The carbon cycle


The carbon cycle describes the exchange of carbon atoms between various reservoirs
within the earth system.  The carbon cycle is one of a number of geochemical
cycles and since it involves the biosphere it is sometimes referred to as a
bio-geochemical cycle.  Other biogeochemical cycles involve oxygen, nitrogen and sulfur.
 

Why study the carbon cycle?


- to learn why atmospheric carbon dioxide has tended to decrease over the lifetime
    of the earth and why it underwent large swings between glacial and interglacial
    periods of the ice ages
- to learn why atmospheric carbon dioxide is increasing only about half the rate
    that one would expect, given the current rate of burning of fossil fuels
- to predict future concentrations of atmospheric carbon dioxide
- to assess the potential of carbon 'sequestration' as a strategy for slowing the
    buildup of atmospheric carbon dioxide
 

Basic concepts


- reservoirs -- forms in which carbon resides within the earth system-- usually
    expressed in terms of the mass of carbon in gigatons (Gt), (billions of metric tons)
- transfer mechanisms -- processes that move carbon between reservoirs - they
    usually involve a physical process and a chemical reaction
- transfer rate -- usually expressed in terms of Gt per year
- residence time for carbon in a reservoir, estimated by dividing the amount of
    carbon in that reservoir by the transfer rates in and out of it.  For example,
    from Fig. 7-7, the residence time for atmospheric carbon dioxide is 760 Gt divided
    by 60 Gt per year or ~13 years.
 

There are three important carbon cycles in the earth system:

- the short term organic carbon cycle, with emphasis on the interactions between
    the atmosphere and the biosphere: it has terrestrial (land) and marine (ocean)
    components
- the long term organic carbon cycle, with emphasis on the formation and
    destruction of fossil fuels and other sediments containing organic carbon
- the inorganic carbon cycle with emphasis on calcium carbonate, by far the
    largest of the carbon reservoirs.
 

The short term organic carbon cycle


The photosynthesis reaction removes carbon atoms from the atmosphere and
incorporates them into the living tissue of green plants.  It requires energy
derived from radiation in the visible part of the electromagnetic spectrum.  The
chemical reaction, which students are responsible for learning, is on p. 133 of
the text.  The respiration (and decay) (p. 133) reaction undoes the work of
photosynthesis, thereby returning carbon atoms to the atmosphere. In contrast to
photosynthesis, respiration and decay involve a release of energy.

The terrestrial biosphere is much more massive than the marine biosphere, largely
because of the presence of trees. Soils also contain a large amount of organic
material.  The influence of the land biosphere is evident in Fig. 7-4.  Each year
during the Northern Hemisphere growing season (spring and summer) atmospheric
carbon dioxide concentrations decrease by ~5 parts per million as carbon is
incorporated into leafy plants.  From October through January, when photosynthesis
is largely confined to the tropics and the relatively small Southern Hemisphere
continents, the respiration and decay reaction dominates and atmospheric carbon
dioxide increases with time.

The marine biosphere operates like a 'biological pump'.  In the sunlit uppermost
100 meters of the ocean, photosynthesis serves as a source of oxygen and a sink
for carbon dioxide and nutrients like phosphorous.  Fecal pellets and dying marine
organisms decay as they settle into the deeper layers of the ocean. consuming
dissolved oxygen and giving off (dissolved) carbon dioxide.  Hence, these layers
have much higher carbon dioxide concentrations and lower oxygen concentrations
than the waters just below the surface as shown on p. 137.  The biological pump
determines the carbon dioxide concentration of the water that is exposed to the
atmosphere.

The marine biosphere is active only in those limited regions of the ocean where
upwelling is bringing up nutrients from below.  Once nutrients reach the sunlit
upper layer of the ocean they are used up in a matter of days by explosive
plankton blooms.
 

The long term organic carbon cycle


Only a tiny fraction of the organic material that is generated by photosynthesis
each year escapes the decay process by being buried and ultimately incorporated
into fossil fuel deposits or sediments containing more dilute fragments of organic
material.  Through this slow process, carbon from both terrestrial and marine
biosphere reservoirs enters into the long term organic carbon cycle.  The rate is
so slow as to be virtually unmeasurable.  Weathering of these same sentiments
releases carbon back into the other reservoirs.  Human society is burning fossil
fuels at a rate many orders of magnitude faster than they were created.

The fossil fuel reservoir in Fig. 7.3 is 4200/760 = 5.5 larger than the
atmospheric reservoir, so if it were all added to the atmospheric reservoir (by
the burning of fossil fuels) without any of it being taken up by the other
reservoirs, the atmospheric concentration of carbon dioxide would increase by a
factor of 6.5.  This, of course, is an upper limit and not an actual prediction.
 

