Scientists have discovered that a well-known 2-billion-year-old chemical signature, long believed to indicate a major shift in Earth's carbon cycle, may have instead been caused by local geological and biological processes. This finding challenges previous assumptions about the global changes that occurred during a critical period in Earth's history. Around 2.5 to 2 billion years ago, Earth experienced its most significant surface chemical transformation. Oxygen began to accumulate in the atmosphere, leading to changes that eventually allowed for the development of complex life forms like plants and animals roughly half a billion years later. As oxygen levels rose for the first time, vast amounts of microbial material were buried beneath the seafloor, trapping carbon in rocks and leaving behind an unusual isotopic signature. For decades, scientists have interpreted this signature as evidence of a dramatic, planet-wide imbalance in Earth's carbon cycle. This interpretation has been supported by drill cores recovered from ancient marine sediments in Karelia, Russia, and the Francevillian Basin in Gabon. A new study led by researchers from the California Institute of Technology (Caltech) questions whether the evidence from Russia truly reflects a global event. The study describes how the researchers reconstructed the sequence of changes preserved in the rocks of Karelia following the first major increase in atmospheric oxygen. Carbon isotopes provide insight into the origins of biological material that accumulated billions of years ago. By measuring the relative amounts of these isotopes in drill cores, researchers can build a record of ancient environmental changes. A notable carbon signal found in both the Zaonega Formation in Karelia and rocks from Gabon is known as the Shunga-Francevillian event. Scientists have often cited this as evidence of a major global disruption of the carbon cycle around 2 billion years ago. The researchers analyzed drill cores stored at the Geological Survey of Norway, focusing on gases trapped 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 forms when buried crude oil or kerogen is subjected to intense heating deep underground. The team suggests that a sheet of magma forced its way through layers of marine sediment at the Zaonega Formation, which was once beneath a prehistoric ocean. Heat from the magma warmed sediments packed with organic material, generating hydrocarbons such as methane and propane. These gases then moved upward through the sediment and 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. Temperature evidence supports this scenario, with a large thermal gradient identified—temperatures reaching approximately 350 degrees Celsius near the magma intrusion and falling to about 72 degrees Celsius at an ancient seafloor asphalt spill roughly 300 meters higher. The researchers acknowledge they cannot completely rule out other processes, but their results suggest the carbon isotope anomaly in the Zaonega Formation was mainly driven by local geological and biological activity rather than a global disturbance. The next step is to determine whether the same explanation applies to 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 rock record in Gabon. In the summer of 2025, a researcher spent four months in Gabon coordinating the drilling campaign. The newly recovered cores arrived at the Geological Survey of Norway in February and are scheduled to be sampled later this year by an international research team representing 18 countries. The study is titled Paleoproterozoic thermogenic hydrocarbon generation, Zaonega Formation, Russia. Additional authors on the study are from a natural isotope analysis laboratory in Germany and a university in Scotland.