Evolution
of the Atmosphere and the rise of oxygen
Link to powerpoint slides of David Catling's lecture: here.
Earth is the only planet in the solar system
whose atmosphere contains substantial amounts of O2. The
atmospheres of Venus and Mars are primarily composed of CO2.
Jupiter and Saturn have massive atmospheres consisting mainly of highly
reduced gases like methane and ammonia, which would be quickly oxidized
if sufficient amounts of oxygen were present.
The earth system can be characterized as
being highly oxidized relative to the other planets. Reactive metals
such as iron in the earth's crust and outer mantle exist in highly oxidized
states like Fe2O3. Hydrogen is only a trace
element in the atmosphere, as are quickly oxidized hydrogen compounds methane
and ammonia. Carbon monoxide, which is also readily oxidized, is
present only in trace amounts. Solid material containing organic
carbon doesn't last long unless it's buried. Hence, there appears
to be plenty of free oxygen in the earth system to have oxidized everything
that's readily oxidizable.
Figure 1. The composition of the Earth's atmosphere has changed
over the last few billion years. [Figure from David Catling's lecture in
class]
Evidence for the rise of oxygen
It's clear that this wasn't always the
case in the earth's history. The oldest fully oxidized soils and 'red beds'
(reddish colored sandy soils and sediments containing ferric (fully oxidized)
iron oxide) date back to 2.2 billion years ago. Banded iron formations,
which contain ferrous (only partially oxidized) iron stopped forming around
1.9 billion years ago. Evidence based on the ages of uranium oxides and
iron pyrite (FeS2) is consistent with these dates. From
this evidence it can be inferred that prior to about 2 billion years ago,
the atmosphere cannot have contained more than a few percent of the amount
of oxygen that it contains today, and that the buildup of oxygen, when
it finally occurred, was rapid.
Figure 2. Banded Iron Formations
Where did all the oxygen come from?
For every molecule of oxygen currently
residing in the atmosphere there are about 10 molecules tied up in oxidized
compounds in the earth system (metal oxides, carbonates, and sulfates).
Where did all this oxygen come from? Some oxygen can be generated by the
photochemical reactions depicted in Fig. 9-1, but nowhere enough to account
for the amount currently observed in the earth system. Scientists are convinced
that the same photosynthesis reaction that we studied in connection with
the carbon cycle must have been the major source.
The prebiotic atmosphere (i.e., the atmosphere
that existed before the advent of life) was substantially different from
today's atmosphere. It was composed mainly of carbon dioxide and nitrogen.
It may have been as much as ten times as massive as the present atmosphere.
For specifics, see Table 1 in the text Table 9-1.
Dating of microfossils of single celled
bacteria indicate that the first life forms on this planet -- single celled
bacteria -- and the presence of organic carbon indicates that life originated
very early in the earth's history-- around (or before) 4 billion years
ago -- just as soon as the intevals between the bombardment by asteroid
size objects became long enough to permit it to happen.
Life as a source of oxygen
Scientists are still debating just how
life began. Perhaps the dominant 'school of thought' is that it evolved
from self replicating RNA molecules. A competing theory is that life was
introduced into the earth system by interplanetary dust particles originating
in extremely cold (10 K) interplanetary dust clouds which provide a favorable
environment for the evolution and survival of complex organic molecules.
[Interest in this possibility has spawned the Astrobiology Program here
at UW.] A third theory is that life originated in or near hydrothermal
vents in the mid-ocean spreading ridges, where water is rich in reduced
compounds FeS and H2S. Fossil evidence indicates that
by 3.5 billion years ago life had evolved to the point where blue green
algae capable of photosythesis were widespread in the oceans. Terrestrial
organisms (life on land) didn't come until much later.
Why did it take so long for oxygen levels
in the atmosphere to rise?
The geological evidence suggests that
once life was established in the world ocean, photosynthesis began producing
oxygen at rates comparable to those observed today. Why did it take
something like 1.5 billion years before atmospheric oxygen levels to begin
their sharp rise to levels comparable to those observed today? Because
oxygen, being a highly reactive gas, did not begin to accumulate in the
form of O2 until it had reacted with (i.e., oxidized) all the
compounds that it comes into contact with in the earth system: i.e., the
atmospheric gases methane, carbon monoxide and H2 and the minerals
in the earth's crust and mantle. [To get an idea of how massive the
mantle is see Fig. 1-10 on p. 110 of the text.] Reactive metals like iron
were converted to oxides; sulfides were converted into sulfates.
The formation of calcium carbonate (limestone) takes up oxygen atoms.
Why do scientists think minerals in the earth's mantle were oxidized as
well? Because if they were not highly oxidized, volcanic emissions
emenanting from the mantle would contain larger fractions of reduced gases
than they do today. How could minerals deep in the earth's mantle
have been oxidized? By the recycling of water through the mantle.
Hydrated (water containing) sediments in the crust get subducted.
As the material heats up the water boils off. Some of this steam
oxidizes ferrous oxide in the mantle, releasing free hydrogen. The
hydrogen and the remaining steam are eventually released in volcanic eruptions
and the hydrogen escapes to space.
The role of hydrogen escape as a source
of oxygen
The slow escape of hydrogen molecules
to space over the lifetime of the earth is an important factor in the evolution
of the earth system. Of the gases in the earth's atmosphere, only
hydrogen and helium are light enough to escape in appreciable amounts.
