Supervolcano eruption in Sumatra deforested India 73,000 years ago

In a new study, scientists have come across what they call "incontrovertible evidence" that the volcanic super-eruption of Toba on the island of Sumatra about 73,000 years ago deforested much of central India, some 3,000 miles from the epicenter.

The volcano ejected an estimated 800 cubic kilometers of ash into the atmosphere, leaving a crater that is 100 kilometers long and 35 kilometers wide.

Ash from the event has been found in India, the Indian Ocean, the Bay of Bengal and the South China Sea.

The bright ash reflected sunlight off the landscape, and volcanic sulfur aerosols impeded solar radiation for six years, initiating an "Instant Ice Age" that - according to evidence in ice cores taken in Greenland - lasted about 1,800 years.

During this instant ice age, temperatures dropped by as much as 16 degrees centigrade (28 degrees Fahrenheit), according to University of Illinois anthropology professor Stanley Ambrose, a principal investigator on the new study with professor Martin A.J. Williams, of the University of Adelaide.

To address the limited evidence of the terrestrial effects of Toba, Ambrose and his colleagues pursued two lines of research.

They analyzed pollen from a marine core in the Bay of Bengal that included a layer of ash from the Toba eruption, and they looked at carbon isotope ratios in fossil soil carbonates taken from directly above and below the Toba ash in three locations in central India.

Carbon isotopes reflect the type of vegetation that existed at a given locale and time.

Heavily forested regions leave carbon isotope fingerprints that are distinct from those of grasses or grassy woodlands.

Both lines of evidence revealed a distinct change in the type of vegetation in India immediately after the Toba eruption, the researchers report.

The pollen analysis indicated a shift to a "more open vegetation cover and reduced representation of ferns, particularly in the first 5 to 7 centimeters above the Toba ash."

The change in vegetation and the loss of ferns, which grow best in humid conditions, "would suggest significantly drier conditions in this region for at least one thousand years after the Toba eruption," according to the researchers.

The dryness probably also indicates a drop in temperature, "because when you turn down the temperature you also turn down the rainfall," Ambrose said.

The carbon isotope analysis showed that forests covered central India when the eruption occurred, but wooded to open grassland predominated for at least 1,000 years after the eruption.

"This is unambiguous evidence that Toba caused deforestation in the tropics for a long time," Ambrose said.