To Peter Johnson and Konrad Gajewski, a lake bottom is a book offering page upon page of information about prehistoric climates and environments.

For the past couple of years, the University of Ottawa geographers have been studying sediments from lakes in the Kluane area of southwestern Yukon in order to learn about the changes the region has undergone since the glaciers receded, more than 12,000 years ago. Their research takes place, with permission, in the traditional territory of the Champagne/Aishihik First Nation.

"The Kluane region is ideal for palaeoenvironment studies because there are a number of lake environments which will give us different types of information," says Johnson.

The area has large glacier-fed lakes where the sediment is mainly rock ground fine by the glaciers, and smaller lakes with surface inflow and outflow whose sediments are primarily organic material, he says.

Some lakes with surface inflow and outflow also receive substantial amounts of groundwater, which leaches carbonates from the surrounding rock and deposits them in the bottom sediment. Finally, there are marl lakes, controlled entirely by groundwater, where the sediment is primarily carbonate.

Each type of sediment provides different kinds of information, and combining the information helps create a picture of what the region was like hundreds or thousands of years ago.

Examining the sediments, however, isn't as simple as digging down and exposing the layers. After all, they lie at the bottom of several metres of icy mountain water.

Johnson and Gajewski drill holes in the lake bottoms and extract cores from the drill-holes.

"On the shallow lakes we use a piston corer," says Johnson. "It takes cores in approximately one-metre intervals and works on the principle of pushing a tube, with an internal piston, into the lake sediments and then pulling the tube out of the lake. The piston holds the sediment core in the tube."

The best time to do the work would be in spring when the lakes are still frozen and the coring equipment could be positioned on the ice, but responsibilities at the university prevent that, says Johnson. Instead, they work in summer and operate the equipment from a boat anchored securely to the shore or the bottom so that it can't change position.

"We usually work with three people in the boat on the small lakes," Johnson says. "The main problem with the coring is that each segment has to be extracted on the boat and wrapped and stored there so that the boat is not moved out of position."

Wind can be a serious problem, making it difficult to keep the boat in position and dangerous to load it with the heavy sediment cores.

"Sometimes it can be completely calm when one starts in the morning but it gets rough during the day," says Johnson.

Once the cores are safely extracted and wrapped, they are shipped back to the University of Ottawa for detailed study. The scientists look at the layers that have been laid down over time, analysing their mineral and chemical content and identifying the small creatures whose remains have become part of the lake sediment.

"For example, marl lakes are dominated by carbonate," says Johnson, "and small changes in the colour indicate differences in the rate of accumulation or the amount of organic or volcanic material."

Those small differences provide information about past geological events, changes in water level, water temperature, and atmospheric conditions. Other materials in the sediment offer more clues to the past.

"Pollen grains can give a picture of the local and regional vegetation," says Johnson. "Molluscs can give a picture of the lake productivity, diatoms can give a picture of water chemistry, and tiny creatures like chironomids and ostracods also give local environmental information. The presence of charcoal in the cores marks past forest fires and helps to develop a forest fire record over longer time periods than the information that can be obtained from trees."

So far most samples have come from smaller lakes in the Kluane region, but Johnson and Gajewski plan to take cores from Kluane Lake itself in the future. Johnson also hopes to study sediments from the Wolf Creek Research Basin near Whitehorse to provide a broader geographic base for the emerging picture of the past.

For more information, contact Peter Johnson, Centre for Research on Cold Environments, Department of Geography, University of Ottawa, Ottawa, On. K1N 6N5.