That Sinking Feeling: Density Currents Lab

Overview

Density currents drive 3D movements within the world’s oceans that dwarf surface currents by volume. Density-driven movements due to temperature/salinity differences keep the world’s oceans well mixed & help to re-distribute heat from tropical areas towards polar areas. Resultant upwelling creates some of the world’s richest ocean ecosystems. Density movements known as turbidity currents are the world’s largest mass wasting events and re-distribute sediment across thousands of kilometers of sea floor.

Objectives

• Students will manipulate three variables affecting water density (Salinity, Temperature, and Turbidity) and observe the effect on water movement.
• Students will associate polar regions with the creation of cold, hypersaline water.
• Students will deduce how sediment accumulating on continental shelves can eventually re-distribute to abyssal plains.
• Students will connect the importance of density currents to nutrient cycling in marine ecosystems.

Lesson Preparation

• It is most helpful if students have already explored the concept of density prior to doing this lab.
• It is also helpful if students have been introduced to ocean currents prior to this lab.
• In addition to the materials above, each group needs something to prop their tubes up with to form about a 20˚ angle from horizontal. I like using a counter edge but boxes, ring stands, or anything else suitable can be used. I just use masking tape to secure the tubes so they won’t roll off.

Procedure

• Demonstrate how to fill tubes nearly full (leave about 4” of headroom) and lean against a counter edge or box so they will repose at about a 20˚ angle. If a counter edge isn’t suitable, use a box, rock, ring stand, etc. to accomplish this. Use masking tape to prevent the tube from rolling off & spilling its contents.
• Explain to students that they will only need one tube-full of water if they sequentially do the ice, then the salt, and then the paint (directions on student sheet.)
• Explain clean-up procedures. The paint will be much easier to rinse out shortly after doing the lab rather than letting it settle overnight.
• Distribute blue ice cubes, dyed salt, and paint to each group. Determine the time allowed to complete the lab & let the students at it! A second day for post-lab discussion may be needed.

Extension

• Students can research deep-sea circulation patterns, which are amazingly complex and cross multiple ocean basins.
• Students can research the immense volume of water moving away from polar regions in bottom currents.
• Students can investigate how a CTD cast can be used to measure salinity & temperature vs. depth, & collect samples from different water masses. For example: http://www.polartrec.com/expeditions/international-continental-shelf-survey/journals/2010-08-10
• Students can investigate how an XBT can quickly yield a temperature profile for a water column. For example: http://www.polartrec.com/expeditions/international-continental-shelf-survey/journals/2010-08-08

Resources

http://www.polartrec.com/expeditions/international-continental-shelf-survey/journals/2010-08-08

Notes on materials used in the lab: Clear PVC pipe, caps, and cement are available online (such as http://www.clearpvcpipe.com) and makes for very durable pipes good for other experiments such as deposition of sediments. Other clear tubes (such as those sold at hardware stores to protect fluorescent lights) may also work but I found them to be harder to cap and not as rugged as PVC.

Red food coloring works but I like crushed fizzy tub tints since they are dry and don’t stain clothes or skin (http://www.stevespanglerscience.com/product/1156.)

“Baby Soda Bottles” (unbreakable, sealable giant test tubes.) (http://www.stevespanglerscience.com/product/1156)

Assessment

Evaluation consists of post-lab discussion and feedback given on sketches and answers to questions. Formal assessment may include test questions relating to density currents.

Credits

Bill Schmoker, Centennial Middle School (bill.schmoker [at] gmail.com)