How To Read A Pollen Diagram

Palynology is the science that studies fossil pollen and other palynomorphs (tiny organic-walled micro-fossils). Archeological palynology examines human use of plants in the past. Pollen analysts study pollen grains preserved in soil sediments, peat bogs, on rocks and even on artifacts. By studying fossil pollen we can correlate climatic change and archaeological evidence to help us to understand Maryland’s environmental history.

Fossil pollen grains are extracted/isolated using various chemical treatments, and mounted on a microscope slide and examined using a light microscope (link to glossary) or a scanning electron microscope (link to glossary). Under magnification, the pollen analyst will identify pollen types and count the number of pollen grains present. This information is used to produce a pollen diagram.

It is helpful to be able interpret the results of palynology by “reading” a stratigraphic pollen diagram. Although they tend to look quite intimidating, pollen diagrams (also called profiles) are just a series of graphs arranged side-by-side.

    
A 4,000-year pollen profile of the major forest taxa in the Magothy River (from Brush 2001)

The graph’s Y axis (running top to bottom) shows the depth of the sample, which usually corresponds to time, with the deepest and oldest deposits at the bottom, and the most shallow or recent deposits at the top. The age of different layers of sediment are determined by measuring the decomposition of carbon through a process called radiocarbon dating (link).

The graph’s X axis (running left to right) plots the percentage of plant pollen types. The greater the height of the graph, the larger the percentage of a given pollen type present. The pollen diagram allows us to see changes through soil depth and time.

For example, in the graph above the percentage of pine pollen comprises about 15% of the sample collected from 1000 centimeters below the surface. The same depth produced a radiocarbon date of 1324 bp +/-96. Radiocarbon dates are reported in years before present. Since “present” is set at 1950 to keep things simple, 1324 bp +/- 96 is approximately A.D. 626.

It is important to understand that the pollen percentages expressed in a pollen diagram are not exactly the same as the percentage of each plant as it existed in a past landscape. This is because not all plants produce the same amount of pollen (an oak tree produces a lot of pollen, while a beet plant produces very little).

Palynologists usually use Latin or scientific names (link) to refer to plants. Groups of related plants often have very similar pollen, and scientific nomenclature allows for greater specificity. Sometimes pollen is identified to a “group” or “type” to reflect a broader range of possible classifications.

         
A grain of maize pollen (from Cornel U website, NEED PERMISSSION)   A grain of maize pollen
http://www.southalabama.edu/
geography/fearn/images/
crnpolb.jpg (NEED PERMISSION)
  Ragweed pollen viewed beneath a
scanning electron microscope. (from http://flickr.com/photos/
90717134@N00/462176889/
NEED PERMISSION)

Studying trends in the pollen record can help us to reconstruct past environments and to identify changes to vegetative landscapes over time. Archaeological palynologists examine pollen data in order to detect cultural activities against a background of naturally occurring pollen “rain”. These specialists try and understand how environmental change affected human groups, and what economic and social factors influenced how people changed their lives to cope with new conditions. Archaeological pollen profiles are particularly useful in this effort as they record ways that human beings have shaped landscape history.

Suggestions for Further Reading on Palynology:

Brush, Grace S.
2001   Forests Before and After the Colonial Encounter. In Discovering the Chesapeake. Edited by P. Curtin, G. Brush and
           G. Fisher. Pp. 40-51. John Hopkins University Press, Baltimore.

Bryant, Vaughn M., and Richard G. Holloway
1983   The Role of Palynology in Archaeology. In Advances in Archaeolgoical Method and Theory (Vol. 6.), edited by             M.S. Schiffer, pp 191-223. Academic Press, New York.

Pearsall, Deborah
2001   Paleoethnobotany: A Handbook of Procedures. Second Edition. Academic Press, New York.




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