Essentials of Oceanography

Seawater Properties II; Activity 2


Activity 2: Is the Chemical Composition of Seawater Constant? Do All Elements Behave Alike?

For this activity we will investigate the behavior of some of the elements in seawater in order to understand their sources, sinks, and cycles through the ocean, and to help us decide whether ocean chemistry has been constant through time. For this purpose we will have you research two different chemical elements (partly of your choosing) to obtain the necessary information.


  1. Choose one element from each of two groups, for a total of two elements (I'm hoping that with 11 of you, they will all get covered, so maybe do a random pick for yourself?)
    1. Group 1: Na, Cl, K, Mg, S
    2. Group 2: P, Ca, Si, N, Ba
  2. To start, go to MBARI for information about chemical elements in seawater. You will find a periodic table (you may need to do a search in the upper right corner), "residence times," and other information on most of the elements.
  3. Find and list the following information for each element you chose from Groups 1 and 2:
    1. concentration in seawater (use g/kg)
    2. total mass in seawater (in metric tonnes or kg)
    3. amount that could be held in seawater at saturation (and percent saturation)
    4. known sources of the element to seawater (e.g. rivers, hydrothermal, meteoritic or cosmic, etc.)
    5. known sinks (extraction from seawater through hydrothermal alteration of seafloor, sedimentation, etc.)
    6. cycling (is the element bio-limiting, bio-intermediate, or bio-inert?)
  4. Do the following calculations and answer the following questions:
    1. Calculate the oceanic residence time for each of the two elements you have chosen to investigate.
    2. How does the Group 1 element you chose differ from the Group 2 element? Why are they different?
    3. Why might the inferences you draw from the "residence time" calculations be in error?
  5. Post your answers in the discussion board on Canvas: Lesson 3, Activity 2: The Residence Time of Salt in the Ocean.
  6. Read the postings made by other EARTH 540 students.
  7. Respond to several posts by asking a follow-up question or by asking for clarification and/or expanding on their theme.

Submitting your work

  • Begin by posting. To respond to another student's posting, use the "reply" link that follows their posting.

Grading criteria

See the grading rubric for specifics on how this assignment will be graded.

You are now ready for the End of Unit Quiz, which will open on Canvas on the dates given in the Course Schedule