Scientists at the University of California, Riverside and Columbia University have found evidence of the release of an enormous quantity of methane gas as ice sheets melted at the end of a global ice age about 600 million years ago, possibly altering the ocean's chemistry, influencing oxygen levels in the ocean and atmosphere, and enhancing climate warming because methane is a powerful greenhouse gas. The study was published in today's issue of the journal Nature.
The global ice age is of particular interest to paleobiologists because it took place shortly before the first appearance of animals in the fossil record, and may have supplied an environmental drive to evolution. The Earth's most severe climate is thought to have occurred about 600 million years ago with ice sheets stretching to the tropics. Some scientists have referred to times of such extreme cold as a "snowball Earth" condition, assuming that the ocean would have been totally ice covered.
The new evidence is based on a chemical fingerprint of the methane gas from rocks in south China, which is strongly enriched in lighter carbon isotope, carbon-12, and which the researchers measured in ancient ocean carbonate sediments that were deposited as the temperature rose. The methane gas was apparently derived from the melting of frozen methane clathrate crystals that had accumulated beneath the seafloor.
"The extremely negative isotopic values from these sediments provide unambiguous evidence for methane-derived carbon," said Ganqing Jiang, a researcher at the University of California, Riverside, and the article's lead author. "The identification of a methane-derived isotope signal and widespread seep-like deposits indicate the massive passage of methane through the sediments," he added. "We now have an important record of the role methane plays in climate change and the global carbon cycle."
Methane clathrates are increasingly thought to play a role in mass extinctions associated with significant climate change in the Earth's history, and they are a large and exceedingly unstable source of greenhouse gas, greater than the equivalent of instantaneously burning all the oil reserves on Earth.
"Linking these dramatic climate events to changes in the methane clathrate pool has important implications for the stability of our current climate," said Martin Kennedy, an associate professor of geology at UC Riverside. "The Earth has a large unstable pool of these clathrates in ocean sediments today, and it is thought that a few degrees of ocean warming could trigger large-scale release into the atmosphere. We now have strong evidence of this doomsday scenario in one of the most important intervals of Earth's biologic history."
The recognition of extreme isotope variability in the rocks examined in south China is expected to stimulate new research.
"This is a very exciting result because the existence of methane seeps and their possible significance in explaining the unusual carbon isotopic signature of the carbonate deposits had been discounted by many on the basis of the lack of expected isotopic heterogeneity," said Nicholas Christie-Blick, a professor of earth and environmental sciences at the Lamont-Doherty Earth Observatory of Columbia University, a member of The Earth Institute. "If the methane hydrate hypothesis is borne out by new studies that are sure to be stimulated by this research, it represents one more reason for questioning why the snowball Earth edifice is needed."
The National Science Foundation's (NSF) division of earth sciences funded the research. NSF is the federal agency responsible for supporting basic science, engineering and education research. NSF is an independent federal agency that supports fundamental research and education across all fields of science and engineering, with an annual budget of nearly $5.3 billion.