Part 1—Interpret Sediment Core Images

Step 1 – Interpret Cores Along the Way to Antarctica

Examples of warm water diatoms.

Sediment cores have been recovered and analyzed from oceans around the world. Marine sediment layers vary from place to place in the ocean, based on the types of materials carried from the land and the organisms that live in the ocean. Rivers, winds, ocean currents, and ice carry material from the land, which eventually settle on the ocean floor. These terrigenous (from the land) sediments vary depending upon where they originated. For instance, volcanic ash carried by the wind and then settling to the ocean floor can be linked to the volcanic eruption of origin. Since the dates of many eruptions are known, the ash from a given eruption can be used to date the layer.

Diatoms are one-celled photosynthetic organisms with a glass silicate (glass-like) shell. Diatoms vary in their tolerance to temperature as well as other ocean conditions. Some species of diatoms thrive in warmer waters, others in cooler waters. When these organisms die their shells become part of the ocean sediment. Scientists can determine past ocean temperatures during the time in which each sediment layer was deposited based on the species of diatoms that are present in the various sediment layers.

The ANDRILL (ANtarctic DRILLing) research team has explored the sedimentary history under the Ross Ice Shelf in Antarctica. This ice shelf has been present for many thousands of years, but has advanced and retreated over time based on the temperature of the Earth's atmosphere and oceans. In this part of the chapter, you are invited to take part in this virtual research expedition.

Map of Antarctica with route of the ANDRILL sediment core team. Note the numbers on the map, they represent the sites of the cores. Click on map for larger view.

The imaginary expedition will leave from the tip of South America, travel along the coast of the Antarctic continent, and arrive at McMurdo research station near the northern edge of the Ross Ice Shelf. Along the way scientists will recover sediment cores from the ocean floor to gain experience in reading the history preserved in the layers of sediment.

There are 6 cards to use in this activity, which you may want to print or download for easy reference. You may also want to download the map of the route of the team. To download individual cards, right-click on the image and choose Save File As... .

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Download the core cards in PPT (PowerPoint 4.7MB Apr24 11) format—best for projection.
Download the core cards in PDF (Acrobat (PDF) 3.5MB Apr24 11) format—best for printing.

On your virtual journey with the ANDRILL team you will practice with the sediment identification. When the team reaches a drill site, you will be asked to select the card which matches the sample from the drill site. (Note: Not all of the cards will be used.)

1. Look at the map (pictured above) and follow the path of the research ship from the tip of South America to the Ross Ice Shelf. Notice that one part of the trip will be over open ocean that rarely sees ice cover (between points 1 and 2), a second part will be over water which experiences seasonal ice cover (between 2 and 3), and a third part will be near ice shelves that cover the ocean for long time periods (area 4). The imaginary team will take a sediment core at each place.
2. Follow the map to drill site #1. This place in the ocean rarely sees ice cover.
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   A microscopic view of Antarctic diatoms.

   
   In the summer months, sunlight, contact with to the atmosphere, and nutrient rich waters allow large populations of diatoms to thrive in in the ice-free waters around Antarctica.
   

   Review the sediment core cards in front of you and decide which card matches the core recovered at this site. Be ready to defend your decision.

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Card #3—Diatomite represents the sediment type from this area of the ocean. The deposits here are mostly composed of the diatom shells mixed with very fine inorganic material, silt and clay particles. Sun and contact with the atmosphere make this area of the ocean a good place for diatoms to thrive. There are no clasts (rocks and pebbles), as there is no mechanism for them to be transported to this place.



Small rocks (clasts) embedded in a glacier.

Remember that an ice shelf is a marine extension of a land glacier that has moved over the ground picking up material as it goes along. In the picture above, rocks are embedded in a glacier. When this wall of ice moves out over the ocean, these rocks will be carried with it and be deposited on the ocean floor when the ice shelf melts. These rocks are called clasts.

3. On the map, continue on the path of the drill ship and find drill site #2. A core is brought up from a place where an ice shelf was known to exist, but it disintegrated (fell apart) last year. This ice shelf has been in the process of disintegrating for several decades and only recently gave way to open ocean.
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   Which card do you think shows the sediment here?

