Palynology is the study of pollen, spores and other plant microfossils. Pollen and spores are part of the male reproductive system (sperm)
of flowering plants, and usually transported by wind or insects to female flowers. Pollen have durable cell walls, made of a substance
called sporopollenin. The tough cell walls help preserve pollen and spores long after the grains are buried in sediment and make them
common fossils. We identify pollen by its shape and structure, often allowing us to identify its family and, in some cases, genus and
species.We need a microscope to look at pollen closely enough for identification because the grains are very small, typically between 10 and 100 µm (less than 1/10th the diameter of the head of a pin). In the lab, we use strong acids and bases to remove the rocks and other plant material from the pollen samples. What remains is a pollen-rich sludge that we smear across microscope slides and place under the microscope. Sometimes every grain on a slide is counted as part of the analysis, but usually we count a subsample on each slide by scrolling the slide up and down in discrete rows, counting and identifying every type we see. Typically 300 to 500 grains are counted from each sample. The results are presented in diagrams that show the relative amounts of each pollen type.
The pollen results tell us what kinds of plants used to grow around our sampling site and allow us to make educated guesses
(hypotheses) about environmental changes.
For example, if one sample has 90% pine tree pollen and only 1% pollen from meadow-plants (like grass and asters), then we'd conclude that pine forest dominated the area. Another sample, from 1000 years later, might have only 3% pine pollen and 70% grass, aster, sage and thistle pollen. This tells us that that the forest was probably gone and that dry meadows had replaced the forest.
Once we have the pollen results and the initial interpretation of vegetation change completed, we look for evidence that climate, fire, agriculture, or some other disturbance caused the changes we see. Abundant ash in a pollen sample is evidence of a fire near the site. Cereal grass and ragweed pollen are evidence of agricultural land use.
Past climate (paleoclimate) change can be hard to identify because the change may be less dramatic than that caused by fire or farming.
Many plants survive climate changes by changing their reproduction method from flowering (which produces pollen) to clonal growth
(which does not). By studying other aspects of the sample, like the diatoms, ostracodes and the sediment itself, we obtain evidence
that supports one interpretation over another.
Lisa Doner is doing the pollen analyses for Bear Lake. Preliminary results suggest that several times in the last 14,000 years climate has caused major vegetation changes, including a cooling over the last 4000 years. Our August 2000 coring efforts are directed at getting a more recent record of changes during the last 3000 years (including the 20th Century).
U.S. Department of the Interior | U.S. Geological Survey
URL: http://esp.cr.usgs.gov/info/lacs/pollen.htm
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Page Last Modified: Fri 28-Jun-2002 19:46:07 MDT