Effect of Life on the Atmosphere:
The Rise of Oxygen and Ozone
Learning Objectives
After reading this chapter, students should be able to:
• Know that methanogens are thought to be among the earliest forms of life on Earth.
• Understand the importance of nitrogen fixers in the nitrogen cycle.
• Know the timing of the evolution of oxygenic cyanobacteria.
• Know the timing of and the evidence for the rise of oxygen.
• Be capable of providing possible explanations for the delay in the O2 rise.
• Discuss the importance of stratospheric O3 to the biosphere.
• Discuss variations on atmospheric O2 concentrations since the Archean.
• Be able to estimate changes in net O2 production based on δ13C data.
• Know and discuss modern controls on atmospheric O2 levels, including forest fires,
oxygenation of the deep oceans, and C:P ratios.
Review Questions:
1.) To which domain of life do methanogens belong? Why are they thought to be
evolutionary ancient?
2.) What is the difference between prokaryotes and eukaryotes? Which are seen first
in the fossil record?
3.) What types of organisms have heterocysts and what are they used for?
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4.) Which organisms were the first to produce O2?
5.) What types of geologic evidence are used to study the rise of atmospheric O2?
There are a number of pieces of evidence used to study the rise of atmospheric O2.
The presence of banded iron formations, the existence of detrital uraninite and
6.) How did plants and algae acquire their ability to photosynthesize?
7.) When did the ozone layer become thick enough to provide an effective UV
screen?
8.) What do carbon isotopes tell us about atmospheric O2 levels?
Carbon isotope values in carbonates indicate the rate of organic carbon burial or,
more precisely, the fraction of outgassed carbon that is buried as organic carbon
9.) What does the fossil charcoal record tell us about atmospheric O2 levels?
10.) How is the atmospheric O2 content maintained at its current level?
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Critical-Thinking Problems:
1.)
a. The atmosphere consists of 78% N2, 21% O2, 1%40Ar, and about 350 ppm CO2.
What is the mean molecular weight of air? Round your answer to three
significant figures. (Note: The atomic weights of N and O are 14 and 16,
respectively.)
We can find the molecular weight of air by the following equation:
b. The total mass of the atmosphere is about 5·1018 kg. How many moles of air does
it contain? How many moles of O2 and CO2 are present? (Note calculate the latter
two answers from the first one, not by computing the mass of O2 and CO2. The
concentrations listed for the various gases are percentages by volume, not by
mass, so you need to work in moles.)
To find how many moles of a gas there are in a given mass of that gas, divide the
c. Forests and soils contain roughly 2160 Gton of carbon in the form of wood,
leaves, and humus. (The actual value is not known this accurately, but this choice
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makes the numbers work out well.) Suppose that all of the world’s forests were to
burn down instantaneously and that all of the soil carbon was oxidized as well.
By how much would atmospheric CO2 increase? By how much would O2
decrease? Express your answers in percentages. Assume that the burning
For each mol of CH2O burnt/oxidized, 1 mol of O2 will be consumed, and 1 mol
of CO2 will be released into the atmosphere. To calculate the % change in CO2
2.) The combined burial rate of organic carbon in marine sediments and in coal is
approximately 0.05 Gton C per year. This burial is the net source of atmospheric
O2. In steady state, this source of oxygen is balanced by the weathering of
reduced materials in rocks (kerogen, sulfides, and iron). If the weathering rate
were to remain constant following the disaster in problem 1c, and if all
photosynthesis were shut off (in the oceans as well as on land), how long would it
take for atmospheric O2 to disappear?
We must first find the rate at which O2 is being consumed by weathering of
reduced materials in terms of mol O2/year. Since the weathering rate currently is
Resource Guide
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Video/Film:
Oxygen: The Poison Gas