Bubbles of gas trapped in rocks for approximately 2 billion years could change how we interpret a major episode in Earth’s history.
Their composition indicates that a chemical signal observed in what is now Russia could be explained mainly by local processes, rather than by a global disruption of the carbon cycle as previously assumed.
The researchers studied the Zaonega Formation in Karelia, northwestern Russia. These ancient marine rocks date back approximately 2 billion years. They formed after the first major increase in oxygen in Earth’s atmosphere.
These rocks contain an unusual proportion of two forms of carbon, carbon-12 and carbon-13. This type of difference can preserve traces of ancient environmental changes. Similar signals were also observed in Gabon, reinforcing the idea that the phenomenon was planetary in scale.
The new study focused on gases that remained trapped in tiny pockets inside the rocks. The researchers analyzed their molecules and isotopes. They found methane and other hydrocarbons formed under the influence of heat.
According to their scenario, a magma intrusion passed deep through sediments rich in organic matter. This heat produced hydrocarbons, including methane, which then rose toward the seafloor that existed at the time. Microbes consuming this methane would then have produced organic matter particularly poor in carbon-13.
The measurements support this mechanism. Reconstructed temperatures reached approximately 350 °C near the ancient magmatic intrusion. They gradually decreased to around 72 °C, some 300 meters higher in the rock.
This succession of geological and biological processes is sufficient to explain the unusual signal observed at Zaonega. It may therefore have originated mainly from this sedimentary basin. However, the researchers point out that they cannot completely rule out the influence of global changes in the carbon cycle.
This nuance is important because Zaonega serves as a reference site for an event known as Shunga–Francevillian. This event has sometimes been interpreted as evidence of a global disturbance that occurred approximately 2 billion years ago. The team now wants to compare these results with new cores collected in Gabon.