Trees and Paleoclimate

Part C: Pollen and Paleoclimate Lab

This lab activity was adapted from: Windows to the Universe and NSTA Climate Change Symposium, March 2011.

In this activity, you will take on the role of a paleobotanist, a scientist that studies ancient pollen from plants. You will analyze samples of "pollen" that are mixed with other materials to represent sediment and rock that might have been located at the bottom of a lake when the pollen was deposited. Based the percentage of pollen types in your sample, you will determine the type of climate that occurred during the same period as the pollen deposition.

First, some basic information about pollen cores:

• All flowering plants produce pollen grains with distinctive shapes.
• Plants are generally distributed based on patterns of temperature and precipitation.
• Plant communities change as climatic factors change.
• By knowing conditions that plants prefer, we can make inferences about past climate.

By examining the pattern of plant changes over time, scientists can: determine how long it took for plant species to migrate into or out of an area due to climate change; predict the speed in which plant communities might change in response to human-induced climate change; and predict which plants will be most likely to thrive if the climate warms or cools again.

The Paleoclimate of Battle Ground Lake, Southern Puget Trough, Washington State

The research site, Battle Ground Lake, is located 30 kilometers (50 miles) north of the Columbia River, in Clark County Washington, near the town of Battle Ground. This area was at the edge of the glacial extent at the end of the last Ice Age.

The lake has been in existence for at least the last 20,000 years and has continuously accumulated sediments through most of that time. Trapped in the sediments are pollen grains from the plants that grew in the general vicinity of the lake at the time the sediments were deposited. These pollen grains were blown into the lake by the wind. They are laid down in annual layers and covered with sediments, preserving them as fossils, much like tree rings. By examining the pollen in different layers of sediment from the bottom layer to the top, one can reconstruct the vegetation changes that have occurred in the area during the lake's existence. Because we know something about the climatic conditions that the plants needed to survive, we can use the vegetation data to reconstruct the past climate in the area for the entire 20,000-year period.

Many layers of sediments have been identified by paleoclimatologists. For the sake of simplicity, in this exercise, the layers have been combined into five major layers. The age of each layer has been established by both radiocarbon dating, and by reference to volcanic ash layers (of known age) from Mt. St. Helens and from the explosion of Mt. Mazama, which is now Crater Lake in Oregon.

View the map (above, left) to find the location where these cores were sampled. You may also want to use Google Earth to see more details of Battle Ground Lake's geographic surroundings today.

Paleoclimate and Pollen Cores Lab Activity

In this activity, you will get a sense of how scientists determine the climate from a sediment core. The data in the activity is based on actual sediment core data from Battle Ground Lake. Follow the instructions below to complete the activity.

Prepare for the investigation

Acquire materials as directed by your teacher. These materials will need to assembled before class begins.

 
Download and print the worksheet needed to record the data. Click the link to download worksheet: Student Worksheet: Pollen and Paleoclimate (Acrobat (PDF) 129kB Jun8 22) 

Get a sense of the species diversity: Preview pollen pictures

Carefully, examine the pictures of the different pollen types pictured right, or distributed by your teacher. Note the structural differences in each type of pollen. Consider those differences and how scientists might use them to identify the plants from which they were shed. Click image for larger view. Note: the USGS site has additional images of pollen.

View the sample sediment column (pictured in the materials list) and discuss the way that sediment is laid down in lakes, how it traps pollen, and how scientists obtain the lake sediment cores.

Be a climate detective: Count the pollen in the sample

Before you begin, your instructor will show you a model of a sediment core. The oldest layers in the core are the ones laid down first, and therefore, are on the bottom of the model. Your instructor will also give you the color code for the various pollen types. You will need to fill this information into the key on your student worksheet.

Gather the following materials from your instructor: one sediment layer sample, paper plate or pie pan, tweezers, and toothpicks or forceps.

Note that each sample contains "pollen" from the species prevalent at the time of deposition. Record the number of your sample bag and then empty the contents of your sample into the paper plate or pie pan. When you have placed your sample on the pie plate, do the following:

• Sift through the sample to separate out the pollen from the sediment
• Determine from the pollen color key in the Student Worksheet: Pollen and Paleoclimate (Acrobat (PDF) 129kB Jun8 22) what species of plants are represented in your sample.
• Record your pollen count on your worksheet.
• Determine what percentage of the total pollen comes from each species in your sample. Use the data and your key to determine the climate at the time the layer was deposited.

Analyze the data: Tell a climate story

If more than one pair of students worked on any sediment layer, share the data between teams. Any teams with the same sample bag should come to a consensus on what plants they have found and the relative abundances (percentage). The table in the student worksheet can be used to keep track of the percentage of plants found in each layer. Based on your data, decide what the climate must have been like at the time of deposition.

After you have analyzed your sample, report your conclusions to the class. Then, as a class, build a consensus on the pattern of climate change represented by this sediment column. To get a fuller picture of the climate history in the sediment core, complete your data table with data provided by those students studying different sediment layers. Then answer the Stop and Think questions below. Your teacher will loan you an activity key so that you can check your answers.

Clean up and put away the lab equipment and then gather for a class discussion of results and explanations, as directed by your teacher.