The Fiske Center Blog

Weblog for the Fiske Center for Archaeological Research at the University of Massachusetts Boston.

After the Excavation: Pollen Analysis


One of the common adages in archaeology is that every day spent in the field leads to three or five days of work in the lab. Well, what are we doing for all of that time in the lab?

Below, Allison Conner, one of the students who worked on the Gore Place project, explains what she will be working on over the coming months.

The field school is over but for many of us our work has just begun. Over the course of our five week excavation at Gore Place we collected over 20 pollen samples from various locations across the site in the hope that they could tell us what sort of plants were being grown in and around the greenhouse. Pollen can be a tricky artifact to deal with archaeologically, because modern pollen and archaeological pollen are indistinguishable from one another. That’s because pollen doesn’t degrade in the same highly visible and obvious way that say a metal artifact rusts when it decays. Hence, pollen samples must be handled very deliberately.

David Landon and Allison Conner collecting a pollen sample.

In the field, pollen samples must be taken carefully to avoid contamination from modern pollen rain. Pollen rain is the cloud of airborne pollen that showers down from modern plant life. The yellow-green film of pollen which covers car windshields and pools of water when pine trees pollenate is one of the most obvious examples of pollen rain. To avoid getting that modern pollen rain in our soil samples several precautions must be taken when gathering pollen samples. The first is to never sample when it’s windy. The second is to maintain as sterile an environment as possible while sampling.

The first step of pollen sampling is to use a trowel to clean off the surface of the wall or floor that you will be sampling from. This scrapes off most of the modern pollen which has fallen on the surface since it has been exposed. The second step is to clean the trowel using distilled water and sterile wipes in order to wipe off the modern pollen on your tool and get as sterile a surface a possible for collection. The third step is to use your freshly cleaned trowel to collect your sample. To process a pollen sample you need at least 30 grams of soil which is collected and placed in a whirlpak bag. Whirlpaks are specially-made collection bags which are completely sterile on the inside and sealed until opened by the archaeologist taking the sample. Once the bag is filled with the sample it is sealed and labeled with the location and number of the sample.

Preparing to the the samples; note the distilled water and labelled whirlpacks.

Pollen samples are best taken from closed archaeological features like buried floors and pits. Pollen samples can also be taken at set increments along the wall of an excavation unit. When taking samples this way it is important to start at the bottom and work your way up so that lower samples are not contaminated from the material above. Though we did not use this method at Gore Place, pollen samples can also be taken from large soil cores driven into the ground.

Once a group of soil samples have been collected, they are refrigerated until they can be processed. Refrigeration prevents the pollen from further decomposing. This is an important step since pollen processing is time consuming and only a maximum of eight samples can be processed at a time. Pollen processing takes a minimum of two days but usually a total of three days to complete. The first day involves using chemicals like hydrofluoric acid and hydrochloric acid to dissolve the minerals and silicon in the soil. On the second day of processing, the chemicals used to dissolve the soil are decanted off and the sample is washed to neutral. The third day of processing involves using several acids to decompose the non-pollen organic materials left in the soil sample. Once that process is completed you are left with a small vial of liquid sample which has the same look and consistency as muddy water.

Cleaning the trowel between samples.

Now that the sample has been processed it can be scanned for pollen grains. A drop of sample solution is placed on a microscope slide with a drop of glycerol, covered with a coverslip and scanned under a 400x dissecting microscope. The glycerol allows the pollen grains to “swim” on the sample so that a gentle tap on the slide will roll the grains and give the scanner a different view of the pollen in question to help aid in identification. As the slide is scanned pollen grains are identified as specifically as possible and counted until the total number of pollen reaches at least 300.

Typically, although with several exceptions, most pollen can only be confidently identified to the Family level. The three pictures of Rosaceae pollen below should give you a hint as to why that’s the case. All three represent very different Genus within the same family, and all three look very similar. Could you tell them apart if you saw them all together on the same slide? Remember also, that individual pollen grains vary in size and appearance and that the pollen grains from soil samples are often broken or degraded in some way.

Thus the results of pollen analysis are often rather general identifications. However, my goal in examining the pollen samples from Gore Place is to identify at least the Rosaceae family to the Genus level so that we can get a more specific sense of what was grown around the Gore Place greenhouse. Even general Family identifications can be extremely valuable as they give us a list of possible plants which can be narrowed down through documentary research.

The Gore samples are currently sitting in the paleoethnobotany lab refrigerator at the Fiske Center awaiting processing and scanning.

–Allison Conner

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