Answers to Assignment 9, Chapter 9
OCG123, Spring 2001
Review Questions
1. Miller and Urey assumed that the earth's ancient atmosphere would have
resembled Jupiter's current one, which is quite reducing. They used a mixture of
methane, ammonia, water vapor, and molecular hydrogen. They then simulated
lightning flashed by electrical discharges. It is now understood that this
atmosphere was too reducing to fully represent early earth.
3. The thermophilic last common ancestor of all extant life is suggested by the fact that thermophilic bacteria (heat-loving, sulfur-metabolizing) of modern vents have changed little during 3.5 billion years and have RNA that resembles that of other organisms. There are two possible explanations for these bacteria being our ancestors: (1) life originated in the hot vents, and (2) that life originated at the surface, colonized the vents, and was extinguished at the surfaced by some sort of huge impact. This would make life in the vents the sole survivor.
5. Prokaryotes are single-celled organisms that lack nuclei and are relatively resistant to UV radiation. Eukaryotes are also single-celled, but have a nucleus and are affected more by UV radiation. The prokaryotes are found earlier in the fossil record.
7. The ozone layer had become thick enough to provide a protective screen by 2.1 billion years ago, or 100 million years after oxygen had risen.
9. The fossil charcoal record, which is reasonably continuous since the late Devonian 360 million years ago, tells us that the atmosphere has contained at least 13% oxygen since then (the minimum required to burn organic material).
Critical-Thinking Problems
2. The logic for this problem is:
(a) 0.3 Gt C is buried per year in coal and marine
sediments.
(b) This burial "produces" atmospheric oxygen
because it prevents the carbon from tying up oxygen by reacting with it to form
carbon dioxide. The amount of free oxygen so "produced" each year is
0.3 Gt (32/12), because one mole of carbon (MW = 12) reacts with one mole of O2
(MW = 32).
(c) On the normal earth, this production of oxygen is
balanced by loss of the same amount by weathering (oxidizing) reduced materials
in exposed rock.
(d) Now if the weathering proceeds but no new oxygen is being
formed (because all the forests have burned down, and no living plants remain to
photosynthesize), atmospheric oxygen will be lost at the rate of 0.3(32/12) Gt
per year.
(e) To find out how long it would take this process to use up
all the atmosphere's oxygen, just divide the total amount of oxygen in the
atmosphere by the amount lost per year, with both numbers being expressed as
grams or moles. It is easier to use moles because you have just calculated the
number of moles of atmospheric oxygen in problem 1 (same as problem 7-4): 3.62 x
1019 moles O2 in the atmosphere.
(f) To get the annual loss of O2 in moles, just
express 0.3(32/12) Gt O2 as grams and divide by 32 grams per mole for
O2. That operation is 0.3(32/12)(1/32) x 1015 g = 2.5 x 1013
moles O2 lost per year.
(g) The calculation from (e) then becomes T = [3.63 x 1019
moles O2]/[2.5 x 1013 moles O2 per year] = 1.45
million years.
The simplified version of this logic is:
(a) 0.3 Gt C is buried per year in coal and marine sediments.
(b) This is equivalent to 2.5 x 1013 moles of C
(0.3 x 1015 g C /12 g C per mole)
(c) This is equivalent to the same number of moles of O2
because of the governing reaction CO2 + H2O = C(H2O)
+ O2.
(d) This burial "produces" atmospheric oxygen
because it prevents the carbon from tying up oxygen by reacting with it to form
carbon dioxide.
(e) On the normal earth, this production of oxygen is
balanced by loss of the same amount by weathering (oxidizing) reduced materials
in exposed rock.
(f) Now if the weathering proceeds but no new oxygen is being
formed (because all the forests have burned down, and no living plants remain to
photosynthesize), atmospheric oxygen will be lost at the rate of 2.5 x 1013
moles per year.
(g) To find out how long it would take this process to use up
all the atmosphere's oxygen, just divide the total number of moles oxygen in the
atmosphere by the number of moles lost per year. Get the total number of number
of moles from problem 1 (same as problem 7-4): 3.62 x 1019 moles O2
in the atmosphere: T = [3.63 x 1019 moles O2]/[2.5 x 1013
moles O2 per year] = 1.45 million years. This is the residence time
of atmospheric oxygen (the amount in the atmospheric reservoir divided by the
amount produced or removed per year).