CHM 475 Introduction to Chemical Oceanography NAME:___________________________________
Dr. S.A. Skrabal
Exam III
7 April 2004
SOLUTIONS
Instructions: Show work on calculations when they are required. Explanations should be
accurate and succinct. Good luck!
1. Consider a sample from the Atlantic Ocean that has an apparent oxygen utilization (AOU) of
85 µmol kg-1. How many moles and milligrams of particulate organic matter (POM) must have been
completely oxidized in this sample according to the Redfield stoichiometry of POM shown below? (12)
(CH2O)106(NH3)16H3PO4 + 138 O2  106 CO2 + 122 H2O + 16 HNO3 + H3PO4
(eq. 1)
The formula weight of this POM is 3553 g mol-1.
(85 mol O2/kg)(1 mol/106 µmol)(1 mol POM/138 mol O2) = 6.2 x 10-7 mol POM/kg
(6.2 x 10-7 mol POM/kg)(3553 g POM/mol POM)(1000 mg/g) = 2.2 mg POM/kg
2. What concentration (µmol kg-1 and mg kg-1) of NO3- would be released from the oxidation of the
biomass in question 2? (8)
(85 µmol O2/kg)(1 mol/106 µmol)(16 mol NO3-/138 mol O2)(106 µmol/mol) = 9.9 µmol NO3-/kg
(9.9 µmol NO3-/kg)(1 mol/106 µmol)(62.01 g NO3-/mol NO3-)(1000 mg/g) = 0.61 mg NO3-/kg
3. Consider equation 1 in the reverse sense, as a photosynthetic equation. How much CO2 (in µmol kg-1
and mg kg-1) would be required to synthesize 7.5 mg (in 1 kg of seawater) of biomass? (10)
(7.5 mg POM/kg)(1 g/1000 mg)(1 mol POM/3553 g POM)(106 mol CO2/1 mol POM)(106 µmol/mol =
224 µmol CO2/kg
(224 µmol CO2/kg)(1 mol/106 µmol)(44.01 g CO2/1 mol CO2)(1000 mg/g) = 9.86 mg CO2/kg
4. What are four chemical forms (species) of inorganic N found in the ocean? (8)
N2 (nitrogen gas), NO3- (nitrate), NO2- (nitrite), NH4+ (ammonium ion), NH3 (ammonia)
5. Name three biointermediate constituents in seawater. (6)
Biointermediate – concentrations affected by biological activity, but usually do not limit it.
Examples: O2, C, S (maybe, but not usually), Ca, Ba. Technically, a lot of trace metals could be
considered biointermediate.
6. Name three biolimiting constituents in seawater. (6)
NO3-, PO43-, Si
7. Certain bioactive trace metals (e.g., Cu, Ni, Zn, Cd) show significant differences in concentration
between the Atlantic and Pacific ocean basins. (A) Sketch a typical vertical profile for one of these
metals in the Atlantic Ocean, showing a typical concentration range. (B) Explain why there are
concentration differences between the ocean basins for these metals, being as detailed as you can. (12)
(A) Nutrient-type profile. See class handouts for examples, including relevant concentration ranges.
(B) Bioactive metals such as Cu, Ni, Zn, and Cd are recycled with organic matter—taken up during
biomass formation and released during degradation. As ocean water ages along the oceanic conveyor
belt, progressive degradation of organic matter leads to progressive accumulation of some bioactive
trace metals in older Pacific waters.
8. (A) Sketch a typical profile of dissolved organic carbon in the open ocean, showing a realistic
concentration range. (B) In which part of the ocean (surface, intermediate, deep) would the highest
concentration of reactive (i.e., readily degradable) POM be found? Explain your answer. (C)
Concentrations of organic matter in coastal sediments tend to be up to 10 times greater than in open
ocean sediments. Explain why this is. (14)
(A) See Figure 11.10(b) in Pilson (1998) for DOC profile.
(B) POM is formed in the surface ocean, primarily as a result of photosynthesis and grazing. Most
degradation occurs at quite shallow depths in the ocean, so that by the time it reaches the deep ocean, it
is mostly degraded.
(C) The coastal ocean generally receives large influxes of nutrients via runoff, leading to high
productivity. This productivity, coupled with shallow water depths, leads to accumulation of organic
matter which reaches the sediments before fully degrading.
9. A typical river water concentration of dissolved titanium (Ti) is 15 x 10-9 mol L-1, whereas the typical
oceanic concentration is 0.2 x 10-9 mol L-1. (A) Given an average riverine input to the oceans of 3.6 x
1016 L y-1 and an oceanic volume of 1.4 x 1021 L, calculate the residence time of Ti in the ocean. (B)
How does this residence time compare to that of most nutrients (N, P, Si) and what does the residence
time of Ti tell about the way it might behave in the ocean? (12)
(A)  = A/dA/dt
A = (0.2 x 10-9 mol Ti/L)(1.4 x 1021 L) = 2.8 x 1011 mol Ti in ocean.
dA/dt = (15 x 10-9 mol Ti/L)(3.6 x 1016 L/y) = 5.4 x 108 mol Ti/y delivered to oceans by rivers
 = A/dA/dt = 2.8 x 1011 mol Ti/5.4 x 108 mol Ti/y = 519 or 520 y
(B) This residence time is considerably shorter than that of nutrients which have residence times on the
order of 10’s of thousands of years (perhaps up to 100,000 y). This indicates that Ti is extremely
reactive, and its behavior is dominated by scavenging onto particles.
10. (A) Show by diagram (preferably) or words the major external and internal sources and sinks of
silica in the ocean. (B) How does the saturation state (i.e., undersaturation, saturation, supersaturation)
of biogenic opal in the open ocean compare to that of CaCO3? (12)
(A)
External sources: riverine, atmospheric
Internal inputs: dissolution of siliceous tests, upwelling, benthic fluxes (dissolution from sediments)
External sinks: not important
Internal sinks: Biological uptake (formation of siliceous tests), loss to sediments
(B)
The ocean is everywhere well undersaturated with respect to biogenic opal. That means conditions are
thermodynamically favorable for opal dissolution to occur throughout the oceanic depths. In contrast, the surface
ocean and a considerable part of the intermediate ocean is supersaturated with respect to CaCO 3 (both calcite
and aragonite). CaCO3 dissolution would only be favored in the deep ocean, where undersaturation occurs.
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