7. The carbon cycle
7.1. Box model of the carbon cycle
Without the greenhouse effect, our planet would experience a permanent ice age and life as weknow it would not be possible. The main contributors to the greenhouse effect are the clouds andgreenhouse gases. The three main greenhouse gases are water vapor, carbon dioxide (CO2), andmethane (CH4). Human activities, particularly the burning of fossil fuels (oil, coal, natural gases),
have led to a significant increase of CO2 in the atmosphere which is warming the planet faster thanwould be expected without the addition of this anthropogenic CO2. To understand the climate andpredict its evolution in the future, it is essential to understand the natural mechanisms controllingthe concentrations of CO2 and CH4 in the atmosphere. We should know where the carbon comesfrom and how it is removed from the atmosphere. We should also be able to quantify how muchcarbon flows in (input flux) and how much flows out (output flux), and how much time on average
it stays in the atmosphere (residence time). The atmosphere is one reservoir of carbon but weneed to identify the other reservoirs and describe how carbon moves from one to another. An
obvious reservoir of carbon is the biosphere. Carbon atoms are essential for life because they form
the backbones of large organic molecules. Other important reservoirs of carbon are the oceans,carbonate rocks (mainly those composed of calcium carbonate, i.e. CaCO3), and fossil fuels (coal,oil, natural gas). In any of these reservoirs, the amount of carbon (reservoir size) has fluctuatedover geological times. When there is a natural balance between what enters and leaves eachreservoir, the system is said to be in a steady state. It means that for any given reservoir the input
flux is equal to the output flux (often expressed in gigaton per year or Gt/yr) and the reservoir size
remains constant. In this condition, the residence time is the reservoir size divided by the input flux
(or the output flux). Hence, the carbon cycle can be represented in a simplified manner by various
reservoirs which exchange certain amounts of carbon at certain rates. This representation of the
carbon cycle is called a box model. The carbon cycle is one example of biogeochemical cycles
that can be represented using a box model. Other examples are the water cycle, the nitrogen cycle,
the sulfur cycle... Box models are particularly useful for modeling biogeochemical cycles using
computer programs.

Figure 65: Reservoir in a steady state condition where the input flux is equal to the output flux and the reservoir
size remains constant.

7.2. The natural carbon cycle
A simplified box model of the natural carbon cycle (without the influence of human activities) isshown in figure 66.

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Figure 66: Simplified box model of the natural carbon cycle. The size of each reservoir is expressed in gigaton (Gt).
Input and output fluxes are expressed in gigaton/year (Gt/yr). DOC means dissolved organic carbon andrepresents organic molecules present marine (oceans) and freshwater (rivers and lakes) systems. Source: partly
derived from Biogeochemistry: An Analysis of Global Change (2013).

If we examine the figure above and examine the fluxes of carbon from one reservoir to another,several interesting observations can be made:-Most carbon is stored in the reservoir “carbonate rocks”.

-

The largest fluxes of carbon are between the land surface (plants and soils) and theatmosphere and between the atmosphere and the ocean (“fast domain”).

-

Soils store three times as much carbon as plants.

-

The smallest fluxes are related to calcium carbonate deposition in the ocean, calcium
carbonate dissolution and weathering of silicate rocks, volcanism, and carbon fluxes in and outof the fossil fuel reservoir (“slow domain”).

-

The residence time of carbon in the atmosphere is very short (3.5 years) whereas in carbonate
rocks the residence time is very long (6x108 years).

7.2.1. Land surface (plants, soils) ↔ atmosphere
The greatest flux of carbon is the exchange of carbon between the terrestrial biosphere and theatmosphere. This involves primarily plant photosynthesis (I), cellular respiration (II), and thedecomposition of organic matter by microbes (III). During photosynthesis, plants take CO2 fromthe atmosphere to build organic molecules using sunlight as a source of energy. Part of this CO2
returns in the atmosphere during cellular respiration, a reaction producing energy. When plants die,organic matter is incorporated in soils where it is decomposed by microbes. This process releases
CO2 in the atmosphere and is one component of soil respiration. Another important component ofsoil respiration is cellular respiration of plant roots which also releases CO2 in the atmosphere.

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7.2.2. Atmosphere ↔ ocean
Understanding how the oceans absorb the atmospheric CO2 is essential to predict the future ofEarth’s climate. The local flux of CO2 between the atmosphere and the ocean depends on various
factors including seawater composition, seawater and air temperature, and winds. Gases are lesssoluble at higher temperature (a soda drink becomes flat faster at room temperature than in thefridge). Increasing temperature reduces CO2 solubility in the ocean. Stronger winds increase theexchange of CO2 between the atmosphere and the ocean because of the formation of air bubbles
in breaking waves and the production of sea spray (droplets of seawater blown from waves).
Bubbles and sea spray increase the contact surface between the air and seawater and therefore
enhance air-sea gas exchange.

