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Last Updated: Tuesday, 12 October, 2004, 10:44 GMT 11:44 UK
Drilling for Africa's climate history
By Roland Pease
BBC science

Looking across Lake Bosumtwi
Calm today but the site one million years ago of a space impact
The still waters of Lake Bosumtwi impose a sense of deep calm on those who visit the sub-Saharan African nation of Ghana. But the silence, punctuated by occasional frogs' croaks and bird calls, belies a violent origin.

"A million years ago, this was the site of an enormous catastrophe," explains Christian Koeberl, a geologist from Vienna University, Austria.

"We had a lush rainforest, filled with animals minding their own business, when a kilometre-sized rock came hurtling down from space, at enormous velocity, and crashed into the ground here.

"Within a few seconds, it opened up a gigantic hole in the ground which later filled with water and became the lake we see today," he told the BBC World Service's Discovery programme.

It's very important to understand what would happen during such an impact; what would happen to mankind, to civilisation
Christian Koeberl, Vienna University
Researchers know of 170 impact craters around the world, including the enormous Chicxulub structure in the Gulf of Mexico formed 65 million years ago, and widely associated with the demise of the dinosaurs.

But Lake Bosumtwi is unique to science, says Professor Koeberl.

"It is the youngest large impact crater we have on Earth, and it's very well preserved. There is no crater of the same size that is younger; and there is no crater that is better exposed or larger."

Earth prepared

This summer, following a 10-year campaign of geological field work and geophysical investigations, Koeberl and a team of collaborators brought a floating drilling platform, to bring up samples of crushed and melted rock from the basement of the crater.

Map of Lake Bosumtwi
One of only 18 confirmed African impact craters
Crater rim about 10km across; maximum water depth 80m
"What we're trying to understand is how exactly the meteorite hit the Earth, from which direction did it come, what type of meteorite it was, and what were the immediate consequences," Professor Koeberl says.

"There are many more rocks of this size out in space - and an impact of this size happens every million years or so. So it's very important to understand what would happen during such an impact; what would happen to mankind, to civilisation."

To appreciate the violence of the collision, you have to understand that it released as much energy in a single second as the eruption of Mount St Helens released in six months in 1980.

Down through the clouds: The crater was made just over a million years ago (Image: Nasa/LPI)

As well as carving out the 11-km bowl of the lake, the impact lifted pulverised and melted rock 20-30 km into the sky, which then rained like fire on the surrounding countryside, incinerating everything in its path.

Tiny frozen droplets of once-molten rock from the impact have been found 1,000km away in Atlantic sediments.

The heat in the crater would have taken 10-20,000 years to dissipate, and during that time the crater floor would have been peppered with Yellowstone-like hot springs, and home to heat-seeking "thermophile" bacteria.

Although an impact of this size would not cause a mass extinction on the Chicxulub scale, today, warns Professor Koeberl, one could devastate a small country.

'Bathring' of data

Scientific interest in the lake does not end with the events of the impact. Climate researchers also have been making the pilgrimage to central Ghana, including John Peck from the University of Akron, US.

"[The impactor] created essentially a very large bowl. And as rainfall increases and evaporation decreases, the bowl fills up; other times the lake decreases, as rainfall goes down and evaporation increases," he explains.

We hope we will understand how wet and how dry west Africa has become in the past, and how quickly, because the answers to those questions have a direct effect on the people living here
John Peck, University of Akron
About 250m of mud at the bottom of the lake record the ups and downs of African climate for the past million years.

In some places, sandy beach deposits replace the mud in the sediments, revealing times when the region was so dry that the lake shrank to a fraction of its current size.

During the wettest times, the lake has overflowed the crater leaving a "bathring" of muddy deposits around the crater rim.

One of the ideal aspects of the lake for palaeo-climate, says Professor Peck, is its stillness.

The surrounding hills shield it from strong winds that would disturb the surface; and no rivers flow through to stir up the lake either. As a result, no oxygen gets mixed into the deeper waters of the lake.

Future climate

"What this means," Professor Peck explains, "is that once you go below about 18m deep, there is no oxygen, there is no life - there are no worms to mix up the mud.

Drilling platform on Lake Bosumtwi
The drilling platform (bottom-right) is after sediments at the bottom of the lake
"What we end up with are very beautiful layers of sediment that record the past seasons. And these layers go all the way down to the bottom of the sediment - a million years ago."

Alternating black and light layers mark out the annual cycles of weather, interrupted occasionally by grey layers of debris washed in from the surrounding hills by rainstorms.

"The black layers represent the wet summertime, when we have lots of algae growing in the lake and dying and settling to the lake bottom; the light brown layers represent the winter time, when you have a lot of dust being blown in from the Sahel and around Lake Chad - it turns the sky yellow."

As a result, Lake Bosumtwi holds an unrivalled record of tropical weather going back through several ice ages.

Half-submerged tree stump
Even the submerged trees hold a record of climate
In combination with climate data drilled from the Antarctic ice sheets, these will give climatologists a detailed picture of global climate since before humans evolved - and against which they can test their models for global warming.

And it is not just a global picture that will emerge. Ghanaians should themselves benefit from the research - the region has been suffering a prolonged drought since the 1960s.

"We hope we will understand how wet and how dry west Africa has become in the past, and how quickly, because the answers to those questions have a direct effect on the people living here," Professor Peck says.

"We know in the future the climate's going to change - but we don't know how that's going to occur. What we hope to do is understand the past as a way to predict the future."

Deep Impact - Ghana was broadcast on the BBC World Service's Discovery Programme on Wednesday 13 October and Thursday 14 October.

Antarctic craters reveal strike
19 Aug 04  |  Science/Nature
Dino crater viewed from space
10 Mar 03  |  Science/Nature
North Pole 'was once subtropical'
07 Sep 04  |  Science/Nature
Country profile: Ghana
15 Sep 04  |  Country profiles


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