Pollen grains are produced by seed plants (gymnosperms and angiosperms), and they range in size from 10 to 100 microns. They contain undeveloped male gametophytes that are surrounded by a complex protective wall. Depending on the species that produces it, the pollen may or may not yet contain sperm. Most insect-pollinated plants produce small quantities of pollen because insects are efficient dispersers of pollen, so relatively little is wasted or released into the air. On the other hand, wind-pollinated plants produce large quantities of pollen because of the low probability that any given grain will land on another flower.

Many forest trees (including pines, oaks, and maples) and prairie plants (including grasses, ragweed, and sage) are wind pollinated, so most of their pollen is dispersed over some distances. In fact, much wind-blown pollen ends up in oceans, lakes, swamps, mangroves, or peat lands, and it accumulates with other deposits, layer by layer, year after year. As a result, the profile of pollen in sediments represents the types of vegetation in the surrounding areas at a given point in time and the proportion of each type of pollen is directly related to the relative abundance of its associated species.

Pollen preserves best if its sedimentary environment lacks oxygen or is acidic, conditions that are unfavorable for decomposing organisms. If the sediments remain moist, pollen can be preserved for thousands or even millions of years. Using fossilized pollen, scientists can reconstruct changes in vegetation over thousands of years and determine when and how rapidly these changes occurred. Since vegetation is sensitive to climate change, a comparison of fossil pollen profiles with modern climate patterns has allowed scientists to reconstruct past climates. The term "pollen analysis" also includes the study of spores. Spores are produced by non-seed plants such as bryophytes and pteridophytes, but they have been included in the discipline of pollen analysis for the sake of convenience.

Pollen analysts concentrate on pollen walls, which are made up of two major layers (Davis, 1963). The outer layer, or exine, is subdivided into the ectexine and endexine, while the inner layer is referred to as the intine. The ectexine is composed primarily of sporopollenin, which is the most decay- and chemical- resistant biopolymer in nature. The endexine and intine are composed of proteins, callose, pectins, and cellulose and are easily degraded by microorganisms and chemicals. Since the structures of ectexines have not changed for thousands of years and they usually appear consistent within a genus or family, they are most useful in the identification of the plants that produced them. Some criteria used in identifying pollen include the presence or absence of apertures, type of apertures (pores or furrows), number of apertures, and surface ornamentations (spines, ridges, bumps, or striations).

To identify and analyze the sequences of fossilized pollen from lake sediments, most scientists obtain cores of sediments using a square-rod piston corer, a device that consists of a meter-long tube fitted with a piston. Several cores from various spots are obtained from a study area, and subsamples are taken at regular intervals along a core. The sediments of virtually any substrate will contain abundant fossil pollen. In fact, a cubic centimeter of lake sediment will typically contain tens or hundreds of thousands of pollen grains. However, since pollen is a very small portion of the sediments, these grains must be concentrated prior to analysis.

Pollen from peats and other substrates presents a particular problem, because contaminating organic materials from other sedementary materials are likely to be present. Therefore, core samples are passed through a series of sieves and washes designed to isolate the pollen. Humic materials are removed with potassium hydroxide. Carbonates are removed with hydrochloric acid (Erdtman, 1969). Silicates are dissolved with hydrofluoric acid. Cellulose is removed with a mixture of sulfuric acid and acetic anhydride. After this series of treatments, most of what remains is pollen. The ability of the pollen wall, specifically the ectexine, to resist strong organic acids and alkaline substances makes this concentration process possible.

The purified and concentrated pollen is mixed with immersion oil and mounted on a glass slide for microscopic analysis. Hundreds of pollen grains are counted from each subsample from a core. The variety of pollen types and quantity of each type are determined. Surface ornamentations and other pollen wall features are used to identify the associated plants. Fossilized pollen grains are also subjected to relative dating techniques to estimate their age. Radiocarbon dating is one such technique. It uses the naturally occurring radioisotope carbon 14 (C14) to establish the age of organic remains up to about fifty thousand years. Plants and animals incorporate a quantity of C14 into their tissues in about the same proportion as that in the atmosphere. When these organisms die, the C14 decays at a fixed exponential rate. A comparison of the remaining C14 from a sample to that expected from atmospheric C14 allows scientists to determine the age of the sample.

Pollen analysis is used in Quaternary studies, such as paleoecology, paleoclimatology, paleogeography, and archaeology. By analyzing fossilized pollen from a broad geographic region, scientists can document changing patterns of vegetation over time and migration of individual taxa. At the largest spatial scale, pollen analysis has been used to reconstruct past climatic and environmental changes, as well as changes in biomes. These data have strengthened predictions of how vegetation is likely to respond to future climatic and environmental conditions. When taken together, all this information provides an indication of the future of agriculture and silviculture.

References

1. Davis, Margaret B. "On the Theory of Pollen Analysis." American Journal of Science 261 (1963): 897- 912.

2. Erdtman, Gunnar. Handbook of Palynology: An Introduction to the Study of Pollen Grains and Spores. New York: Hafner, 1969.

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