Climate Clues from Sand and Mud

Part B: Modeling Deposition by Icebergs in Coastal Ocean Margins

Introduction

Sediments are deposited into the ocean primarily by several mechanisms: carried by running water in streams and rivers, blown by the wind, and produced by microorganisms in the water. But ice sheets a dome-shaped mass of glacier ice that covers surrounding terrain and is greater than 50,000 square kilometers (12 million acres). e.g., the Greenland and Antarctic ice sheets. and glaciers a mass of ice that originates on land, usually having an area larger than one tenth of a square kilometer. that reach the sea in coastal areas supply an additional and significant source of sediments to the ocean basin.

When an ice sheet is present in a region it causes more erosion on land that drastically increases the rate at which sediments are deposited over time. As mountains rise and glaciers advance, more and more material is eroded as the ice slowly flows downhill to the sea. As mountains grow higher, glaciers erode more land which accelerates erosion rates and leads to an even higher sedimentation rate.

As ice sheets advance they sometimes encounter ocean margins. When this occurs, chunks of the ice sheet break off (called calving the process by which an iceberg breaks off from an ice shelf or glacier. ) and float away to form icebergs.

Watch the video of the calving process occurring at a glacier's leading edge in Alaska.

When ice reaches the sea, retreating ice sheets, and the massive numbers of icebergs they produce, act as sediment delivery systems. The material they deposit in ocean basins is called ice-rafted debris when icebergs melt, a certain amount of drifting sediments of different sizes are deposited onto the bottom of the ocean. These pieces of sediment are normally larger than the surrounding fine-grained mud found there.. These sediments (larger chunks of rock are called dropstones isolated fragments of rock found within finer-grained water-deposited sedimentary rocks. They range in size from small pebbles to boulders.) are incorporated into the oceanic sediments, which are typically fine grained sand and mud. Melting icebergs also increase the sedimentation rates on the continental shelf, slope, and in deep water. Because sedimentation rates change as the amount of ice changes, scientists can determine the relative nature of the climate (and temperature) by measuring the thickness of the sediments deposited over a certain period of time. So low sedimentation rates indicate a warmer climate since only rivers, streams, blowing dust, and microorganisms were responsible for depositing ocean sediments. High sedimentation rates indicate a cooler climate because a lot of additional sediments were also deposited by melting icebergs.

Explore glacier calving and ice-rafted debris

It's important to keep in mind that other geologic processes, such as the uplifting of mountains, can also increase sedimentation rates. In this activity, you will construct a model to investigate the types of sediment deposition to expect when the climate is cold and ice sheets are found at ocean margins.

Assemble the following materials and review the lab procedure below.

Materials

• play sand
• aquarium gravel
• cocktail or regular straws
• ice cube tray
• plastic container (shoebox size)
• white paper
• several bricks or paver stones

Procedure

1. Build model glaciers/icebergs

Put a ¼ teaspoon of sand and a few pieces of gravel in each compartment of an ice cube tray. Fill with water and freeze. NOTE: If you add too much sand and gravel, the glacier will sink since the ice will have a density greater than 1! (The ice cubes may already be made by your teacher.)

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A model iceberg carrying sediment. Source: J. Lockwood

2. Prepare the land-ocean interface.

To simulate land, put a brick or a number of rocks on one side of a plastic box. Put white paper under the box and on top of the brick so the sediment particles will be more visible when glaciers and icebergs melt. Fill the box with several inches of water.

3. Complete the demonstration.

One ice cube will be left to melt on land (glacier) while one floats into the model ocean (iceberg). Predict how the pattern of deposited sediments will differ between them.

Place one of the model icebergs on the white sheet of paper on the brick. It will be allowed to melt in place. Place several other model icebergs at the edge of the brick and let them "calve" and move away from the shoreline. Use gentle puffs of air through a straw to move the icebergs slowly until they reach the other side of the box or disappear. This action models how wind and ocean currents actually moves them.

4. Observe your demonstration

After both ice cubes completely melt, draw a sketch that shows the deposits of sediment left behind by the two glaciers.

Answer the following questions.