating photosynthesis evolved only shortly before the Great Oxygenation Event.
Slotznick said that they expect the research to generate interest from both Earth and other planets. We hope that it will encourage further discussion about how we analyze rocks that are billions of years old.
The University of Western Australia and the China University of Geosciences have several people associated with them.
Sarah P. Slotznick states in Science Advances that the oxygen interval points to anoxic ocean before GOE. Science.org has a DOI of/10.1126/sciadv.abj7190.
The original research papers suggested a "whiff of oxygen".
A. D. Anbar, Y. Duan, T.W. Lyons, G. L. Arnold, B. Kendall, R. A. Creaser, A. J. Kaufman, G. W. Gordon, C. Scott, J. Garvin, R. Buick Science, 1903-1906.
J. Farquhar, A. L. Masterson, T. W. Lyons, A. D. Anbar, G. L. Arnold, J. Garvin, R. Buick. Science, 1900-1903.
Science Advances is a journal.
There are new research questions about 'whiff of oxygen' in Earth's early history.
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