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14 March 2007

500,000 Years of Climate History Stored Year by Year

The bottom of Turkey’s Lake Van is covered by a layer of mud several
hundreds of metres deep. For climatologists this unprepossessing slime
is worth its weight in gold: summer by summer pollen has been
deposited from times long past. From it they can detect right down to
a specific year what climatic conditions prevailed at the time of the
Neanderthals, for example. These archives may go back as much as half
a million years. An international team of researchers headed by the
University of Bonn now wants to tap this treasure. Preliminary
investigations have been a complete success: the researchers were able
to prove that the climate has occasionally changed quite suddenly –
sometimes within ten or twenty years.

Every summer an inch-thick layer of lime – calcium carbonate –
trickles down to find its final resting place at the bottom of Lake
Van. Day by day during this period millions and millions of pollen
grains float down to the depths. Together with lime they form a
light-coloured layer of sediment, what is known as the summer
sediment.

In winter the continual ‘snowdrift’ beneath the surface changes its
colour: now clay is the main ingredient in the sediment, which is
deposited as a dark brown winter sediment on top of the pollen-lime
mix. At a depth of 400 metres no storm or waves disturb this
process. These ‘annual rings’ in the sediment can be traced back for
hundreds of thousands of years. ‘In some places the layer of sediment
is up to 400 metres thick,’ the Bonn palaeontologist Professor Thomas
Litt explains. ‘There are about 20,000 annual strata to every 10
metres,’ he calculates. ‘We presume that the bottom of Lake Van stores
the climate history of the last 800,000 years – an incomparable
treasure house of data which we want to tap for at least the last
500,000 years.’

250 metres of sediment = 500,000 years’ worth of climate archives

Professor Litt is the spokesman of an international consortium of
scientists that wants to get stuck into a thorny problem: using high
tech equipment they want to cut drill cores as thick as a man’s arm
out of the lakebed sediment from a big floating platform – not an easy
task at depths of 380 metres. The researchers want to drill down to a
sediment depth of 250 metres. For this they have applied for funding
by the International Continental Drilling Programme (ICDP). This would
be the first time that an ICDP drilling was headed by a German. The
prospects of this happening are not bad. A preliminary application was
assessed as very good by the ICDP Executive Committee – above all
thanks to a successful preliminary investigation which the researchers
had carried out at Lake Van in 2004. The German Research Council
(Deutsche Forschungsgemeinschaft, DFG) financed this. It has just
extended the project for two more years.

The sediment promises to deliver a host of exciting results. For
example vulcanologists can determine exactly when volcanoes near the
lake erupted. In this case there will suddenly be a black layer of ash
between the annual layers. ‘With our test drill we counted 15
outbreaks in the past 20,000 years,’ Prof. Litt says. ‘The composition
of the ash even reveals which nearby volcano it originates from.’

Chubby-cheeked pollen

Even earthquakes in this area of high geological activity are
painstakingly stored in these archives. What is the most interesting
aspect for Thomas Litt, however, is the biological filling contained
in the summer layers, especially. The microscopically small pollen
tells the palaeobotanist what sorts of things used to flourish on the
shores of the lake. In a piece of sediment the size of a sugar cube up
to 200,000 grains of pollen can be trapped. Under the microscope the
fine dust reveals a very special kind of beauty. The pollen of yarrow
is as prickly as a hedgehog, the pollen of pine with its air sacs
resembles the chubby-cheeked face of a hamster, ‘and look at the olive
tree,’ Professor Litt enthuses, ‘it’s also got a very nice pollen
grain.’

The researcher normally recognises at once what genus or species the
finds belong to – even when they are several thousands of years old,
since the exine, the outer coat of the grain, successfully resists the
ravages of time. ‘The material is extremely resistant to environmental
influences and even withstands strong acids or bases,’ Professor Litt
explains. Using hydrofluoric acid or potassium hydroxide he dissolves
the pollen grains from the sediment samples; the grains prove to be
completely impervious to such rough treatment. Under the microscope
the botanists then assess how much pollen of which species is present
in the layer in question. ‘At interesting points we take every
centimetre of material from the drill cores; in this way we achieve a
chronological resolution of a few years.’

The pollen permits pretty precise statements to be made about
temperature and average amount of precipitation for the period covered
by the finds, as every species makes different demands on its
environment. ‘If we find pollen in a specimen from different species,
whose demands on its habitat are known, we can make a plausibility
statement about the nature of the climate of the time,’ he adds. ‘Lake
Van promises to provide unique insights into the development of the
climate in Eurasia – and thus for assessing the current warm period.’

© AlphaGalileo Foundation 2003