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Pollen Analysis Activity
How do Palynologists use ancient pollen to reconstruct ancient plant communities, and what do these communities tell them about past climates in Illinois?
For the assessment activity: graph paper.
Time: One 50-minute class period.
Construct the core by making a tube from mylar or acetate sheets, taping the long edges together and taping over the bottom. The tube should be long enough to accommodate the nine layers, roughly in proportion to the length of time they represent on the time scale (Student Page 1). An easier method is to use a graduated cylinder. Designate certain sediment materials to represent certain layers (e.g., rice represents sediment from 8,000-6,000 years ago, etc). Use a different material for each layer and keep a list of the material (rice, beans, sand) that corresponds to the sediment of each age. Place the oldest layer at the bottom of the tube.
2). When preparing cores for student use, keep the sediment of each age level constant. Referring to Student Page 2, mix paper pollen grains of the appropriate colors within each sediment layer. These should be added in approximately the proportions for each time period; e.g., 14,000-10,900 years ago requires 60% spruce pollen, 10% larch, 10% hemlock, and 20% grasses. The proportions do not need to be exact, but they should be approximate. Prepare a core for each group of three or four students; however, do not include all nine layers in each core. Make one that has only three layers, another that has five layers, etc. Make sure that each layer is constructed from the material used for that layer in the master core, has the same thickness and the same relative proportions of pollen as were used in that layer in the master core, and is in the correct chronological order (oldest at the bottom). You may leave out certain layers, but be sure that all nine layers are represented among the cores you prepare for the class. It might be helpful if you insert a cardboard disk the same diameter as the cylinder between each layer. These disks help to keep the layers separate when students scoop out their cores, layer by layer, into pie tins (Procedure, step 4).
Note: the time periods on Student Page 2 do not correspond exactly to the nine periods on Student Page 1 because periods with the same pollen proportions were combined on Student Page 2.
Introducing the Activity
Present the following information in an informal manner. Pollen is produced by male plant structures and enables the plant to reproduce. Certain angiosperms, such as grasses, trees, and weeds, produce small, light dry pollen grains that are distributed by the air. Because airborne pollen travels far beyond the parent plant, it can be used to survey an area to determine the historic distribution of plants. Paleoclimatologists rely on the expertise of palynologists to identify and count pollen grains contained in sediment samples. These sediment samples are obtained from cores taken from lakes and bogs, and can be dated (typically by the Carbon-14 method); the pollen information is used to reconstruct the vegetational communities and show how they have have changed through time.
1). Divide the class into groups of three or four.
2). Display the master core, noting the nine layers. Explain that each layer was deposited at a different time in the past. Make sure that students pay close attention to the color and texture of each layer because they will need to identify these layers in their own cores. They will use this complete sediment core as a key for defining the layers in their own cores.
3). Give each group a sediment core with x number of layers, a pie tin, and copy of Student Pages 1, 2, and 3. Make sure that each group determines the ages of its sediment layers by comparing them with the examples of other groups and with the master core. They must do this before they dump their core samples into the pie tin. The thickness of a sediment layer represents the time period of a particular vegetation type (thicker = longer time, thinner = shorter time). Each layer contains pollen (colored paper dots), with each color representing pollen from a different species of plant (see the color code on Student Page 2). These plants grew in Illinois during the time the sediment was deposited.
4). Each group now separates the pollen from the sediment by emptying the sediments from their core, one layer at a time, into the pie tin. Students sift and dig until they find all of the pollen grains in a given layer (separated by color and by layer). They then count the number of grains of each color in each layer and compute the percentages of each type of pollen for each layer (number of one type of pollen divided by the total pollen grains in that layer x 100 = % of that pollen type).
5). Students use the pollen key to determine the plant species represented in each sample and the percentage of the total pollen that is accounted for by each species, completing Student Page 3 f or each layer of sediment in their groups core.
6). Students refer to Student Page 1 to determine the climate when each layer was deposited and record this information on Student Page 3.
7). Compare each groups conclusions with those of the other groups. Did everyone find the same plants in the same proportions in the same layer? Can the class, as a group, reconstruct the entire nine-layer core from their individual cores? Hint: They will need to match their cores up with the master core to determine what time periods are missing from a given student core. By combining data from all the groups, students should be able to reconstruct the entire sequence of events since the last glaciation. Do all agree on the climate that probably existed at each time period?
8). As a class, review the species of plants found in each layer and the climate that probably existed in Illinois at the time. Can students discover an overall pattern of climate change during the past 15,000 years? Can they infer what might have caused these changes? Which sediment layer first shows signs of human influence? Besides climate, what is another significant factor in prairie formation?
Assessing the Activity
Extending the Activity
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