By Sid Perkins
When the tectonic plate carrying India slammed into Asia about 50 million years ago, the ensuing geological changes triggered a long-term cooling trend. That trend 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 shoved below Asia by tectonic movements, says Dennis V. Kent, an earth scientist at RutgersUniversity in Piscataway, N.J. Carbon in those sediments soon reappeared in the atmosphere as the carbon dioxide spewing from volcanoes. When the India-Asia collision removed those seafloor sediments, that source of carbon dioxide disappeared, Kent and his colleagues argue in an upcoming 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 carbon dioxide. That double whammy, the researchers speculate, caused atmospheric concentrations of carbon dioxide to plummet, cooling Earth significantly.
About 120 million years ago, the tectonic plate that carried what is now the Indian subcontinent split from Gondwana, the supercontinent that sat astride the South Pole. The subcontinent began to move quickly northward, at times migrating about 25 centimeters per year, says Kent.
About 65 million years ago, before the subcontinent reached the tropics, a spate of volcanic activity on the subcontinent 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 carbon dioxide levels sat well above 1,000 parts per million. After the collision, subduction of carbonate-rich ocean crust beneath Asia ceased, so carbon dioxide levels began to drop.
Erosion of India’s volcanic basalts caused carbon dioxide levels to drop even farther, the researchers propose. When those eruptions had occurred, only 3 percent of Earth’s continental land area sat between 10°N and 10°S. When tectonic motions carried India into the tropics, however, that proportion rose to about 20 percent. The high temperature and rainfall of the tropics increased erosion on the landmass, essentially soaking up large amounts of carbon dioxide from the atmosphere and cooling Earth significantly, he adds.
Between 50 million and 34 million years ago, as erosion and other geological processes sapped the greenhouse gas from the atmosphere, CO2 levels dropped to modern-day, pre-industrial 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 findings “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 past 65 million years. 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 YaleUniversity. He notes, however, that high CO2 concentrations and temperatures worldwide 50 million years ago ensured the extensive erosion of continental rock everywhere, not just in India, casting some doubt on Kent’s estimate of India’s importance in cooling Earth.
The scenario set out in the new paper “is plausible,” says William F. Ruddiman of the University of Virginia in Charlottesville. Like Turekian, he notes that erosion of rocks in India wasn’t the only process sequestering carbon dioxide 50 million years ago. The relative contributions of all of these carbon-dioxide–sopping processes are yet to be determined, he adds.
