The bottom of Turkey’s Lake Van is covered by a layer of mud several hundreds of yards deep. For climatologists, this unprepossessing slime is worth its weight in gold: Each summer, pollen has been deposited from times long past. From it, they can detect 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 10 or 20 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-colored layer of sediment, known as the summer sediment.
In winter, the continual snowdrift beneath the surface changes its color: Now clay is the main ingredient in the sediment, which is deposited as dark brown winter sediment on top of the pollen-lime mix. At a depth of 1,300 feet, 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 [1,312 feet] thick,” Bonn paleontologist Professor Thomas Litt explains. “There are about 20,000 annual strata to every [32 feet],” 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.”
Drilling into SedimentLitt is the spokesman of an international consortium of scientists that wants to get stuck into a murky 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 nearly 1,250 feet. The researchers want to drill down to a sediment depth of 820 feet. For this, they have applied for funding by the International Continental Drilling Programme (ICDP).
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 suddenly will be a black layer of ash between the annual layers. “With our test drill, we counted 15 outbreaks in the past 20,000 years,” Litt says. “The composition of the ash even reveals which nearby volcano it originates from.”
What the Drill Cores RevealEven earthquakes in this area of high geological activity are painstakingly stored in these archives. What is the most interesting aspect for Litt, however, is the biological filling contained in the summer layers. The microscopically small pollen tells the paleobotanist 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.
The researcher normally recognizes 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,” 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 centimeter 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.”