Climatic Data, Lake Records

SEDIMENTS THAT ACCUMULATE in lakes and ponds are composed of many kinds of materials. Some, such as windborne dust, pollen, and soot, fall into lakes from the atmosphere. Others, such as clay, silt, sand, gravel, and large organic debris, such as charcoal and leaves, are washed into lakes by running water.

A third group of materials originates in lakes: the remains of aquatic organisms, most importantly the silica-based shells of diatoms (algae) and the calcium-based shells of zooplankton and some invertebrate animals. Core samples extracted with hollow drills from lakebeds show that sedimnt composition varies over time, suggesting that the materials accumulated under different environmental conditions. Sometimes, even seasonal variations are significant enough to produce annual layers, called varves. By extrapolating from modern observations of events ...