Box and arrow diagrams are often used to show the flux of carbon between and sto

Question

Box and arrow diagrams are often used to show the flux of carbon between and storage in reservoirs of the carbon cycle and may be used to consider a carbon ‘budget’.  We can also consider the residence time (turnover) of carbon in reservoirs, which is calculated using the following formula:

                        Residence time = Amount of carbon in reservoir/rate at which element is added (or removed)

In a system that is in a steady state material added = material removed so you only need to consider one or the other

Units used to consider carbon fluxes and residence times of carbon are Gt C/yr where:

Gt is a gigaton

The giga prefix is 109

One ton is a metric ton (1,000 kg).

Figure two: simplified carbon cycle showing fluxes and reservoirs

Ketter and Goldstien - Introductory Modeling: The global carbon cycle and clmate change. http://www.ableweb.org/volumes/vol-24/mini.9.teareketter.pdf

Using Fig. 2 calculate the residence time and complete the table below (6 points)

Reservoir

Amount in reservoir (Gt C)

Rate of flux into (or out of) reservoir (Gt C/yr)

Residence time (yrs)

Rank (1 = longest)

Atmosphere

Vegetation/soil/detritus

Surface ocean

Marine biota

Dissolved organic C

(DOC)

Intermediate/deep ocean

Using the residence times you calculated and what you have discovered about the carbon cycle answer the following questions:

1. Which global carbon reservoir has the longest residence time?

2. Suggest why the residence is so long for this reservoir:

HINT: think of what happens to carbon here

3. Given what you already know about soil, explain why the residence time for the vegetation/soil/detritus reservoir is so long. How would deforestation affect the carbon storage of this reservoir and the fluxes into and out of it?

4. How would residence time for coal/oil/gas compare to those you calculated for the carbon cycle reservoirs considered above? Explain your reasoning.

5. The flux for fossil fuels and cement production is only shown as outgoing suggesting the reservoir is not in a steady state. Why is the reservoir not in a steady state? Why is there no input flux in this box and arrow diagram?

Reservoir

Amount in reservoir (Gt C)

Rate of flux into (or out of) reservoir (Gt C/yr)

Residence time (yrs)

Rank (1 = longest)

Atmosphere

Vegetation/soil/detritus

Surface ocean

Marine biota

Dissolved organic C

(DOC)

Intermediate/deep ocean

Atmosphere 750 5.5 Fossil fuels and cement production Vegetation 610 Soils and detritus 1580 2190 92 Surface Ocean 1020 50 ?4. Marine biota DOC

Explanation / Answer

First of all calculate the amount in reservoirs by adding the amount of carbon in the given reservoirs. Then calculate the rate of flux adding the amounts added to the reservoirs and subtracting the amount of carbon lost from them. Then divide the amount in reservoir by rate of flux to get the residence time.

Amount in reservoir(GtC)

1. Marine biota and dissolved organic carbon are the reservoirs having the longest residence times.

2. Their longest residence time is because there is no net addition or removal of carbon. Carbon will be present in the same quantity.

3. The long residence time of carbon in soil and deteritus pool is because carbon is constantly being added from the tree leaves and remains of animals and is stored inside the soil by the action of microbes which degrage these remains and store the carbon as soil carbon.

Deforestation will cause increase in temperature of the soil and reduction in carbon input to the soil. Further aerobic microbe will start oxidising the soil carbon and it will be lost to the atmosphere as carbon dioxide.

4. Residence time for coal/gas will also be infinite iff they are not extracted out of the earth crust, because they are trapped inside the crust. And there is no addition or removal of carbon from them.

Reservoir

Amount in reservoir(GtC)

Rate of flux(GtC/yr) Residence time(yr) Rank Atmosphere 750 3.7 202.702 6 Vegetation/soil/deteritus 4380 -0.2 21900 4 Surface ocean 1020 0.4 2550 5 Marine biota 3 0 infinite 2 Dissolved organic carbon 700 0 infinite 1 Intermediate/deep ocean 38250 1.6 23906.25 3

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