7.2.3. Oceans ↔ marine life ↔ carbonate rocks
Marine photosynthetic organisms use the dissolved CO2 in the ocean as a source of carbon they
incorporate in their tissue. Marine organisms also perform cellular respiration which release CO2 inthe environment. Some of the organic carbon is recycled in the surface ocean and some istransferred to the deep ocean (“marine snow”). Much like on land organic matter is decomposed by
microbes which releases CO2 in the environment. In addition, many marine organisms are able toprecipitate CaCO3 (calcium carbonate) to build a hard shell or skeleton (Fig. 67). Hard parts ofmarine organisms made of CaCO3 accumulate on the sea floor and enter the carbonate reservoir(but see slides for a definition of lysocline and carbonate compensation depth). The process ofcalcification releases CO2 in the environment as shown by the following reaction:
Ca2+ + 2HCO3- → CaCO3 + H2O + CO2
The rate at which carbon enters the carbonate reservoir is very low (0.2Gt/yr). However, thecarbon stored in this reservoir may remain there for millions of years. In addition, the organic
matter that escaped decomposition can be buried and stored in the lithosphere for a very longtime.

Figure 67: Examples of marine organisms producing a shell or skeleton made of calcium carbonate. (A) Livebenthic foraminifera Calcarina (source: Catalogue of organisms, http://coo.fieldofscience.com), (B) sediment
composed of Calcarina shells (source: Wikipedia), (C) live calcareous green algae Halimeda (source:
www.algaebase.org), (D) sediment composed of Halimeda segments, (E) coral reef built primarily by corals andcoralline algae (Great Barrier Reef of Australia, source: AIMS), (F) encrusting coralline algae (source: Encyclopediaof Earth, www.eoearth.org), and (G) detail of a coral skeleton.

417.2.4. Lithosphere ↔ atmosphere / ocean
The lithosphere is the largest reservoir of carbon. The carbon present in the lithosphere occurs
primarily in carbonate rocks, organic matter, and carbon dioxide. Some important processes
behind the exchange of carbon between the lithosphere and the atmosphere (or the ocean) are
explained below.
a. Dissolution / precipitation of calcium carbonate
Exchange of carbon between the atmosphere and the lithosphere occurs during dissolution andprecipitation of CaCO3. Rainwater and CO2 combine in soils to form the weak carbonic acid H2CO3.
In contact with carbonate rocks, the carbonic acid dissolves CaCO3. The products of this reaction,
Ca2+, HCO3- and CO32-, are transported by rivers to the ocean. In the ocean, calcifying marineorganisms use these ions to build their hard parts (shells, skeletons…). The process of dissolutionof calcium carbonate removes CO2 from the atmosphere. However, the same amount of CO2 isreleased during calcification. Therefore, the overall process of dissolution-calcification does not
result in a net loss (nor gain) of CO2 in the atmosphere as shown by the following simplified
reactions (from Ruddiman, 2001):

b. Weathering of silicate rocks
An important process controlling the long-term concentration of atmospheric CO2 (over millions ofyears) is the weathering of silicate rocks. Silicate minerals are the most abundant minerals in the
Earth’s crust (see section 2, page 3). Here we are concerned more specifically with Ca-bearing
silicate minerals. Like carbonate rocks, Ca-bearing silicate minerals react with acidic groundwater.
The products of this weathering reaction include dissolved ions (e.g. Ca2+, Si4+, Fe2+, HCO3-, CO32-,
H+) and clays. These ions are transported by rivers to the ocean and some can be used by marine42

organisms to build hard parts made of CaCO3 and SiO2. We can use the simple formula CaSiO3 torepresent Ca-bearing silicate minerals and the overall process of silicate weathering combinedwith calcification and silification in the ocean can be summarized by the following reactions (from
Ruddiman, 2001):

Most importantly, the above reactions show that the weathering of silicate rocks results in a net
removal of CO2 from the atmosphere! This slow removal of CO2 must be balanced by an equallyslow input of CO2. This input of CO2 is provided by volcanoes and hot springs.
c. Volcanism
The CO2 from Earth’s interior is released by volcanoes and hot springs. The rate of CO2 input by
volcanic activity is variable and depends on the frequency and intensity of volcanic eruptions. The
geological record shows evidence of past volcanic eruptions which were far more intense andlasted far longer than those recorded in human history. Such events can release a huge amount of
CO2 in the atmosphere and potentially alter the Earth’s climate for an extensive period of time. An
example of exceptionally intense period of volcanism is documented in the Siberian Traps, a
succession of huge basaltic lava flows covering millions of km 2 and related to an eruptive event
spanning one million year that happened 250-251 million years ago. This event may have been
one of the factors which triggered the largest mass extinction in life history, i.e. the end-Permianmass extinction.



