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Notes for the lectures on Tuesday and Wednesday Oct. 29 and 30
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 at 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' (planting new trees) 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 long term inorganic carbon cycle with emphasis on calcium carbonate (CaCO3, limestone), by far the largest of the carbon reservoirs.  This cycle is linked to the carbonate-silicate cycle, supplying the calcium ions necessary for the formation of limestone. 
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. 

All three of these cycles are linked together as part of the global carbon cycle, but we examine them separately because they control atmospheric levels of CO2 on different timescales ranging from months (short term organic carbon cycle), to tens of millions of years (long term inorganic carbon cycle).  Dividing the carbon cycle in these individual pieces help us represent this system in simple terms.

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:

CO2 + H2O --> CH2O + O2

The respiration (and decay) (p. 133) reaction undoes the work of photosynthesis, thereby returning carbon atoms to the atmosphere:

CH2O + O2 --> CO2 + H2O

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.

Figure 1.  Diagram of the organic carbon cycle.

The long term 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 (CaCO3) is formed from bicarbonate (HCO3-) 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. 135-138 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 linked to the carbonate-silicate cycle, which controls availability of the calcium ions that are required to form limestone. 

The carbonate-silicate cycle
An 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, SiO2) 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 (CaSiO3).  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 carbonate-silicate cycle serves as a negative feedback on the temperature of the earth system.

Figure 2. Diagram of the long-term inorganic carbon cycle.
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. 


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