Sampling the Ocean Floor
Introduction: Different kinds of sediments are found in varying amounts on different parts of the ocean floor. Some of these sediments are soil and rock particles carried from shore, some are skeletal remains of the critters that live in the ocean, and some are precipitates from the sea water itself. Oceanographers make use of ocean floor sediments to pursue questions about the organisms that compose them, about the currents that carried them, about climate changes over time, and about the formation of the ocean crust.
Samples of deep ocean sediments are difficult to obtain, and inferences about ocean processes based on information from the sediments must be made carefully. Samples are collected by grabs, which scoop out small chunks of sediments; and cores, which yield a tube of sediments from a meter or two to tens of meters in length. Geological Oceanographer Dr. Susan Humphris describes the sampling process this way: A...imagine you are from another planet and you want to investigate the earth. You take your spaceship and fly outside the atmosphere. You can't see anything because all these clouds are in the way, so you throw down an enormous garbage-can on the end of a string and drag it across the earth. You bring it back again and end up with a tin can, a window frame, a policeman, and a church steeple. From these completely separate items, you try to draw a picture of what the earth looks like.
What to Expect: This exercise dramatizes the importance of sample size in obtaining an accurate picture of what occurs in nature. Younger children can use symbols or icons to keep track of the different sediment types as they sample, and make graphs of the total numbers of each type of sediment. Older students can compute percentages and compare the accuracy of each sample group (for example, if results are computed after every ten students sample) to the actual composition of the ocean floor. Each group of ten students can be a separate sample (if you return the samples to the ocean) or samples can be cumulative, with the final results indicating the original composition of the ocean floor. Try it both ways to highlight the difference; allow plenty of time for discussion of experimental design.
- Small, wrapped candies (or cookies or game markers): four or five different kinds with a total amount of four or five times as many candies as number of students. Vary the quantities of each type of candy so that one or two kinds are very common, others less common, and one type very rare.
- Large box or bowl, at least the size of a copy-paper box
- Covering for the box: a cloth, towel, or crepe paper strips, woven loosely across top of box
- Long-handled tongs (olive, spaghetti, or salad tongs)
- Real sediment samples for students to handle
1. Put candies in the bottom of the box.
2. Have each student extend the tongs under or through the covering, grab a sediment, and retrieve it. Any hand through the covering is a man overboard and lost sailors have no sediment samples.
3. As the students sample, collect and analyze data. Mark the type of sediment obtained on the board, creating new categories when a new type of sample is retrieved. Keep a tally of each type.
4. After every few samples, work out the relative percentages of each material.
5. Ask students to infer what the sediments in the box are like, based on the samples the group has retrieved. Compare inferences. Students should be able to explain and defend their inferences.
6. Continue sampling until each student has sampled once. Discuss the usefulness of obtaining a second full set of samples. Students may choose to carry this out.
7. Only after thorough discussion should the data be compared to what is really there. Students should be able to explain why there are discrepancies.
Evaluation: Students submit a written or diagram description of the activity, an organized and complete data table, an inference about the actual sediment in the box and the reason behind that inference, and a reasonable explanation for different inferences.
1. Divide students into groups of 4-6. Each group may sample only 6 times, due to the cost of oceanographic research vessels ($1,000 to more than $10,000 per day). Have each group devise its own sampling plan. Students retrieve their samples and keep the data secret. Each group describes the sea floor based on its own data. Compare each group's inferences. Some possible questions are: Why are these so different? Which one is right? Then compile all data and work on inferences again. Again question the students: Do you think these are better inferences? What makes them better? How do oceanographers decide what is true? Considering the cost to run an oceanographic research vessel, how many samples are enough?
2. Provide real sediment samples for students to examine. Actual samples from the ocean are ideal, but local samples from many different formations are just as engaging. Use the samples to introduce concepts in geological oceanography or local geology.
Source: Adapted from ideas by Melissa Warner, SEA Experience 1994; based on SEA oceanography faculty presentations.
Copyright 1998-2008 by Sea Education Association, all rights reserved.
Compiled and edited by Pat Harcourt & Teri Stanley.
This project was supported, in part, by the National Science Foundation (Proposals # TEI-8652383, TPE-8955214, and ESI-925324), the Henry L. and Grace Doherty Foundation, the Donner Foundation and the Pew Charitable Trusts. Opinions, findings, conclusions or recommendations expressed are those of the authors and not necessarily of the Foundations.