Silica, i.e. SiO2, is with CaCO3 a major component of mineral hard parts produced by marine organisms. Examples of organisms
producing hard parts made of SiO2 are diatoms (phytoplankton), radiolarians (zooplankton) and siliceous sponges.

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d. Fossil fuels
Another reservoir of carbon which is part of the lithosphere is the organic matter preserved in rockstrata. A common sedimentary rock composed of the accumulation of organic matter is coal. Other
products of the accumulation of organic matter are oil and natural gas (mostly methane). In thenatural carbon cycle (without the influence of human activities), the transfer of carbon from thereservoir of fossil organic carbon to the atmosphere is extremely slow. The organic carbon present
in the lithosphere can return into the atmosphere via volcanic activity, natural fire and oxidation(biological or not).
In conclusion, unlike the exchange of carbon between the biosphere and the atmosphere, thenatural rate at which carbon is exchanged between the lithosphere and the atmosphere is
extremely slow. The two main processes controlling the flux of carbon between the lithosphere andthe atmosphere are silicate weathering and volcanism. These very slow exchanges affect theconcentration of atmospheric CO2 (hence climate!) over the very long term (millions of years) andare controlled by plate tectonics (see slides and text of chapter 10).

7.3. Anthropogenic perturbations, global warming, and ocean acidification
In May 2013, the level of atmospheric CO2 has reached 400 ppm (parts per million) for the firsttime since accurate measurements began in 1958 (Fig. 68). The rapid increase in atmospheric
CO2 is related to human activities, primarily the combustion of fossil fuels. Figure 69 shows theimpact of human activities on the carbon cycle. Besides fossil fuel burning, other human-induced
sources of atmospheric CO2 are related to deforestation and cement production. The figure also
indicates that natural processes removing CO2 from the atmosphere (plant growth and ocean
uptake) do not counterbalanced human emissions, leading to a net increase in atmospheric CO2(3.2 ± 0.1 Gt/yr for the 1990s and 4.1 ± 0.1 Gt/yr for 2000-2005, source: IPCC 2007 report).
We know with a high level of certainty that human emissions of CO2 contributes in part to thepresent-day global warming. What we don’t know for sure is exactly how much warming is causedby anthropogenic CO2. As we will see in the next chapter, the Earth has experienced many
cold/glacial and warm/interglacial periods over the last 2.5 million years. The last glacial period
reached its coldest peak 20,000 years ago when the extent of ice sheets was maximum. Since thislast ice age, the Earth is warming again and ice sheets are retreating. Warming caused by humans
is accelerating this natural trend.

Figure 68: Evolution of the concentration of atmospheric CO2 for the past 55 years. Source: Scripps Institution of
Oceanography, CO2 Program, Mauna Loa Observatory.

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Figure 69: Impact of human activities on the carbon cycle. Carbon fluxes related to human activities are partly
derived from Biogeochemistry: An Analysis of Global Change (2013).

We will discuss more about the impact of human activities on climate later in this course. Now let’s
briefly examine another consequence of the increase of atmospheric CO2: ocean acidification.
In the ocean, CO2 reacts with H2O to form carbonic acid (H2CO3). Most of the carbonic aciddissociates to hydrogen ions (H+) and bicarbonate ions (HCO3-). Some of the bicarbonate ionsdissociate to hydrogen ions and carbonate ions (CO32-). Adding CO2 in seawater increases theconcentration of hydrogen ions and therefore decreases the pH. The chemical reactions involved
can be expressed as follows:
CO2 + H2O ↔ H2CO3
H2CO3 ↔ H+ + HCO3HCO3- ↔ H+ + CO32Since these reactions involves the concentration of bicarbonate and carbonate ions in the ocean, it
is clear that increasing the amount of CO2 absorbed by the ocean has an impact on biologicalcalcification (e.g. corals and coral reef ecosystems). One way to look at the problem is to consider
that organisms combine calcium ions (Ca2+) and carbonate ions (CO32-) to produce their shell or
skeleton made of calcium carbonate (CaCO3). What would happen to the concentration ofcarbonate ions if more CO2 was added in the ocean? As more CO2 is added, more hydrogen ionsis produced and pH would decrease. Some of these hydrogen ions would tend to bond withcarbonate ions to form bicarbonate. Therefore, it would also lead to a decrease in theconcentration of carbonate ions and would potentially make calcification more difficult (sincecarbonate ions are needed for calcification!). The relationship between pH and the concentration ofdissolved CO2, HCO3- and CO32- is shown in figure 70. It clearly shows the decrease inconcentration of CO32- with decreasing pH. Several experiments have shown that adding CO2 tothe ocean would be detrimental to calcifying organisms. However, biological calcification is acomplex process which is far from being well understood. More work is needed to predict the
impact of ocean acidification on calcifying organisms and related ecosystems.

Figure 70: Concentrations of the dissolved CO2, bicarbonate and carbonate ions as a function of pH. Source:
Ridgwell and Zeebe (2005).

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