Around 73,000 years ago, the Toba volcano on the Indonesian island of Sumatra erupted, sending an estimated 800 cubic kilometres of ash into the atmosphere and producing a 100-kilometre-long crater that now hosts the world’s largest volcanic lake (right). The ash and sulphur produced in the eruption triggered an ‘instant ice age’ that lasted for around 1,800 years, with temperatures dropping by as much as 16°C.

Previous research suggested that the eruption may have caused a ‘genetic bottleneck’ in human populations – the lack of genetic diversity found in modern humans suggests that at some point between 50,000 and 100,000 years ago, humans came close to going extinct.

In the new study, Professor Martin Williams of the University of Adelaide and Professor Stanley Ambrose of the University of Illinois analysed pollen from a marine core taken from the Bay of Bengal that included a layer of ash produced by the Toba eruption. They also analysed carbon isotope ratios in fossil soil carbonates located above and below the Toba ash from three locations in central India. The latter technique can be used to determine the type of vegetation present at the time, as each vegetation type has a distinct isotope ‘fingerprint’.

Both analyses pointed to a significant change in vegetation following the eruption; the pollen study suggested a shift to a more open habitat, with a reduction in ferns, while the isotope analysis indicated that forests had given way to woods and grasslands.

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

February 2010