These exercises were created by Dr. Karen Grove (© 1998 Grove) for use in the Introduction to Oceanography course offered by the Department of Geosciences at San Francisco State University. Other educators are welcome to use these exercises in their own classes. Please send comments to kgrove@sfsu.edu. Course home page: http://geosci.sfsu.edu/courses/geol102/home.html Commercial use is prohibited.
VOYAGE 8: Sediment Stories and Resource Riches
Instructions: First print copies of Voyage 8, Lag-time questions, and Real-time questions. Read the text and examine the images. Write answers to the questions on your copies. When you are satisfied with your answers, type them into the computer and submit. Keep the copies as a study guide.
Don't forget to check out the course web site where you can obtain preparatory information before each class and summaries that are posted after the class meets. Click here to bring up a new browser window with the course site. You can also use this site to display the text and images part of the voyage on a second window so that you don't have to print anything.
Most sediment in the ocean comes from either the recycling of rock materials (lithogenous sediment), or from the shells of organisms that live in the ocean (biogenous sediment). Sediments fall through the water and then come to rest on the ocean (or bay) bottom. Once on the bottom, the sediments can be moved around by currents, or they can be buried by younger sediments that fall down on top of them. In some parts of the ocean floor, several kilometers of sediment have accumulated on top of the oceanic crust. These sediments provide a record of biological and geological conditions in the ocean. In this voyage we will explore some of the information that can be learned from sediments.
In some places the sediment surface in the ocean is extremely flat. In other places, the sediments are disrupted by rocks or formed into ripples by currents. Oceanographers use sonar surveys to investigate the sediment surface. They use these data to interpret aspects of the environment, such as the amount of current activity.
Figure 1. This perspective map of San Francisco Bay near the mouth at the Golden Gate (looking north from San Francisco) is based on images obtained from sonar surveys. The rippled surface consists of large sand waves (underwater sand dunes) produced by currents moving over the Bay bottom. NOTE: this image may take a minute to load.
Perturbations of the sediment surface (like those depicted in Figure 1) tell us about processes operating in the Bay TODAY. We can also collect cores of sediment that provide evidence about what the Bay was like in the past. In the bay these cores may extend just a meter (few feet) below the sea floor or they may extend for many meters below the sea floor (10s of feet). In the open ocean cores may extend a few kilometers below the sea floor!
Link A1. Click on Link A1 to link to a U.S. Geological Survey (USGS) web site about organic compounds in San Francisco Bay that commonly act as pollutants (that is, they are harmful to living organisms). In the Introduction you can see some of the sources of these pollutants into the bay.
Link A2. Click on Link A2 to go to another part of the USGS web site about organic compounds in the bay. Pollutants that get into the bay may stick to sediment and then fall to the bottom of the bay where they collect over time. The USGS drilled cores into the bay mud to see how the accumulation of certain chemicals that act as pollutants in the bay have changed through time. This diagram shows the concentration of DDT (an organic compounds that has been used as an agricultural insecticide) in bay sediments. The vertical axis shows depth: 0 is at the surface of the bay sediments, and -250 cm is 250 cm (2.5 meters) below the sediment surface.
The sediment cores viewed in Link A2 only go several meters below the bay bottom. The next figure shows cores that were drilled much deeper and that can provide information about past environments in the bay.
Figure 2. This figure is a profile of sediments beneath San Francisco Bay that is based on sediment cores collected from the Bay bottom for engineering purposes when the San Mateo Bridge was built. The upper layer of mud is sediment deposited as sea level rose in the Bay during the past 10,000 years. The mud, sand and gravel layers beneath were deposited during previous cycles of sea-level change.
In class we will discuss how sediments can be used to tell about past conditions in the ocean and the atmosphere. Of particular importance is the information sediments provide about past climates on earth. For example, we will look at how sediments can be used to explore El Niños.
Link B1. In preparation for our discussion about El Niños, click on Link B1 to go to a site that the instructor created to explain El Niños. Look at the diagram (under 5.(a)) that shows sea-surface temperatures along the equator in the Pacific Ocean during normal conditions and El Niño conditions. The red-yellow colors are warm temperatures and the blue-green colors are cold temperatures. Remember the diagram we looked at in Voyage 4 that showed the band of high sea-surface temperatures widening toward the west along the equator (Normal Condition diagram on this link's figure). You may wish to explore more of this site to prepare yourself for our in-class discussion about El Niños.
In this part of the exercise, we will do something a bit different. Here you will be able to choose a topic dealing with marine resources and write a review of what you are able to find. One of the reasons people have interest in the ocean is because it is such a rich source of products that humans find usable. Below is a list of some of the major resources available in the ocean.
Don Reed, a professor of marine science at San Jose State University, compiles an archive of news articles that pertain to the ocean. Click here to go to the news archive for marine resources. Choose one of the resources from the list above (for example, minerals, fisheries, recreation, or legal aspects) and then choose an article from the archive about this resource. You may instead choose to read an article from some other source (either on the web or in a newspaper or magazine), but you must list the source of your information. In the lag-time question box, write an analysis of the article you read, including the following items.