Talk about global warming. About 55 million years ago, the planet's temperature jumped by as much as 5˚C and remained that way for about 170,000 years. Thousands of primitive marine species vanished. But the event also coincided with an unsurpassed era of plant diversity as well as the rise of mammals. Now, researchers think they've figured out what caused the hothouse: A series of massive undersea eruptions may have saturated the air with perhaps trillions of tons of methane—a much more potent greenhouse gas than carbon dioxide.
Tracing the causes of mass extinctions and ancient climate upheavals has kept investigators busy for decades. And the particular event in question, called the Paleocene-Eocene Thermal Maximum (PETM) is no exception.
Five years ago, a possible answer arrived in drill cores and seismic data taken from the Norwegian Sea, coincidentally not too far from Iceland, where eruptions of the Eyjafjallajökull volcano are currently attracting worldwide attention. The cores and seismic data indicated that as many as 700 kilometers-wide craters once dotted the sea floor, suggesting that the area experienced volcanic upheaval on a scale dwarfing anything in human history. The craters are also located in areas that contain huge reservoirs of methane.
Geologist Henrik Svensen of the University of Oslo and colleagues have been studying contents of the drill cores since the discovery. In particular, they have focused on tiny zircon crystals in the sediments. Based on analyses of the isotopes of uranium and lead contained in the crystals—published today in the Journal of the Geological Society—the researchers conclude that the crystals formed 55 million years ago, exactly when PETM began. "I can still recall the excitement," Svensen says, when his colleague and co-author Fernando Corfu contacted him to report that he had established the age of the zircons.
In their paper, Svensen and colleagues argue that the magma that created the craters heated the overlying sediments, which then released massive amounts of methane. That methane bubbled up to the surface and into the atmosphere, creating a powerful greenhouse effect that lasted for nearly 200 millennia.
It's an impressive study about forces operating on an amazing scale, says climate dynamicist Matthew Huber of Purdue University in West Lafayette, Indiana. The data make a convincing argument, he says. "It's hard to get dates this good this far back in time."
Not so fast, says geochemist Alan Jay Kaufman of the University of Maryland, College Park. The researchers show that the volcanic eruptions coincided with the global warming during PETM, "but their assertion that the one caused the other remains untested." The problem is that their data don't quantify the amount of methane that could be released from the sediments, he says, so the idea of a massive expulsion remains unclear.
Geoscientist Klaus Keller of Pennsylvania State University, University Park, agrees and says the next challenge will be determining whether the eruptions really were the source of the greenhouse gas and PETM. "Many exciting and important questions remain," he says.