A two-billion-year-old clue about Earth may have been misread

One of the most famous chemical signals in Earth's ancient rocks may not mean what scientists have long believed, according to new research from the California Institute of Technology.

The signal, known as the Shunga-Francevillian event, is a distinctive shift in carbon isotopes preserved in rocks about two billion years old. For decades it has been read as evidence of a dramatic, worldwide disruption to the planet's carbon cycle at a time when oxygen was transforming the young Earth.

A local story

The Caltech team argues that the signal can instead be explained by events in one place. "It can be explained by local phenomena that occurred in a several-hundred-square-kilometer sedimentary basin rather than across the entire globe," said the lead researcher, Nivedita Thiagarajan, a senior scientific researcher at the institute.

The researchers propose that magma forced its way into sediments on the ancient sea floor that were rich in organic matter. The heat cooked that material and produced hydrocarbons, including methane. Microbes near the sea floor that feed on methane then grew in large numbers, and their remains carried an unusually light carbon signature, which is what the rocks still record today.

The team found evidence of temperatures of about 350 degrees Celsius beside the magma intrusion, falling to about 72 degrees Celsius at an asphalt seep some 300 metres higher up.

Who was involved

The study, published in the journal Geology, also involved Aivo Lepland of the Geological Survey of Norway, John Eiler of Caltech, Florian Eichinger of Hydroisotop GmbH in Germany and Anthony Prave of the University of St Andrews in Scotland.

What happens next

The finding matters because the Shunga-Francevillian signal has been used as a marker for how Earth's environment changed after the Great Oxidation Event, when oxygen first built up in the atmosphere. If the signal is local rather than global, some of those conclusions may need to be revisited.

The scientists now plan to analyse rock samples from Gabon, collected by the GOE-DEEP project, to see whether the same local processes can explain similar signals at that key reference site.