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 sediment 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 and conditions that produce different kinds of sediments, and by determining the relative amounts of different chemical compounds present in different sediment layers, paleolimnologists have assembled local records that help show how climates have changed worldwide.
The presence of a particular material in a lake sediment sample represents a proxy, or stand-in, for some event that helped precipitate its deposition. Dating lake sediments usually involves analyzing and comparing several materials to produce a multi-proxy account. Relative dating, or chronostratigraphy, is a straightforward process, because younger sediments almost always overlie older ones. However, the action of waves, currents, and lake bottom springs can locally mix material from different strata. Even without such confounding effects, each type of material poses a different set of analytical challenges.
Windborne or aeolian materials that fall into a lake may have been blown overland for considerable distances. Fine, inorganic particles like dust and sand carried by winds are difficult to distinguish from similar, waterborne materials unless they are chemically or geologically unique, representing particular outcroppings exposed at a lower elevation than the lake, or otherwise outside its watershed. Abundant aeolian sediments can indicate drying climates, because they are more likely to be mobilized by winds when soils are dry and plant cover is sparse. Soot is deposited during fires, which are more common under drying conditions.
Pollen grains are often distinct enough that plant families, genera, and even species can be identified from pollens preserved in lake sediments. An abundance of particular pollens at any given sediment level can be taken as a proxy for climatic conditions favoring the plant species that produced them. However, not all pollens become windborne (particularly those adapted for insect borne dispersal), and those that are windborne are not equally durable, so sediment pollen records are always incomplete. Finally, unusual short-term weather patterns and major storms can produce unusual aeolian deposits, resulting in inaccurate or imprecise interpretations.
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