Introduction to Marine
Chemistry and Geochemistry
EOSC 472
Winter 2008
Kristin J. Orians
Midterm Exam - February 27, 2008
(25% of total course marks)
Show all your work, explain each step and use proper terminology!
1. (14 pts) The "Big Four" cations in the oceans are Na+, Mg+2, K+, and Ca+2, with residence
times of 108, 14, 4.5 and 1 million years, respectively.
a) (2) Describe (with both words and an equation) what is meant by the term "residence
time".
b) (8) Discuss the reasons for the different residence times listed above.
(Be specific - describe the relevant processes for each element)
c) (4) State whether each element is conservative or, if not, what kind of distribution it has
in the oceans (be sure to define the term “conservative”).
2. (10 pts) Given that boric acid, H3BO3, has an acid dissociation constant (pKa) of 9.3, answer
the following questions:
a) In distilled water, at a pH of 8.3, what is the expected ratio of [H3BO3] to [H2BO3-]?
b) Do you expect this ratio to be the same, higher, or lower in surface seawater at pH 8.3?
(Explain your answer with as much detail as you can – if you expect a difference, why?)
c) What additional information would you need to calculate the ratio in seawater? How
would you set up the calculation?
3. (6 pts) Surface seawater is typically observed to be supersaturated with O2. Explain why this
is the case. (If you know of more than one contributing factor, describe them all)
4. (8 pts) The figure on the back of this page shows the distribution of nitrate in the deep waters
of the global ocean. Discuss the processes which lead to this distribution.