Fun with Science Club®


			Examining rainfall patterns from lake sediments F. A. Jones 01.30.01 How do you tell how much rain has fallen in South America over the past 25,000 years? By looking at lake sediments, of course. Scientists from several US universities have reconstructed rainfall patterns for the past 25,000 years by examining sediment cores from Lake Titicaca. Perviously, most people thought that South America was a very dry place during the last ice age over 15,000 years ago. However, cores taken from Lake Titicaca showed that South America was much wetter than previously thought during the last ice age. Sediment cores are made by drilling a long tube into the bottom of lake. Lake sediments contain dirt, dead algae, and other materials that sink to the bottom of lakes. By examining a core, scientists can peer into the history of a lake by knowing how fast the sediment is deposited. The deeper the core, the older the sediment. Many things are found in lake sediments, including species of fossilized algae. During times of low rainfall, Lake Titicaca dries up and the amount of water in the lake decreases. With less water in the lake, the lake becomes salty. Because some species of algae grow more quickly and prefer to live under salty conditions, their numbers increase during times of low rainfall. When rainfall is high, different species of algae that prefer freshwater are more abundant. By knowing which species prefer salty conditions and which prefer freshwater conditions and comparing these at different depths of the core, scientists can describe how lake levels changed through history.


			Paul A. Baker, et al. 2001. The History of South American Tropical Precipitation for the Past 25,000 Years. Science 291: 640-643.