Carbon Cycle Midterm Exam April 1, 2008

Answer Key

1. a. What process dominates the seasonal cycle in atmospheric O2 at 41˚S? Southern summer

release and southern winter uptake by the ocean. Part due to warming-cooling and part due to respiration.

b. What is the principal reason why the atmospheric O2 content declines at only 85 percent

that expected from the consumption by fossil fuel burning? Excess photosynthesis over respiration plus forest and soil organic matter destruction in the Northern Hemisphere. (Note: Ocean O2 release is secondary)

2. a. Why is the negative charge deficit a bit larger in the Pacific Ocean deep water than in tropical surface water? Ca removal (as CaCO3) from surface water and Ca addition to deep water (CaCO3 dissolution).

b. Why is the carbonate ion contribution to satisfying the negative charge deficit larger in

tropical surface water than in Pacific Ocean deep water? Photosynthesis in surface water and sinking of organic matter thus formed removes CO2 and hence raises CO3

2-. The reverse is due to respiration in deep water.

(Note: SO4++ in figure should be SO4

= instead.)

3. a. Note that no measurement points constrain the biosphere uptake curve. This being the case, how was it constructed? Based on amounts of carbon stored in organic reservoirs (i.e., wood, soil, small plants and rootlets…) and estimates of turnover times for each reservoir.

b. Whereas the inventory of excess 14C will decline only very slowly (i.e., one percent every

80 years), the 14C to C ratio in atmospheric CO2 will drop below its preindustrial value within a few tens of years. Why? Because fossil fuel-derived CO2 has no 14C.

Biosphere

4. a. Why do cultivated soils have lower 14C to C ratios than native soils? (see left-hand diagram) Because reduced plant input and oxygenation by tillage reduces short-lived

(fast) reservoir relative to long-term (slow) reservoir. b. What is the point of the right-hand diagram? Agriculture has reduced soil organic

content by an average of 15% (at the time of this survey).

5. a. What important message does the relationship between the CO2 and δD records in Antarctic ice send us? That something happening in the Southern Hemisphere dictated the warming of Antarctica and the release of CO2 from the ocean. Suspect: sea ice cover around Antarctica.

b. What important message does the methane record in Antarctic ice send us? That

something happening in the Northern Hemisphere dictated the rise in methane. Suspect: strength of monsoons.

6. a. Name three pieces of information which can be derived from measurements of nitrogen and argon isotopes on gases trapped in Greenland ice. Any three of these four:

1. Gas age-ice age offset 2. Extent of warming 3. Time lag for methane rise 4. Contribution of firn thickness change b. Why is it important to measure argon isotopes as well as nitrogen isotopes? To

distinguish thermal diffusion contribution from gravitational “settling” contribution.

7. a. In what pH range are boron isotope-based pH estimates useful? ~7.9 to ~10.1. b. Why is it that the δ11B value for surface water can be assumed to have been the same during the last glacial period as it is today? Boron residence time in sea 5 to

10 x 106 years (i.e. far longer than a glacial cycle).

8. a. What are the two types of measurements which allowed Schrag and Adkins to reconstruct the temperature of glacial age deep ocean water?

1) 18O to 16O in pore waters 2) 18O to 16O in benthic forams b. What do these reconstructions tell us about the conditions which must have prevailed in

the glacial North Atlantic? Deep water was generated near the freezing point of sea water (i.e., -2˚C), hence in association with sea ice!!

9. a. What is the point of the upper NO3- anomaly section? NO3 used as oxidant in low-O2

waters beneath upwelling zones. b. What is the point of the lower NO3

- anomaly secion? NO3 produced by N2-fixing bacteria dwelling in the thermocline.

10. a. What are the two inorganic means by which the CO2 produced by fossil fuel burning might be captured? (You don’t need to list specific reactions here, just general devices or settings for capturing CO2.)

1) Coal gasification plants equipped with CO2 capture devices 2) Directly from the atmosphere by a Lackner device b. Name four strategies for disposing of this captured CO2. 1) Saline aquifers 2) Deep sea 3) Basalt provinces 4) React with Mg from ultra basic rock 5) Under ice caps