CHM 475 Chemical Oceanography
Dr. S.A. Skrabal
Final Exam
4 May 2002
NAME:________SOLUTIONS______________
Instructions: Show work on calculations when they are required. The backs of the pages may be used for
space, as needed. A periodic table is located on the last page. Good luck!
1. Describe the trends that are observed for radiocarbon (14C) distributions in the ocean (A) within
vertical profiles and (B) between the Atlantic and Pacific ocean basins. You may describe these
distributions in terms of Δ14C notation. Please be as detailed as possible in your descriptions and use
sketches if necessary. (10)
(A) A reasonable distribution would show relatively more positive 14C values in surface waters and
more negative values with depth. The more negative values reflect the decay of 14C with time from when
it first enters the ocean.
(B) Atlantic 14C values are relatively more positive than Pacific 14C values with depth. This is a
consequence of the youngest ocean water originating in the North Atlantic, where it equilibrates with
14
C in the atmosphere. Aging of the water as it travels to the deep Pacific depletes the 14C via
radioactive decay.
2. Consider the upper 15 cm of a coastal sediment. Assume that the upper 5 cm is oxic, and below this
the sediment become anoxic. Sketch reasonable depth profiles of (A) oxygen (O2); (B) CO2; and (C)
ammonium (NH4+). Please clearly label each profile. (9)
Figures left to right are O2, CO2, and NH4+. Concentrations increase to right. Depth bars are in 5 cm
increments.
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3. Consider a 10 cm deep sediment column, with the top 1 cm oxic, and the next 9 cm anoxic. Such a
situation might be found in the central part of Chesapeake Bay. Sketch reasonable profiles of dissolved
(A) sulfate (SO42-); (B) sulfide (H2S); and (C) Fe2+ with depth. (9)
Profiles left to right are SO42-. H2S, and Fe2+.
4. In the spring in the Cape Fear estuary, nutrients such as nitrate (NO3-) and phosphate (PO43-) are
conservative at low salinities, but are non-conservatively utilized at higher salinities by phytoplankton.
Sketch reasonable estuarine distributions of NO3- and PO43- as a function of salinity (0 to 35) in the
Cape Fear estuary. In this sketch, please indicate what you think are reasonable concentrations of these
two nutrients. (10)
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5. Cesium-137 (137Cs) is a bomb-produced radionuclide with a half-life of 30.07 y. Sketch what you think
the shape of the vertical profile of this radionuclide would look like in the open ocean. Would this be a
useful radionuclide for examining the mixing time of water in the entire ocean? Why or why not? (8)
Activity of Cs-137 
250 m
500
750
Cs-137 would not be detectable in deep water because its half-life is too short. This would not be
a useful radionuclide for looking at mixing time of the ocean because its half-life is so much shorter that
the mixing time (1000 y) of the ocean. However, it could be used to look at surface water mixing, which
occurs on much shorter time scales.
6. Explain how the parent-daughter pair, 238U (half-life = 4.47 x 109 y) and 234Th (half-life = 24.1 d) is
used to examine scavenging processes in estuaries and the ocean. (10)
U-238 is long-lived, soluble, and relatively conservative in seawater. Th-234 is a daughter of U238 that should be in secular equilibrium with U-238, meaning that their activities might be expected to
be the same. However, Th-234 is very particle reactive. A “deficit” of Th-234 in the dissolved phase
would indicate scavenging onto particles.
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7. Describe whether or not each of the following quantities or constituents would exhibit fractionation
between the Atlantic and Pacific Oceans. If they do show fractionation, specify how this fractionation
occurs. Please justify your answer as completely as possible, using chemical equations if possible. (15)
(A) alkalinity
Although alkalinity is nearly conservative, it does increase slightly from Atlantic to deep Pacific. This
increase occurs mainly by dissolution of CaCO3:
CaCO3 + CO2 +H2O  Ca2+ + 2 HCO3-.
(B) TCO2
TCO2 increases from surface waters to deep Atlantic to deep Pacific because of the progressive
remineralization of organic matter:
(CH2O)x + O2  H2O + CO2.
(C) Cu
Dissolved Cu increases from deep Atlantic to deep Pacific because, as a bioactive trace metal, its cycling
is associated with that of organic matter.
(D) Ca
Calcium is quite conservative in seawater (with some very slight depletion in surface water). The amount
added and removed by CaCO3 uptake and dissolution does not significantly affect its distribution in the
bulk of the ocean. No interocean fractionation.
(E) nitrate
Nitrate accumulates in deep Pacific waters relative to the deep Atlantic because of progressive
remineralization of organic matter into its inorganic constituents.
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8. The respiration of a mole of particulate organic matter by manganese reduction can be represented by
the equation shown below.
(CH2O)80(CH2)42(NH3)16H3PO4 + 286 MnO2 + 572 H+ —> 122 CO2 + 286 Mn2+ + 16 NH3 + 408 H2O + H3PO4
The POM shown here has a formula weight of 3362 g mol-1. What is the maximum mass (in milligrams)
of POM that could be completely oxidized by 10.0 mg of MnO2? How many milligrams of Mn2+ would
be released by this processed? In what type of environment would you expect this reaction to occur? (10)
(10.0 mg MnO2)(1 g/1000 mg)(1 mol MnO2/86.94 g MnO2)(1 mol POM/286 mol MnO2)
(3362 g POM/1 mol POM)(1000 mg/g) = 1.35 mg POM
(10.0 mg MnO2)(1 g/1000 mg)(1 mol MnO2/86.94 g MnO2)(286 mol Mn2+/286 mol MnO2)(54.94 g Mn2+/1
mol Mn2+)(1000 mg/g) = 6.32 mg Mn2+
Reaction would occur in hypoxic environments.
9. Many areas of the ocean which experience intense, sustained upwelling have hypoxic or anoxic water
columns. Explain. (9)
Sustained upwelling brings nutrient-rich deeper waters to surface waters, where photosynthesis by
phytoplankton can occur. This high productivity results in a large amount of organic matter which
degrades by respiration. Oxygen can be used up during aerobic respiration, leaving other processes (e.g.,
denitrification) as important respiration pathways.
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10. Give two examples for each of the following items: (10)
(A) A conservative anion in seawater
SO42-, Cl-, F-, Br-
(B) A conservative cation in seawater
Na+, Mg2+, Ca2+, K+
(C) A trace metal which exhibits a large degree of organic complexation in seawater
Cu, Fe, Zn, Cd, Co
(D) A constituent of seawater whose concentration is significantly affected by biological activity, but
which is not limiting (i.e., a biointermediate constituent)
O2, CO2, HCO3-, CO32-
(E) A constituent which contributes to total alkalinity
H+, OH-, HCO3-, CO32-, B(OH)4-
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