Researchers found carbon isotopes in rocks on Washington state's South Lopez Island that suggest the minerals grew from fluids flush with microbial methane. Methane from living creatures has distinct levels of carbon isotopes that distinguish it from methane gas that arises from rocks. (Isotopes are atoms of the same element with different numbers of neutrons in their nuclei.)

In a calcium carbonate mineral called aragonite, the standard mix of carbon isotopes was radically shifted toward lighter carbon isotopes (by about 50 per mil, or parts per thousand). This ratio is characteristic of methane gas made by microorganisms, said Philippa Stoddard, an undergraduate student at Yale University who presented the research Tuesday (Oct. 21) at the Geological Society of America's annual meeting in Vancouver, British Columbia. "These really light signals are only observed when you have biological processes," she told Live Science.