2-Billion-Year-Old Rock Signal May Be Local, Not Global, Study Suggests

Between roughly 2.5 and 2 billion years ago, oxygen began accumulating in Earth's atmosphere, the greatest chemical transformation known from the planet's surface history, which eventually helped create conditions for complex organisms such as plants and animals. During that period, large amounts of microbial material were buried beneath ancient seafloors, leaving rocks with an unusual carbon-isotope signature -- a chemical fingerprint of where carbon originated -- long interpreted as evidence of a global disruption to Earth's carbon cycle.

Much of the evidence for that global event, known as the Shunga-Francevillian event, comes from drill cores taken from ancient seafloor deposits in Karelia, Russia, and the Francevillian Basin of Gabon. A study led by Caltech researcher Nivedita Thiagarajan, published in Geology, examined gases trapped in microscopic pockets within pyrobitumen-rich rocks from the Zaonega Formation in Karelia, one of the world's oldest known fossil oil fields. Pyrobitumen is a solid form of carbon left behind when buried oil or gas-forming material is exposed to intense heat.

The researchers propose that a sheet of magma pushed into marine sediment at the site while it lay beneath a prehistoric ocean, heating organic-rich material and generating methane and other hydrocarbons. Those compounds rose toward the seafloor, where methane-consuming microbes produced biomass carrying a light carbon-isotope signature -- the same kind of signal previously read as evidence of a worldwide event. Measurements showed a temperature gradient from about 350 degrees Celsius near the magma to about 72 degrees Celsius at an ancient seafloor asphalt spill roughly 300 meters higher.

The researchers say their measurements indicate the Zaonega signal was driven mainly by processes within that single basin rather than a planet-wide change, though they cannot fully rule out other contributions. Because the Zaonega site is a key reference point for the Shunga-Francevillian event, the team plans to test the finding against similar rock records in Gabon through the GOE-DEEP drilling project.