Caltech Study Suggests Ancient Carbon Signal Was Local, Not Global

Researchers at Caltech have found that an unusual carbon-isotope signal in ancient rocks from Karelia, Russia, likely resulted from local geological activity rather than a global environmental change. The signal, found in the Zaonega Formation, has long been cited as evidence for the Shunga–Francevillian event, a hypothesized planet-wide disruption of the carbon cycle roughly 2 billion years ago.

The team analyzed microscopic gas bubbles trapped in pyrobitumen, a form of organic carbon, within drill cores from the formation. Lead author Nivedita Thiagarajan, a senior scientific researcher at Caltech, said the isotope ratios in these gases match a process where a magma sheet intruded into marine sediments, heating them to produce hydrocarbons like methane and propane.

Those hydrocarbons migrated upward and fed methane-consuming microbes near the ancient seafloor, which produced biomass with a distinctively light carbon signature. Measurements revealed a temperature gradient from roughly 350 degrees Celsius near the intrusion to 72 degrees Celsius at an ancient asphalt spill about 300 meters above.

Co-author Aivo Lepland of the Geological Survey of Norway noted that the Zaonega Formation is a primary reference site for the Shunga–Francevillian event. If the anomaly there is local, the case for a synchronous global event weakens significantly.

The researchers cannot entirely rule out other contributing processes, but their data support a predominantly local driver. The collaboration combined Lepland's dataset of trapped-gas isotopes with a theoretical framework for natural-gas formation developed by Thiagarajan and Caltech professor John Eiler.

The team plans to test whether the same local mechanisms explain similar isotopic signals in rocks from the Francevillian Basin in Gabon. Cores from a 2025 drilling campaign there arrived at the Geological Survey of Norway in February and will be analyzed by an international team later this year.

The study, titled "Paleoproterozoic thermogenic hydrocarbon generation, Zaonega Formation, Russia," was published in the journal Geology. Additional authors include Florian Eichinger of Hydroisotop GmbH and Anthony Prave of the University of St. Andrews.