Martin Kennedy has been examining cap carbonates — layers of sedimentary rocks that overlay glacial deposits from around 635 million years ago — since 1990, when he was a graduate student at the University of Adelaide in Australia. “It probably seems strange that you can study a couple of metres of rocks like this for close to two decades,” laughs the geologist, who is now based at the University of California, Riverside. But the rocks may hold the key to understanding what brought Earth's most severe ice age so far — the Marinoan — to an end. And, Kennedy warns, mechanisms similar to that which he and his colleagues have uncovered could play havoc with Earth's climate today.

Cap carbonates have long fascinated geologists. Their chemical signatures indicate the occurrence of an abrupt and massive change in Earth's climate. And sedimentary deposits just above the carbonate layer contain the first fossils of complex animals. “The changes in the climate system and the first appearance of animal life in the fossil record occurred in the time interval that began with these cap carbonates,” says Kennedy.

As early as 2000, he began to suspect that the cap carbonates had formed when methane clathrates — a form of ice rich in frozen methane gas — melted, releasing the gas. In 2003, he and his co-workers proposed that the extreme variety of carbon and oxygen isotopes that they had found in cap-carbonate deposits in southern China were due to a methane clathrate source (G. Jiang et al. Nature 426, 822–826; 2003).

Kennedy looked for similar evidence in other regions. “I spent a lot of time walking over cap-carbonate horizons in the far corners of the world looking for these methane induced structures,” he says. Ironically, he found the best examples of ancient methane seeps exposed in sea cliffs literally underneath co-author Chris von Borch's house in Adelaide. The structures had formed when methane released from melting clathrate pushed its way up through overlaying sediments.

Chemical analysis of rock samples taken from just below the cap-carbonate layer revealed an unusual mix of heavy and light oxygen isotopes. “The heavy oxygen is enriched in pore fluids during methane-clathrate formation, whereas the lightest oxygen is indicative of water derived from a melting ice sheet,” Kennedy explains.

Methane clathrates remain stable in permafrost that is under pressure from a thick overlying ice sheet. But the inherent instability of ice sheets and melting at certain locations would have destabilized the clathrates, releasing methane — a greenhouse gas 30–60 times as efficient as carbon dioxide — into the atmosphere. This would have triggered a cycle of increasing temperatures, melting ice and further methane release (see page 642).

“These results suggest that the greatest global-warming event in Earth's history was caused by methane release,” says Kennedy. And, he adds, because methane clathrates are trapped in Earth's permafrost today, the finding has important implications. “We are pushing the climate harder than at any other time in history,” he says. “What will it take for methane to be released or for other types of feedback system that affect the climate to be activated?”

Kennedy attributes the success of this and earlier studies to meticulous sampling. “It is not a matter of knocking off a few samples and then going home. We work in great detail in the field to collect the most important and telling samples,” he says. “It can take years to get to grips with a single region.”