Around 2 billion years ago, Earth was going through one of the most important transformations in its history.
Oxygen was becoming abundant in the atmosphere, helping create conditions that would eventually allow complex life to evolve.
Scientists have long believed that an unusual chemical signal preserved in rocks from this period reflects a dramatic change in the global carbon cycle. But new research from Caltech suggests that at least some of this evidence may instead have been created by local geological and biological activity.
The findings, published in the journal Geology, could change how scientists interpret an important chapter in Earth’s early history.
Between roughly 2.5 billion and 2 billion years ago, atmospheric oxygen increased dramatically during a period associated with the Great Oxidation Event. At around the same time, large amounts of microbial material were buried on ancient seafloors, leaving unusual carbon isotope signatures in rocks.
Isotopes are different forms of the same element. By measuring the ratios of heavier and lighter carbon isotopes in ancient rocks, scientists can reconstruct environmental changes that occurred billions of years ago.
A particularly unusual carbon signature has been found in rocks from Karelia in Russia and the Francevillian Basin in Gabon. Known as the Shunga–Francevillian event, it has often been interpreted as evidence that Earth’s entire carbon cycle underwent a major disruption around 2 billion years ago.
The new research questions that interpretation for the Russian site.
Scientists studied tiny pockets of gas trapped inside rocks from the Zaonega Formation in Karelia, an ancient marine sedimentary basin that also contains one of the world’s oldest known fossil oil fields.
By analyzing the molecules and isotopes inside these microscopic pockets, the researchers reconstructed what may have happened there billions of years ago.
Their evidence points to a chain of local events beginning when a sheet of hot magma pushed into layers of organic-rich sediment beneath an ancient ocean.
The intense heat transformed buried organic material and produced hydrocarbons such as methane and propane. These gases then moved upward through the sediments toward the seafloor.
Microorganisms living near the seafloor consumed the methane and produced biomass containing unusually light carbon isotopes. When this biological material was eventually preserved in rock, it left behind the unusual chemical signature that scientists later interpreted as evidence of a global event.
The researchers found evidence of a striking temperature gradient supporting this explanation. Temperatures reached about 350°C close to the ancient magma intrusion but fell to around 72°C roughly 300 meters above it, near what was once a seafloor asphalt spill.
Remarkably, the gases trapped inside these 2-billion-year-old rocks also preserve chemical patterns similar to those found in modern oil and gas basins.
The researchers cannot completely rule out other processes or conclude that the broader Shunga–Francevillian event was entirely local. However, their findings suggest that the important Russian evidence can largely be explained without requiring a planet-wide change in the carbon cycle.
Attention will now turn to Gabon. Scientists have recently collected new drill cores there and plan to investigate whether similar local processes could explain the carbon isotope signals found at that site.
If they can, scientists may need to rethink a long-standing explanation of what was happening on Earth as oxygen transformed the planet billions of years ago.