The inorganic carbon cycle


Over the lifetime of the earth, roughly 75% of the carbon injected into the
atmosphere by volcanoes has found its way into deposits of calcium carbonate
(limestone) deposits which constitute by far the largest reservoir in the carbon
cycle.  Limestone is formed from bicarbonate (HCO-) ions dissolved in the ocean
and it is destroyed by 'chemical weathering' as described in the text.  It tends
to accumulate on the beds of shallow seas where the acidity of sea water is
reduced by the biological pump' described on p. xx of the text. (On the floor of
the deep ocean, where the acidity is higher, shells and skeletons dissolve as fast
as they precipitate.)  Limestone formation involves a series of chemical reactions
that has the net effect of removing carbon dioxide from the atmosphere.
Weathering of limestone deposits by rain tends to return carbon atoms to the short
term reservoirs, thereby replenishing the concentration of atmospheric carbon
dioxide.  The inorganic carbon cycle is lined to the carbonate-silicate cycle,
which controls availability of the calcium ions that are required to form limestone.

The carbonate-silicate cycleAn important family of rocks in the earth's crust is
made up of molecules in which calcium occurs in combination with silicon.  When
these calcium silicate rocks weather, the silicon atoms in them combine with
oxygen to form quartz-like (silicon dioxide) minerals and the calcium ions become
available to form limestone.  As noted in the previous section, the formation of
limestone deposits has the net effect of removing carbon atoms from the other
reservoirs in the earth system (including the atmosphere).

However, limestone deposits don't last forever.  Eventually they get subducted
(drawn down) deep into the earth's crust where temperatures are high enough to
cause calcium carbonate to undergo a metamorphosis (a change in form) into
calcium-silicate rock.  For each calcium carbonate molecule that that gets
transformed a carbon dioxide molecule is released.  These carbon dioxide molecules
eventually they find their way back to the earth's surface in the emissions from
volcanic eruptions or hydrothermal vents.  The slow motion of the earth's crust
that occurs in association with plate tectonics is responsible for both the
subduction of the limestone layers and the volcanic activity that releases the
carbon dioxide to the atmosphere. Subduction is currently occurring the
mid-Pacific, while new crust is emerging from the sea floor and spreading apart
along a seam in the mid Atlantic.  The calcium-silicate rocks in the emerging
crust will eventually be lifted onto land where it will be subject to weathering,
thus completing the cycle.

The processes involved in the carbonate-silicate cycle are pictured in cartoon
form in Fig. 7-17 of the text.  If the various processes in this cycle were all
proceeding at the same rate, there would be no change in the amount of carbon
stored in the various reservoirs. However if something happens that makes one of
the reactions proceed at a faster rate than the reverse reaction, then the
storages can change.

It is known that weathering of rocks proceeds faster in a warmer climate because
rainfall amounts tend to be greater.  By providing calcium ions, weathering
promotes limestone formation and removal of carbon dioxide from the atmosphere.
Hence, a perturbation of the earth's climate toward the warm side would favor
decreasing atmospheric carbon dioxide concentrations, which would tend to return
the climate to its original state.  In this way, the the carbonate-silicate cycle
serves as a negative feedback on the temperature of the earth system.
 

Chapter Summary


We considered four different geochemical cycles, three of them involving carbon
and the fourth involving the cycling of calcium atoms between carbonate and
silicate rocks.  The four cycles can be summarized schematically as

the short term organic carbon cycle
    atmosphere <------->  biosphere

the long term organic carbon cycle
    short term carbon reservoirs <------->  fossil fuels, oil shales...

the long term inorganic carbon cycle
    short term carbon reservoirs <------->  limestone deposits

the carbonate - silicate cycle (of calcium atoms)
    limestone deposits <------->  calcium silicate rocks

In the the short term organic carbon cycle, the transfer rates are large but the
biospheric reservoir is relatively small, whereas in the long term cycles the
reverse is true.
 

Review Questions

1) Name and compare the sizes of the major reservoirs of carbon in the earth
    system.
2) Describe the biological pump.
3) Describe the role of plate tectonics in the the carbonate - silicate cycle.
4) How are the the long term inorganic carbon cycle and the carbonate - silicate
    cycles linked?
5) In what way does the carbonate-silicate cycle serve stabilize the temperature
    of the climate system?
6) Weathering of silicate rocks removes carbon dioxide from the atmosphere,
    whereas the weathering of carbonate rocks does not. Explain.

Critical Thinking Questions

1) Are any of the issues raised in this chapter of concern in terms of the future
    health of the world's coral reefs?
2)  Text Question #1
3)  Text Question #2 a, d