Paleontologists searching for an explanation for why modern mammals suddenly appeared in North America 55 million years ago had never thought to probe the deep sea. But according to a report in the March issue of Geology, the global warming that drove the mammals' transition had roots beneath the ocean floor.
Tipped off by an extraordinary shift in the isotopes of carbon, Gerald Dickens and his colleagues at the University of Michigan have shown in a mathematical model how a reversal of ocean circulation could have unleashed a 10,000-year gush of methane from sea-floor sediments and warmed the world with its greenhouse effect. The calculations don't prove the methane scenario, but "the results match what we see in the sedimentary record," says paleoceanographer James Zachos of the University of California, Santa Cruz. "They confirm what a lot of us had been suspecting."
Early this decade, oceanographers and paleontologists found a striking coincidence recorded in 55-million-year-old deep-sea sediments: a 10,000-year-long warming of bottom waters, an equally abrupt mass extinction of tiny bottom-dwelling organisms called forams, and a shift in the relative abundance of carbon isotopes in the forams. Then the isotopic spike showed up in fossil mammal teeth from North America, just when modern mammals such as primates and rodents first appeared, presumably after a warming climate made Arctic land bridges hospitable enough for mammals to migrate.
Researchers reasoned that a change in ocean circulation, in which warm, salty water from the tropics sank to the sea floor and displaced cold, polar bottom waters, killed off the forams. This also would have warmed bottom sediments, where vast amounts of methane lie trapped in an icy brew called methane hydrate. As the hydrate melted, the methane--which is produced by sediment bacteria and is isotopically extremely light--would be released to the ocean and ultimately the atmosphere, driving a greenhouse warming.
That's a nice story, but would it really work? Dickens's team put it to the test in a mathematical model of how carbon cycles through ocean water, sediment, atmosphere, land, and plants. They were able to produce the isotopic spike with just 8% of the methane hydrate assumed to exist at the time. And the gas blast would have warmed global climate more than half of the observed 4 degrees Celsius. Experts are impressed. "I really like the idea," says paleoceanographer Ellen Thomas of Wesleyan University in Middletown, Connecticut, who is probing for evidence of other fateful deep-sea belches.