Around 2.5 to 2 billion years ago, Earth underwent the largest chemical transformation ever recorded at its surface. Oxygen began accumulating in the atmosphere, setting in motion changes that eventually helped make possible the rise of complex organisms such as plants and animals about half a billion years ago.
As oxygen levels increased for the first time, enormous amounts of microbial material were buried beneath the seafloor. That burial trapped carbon in rocks and left behind an unusual isotopic signature. For decades, many scientists have interpreted that signature as evidence that Earth’s carbon cycle became dramatically unbalanced on a global scale.
A Famous Signal From Early Earth
Evidence supporting that interpretation has come from drill cores — long cylinders of solid rock pulled from deep underground — recovered from ancient marine sediments in Karelia, Russia, and the Francevillian Basin in Gabon. A new study led by Caltech researchers, however, is raising questions about whether the Russian evidence really records a planetwide event.
“One major debate centers on an unusual carbon-isotope signal that has often been interpreted as evidence of a worldwide environmental change,” says Nivedita Thiagarajan (PhD ’12), a senior scientific researcher at Caltech who works in the lab of John Eiler, the Robert P. Sharp Professor of Geology and Geochemistry and Ted and Ginger Jenkins Leadership Chair of the Division of Geological and Planetary Sciences. “We studied gases trapped in microscopic pockets within rocks from the Zaonega Formation in Karelia, Russia, one of the world’s oldest known fossil oil fields, and found that the carbon-isotope signal at this key site can be explained by local phenomena that occurred in a several-hundred-square-kilometer sedimentary basin rather than across the entire globe.”
Thiagarajan is the lead author of a recent paper published in Geology. The study describes how the researchers reconstructed the sequence of changes preserved in Karelia’s rocks following the first major increase in atmospheric oxygen.
Carbon isotopes — heavier or lighter forms of the element — provide information about the origins of biological material that accumulated billions of years ago. By measuring the relative amounts of these isotopes in drill cores (or carbon-isotope signals), researchers can build a record of ancient environmental change, somewhat like reading the growth rings of a tree.
An unusual carbon signal found in both the Zaonega Formation and rocks from Gabon is known as the Shunga-Francevillian event. Scientists have often pointed to it as evidence that Earth experienced a major global disruption of the carbon cycle around 2 billion years ago.
“Earth, in a way, went crazy during that time interval when oxygen appeared in the atmosphere. What we are trying to assess are the causes and consequences of Earth oxygenation,” explains Aivo Lepland, a researcher at the Geological Survey of Norway (NGU) in Trondheim and a co-author on the study. “This information is archived in the rocks, so, in order to study what happened, you have to study rocks.”
Gases Trapped for 2 Billion Years
To examine the Shunga-Francevillian event from another perspective, the researchers analyzed drill cores stored at NGU. They focused on gases sealed inside microscopic fluid inclusions within samples from the Zaonega Formation that are rich in pyrobitumen.
The Zaonega Formation was once part of a marine sedimentary basin. Pyrobitumen is an insoluble form of organic carbon that develops when buried crude oil or kerogen — a source material for natural gas — is subjected to intense heating deep below the surface.
The project took shape after Lepland arrived at Caltech for a sabbatical. He brought with him a new collection of isotope measurements from gases trapped in Zaonega rocks that had not yet been fully interpreted.
At the same time, Thiagarajan and Eiler had recently finished research measuring isotope ratios in natural gases. That work had helped them develop a broader framework for understanding how natural gas forms.
When the researchers combined the two sets of expertise and data, a different explanation for the ancient isotope signatures began to emerge.
Magma, Methane, and Microbes
The team proposes that a sheet of magma forced its way through layers of marine sediment at the Zaonega Formation, which at the time lay beneath a prehistoric ocean. Heat from the magma warmed sediments packed with organic material.
That heating generated hydrocarbons including methane and propane. The gases then moved upward through the sediment and eventually reached microbes living near the seafloor that consumed methane.
Those microbes produced biomass carrying a light carbon isotope signature, potentially explaining the unusual signal preserved in the rocks.
The temperature evidence supports this scenario. The researchers identified a large thermal gradient, with temperatures reaching approximately 350 degrees Celsius beside the magma intrusion and falling to about 72 degrees Celsius at an ancient seafloor asphalt spill roughly 300 meters higher.
“This chain of geological and biological processes can account for the unusual carbon-isotope signal recorded at the Zaonega Formation,” Thiagarajan says. “It was interesting to see that some of the same signatures that we observe in modern oil and gas basins are also there and preserved in 2-billion-year-old samples.”
The researchers emphasize that they cannot completely rule out contributions from other processes. Even so, their results indicate that the carbon isotope anomaly preserved at Zaonega was driven mainly by events within the local sedimentary basin rather than by a global disturbance.
“Because Zaonega is a reference site for the Shunga-Francevillian event, our findings raise important questions about whether it should be considered a worldwide event,” says Thiagarajan.
Testing the Theory in Gabon
The next step will be to determine whether the same explanation can account for the similar isotope signal found in Gabon.
Researchers plan to analyze samples collected through the GOE-DEEP project, co-funded by the International Continental Scientific Drilling Program. The goal is to test whether local geological and biological processes like those identified in Russia also shaped the Gabonese rock record.
In the summer of 2025, Lepland spent four months in Gabon coordinating the drilling campaign. The newly recovered cores arrived at NGU in February and are scheduled to be sampled later this year by an international research team representing 18 countries.
“Now we can really put things together by doing a similar type of study on the Gabonese rocks to compare the two sites,” Lepland says. “This is how science moves forward.”
The Geology paper is titled “Paleoproterozoic thermogenic hydrocarbon generation, Zaonega Formation, Russia.” Additional authors on the study are Florian Eichinger of Hydroisotop GmbH, a natural isotope analysis laboratory in Germany, and Anthony Prave of the University of St. Andrews in Scotland.
