Ocean 400 Name:_________________ Chemical Oceanography 7 February 2012 Winter 2012 Points are in parentheses (show all your work) (use back if necessary) Mid-Term – Answer Key 1. Carbonate System Processes As a result of respiration of organic matter in seawater (true/false; state why in each case): a. PCO2 goes down (5) b. pH goes up (5) c. alkalinity goes up (5) d. DIC goes down (5) 2. What controls seawater composition? a) Describe the concept of the kinetic model as a control on the composition of seawater. (5) b) What controls the concentration of Ca2+ in seawater? (5).3. Ocean carbonate reactions a) The concentration of carbonate ion (CO32-) is important to know to understand carbonate mineral solubility reactions. Calculate the concentration of CO32- in surface seawater if you know the alkalinity and PCO2. Assume the apparent equilibrium constants for seawater of KH’ = 10-1.5, K’1 = 10-6.0 and K’2 = 10-9.0 Assume Alkalinity = 2.000 x 10-3 and PCO2 = 390 (10 pts) b) We can write the solubility of CaCO3 in terms of Ca+2 and CO32- as follows: CaCO3 = Ca2+ + CO3-2 The equilibrium constant (Ks’) for this reaction = 4.26 x 10-7 or 10-6.37 Calculate the equilibrium concentration of CO3-2 if you know the concentration of Ca2+ in seawater. Assume that the concentration of Ca2+ = 10.5 x 10-3 M. (10 pts) c) What is the degree of saturation (Ω) of calcite in surface seawater. (10)4. Sarmiento and Gruber (2002) Remember the box model for the carbon cycle from Figure 1 of Sarmiento and Gruber (2002). Calculate the residence time (in years) of C in the surface ocean for the present (black plus red arrows) accounting for all a) Sources (5) b) Sinks. (5) b) Calculate the residence time of C in marine biota. (10 pts) c) Has the residence time of C in marine biota been changed by anthropogenic effects? (5 pts)5. Major Ions (true/false) A conservative element in seawater a. has the same concentration everywhere in the ocean (2) b. will have a constant ratio to other conservative elements (2) c. can occur in seawater in more than one form or species (2) d. does not contribute to the salinity of seawater (2) 6. Radioactive decay 232Th90 decays by alpha (α) decay. This isotope has an atomic number (protons) of 90 and an atomic weight (protons + neutrons) of 232. What is the atomic number and atomic weight of its product. (5). 7. Secular Equilibrium Explain (using words and graphs only) the meaning of secular equilibrium and explain why it is important to the application of U-Th isotopes in oceanography.
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