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‘Super-eruption’ linked to accelerated climate cooling

The largest volcanic eruption on Earth in the last two million years took place about 74,000 years ago in northern Sumatra. This ‘super-eruption’ created Lake Toba, 100km long, 30km wide and 500m deep.

But the exact timing of this mega-explosion and its impact on the world’s climate, plants and animals -- including the fate of people living at the time -- has proved elusive.

A new study, published online today in the Proceedings of the National Academy of Sciences (PNAS) of the USA, pinpoints the timing of the Toba eruption at 73,880 years ago, with a margin of error of just a few centuries.

“This represents a 10-fold improvement in precision on previous age estimates, which allows the time of the eruption to be clearly linked to a period of dramatic climatic cooling,” according to team leader Dr Michael Storey, from Roskilde University in Denmark. A co-author on the paper is Professor Richard ‘Bert’ Roberts from the University of Wollongong’s Centre for Archaeological Science.

The breakthrough in dating was achieved using the latest advances in instrumentation to measure tiny amounts of argon gas produced inside individual crystals of sanidine, a natural mineral packed with potassium. The time elapsed since an eruption is calculated by measuring the amount of argon gas that has built up inside the crystals as a result of the natural radioactive decay of the isotope potassium-40.

“The Roskilde instruments have been specially designed to measure trace amounts of argon gas with increased accuracy and precision and this has allowed us to nail the timing of the Toba eruption with unprecedented resolution,” Dr Storey said.

The high-precision age was obtained from volcanic ash deposits in Malaysia’s Lenggong Valley, where stone tools are buried beneath Toba ash at the archaeological site of Kota Tampan.

“We’ve been waiting a long time to obtain a reliable and absolute date for these stone tools, which were discovered more than 70 years ago,” co-author Professor Mokhtar Saidin from Universiti Sains Malaysia, an expert on the archaeology of Malaysia, said.

The new study shows that the super-eruption took place when early modern humans were spreading out of Africa and east across Asia.

“It now seems likely that our ancestors had reached southeast Asia by 74,000 years ago, in line with recent revisions to the ‘molecular clock’ for human evolution and with archaeological evidence from Arabia and India for a pre-Toba exit of modern humans from Africa,” according to Professor Roberts.

The Toba explosion released more than 2800 km3 of magma, of which 800 km3 was spewed into the atmosphere as ash that spread as far west as the Arabian Sea. Chemical aerosols were transported even further and have been detected in ice cores in Greenland and Antarctica. Volcanic dust and gases, such as sulfur dioxide, can exert a dramatic effect on the Earth’s climate.

Sulfur dioxide combines with water to form tiny droplets (or aerosols) of sulfuric acid, which can create a haze that reduces the amount of sunlight reaching the surface of the Earth—cooling the climate and triggering a so-called ‘volcanic winter’. The Toba super-eruption injected 100 times more sulfur dioxide and dust into the atmosphere than the 1991 eruption of Mt Pinatubo in the Philippines, which led to a fall in global temperature of about 0.4 °C in 1992.

The new study shows that the Toba eruption coincides with a spike in sulfate concentration in ice cores from Greenland, and this was followed by a century or so of rapid cooling during which temperatures fell by about 10 °C.

“Our precise new age for Toba supports the idea that it triggered a period of accelerated cooling of the climate that preceded a millennium or more of extreme cold,” Dr Storey said.

It has previously been speculated that the super-eruption had a catastrophic impact on the climate and ecosystems at that time, and may even have contributed to a human population ‘bottleneck’ that threatened the survival of our species.

“The legacy of this eruption might still be in our genes, given that our ancestors -- or those of the enigmatic ‘Denisovans’-- were living in southeast Asia at the time of the blast,” Professor Roberts said.

Last reviewed: 30 October, 2012

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