Evolution of the Atmosphere


Earth is the only planet in the solar system whose atmosphere contains
substantial amounts of O2.  The atmospheres of Venus and Mars and 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.

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.

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.

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.

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 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.

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 buildup of oxygen in the earth's atmosphere led to the formation of the
ozone layer.  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.

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

1) Describe the composition of the earth's atmosphere as it was though to exist
    before the advent of life.  What is this assessment based on?
2) Describe this evolution of oxygen levels in the earth's atmosphere. What is
    the evidence in support of this view?
3) 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?  [Hint: Life in this phase of the earth's history was largely confined to
    the oceans and it need not have been right at the surface.]
2) In what way might the evolution of the earth system have been different if
    hydrogen had not been escaping to space?
3) If all the carbon in the fossil fuel reservoir (Fig. 7-2) were instantly
    burned (i.e., converted into carbon dioxide), by how much would the oxygen level
    of the atmosphere be depleted?  [Hints will be forthcoming]
 

Two Quantitative Problems:

1) Suppose that all the fossil fuels in the earth's crust were instantly burned
    and that the carbon dioxide resulting from the combustion remained in the
    atmosphere.  By how much would atmospheric carbon dioxide levels rise and by how
    much would atmospheric oxygen levels drop?

Data:
size of the fossil fuel reservoir: 4200 Gt(C)  (Fig. 7-3)
present atmospheric CO2 concentration: 760 Gt(C)  (Fig. 7-3)
mass of atmosphere: 5.14 million Gt
present atmospheric O2 concentration: 23% by mass
atomic weights: C = 12,  O = 16

The rise in CO2 is calculated simply by adding all the carbon in the fossil fuel
reservoir to the carbon in the atmosphere.  The result is 760 + 4200 = 4960 Gt(C)
the percentage increase is (4200/760) x 100% = 553%.  Another way of
stating it is that it would increase by a factor of 4960/760 = 6.5. The current
level 760Gt(C) already represents an increase of a factor of 1.32 relative to
pre-industrial concentrations.  Hence if all the fossil fuels were burned and if
all the carbon dioxide stayed in the atmosphere, the increase relative to
pre-industrial concentrations would be a factor of 1.32 x 6.5 = 8.6.

Each carbon atom (of atomic weight 12) burned takes up two oxygen atoms (of
atomic weight 16 each).  Hence, burning the carbon reduces the mass of oxygen in
the atmosphere by 4200 Gt(C) times (16 + 16) Gt(O2) per 12 Gt(C) burned, or 4200
x (32/12) = 11,200 Gt

To put this oxygen loss into context, we need to compare it to the mass of O2
that's there to begin with, which is 23% of the entire mass of the atmosphere,
or 1.18 million Gt.

Hence the loss of oxygen amounts to roughly 11,200 out of an original 1,180,000 Gt,
or 1/100th of the amount currently in the atmosphere: hardly enough to notice.

To seriously deplete the oxygen level it would be be necessary to burn an
appreciable fraction of the organic carbon stored in sedimentary rocks, which is
larger than the carbon in the fossil fuel reservoir by a factor of more than 2000.
 

2) Estimate the rate of uptake of carbon between by the biosphere during spring
   and early summer and compare it with the rate at which humans are liberating
   carbon atoms by burning fossil fuels.

If we assume that data for Mauna Loa are representative of the atmosphere as a
whole, we can estimate the rate of uptake from Fig. 7.4. Inspection of this
figure reveals that each year the CO2 concentration drops by about 6 ppm from
early spring to midsummer. Noting that the mean concentration is roughly 370
ppm, which is equivalent to 760 Gt (C), we can express the biospheric uptake in
units of Gt(C) simply by multiplying 6 ppm by the ratio (760 Gt / 370 ppm),
which yields roughly 12Gt(C) over a 6-month period.

It is estimated that human activity consumes 6 Gt(C) per year (e.g., see
Fig. 13.1 of the text) : this rate is approximately 1/2 as large as the annual
uptake by the biosphere estimated in the previous paragraph and 1/4 as large if
we allow for the fact that the uptake by the biosphere occurs during a 6-month
period whereas human consumption of fossil fuels goes on all year.