Researchers Challenge Global Carbon Cycle Disruption Theory During Earth's Great Oxidation Event

Questioning a Planetary Narrative

For decades, scientists have pointed to an unusual carbon-isotope signature preserved in ancient rock formations as evidence that Earth experienced a dramatic global disruption of its carbon cycle approximately 2 billion years ago, coinciding with the planet's first major accumulation of atmospheric oxygen. This anomalous signal, known as the Shunga-Francevillian event, has been identified in drill cores from two geographically distant locations: the Zaonega Formation in Karelia, Russia, and the Francevillian Basin in Gabon. The prevailing interpretation suggested these sites recorded evidence of a worldwide environmental transformation. However, new research from Caltech scientists is challenging this narrative by demonstrating that at least one of these sites may reflect local geological phenomena rather than global-scale changes.

The New Explanation: Local Heat and Microbial Activity

The Caltech team, led by Nivedita Thiagarajan in the laboratory of geochemistry professor John Eiler, examined gases trapped within microscopic pockets inside rocks from the Zaonega Formation. Rather than interpreting the carbon-isotope anomaly as evidence of worldwide disruption, they propose an alternative mechanism centered on local heating and biological activity within a sedimentary basin spanning several hundred square kilometers. The researchers theorize that magma intruded through marine sediment layers, generating intense heat that reached approximately 350 degrees Celsius near the intrusion point and gradually cooled to about 72 degrees Celsius at distances around 300 meters away. This thermal gradient drove the generation of hydrocarbons including methane and propane from buried organic material. The rising methane then encountered microbes living near the ancient seafloor, which consumed the gas and produced biomass bearing a distinctive light carbon-isotope signature—the very anomaly previously interpreted as evidence of global-scale disruption.

Bridging Ancient and Modern Geochemistry

The research methodology combined two complementary expertise streams. Thiagarajan and Eiler had previously developed a framework for understanding how natural gas forms through isotope ratio analysis in modern systems. When collaborator Aivo Lepland from Norway's Geological Survey arrived at Caltech on sabbatical bringing newly measured isotope data from Zaonega rocks, the team recognized they could apply their modern gas-formation insights to ancient samples. This cross-disciplinary approach proved productive: the researchers identified signatures in 2-billion-year-old samples that closely paralleled patterns observed in contemporary oil and gas basins. The thermal evidence strongly supported their thermogenic hydrocarbon hypothesis, with temperature gradients preserved in the rock record serving as a reliable guide to reconstructing the subsurface processes that generated the isotopic anomalies.

Implications and Next Steps

While the researchers acknowledge they cannot completely eliminate the possibility that other processes contributed to the observed isotopic signal, their findings fundamentally reshape how scientists should interpret the Zaonega Formation evidence. Because Zaonega has long served as a reference site for the Shunga-Francevillian event, the implications are substantial: the carbon-isotope anomaly may not indicate a worldwide environmental disruption but rather a local geological and biological sequence driven by magmatic heating. The team's findings raise urgent questions about whether the Shunga-Francevillian event itself should continue to be classified as a global phenomenon. To test whether similar local processes might explain the isotopic patterns observed in Gabon's rocks, researchers have begun analyzing cores recovered through the GOE-DEEP project, an international drilling initiative co-funded through the International Continental Scientific Drilling Program. Lepland spent four months in Gabon during summer 2025 coordinating the drilling operation. The newly recovered cores arrived in Norway in February and are scheduled for detailed analysis later this year by an international team representing 18 countries. This comparative approach between the Russian and Gabonese sites will clarify whether the carbon-cycle anomaly of 2 billion years ago represents a true planetary event or separate regional phenomena that happen to have left similar geochemical signatures.