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How Do We Know?
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|So what does
a pollen grain look like?
The pollen grain has three walls or layers. The outer wall, the ektexine is most often sculptured - it may be covered with any combination of spines (echinae), pores, ridges (rugulae) or bumps (scabrae) that help in identifying the grain. The middle layer contains the enzymes necessary for plant recognition in pollination. These protiens can also provoke an allergic response in people who suffer from seasonal allergies like hayfever. The third layer is a bottom wall and sometimes has columns rising up to the outer wall . When pollen is preserved in lake sediments or any other oxygen-free environment, the middle, biologically-active layer (the enzymes) is not preserved. Only the outer wall and the bottom-most wall are preserved.
Subsamples (usually 0.5-1.0 cc) are taken at regular intervals down the core, typically every 4-16 cm. Seeds, charcoal, and pieces of wood are sieved from the core for identification and for radiocarbon dating.
Because the pollen is a very small portion of the sediment, it must be concentrated for analysis with a series of sieving and chemical steps. Most pollen grains range from 10-100 microns in size. A micron is 1/1000 of a centimeter. Very fine material, mainly clays, are sieved out with a sieve having 7 micron openings, and larger material is removed with a sieve having 500 micron openings. Humic materials from partial decomposition of organic matter are removed with hot 10% potassium hydroxide. Carbonates are removed with hot 10% hydrochloric acid. Silicates (silt and sand) are dissolved with hot hydrofluoric acid, and more easily dissovled organic matter is removed with acetolysis solution, a mixture of concentrated sulfuric acid and acetic anhydride. Sometimes pyrite, an iron sulfide, is removed with 10% nitric acid. The material left after this series of treatments consists of pollen, charcoal, other tough organic matter, and a few resistant minerals. The fortunate chemical resistance of the pollen grain makes this whole concentration process possible.
The pollen concentrate is then mixed with silicon oil and mounted on a microscope slide for counting. The analyst makes transects across the slide, usually at 0.5 or 1.0 mm intervals and tabulates every pollen grain encountered. Several hundred grains will be counted from each sample
The morphology of pollen grains has not changed over thousands of years, and most pollen is readily identifiable. Thus, ancient pollen grains can be compared to modern pollen in the Museum's pollen reference collection for identification. The reference collection was developed from plants collected in the field or from herbarium specimens. Once the plant is positively identified, the pollen may be collected from the flowers, processed, and mounted on permanent microscope slides. The taxonomic resolution of pollen identification varies. Some plants are identifiable to species, others to genus, and some to just family.
A "pollen diagram" displays the results from the analysis of a site. For each sample or level, the percentages of the various pollen types are calculated, and these are displayed on the pollen diagram. The pollen diagram consists of a graph of the percentages of each pollen type plotted against depth. The graphs are placed side by side to facilitate comparison. A pollen diagram from a single site illustrates changes that occurred in the composition of the vegetation over time. By analyzing pollen data over a broad geographic region, scientists are able to document changing patterns of vegetation over time and the migrations of individual taxa. By comparing modern climate patterns with modern pollen assemblages, scientists can reconstruct past climates from fossil pollen assemblages.
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