Worksheet #18 ATOC 3600 October 31, 2000
NAME: ____________________


1.  If 100 kg of carbon dioxide is photosynthesized, does this contribute to the amount of free oxygen (O or O2) in the atmosphere or deduct from it? How much oxygen is produced or destroyed? How much carbon is produced or destroyed?
Photosynthesis generates free oxygen. The atomic weight of CO2 is 44 with 12 AMU from Carbon and 32 AMU from Oxygen. If 100 kg of CO2 is photosynthesized, 72.7 kg of free oxygen (2 atoms for every one molecule of CO2) is produced and 27.3 kg of Carbon (in the form of carbohydrates) is produced. (The carbohydrates would weight 68.25 kg.)

The reaction for photosynthesis is:
CO2 + H2O + hv --> CH2O + O2

2. During the early evolution of the earth, what was an important source of free oxygen? Did it eventually help sustain a new processes of oxygen production? What was that new process?
Free oxygen may have been created by photodissociation of water vapor in the early Earth. This created free oxygen and eventually ozone which enabled plantlife and photosynthesis to exist on land. It is probably that photosynthesis already existed in the oceans before the ozone layer.

3. If under global warming, the amount of photosynthesis decreases (due to reduced extent of boreal forests) from 200 Gt-O/yr to 100 Gt-O/yr, how long will the oxygen in the atmosphere last?
In steady state, photosynthesis and respiration are in balance. If photosynthesis were to decrease by 50%, this represents a net sink of 100 Gt-O/yr. Given that the reservoir is 1.1x106 Gt. Oxygen would last 1.1x106/100 years or about 10,000 years.

4. Identify at least one rapid process not mentioned in class that may constrain the amount of atmospheric oxygen  between certain minimum and maximum bounds.
Fire may help constrain the levels of atmospheric oxygen. If oxygen levels become too high, fires will burn stronger and longer thus depleting atmospheric O2. If oxygen levels decrease, fewer fires will occur and they will not burn as strongly. It is believed that this has helped constrain levels of oxygen in recent millenia to between roughly 10% and 30%.

5. Why is the amount of "free" oxygen in the atmosphere related to the amount of carbon in the atmosphere? If one increases the amount of carbon in the atmosphere does this increase or decrease the amount of "free" oxygen? 
Carbon in the atmosphere is precominantly found in CO2. If carbon reacts (to form sediments for example) this releases free oxygen in the atmosphere. If carbon reacts to re-enter the atmosphere (through respiration for example) this removes oxygen from the atmosphere. If one were to increase the amount of carbon in the atmosphere, this would decrease the amount of free oxygen.