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Tiny Creatures Give Clues to Past Climate

Floating in every body of water on Earth, diatoms are microscopic algae so plentiful that you swallowed millions of them the last time you a gulped a mouthful of lake water. Though among the world's smallest organisms, diatoms have a large role in helping scientists reconstruct the climate of the past. With more than 10,000 species that thrive in both fresh and salt water, diatoms are the favorite meals of water fleas and other tiny aquatic organisms.

Diatoms are single-celled algae with cell walls made of silicon dioxide-glass. Their construction makes them heavy enough to sink when they die, sometimes after a life that lasts only 24 hours. Graveyards of diatoms are the main ingredient of the muck at lake bottoms. Besides their resiliency, another useful feature of diatoms is their finicky nature-different types of diatoms prefer different levels of water acidity, temperature, or nutrient content. By checking what species of diatom lived in a lake during a certain time, scientists can decipher the climate surrounding the lake at the time. "It's a lot like being a forensic scientist, looking for clues," said John Smol, a professor at Queen's University in Ontario who recently visited Fairbanks. Smol has pulled diatom skeletons from lake bottoms for years. He first realized the usefulness of diatoms when acid rain made headlines in the 1980s. At the time, fish in upstate New York and Canada lakes were dying in alarming numbers.

Researchers found the lakes were acidic, possibly due to emissions from coal-burning power plants in the midwestern United States. A debate raged: were the lakes acidic to begin with or was burning fossil fuels making them that way? Smol and others took sediment cores of lakes in the Adirondack Mountains and found that the diatoms that lived there in the past were not the type that preferred acidic water. Since then, Smol and his colleagues have taken sediment samples from lakes all over the world. He now performs much of his work in the Arctic because of the area's sensitivity to climate change.

In lakes on the Ellesmere Islands in northern Canada, Smol has found a striking pattern. For thousands of years, the diatoms that lived in the lakes and ponds were species that preferred cool water. In the mid-1800s, a "100 percent shift in species" occurred in some lakes, Smol said. Warmer-water loving diatoms were suddenly the norm in the lakes, a trend that has continued since. Smol said the change in diatoms indicates a rapid period of warming that coincides with widespread burning of coal and the beginning of the Industrial Revolution. Smol is now working with Bruce Finney, a paleoceanographer at the University of Alaska's Institute of Marine Science. Finney uses the nitrogen left behind from salmon carcasses to track hundreds of years of salmon runs in Alaska and the Pacific Northwest. When Smol and Finney compare diatoms with nitrogen levels, they find a correlation between years of high salmon runs and the abundance of diatoms that love nutrient-rich water.

Smol's colleagues at Queen's University also have used diatoms to check the former salinity of water bodies in the Great Plains. Salinity is an indicator of massive droughts in the past. Some researchers have used salt-water diatoms found in freshwater lakes to determine the dates of past tsunamis that have crashed inland. Smol said diatoms can even be used to help date ancient bits of pottery, and to find the water sources with which the pots were made. Not too bad for creatures so small that hundreds of them could sit on the head of a pin without bumping elbows.