The cores sit in storage at the Geological Survey of Norway (NGU) in Trondheim. Co-author Aivo Lepland, a researcher at NGU, brought fresh isotope measurements from those rocks to Caltech during a sabbatical. Caltech's Thiagarajan and John Eiler, the Robert P. Sharp Professor of Geology and Geochemistry, had just finished related work on natural gas. Combining the two datasets pointed toward a different explanation for the ancient signal.

"Earth, in a way, went crazy during that time interval when oxygen appeared in the atmosphere," Lepland says. "What we are trying to assess are the causes and consequences of Earth oxygenation. This information is archived in the rocks, so, in order to study what happened, you have to study rocks."

What local processes could explain the signal?

The researchers propose a four-step chain of events:

• A sheet of magma pushed through marine sediment that once lay beneath a prehistoric ocean.
• Heat from the intrusion warmed organic-rich sediments, generating hydrocarbons such as methane and propane.
• Those gases migrated upward toward microbes living near the seafloor that consumed methane.
• The microbes produced biomass carrying a light carbon-isotope signature — the same unusual signal preserved in the rocks.

Temperature data supports the picture. The team identified a large thermal gradient, with temperatures reaching roughly 350 °C next to the magma intrusion and dropping to about 72 °C at an ancient seafloor asphalt spill about 300 meters higher in the section.

"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."

How confident are the researchers?

The team cautions that it cannot fully exclude contributions from other processes. Even so, the new analysis indicates that the anomaly at Zaonega was driven mainly by events within a 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," Thiagarajan adds.

What happens next?

Researchers will test whether local processes can also explain the matching signal in Gabon. They plan to analyze new cores from the GOE-DEEP project, co-funded by the International Continental Scientific Drilling Program.

Lepland spent four months in Gabon during summer 2025 coordinating the drilling campaign. The cores arrived at NGU in February 2026. An international team from 18 countries is scheduled to sample them later this year.

"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 co-authors are Florian Eichinger of Hydroisotop GmbH in Germany and Anthony Prave of the University of St. Andrews in Scotland.