When the tectonic plate carrying India slammed into Asia about 50 million years ago, the ensuing geological changes triggered a long-term cooling trend--a trend that later enabled Antarctic ice sheets to grow, a new study suggests.

Before the collision, volcanoes along the rim of southern

Asia spewed immense quantities of carbon dioxide into the atmosphere. Much of that planet-warming greenhouse gas came from seafloor, carbonate-rich sediments that were being shoved below Asia by tectonic movements, says Dennis V. Kent, an earth scientist at Rutgers University in Piscataway, N.J.

The tectonic plate that carried what is now the Indian subcontinent split from Gondwana, the supercontinent that sat astride the South Pole, about 120 million years ago. The subcontinent began to move quickly northward, at times migrating 25 centimeters per year, Kent says.

Before India reached the tropics, a spate of volcanic activity lasting a million years spewed about 4 million cubic kilometers of basalt lava--an outpouring that contributed to the demise of the dinosaurs, some scientists propose.

By about 50 million years ago, when India crashed into Asia, atmospheric C[O.sub.2] levels sat well above 1,000 parts per million. But after the collision, seafloor sediments were no longer a volcanic source of C[O.sub.2], so levels began to drop, Kent and his colleagues argue in a paper published online September 22 in the Proceedings of the National Academy of Sciences.

Simultaneously, erosion of rocks on the Indian subcontinent--in particular, the chemical weathering of a large amount of basaltic rocks formed from volcanic eruptions just a few million years earlier--consumed large volumes of C[O.sub.2].

When the volcanic basalts formed, only 3 percent of Earth's continental land area sat within 10 degrees of the equator. But as tectonic motions carried India into the tropics and shifted other land masses, that proportion increased to about 20 percent. The high temperature and rainfall of the tropics increased erosion on the landmass, essentially soaking up large amounts of C[O.sub.2] from the atmosphere.

This double whammy, the researchers speculate, caused atmospheric concentrations of C[O.sub.2] to plummet, cooling Earth significantly.

Between 50 million and 34 million years ago, as erosion and other geological processes sapped the greenhouse gas from the atmosphere, C[O.sub.2] levels dropped close to modern-day, preindustrial levels of about 300 parts per million.

Other changes in landmass distribution in the Southern Hemisphere resulted in changes in ocean currents in the region, which led to further cooling and the development of permanent ice sheets on Antarctica.

The new analyses "describe a perfect storm of carbon cycling," says Mimi Katz, a paleooceanographer at Rensselaer Polytechnic Institute in Troy, N.Y.

Pre-collision volcanism and other geological events contributed to the warmest climates of the 65 million years before today. But, she notes, when the India-Asia collision shut those processes down, the climate ended up in the icehouse.

"This is a very interesting and imaginative paper," says Karl Turekian, a geochemist at Yale University.

He notes, however, that high C[O.sub.2] concentrations and temperatures worldwide 50 million years ago ensured extensive erosion of continental rock everywhere, not just in India, casting some doubt on Kent's estimate of India's importance in the cooling.

William F. Ruddiman of the University of Virginia in Charlottesville agrees that the relative contributions of all of these carbon-dioxide-sopping processes have yet to be determined.