As hydrogen escapes, oxygen that might otherwise be bound up in water molecules
and/or used to oxidize methane (CH4) and ammonia (NH3)
is freed up. Venus is believed to have lost all its hydrogen (and
hence its water) because it is too hot. Jupiter and Saturn have lost
none of their hydrogen because they are too cold, and therefore their atmospheres
are full of highly reduced gases methane and ammonia. As in the Goldilocks
fable, the temperature of the earth is just right so that the escape
of hydrogen was fast enough free up oxygen but not large enough to produce
significant losses of water.
Oxygen and the carbon cycle
From the photosynthesis reaction
CO2+ H20 -->
CH20 +O2
it is evident that for each molecule of
oxygen that is produced a carbon atom must be buried (as part of an organic
carbon molecule) in sedimentary rock. Hence the amount of carbon in this
reservoir is a measure of the net production of O2 over the
lifetime of the earth. Based on estimates of the size of the organic
carbon reservoir, this amount is large enough to account for the reactions
listed in the previous paragraph.
The formation of the ozone layer
The buildup of oxygen in the earth's atmosphere
led to the formation of the ozone layer (ozone has three oxygen atoms,
and its chemical formula is O3). Chemical models indicate
that shouldn't have taken very much O2 (perhaps as little as
a percent of the levels observed today) for photochemical processes in
the stratosphere to produce an ozone layer thick enough to shield life
on the surface of the planet from the harmful effects of UV radiations
described in Fig. 9-12 and the accompanying discussion in the text.
The other planets don't have ozone layers because their atmospheres don't
contain appreciable amounts of oxygen. The specifics of how the ozone
layer was formed and how it is constantly being renewed are reserved for
Chapter 14.
Present level of oxygen in the atmosphere:
trial by fire
Just how far back in the earth's history
oxygen levels rose to their present values is difficult to say. During
the past 360 million years, when forests and occasional forest fires are
known to have existed more or less continuously, oxygen levels cannot have
exceeded 35% (the level at which recurrent fires would have destroyed them,
and they cannot have dropped below 13% (the level below which fires could
not have been prevalent enough to account for the amount of burned material
evident in the fossil remains of trees). Just why oxygen levels have
remained within this range for such a long time is not fully understood.
Review Questions
-
Describe the composition of the earth's atmosphere
as it was thought to exist before the advent of life. What is this
assessment based on?
-
Describe this evolution of oxygen levels in
the earth's atmosphere. What is the evidence in support of this view?
-
How does the amount of O2 in today's
atmosphere compare with the amount that has been produced by photosynthesis
over the lifetime of the earth? Where did the O2 produced by
photosynthesis that is not still in the atmosphere end up?
Critical Thinking Questions
1) How could life have existed on
earth prior to the formation of the ozone layer?
2) Does the burning of fossil fuels
affect atmospheric oxygen?
3) Does the destruction of tropical
rainforests menace the supply of atmospheric oxygen?
We discussed this question in class.
Tropical rainforests are not a net source of oxygen because most of the
O2 produced during the production of organic carbon by photosynthesis is
eventually consummed by oxidation of this organic carbon by respiration
and decay (which both consume oxygen). So the answer is no: destroying
rainforests will not affect O2.
4) A developer in Amazonia has a
plan to raise the levels of atmospheric oxygen by cutting down the rainforest
and replacing it with a managed forest. The managed forest would be cut
every 20 years, the cut trees would be sealed in plastic bags loaded with
weights, and the bags would be dumped to the bottom of the ocean. What
is the developer's reasoning? Would the plan work? Why or why not?
This is another question we discussed
in class. The reasoning of the developper is that cutting down mature
trees and sealing them against oxidation will prevent O2 generated during
the growth of these trees to be consumed by oxidizing organic carbon when
the tree decays. So, yes, O2 levels would increase in theory. The
problem in this reasoning is that the developer would have gone through
a lot of effort for a very small result: the increase in O2 will be limited
by the amount of atmospheric CO2 available for photosynthesis. Atmospheric
CO2 makes up only 0.036% of atmospheric composition, while O2 is 21%. So
atmospheric CO2 is less than 0.2% of atmospheric O2 (0.036/21*100).
The plan would not work.
5) Do you expect atmospheric O2
to have a seasonal cycle? Why? How large would it be?
Answer. In the same way as CO2 has
a seasonal cycle due to photosynthesis and respiration (Fig. 7-4), so does
O2. The source of O2 from photosynthesis is limited to spring and
summer, while the sink of O2 by oxidation of dead biomass is more evenly
spread during the year. The seasonal cycle for O2 (increase in spring
and summer/decrease in fall and winter) is thus the reverse of that of
CO2 since the source of O2 is a sink of CO2 and vice versa.
Fig. 7-4 shows us the seasonal cycle
of CO2 at Mauna Loa in Hawaii. The amplitutude of the variation (difference
between maximum and minimum CO2 over a year) is close to 5 parts per million.
The same amplitude should apply to O2.
A little more fuel for thought (for
those of you who are interested): For CO2 this is a significant seasonal
cycle, corresponding to a 5 ppm/365 ppm x 100 = 1% variation. For O2 which
is much more abundant, this only corresponds to a tiny perturbation.
If we convert 21% to ppmv, we find that the concentration of O2 is 21/100x106=210,000
ppm, and thus the seasonal variation represents only a change by 5/210,000x100=0.002%.
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