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   Card #1—Mudstone with Clasts, represents the kind of sediment deposited in this environment. As the ice shelf covered this site for several thousand years, no sunlight was able to penetrate the water. Therefore, diatoms are not present in this sediment. When the ice began to melt and the shelf retreated, the clasts (small rocks) in the ice fell to the ocean floor and were embedded in the mud.
4. At this same drill site the crew decided to drill deeper and see what sediment was under the Mudstone with clasts. They wondered, what was happening before the ice shelf began to break up? What they found was a layer similar to card # 2, Diamictite.

   What was happening in this location when this layer was laid down?

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   As a glacier moves off the land, some of it floats, but the glacier may be large enough to be grounded on the ocean floor. As glaciers move they push piles of rocks in front of them and compact the rocks underneath them. The appearance of the diamictite layer indicates the glacier was grounded in this place at some time in history.

   Do you think the climate was colder or warmer when this event happened?

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   Answer: It was colder because the glacier was larger than it was in recent times.
5. On the map, continue on the path of the drill ship and find drill site #3. The ship stops near an ice shelf which has recently lost a large chunk from the edge, called an iceberg. This area of ocean has been covered by the ice shelf, and cut off from sunlight, atmospheric gases and nutrient rich waters for several thousand years. Diatoms can not thrive without sunlight and nutrients. In this location, the drill brings up a sample of the ocean floor.
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   Iceberg.

   This is an example of a tabular iceberg off the coast of the Ross Ice Shelf. Icebergs can be hundreds of meters thick; just compare this iceberg to the size of the zodiac raft in the foreground!
   
   
   

   Which of the sediment cards indicates the kind of layers you would expect to find here?

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   Card #4—Mudstone represents the sediment from this area. The ocean has been under an ice shelf for thousands of years where very little light and no contact with the atmosphere. Few organisms thrive in this environment so the sediment is largely sand, silt, clay and mud. There are no larger rocks present in the sample as the ice shelf did not retreat by melting, part of it broke off.
6. At the last stop before reaching the Ross Ice Shelf, site #4, the ship takes a core from an area that is close to the shelf. This location has experienced icebergs floating by over the last several thousand years, but has not actually been under the shelf. This location receives sunlight during the summer, is in contact with atmospheric gases, and has plenty of nutrients in the water.
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   Which card would represent the sediment core from this site?

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Card #6—Conglomerate represents the type of sediment found in an area of floating icebergs. Here the ocean receives sunlight, which allows organisms to thrive, and when they die they sink to the sea floor, creating sediment layers. As the icebergs fall away from the shelf and float out to sea and melt, deposits of rock and pebbles (clasts) fall to the ocean. Notice the small rocks in the sediment, as well as material that includes diatoms.

Glacier foot.
In the picture to the right you see the leading edge of a glacier, also known as the glacier foot. It is pushing a large pile of rock in front of it as it advances. This pile will extend into the water as the glacier moves out. Rock underneath the glacier is being compacted and becomes the diamictite layer that you saw in card #2.

Step 2 – Create a Climate Change Cycle with the Cards

1. Now that you have worked with the sediment cards and the climate records they contain, place them in order according to a full climate cycle of Earth. Start with a warm climate period with an open ocean and follow with cards showing a decreasing atmospheric temperature, which would cause an advancing glacier/ice shelf. End the cycle with cards showing an increasing atmospheric temperature causing a retreating glacier/ice shelf.
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The cards should be in this order:
1. Diatomite #3. Description: The ocean is open, many ocean organisms are thriving, the sediment layer is full of diatoms, with little to no rocks or pebbles.
2. Mudstone #4. Description: The climate is cooling and the ice shelf grows. It grows to the extent that no light can reach the water underneath. Since the ice shelf is advancing, melting is not taking place.
3. Mudstone with clasts #1. Description: Earth is now warming, but the ocean is still under the ice shelf. As the ice shelf retreats it drops small rocks, pebbles and stones into the mud.
4. Diamictite. #2. Description: As Earth warms the glacier retreats leaving a new surface for the sediment layers to form. Since the ice shelf is melting the rocks embedded in the ice are falling to the ocean floor and creating this layer. There is no sunlight or connection to the atmosphere yet.
5. The other two cards are sediment layers in the ocean sea floor but will not be used in the rest of this activity.
2. You have now completed the warm-up activity for this project. Return to the questions at the beginning of the page and answer them in your notebook or on a piece of paper. Save the cards for use in Part 3.