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Eruption of Sumatra's Mt Toba -

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Eruption of Sumatra's Mt Toba

Martin Williams found volcanic ash in India. It came from Mt Toba in Sumatra 73,000 years ago. The
explosion of Mt Toba was the biggest bang in 2 million years. For comparison, Krakatoa ejected
about 18 cubic kilometres of ash and rock. It's thought Toba ejected 3,000 cubic kilometres of ash
and rock. The eruption of Toba changed world climate. Temperatures in Greenland dropped 16 degrees.

Around 100,000 to 50,000 years ago, world population plummeted to just a few thousand individuals
prior to recolonisation out of Africa. Martin Williams speculates the eruption forced people to
develop social networks and reciprocal relationships and this assisted in the movement back out of
Africa.

Transcript

Robyn Williams: Professor Martin Williams from Cambridge but who is now an emeritus professor at
the University of Adelaide, has been in pursuit of one of the biggest bangs in history which made
us who we are. Martin, do tell.

Martin Williams: Quite by accident, in February 1980, I was in north central India and we'd logged
70 kilometres length of cliff sections in the Son Valley, beautiful valley, Siddhartha country, and
we spotted something unusual halfway up this 30-metre cliff. I turned to Keith and said, 'Were this
the Ethiopian highlands I'd call that volcanic ash.' Since no volcanic ash has ever been recorded
in geologically recent times in India, we sampled it every centimetre.

So we fingerprinted it. Every eruption, as you know, has its own individualistic signal, its own
geochemical fingerprint, and it turned out to come from northern Sumatra, 73,000 years ago, and
Sumatra or Toba, is now just a big, beautiful lake. It ejected something like 3,000 cubic
kilometres of rock equivalent in the form of ash. Compare that to Krakatoa which is about 14 to 18
(cubic kilometres of rock), so we're talking about orders of magnitude difference. Krakatoa of
course killed 42,000 people.

So what was the impact of this huge eruption, the biggest explosive eruption in at least the last
two million years, probably a great deal longer? Once I'd put it on record, the lads and lasses of
the Indian geological survey found it everywhere, it was all over India. It was in the Bay of
Bengal, it turned up in the Arabian Sea, it turned up 12 degrees south of the equator in the Indian
Ocean. Then it turned up in the East China Sea and in Greenland. In the sulphur spike in Greenland
associated with it you have a 16-degree drop in temperature, which is quite dramatic...

Robyn Williams: It's gigantic, isn't it!

Martin Williams: Yes. And a decade-and-a-half ago a number of very cluey geneticists were looking
at the mitochondrial DNA inherited through the female in human populations, and they speculated
that somewhere between 100,000 and 50,000 years ago the world population plummeted down to a few
thousand males and females.

Robyn Williams: It was about 20,000 they said, roughly.

Martin Williams: It varies. Some estimates say 2,000, some say 5,000, some say 10,000. So 20,000 is
not a lot. And then re-colonisation out of Africa. Since that time there's been a raging...no,
academics don't rage, they're too polite for that, but there's been some fairly brutal debate. One
school says there was no impact whatsoever because when you look at the artefacts in southern India
above and beneath the ash they're the same, they're middle Palaeolithic, middle stone age,
therefore no impact.

So I thought I'd take a different approach and did a transect across central India, about 500
kilometres, and we sampled soils beneath the ash and above and the carbonate nodules in them, and
we analysed the carbon isotopes 12 and 13. And so we had a fix on the type of vegetation that was
growing at the time, what photosynthetic pathway it followed so we could distinguish C3 from C4.
And in essence beneath the ash, right across central India, forest, and above the ash, mixed
woodland and grassland. So that's a fairly indirect form of evidence.

So then we...in a couple of marine cores, one just off north Sumatra, one in the Bay of Bengal, we
looked in fine resolution at the pollen, and the pollen shows initially dramatic cooling, quite
significant cooling, but it didn't persist. But the more interesting thing is that what you then
see is prolonged drought lasing about 1,200-1,500 years. That would have been pretty tough on human
societies of the day. They would have had to adapt. Some of our work in the west Kenya rift...we're
trying to document there the transition from middle to late Stone Age time.

Putting it in modern terms, would you give your wife a frying pan for her birthday or would you
give her a pearl necklace? Well, middle Stone Age people opted for frying pans, late Stone Age for
necklaces. And what you see in late Stone Age times is much more care being taken in selection of
the raw materials to make stone tools, you see the first evidence of objects of great beauty being
made which had no utilitarian value, on the face of it. So we found and excavated, for example, a
factory which was ostrich eggshell beads for making necklaces for the women.

And one can speculate here (and it's purely speculative) that the impact of this catastrophic
eruption and the effect it had both on the vegetation, both directly in terms of blasting the
vegetation and indirectly in terms of the effect on climate, forced people to develop social
networks, forced them to develop reciprocal relationships. And that assisted then in the movement
back out of Africa, across the deserts of Arabia and Pakistan, Afghanistan and back into India and
south Asia generally.

Robyn Williams: What a story! Some people have said that the reason that we're one race is that
having been reduced to such a small bunch of people who, if you like, were the originals of Eves
and Adams and led to all of us, means that we have less variety...well, we've got no variety, we're
one species, and it stems from that explosion, it stems from that reduction in the population. Is
that controversial? You said that there was some barnies about it, but is it..?

Martin Williams: Oh yes, a lot of people are yet to be convinced. In fact, more people are happy to
accept the impacts of small volcanic eruptions...Pinatubo, for example, June '91, global
temperatures, as you know, dropped nearly three-quarters of a degree Celsius over the next 12
months, which was equal to the net average global warming of the preceding 100 years. And that was
a fairly minor eruption. I've seen the impact at first hand of Mt St Helens near Seattle and that's
pretty dramatic, but again, very small beer compared to Krakatoa. Krakatoa is...

Robyn Williams: ...a quarter of Toba.

Martin Williams: Well, quite, yes. So Toba was quite something. And I think no one research team
will resolve this. We need a whole variety of approaches. But a number of geneticists now are
looking very carefully at the so-called bottlenecks or founder populations in tigers, in gorillas,
in macaque monkeys, and once again you're finding this curious bottleneck around about this time.

Robyn Williams: The full fascinating story is on In Conversation with Professor Martin Williams
next Thursday 11th September on ABC Radio National at 7.30pm. He's Professor Emeritus at the
University of Adelaide. Do avoid big bangs.