The Methane Budget According to Carbon-14 (IMAGE) University of Rochester Caption Methane emitted into the atmosphere can be sorted into two categories, based on its signature of carbon-14, a rare radioactive isotope. Fossil methane has been sequestered for millions of years in ancient hydrocarbon deposits and no longer contains carbon-14; biological methane is in contact with plants and wildlife on the planet's surface and does contain carbon-14. Biological methane can be released naturally from sources such as wetlands or via anthropogenic sources such as landfills, rice fields, and livestock. Fossil methane can be emitted via natural geologic seeps or as a result of humans extracting and using fossil fuels. Credit University of Rochester illustration / Michael Osadciw Usage Restrictions None License Licensed content Disclaimer: AAAS and EurekAlert! are not responsible for the accuracy of news releases posted to EurekAlert! by contributing institutions or for the use of any information through the EurekAlert system.