The Carbon Cycle


The carbon cycle is modelled as exchanges of carbon between various reservoirs.  The short term carbon cycle that we hear about on the news involves exchanges between the atmosphere, oceans, soils, and organisms; we expect changes to occur over timescales of human generations.  These reservoirs may be collectively referred to as the surficial reservoir.
Weathering is shown in red; the dotted red line refers to abiotic weathering while plant weathering is shown as the solid double arrow between the "terrestrial organisms" and  "regolith" (soil) reservoirs. Weathering products and carbon are dissolved in water and end up at sea. Amounts of carbon in each reservoir are shown; the units are examols (1018 moles). The long-term carbon cycle involves two additional reservoirs.  Oil and coal are found in the organic reservoir, while limestones and dolomites form the bulk of the carbonate reservoir.  My project concerns exchanges between the surficial and carbonate reservoirs, as controlled by weathering. Note that these two reservoirs contain much more carbon than the surficial reservoir. In fact, the value given for the carbonate reservoir is probably an underestimate.  Values are from Berner (2004).

reservoirs
 

An easy way to see how silicates fit into the carbon cycle is to follow the carbon around as it is exchanged along the red lines in the surficial reservoir, and then between the surficial and carbonate reservoirs.
  1. We start with carbon in the atmosphere. This is going to enter the soil dissolved in rainwater, where it may have an abiotic weathering effect, or it will be fixed by plants, driving a variety of weathering processes.
  2. Some rock types (like limestone) are already in the carbonate reservoir. Silicates are NOT part of the carbonate reservoir. They contain no carbon, but any calcium they contain can be combined with CO2 from the surficial reservoir to make limestone. (Magnesium leads to the same result by a more complicated process.)
  3. The rocks are weathered, and the weathering products are transported out to sea. The carbon is found in bicarbonate ions; two per calcium ion.
  4. At sea, our two bicarbonate ions will react with our calcium to form a new carbonate molecule containing ONE carbon.  The other carbon will be freed; it remains in the surficial reservoir.  So: no net change in surficial carbon if we remove carbon from the carbonate reservoir and then put it back.  But if the rock contained no carbon, we have a net LOSS of one carbon from the surficial reservoir.
  5. The new carbonate rocks may remain in the carbonate reservoir for millions of years before they are either uplifted and weathered, or subjected to metamorphosis.   In either case, they will be destroyed and return their one carbon to the surficial reservoir.

following carbon around the carbon cycle
   


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