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Focus question

How can pollen records and core samples be used by scientists to understand climatic conditions in the past?

Tools and materials

• Modeling clay in four different colors
• Small, shallow, opaque dish, approximately 15 cm by 10 cm by 6 cm (6 in by 4 in by 2.5 in)
• Plastic or metal pipe with a 2-cm (0.8-in) diameter, approximately 15 cm (6 in) in length
• Wooden dowel or stick that fits inside the pipe and is longer than the pipe
• Ruler
• Waxed paper (optional)
• Rolling pin (optional)
• Paper ball or piece of rag that fits snugly into the plastic or metal pipe (optional)

Main ideas and background information

• A paleoclimate is a climate that existed sometime in the past (as recently as just a few centuries ago or as long as billions of years ago).

• The study of past climates is called paleoclimatology. 

• Records of past climate change from human measurements generally cover fewer than 200 years and are too short to examine the full range of climatic variability.

• Scientists examine climate change going back hundreds and thousands of years using climate proxies. These are paleoclimate records from sources such as trees, corals, sediments, glaciers, and fossils.

• Tree rings can be used to study past climates. They are counted to determine age. The thickness of each ring can be used to infer fluctuations in temperature and precipitation, since optimal conditions for the particular species will result in more growth, and thus thicker rings for a given year.

• Scientists use ice core records to reconstruct past climates. Deep ice cores can be analyzed for trapped gas, stable isotope ratios, and pollen trapped within the layers to infer past climate.

• An important principle in paleoclimatology is: If plants or animals today need certain conditions to live, then similar plants and animals in the past also required those conditions.

• Scientists analyze the remains of microorganisms in sediment cores to understand past climates. Diatoms, foraminifera, microbiota, and fossilized pollen within sediment can indicate changes in past climate, since each species has a limited range of habitable conditions. When these organisms and pollen sink to the bottom of a lake or ocean, they can become buried within the sediment. Thus, climate change can be inferred by species composition within the sediment.

• Scientists use the law of superposition, which states that sedimentary layers are deposited in sequence, with older layers at the bottom and younger layers at the top. This is illustrated in the activity by the sequence in which students place the layers of clay in their containers. 

• Sediment core samples follow the law of superposition, which states the first layer deposited (the bottom layer) must be the oldest and the last layer deposited (the top layer) must be the youngest.

• Scientists often use combinations of evidence, including tree rings, to determine past and ancient climates.

Procedural tips

• In step 2, students create a model of the sediments at the bottom of a lake or pond by layering different colors of modeling clay in a small container. They put the colors down in any order and thickness. It is easier to make the layers first and then put them in the container; students can flatten lumps of clay between their hands or by rolling the clay with a kitchen rolling pin. The best way to do that is to put the clay between sheets of waxed paper, an open plastic bag, or sheets of plastic wrap so that the clay does not stick to the rolling pin or the table.

• Point out to the students that the law of superposition applies here, with the older layers at the bottom and the younger layers at the top.

• In step 2, we found it useful to turn the pipe as the students push it through the layers. When the pipe reaches the bottom of the container, make sure the students press down hard in order to cut through the bottom layer.

Discussion

What concepts do scientists apply when using fossil evidence to study ancient climates?

When using fossils to study ancient climates, scientists apply two important concepts. One is that ancient plants and animals probably required the same living conditions as similar plants and animals living now. For example, palm trees grow in warm, moist climates, so ancestors of palm trees probably required similar conditions. This means that 70-million-year-old fossils of these trees in Northern Canada are good evidence that the climate of Northern Canada was warm and moist 70 million years ago. The other states that sedimentary layers are deposited in sequence, with older layers at the bottom and younger layers at the top. This is the law of superposition.

Assessment

• Have students use colored pencils and a ruler to draw a hypothetical series of lake-bottom sediments representing the sequence of climates given below. They should use the same colors they used in the activity to represent layers containing different kinds of pollen. Tell them to assume that it takes 1,000 years to deposit 1 cm of sediment.
  • 7,000‒5,000 years ago: climate supporting grasses and oaks
  • 5,000‒3,000 years ago: climate supporting mostly grasses and oaks, with some spruce and alder
  • 3,000‒2,000 years ago: climate supporting mostly spruce and alder, with some grasses and oaks
  • 2,000‒1,500 years ago: climate supporting mostly grasses and oaks, with some spruce and alder
  • 1,500 years ago to the present: climate supporting grasses and oaks

• Ask students to complete the following:
  
  1. Describe what the climate around the lake was like when each layer of sediment was deposited.
  2. From these data, what time period marks the coldest climate recorded in the lake-bottom sediments?
  3. Did the climate cool at the same rate as it warmed?

• Ask students to research the Greenland Ice Sheet Project Two (GISP2), a project designed to collect and analyze ice cores. Have them answer the following:
  
  1. How many ice cores have been collected?
  2. How many have been analyzed?
  3. How far into the past do the data currently reach?
  4. What discoveries have been made?

Glossary/Vocabulary

climate

the general pattern of weather conditions for a region over a long period of time. 

climate proxy

any feature or set of data that has a predictable relationship to climatic factors and can therefore be used to indirectly measure those factors.

fossilized pollen

remains of pollen grains found in some sedimentary rocks.

global climate

the mean climatic conditions over the surface of the Earth as determined by averaging of a large number of observations spatially distributed throughout Earth.

interglacial

a period of time during which Earth experiences temperatures warmer than during glacial intervals and glaciers recede. 

law of superposition

a general law stating that in an undisturbed sequence of sediments, the oldest sediments are found on the bottom and the youngest sediments are found on top. 

pollen

minute grains produced by certain plants when they reproduce; contain the male reproductive cells.

Adapted from: American Geological Institute, EarthComm: Project-Based Space and Earth System Science. 2^d ed., Armonk, NY: It’s About Time, 2012.

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