Van-See: Climate caprioles since the last ice age

Climate history of the last 800, 000 years stored in the lake floor

Pine pollen. © Prof. dr. Thomas Litt / Uni Bonn
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At the bottom of the Turkish Van Lake rests a layer of mud several hundred meters thick, which is worth gold for climate researchers: summer after summer, pollen from days gone by have deposited there. Up to the year you can see exactly what kind of climate prevailed during the Neanderthal era or earlier. An international research team now wants to raise this treasure. However, initial research has already shown that the climate has changed very rapidly since the last ice age - sometimes within ten or twenty years.

The lime trickles gently: every summer, a thick layer of calcium carbonate on the bottom of the lake of Van settles to rest. Every day, millions of pollen grains float to the ground. Together with lime they form a light sediment layer, the so-called summer position.

In winter, the permanent "snow drift" under the lake level changes its color: Now, clay is the main constituent of the deposits, which lays as a dark brown winter over the pollen-lime mixture. At a depth of 400 meters no storm disturbs, no wave disturbs this process. For hundreds of thousands of years, one can follow the "annual rings" in the soil mud. "In some places, the sediment layer is up to 400 meters thick, " explains Bonn paleontologist Professor Thomas Litt from the University of Bonn. "At ten meters there are about 20, 000 annual shifts, " he calculates. "The bottom of Lake Van probably contains the climate history of the last 800, 000 years - an incomparable treasure trove that we now want to raise, at least for the last 500, 000 years."

250 meters of sediment = 500, 000 years of climate archive

Litt is the spokesman of an international research consortium, which wants to drill the proverbial "thick boards" at Lake Van: With high-tech equipment, scientists want to dig arm-thick cores out of the soil sludge from a large, buoyant platform - and not an easy task at 380 meters water coverage. The researchers want to penetrate up to 250 meters of sediment depth. In addition they have a promotion through the international continental drilling program

(ICDP). It would be the first time that an ICDP hole would be led by a German. display

The signs that this happens are not bad. A pre-proposal was rated very well by the ICDP Executive Committee, thanks in particular to a successful preliminary study carried out by the scientists at Lake Van in 2004. The German Research Foundation (DFG) funded the excursion. She just extended the project for another two years.

The sediment promises several exciting discoveries: For example, volcanologists can determine exactly when the volcanoes erupted in the vicinity of the lake. Then suddenly there is a thick black ash layer between the annual layers. "We have counted 15 outbursts in the last 20, 000 years of the trial drilling, " says Litt. "The composition of the ashes even lapses from which volcano in the area it comes from."

Pollen with chubby cheeks

Even earthquakes in the geologically very active area are meticulously recorded in the archive. The most interesting for Litt, however, is the biological filling, which is mainly responsible for the summer strata: the microscopically small pollen reveal to the pal obotanist what used to grow on the shores of the lake. In a sugar-sugar bag, up to 200, 000 grains are trapped. Under the microscope, the fine dust unfolds a very special aesthetic: The yarrow grain is prickly like a hedgehog, the pine pollen resembles a pudgy face with its air sockets, "and see the Lbaum, "enthuses Litt, " he also has a very nice pollen grain. "

The researcher usually recognizes at first glance, of which genus or species the findings originate - even if they are already several thousand years old. Because the Pollenh lle successfully defies the ravages of time. "The material is extremely resistant to environmental influences and even resists strong acids or alkalis, " explains Professor Litt. He releases the pollen grains from the sediment samples with hydrofluoric acid or potassium hydroxide solution; the grains are completely unimpressed by the rabid treatment. Under the microscope, the botanists then evaluate how much pollen of which species is present in each layer. "At interesting points, we take every inch of material from the cores; That's how we achieve a temporal resolution of a few years. "

The pollen allows quite accurate statements about temperature and average rainfall at the time of finding. Because each species places very individual demands on its environment. "If we find pollen of several species whose location requirements we know in a specimen, we can make a probability statement about the climate at that time, " explains the scientist. "Lake Van promises unique insights into the development of the climate in Eurasia - and thus also the classification of the current warming period."

(idw - University of Bonn, 13.03.2007 - DLO)