The Carbon Cycle

Most of the exchanges between reservoirs in the hydrologic cycle considered in the previous section involve phase changes and transports of a single chemical species, H2O. In contrast, the cycling of carbon involves chemical transformations. The carbon cycle is of interest from the point of view of climate because it regulates the concentrations of two of the atmosphere's two most important greenhouse gases: carbon dioxide (CO2) and methane (CH4).

The important carbon reservoirs in the Earth system are listed in Table 2.3 together with their masses and the residence times, in the same units as in Table 2.2. The atmospheric CO2 reservoir is intermediate in size between the active biospheric reservoir (green plants, plankton, and the entire food web) and the gigantic reservoirs in the Earth's crust. The exchange rates into and out of the small reservoirs are many orders of magnitude faster than those that involve the large reservoirs. The carbon reservoirs in the Earth's crust have residence times many orders of magnitude longer than the atmospheric reservoirs, reflecting not only their larger sizes, but also the much slower rates at which they exchange carbon with the other components of the Earth system. Figure 2.23 provides an overview of the cycling of carbon between the various carbon reservoirs.

Table 2.3 Major carbon reservoirs in the Earth system and their present capacities in units of kg m-2 averaged over the Earth's surface and their residence times"

Reservoir

Capacity

Residence time

Atmospheric CO2

1.6

10 years

Atmospheric CH4

0.02

9 years

Green part of the biosphere

0.2

Days to seasons

Tree trunks and roots

1.2

Up to centuries

Soils and sediments

3

Decades to millennia

Fossil fuels

10

-

Organic C in sedimentary rocks

20,000

2 X 108 years

Ocean: dissolved CO2

1.5

Ocean CO3

2.5

6,500 years

Ocean HCO—

70

200,000 years

Inorganic C in sedimentary rocks

80,000

108 years

a Capacities based on data in Fig. 8.3 (p. 150) of Kump, Lee R.; Kasting, James F.; Crane, Robert G., The Earth System, 2nd Edition, © 2004. Adapted by permission of Pearson Education, Inc., Upper Saddle River, NJ.

a Capacities based on data in Fig. 8.3 (p. 150) of Kump, Lee R.; Kasting, James F.; Crane, Robert G., The Earth System, 2nd Edition, © 2004. Adapted by permission of Pearson Education, Inc., Upper Saddle River, NJ.

Exercise 2.3 Carbon inventories are often expressed in terms of gigatons of carbon (Gt C), where the prefix giga indicates 109 and t indicates a metric ton or 103 kg. (Gt is equivalent to Pg in cgs units, where the prefix peta denotes 1015.) What is the conversion factor between these units and the units used in Table 2.3?

Fig. 2.23 Processes responsible for the cycling of carbon between the various reservoirs in the Earth system.

Year

42 The Earth System

Solution: The surface area of the Earth is 4-nR|, where RE = 6.37 X 106 m is the mean radius of the Earth, or 5.10 X 1014 m2. To convert mass per unit area in units of kg m-2 averaged over the surface of the Earth to Gt C or Pg C, we multiply by 5.10 X 1014 m2 to obtain the mass in kg and divide by 1012 to get the mass in units of Gt (or Pg). Hence, the conversion factor is 510 Gt (or Pg) C per kg m-2. ■

where the i subscripts refer to the major constituents of air, ma is the mass of the atmosphere per m2, and MC is the molecular weight of the carbon atom. In carrying out this calculation, we find that it is sufficient to take into account the three major constituents: N2, O2, and A. Substituting data from Table 1.1 into this expression (taking care to use the molecular weight of just the carbon atom rather than that of the CO2 molecule), we obtain

2.3.1 Carbon in the Atmosphere

Most of the carbon in the atmospheric reservoir is in the form of CO2. Because of its chemical inertness, CO2 is relatively well mixed within the atmosphere: away from forest canopies and other sites in close contact with vegetation, CO2 concentrations vary by only —1% over the surface of the Earth (e.g., compare the concentrations at Mauna Loa and the South Pole in Fig. 1.3).

Methane (CH4) is present only in trace concentrations in the Earth's atmosphere, but it contributes to the greenhouse effect and is chemically active. It enters the atmosphere mainly through the escape of natural gas in mining operations and pipelines and through the anaerobic breakdown of organic matter, much of which is also human induced through activities such as the production of rice and livestock.13 Methane has a —9-year residence time in the atmosphere: it is removed by the oxidation reaction m (CO2)

and by the oxidation reactions described in Section 5.3.14

ma X

a' ' [(0.7808 X 28.016) + (0.2095 X 32.00) + (0.0093 X 39.94)]

which yields m (CO2)

ma X

ma X

28.94

Substituting ma = 1.004 X 104 kg m-2 from Exercise 1.1, we obtain m (CO2) = 1.58 kg m-2, in agreement with Table 2.3. Multiplying by the area of the Earth's surface (5.10 X 1014 m2), we obtain an atmospheric mass of 8.06 X 1014 kg, or 806 GtC. ■

2.3.2 Carbon in the Biosphere

On short timescales, large quantities of carbon pass back and forth between the atmosphere and the biosphere. These exchanges involve the photosynthesis reaction:

Exercise 2.4 Reconcile the present atmospheric CO2 concentration of —380ppmv with the mass concentration of elemental carbon in CO2 given in Table 2.3.

Solution: Making use of (1.7) with the volume concentration for the ith constituent ci = ni/n, where n is the total number of molecules, we can write m (CO2) = ma X

which removes carbon from the atmosphere and stores it in organic molecules in phytoplankton and leafy plants, and the respiration and decay reaction:

which oxidizes organic matter and returns the CO2 to the atmosphere. Photosynthesis involves the absorption of energy in the form of visible light at

13 Ruminants, such as cows, release (burp) methane as they digest the cellulose in grass.

14 The oxidation of methane is an important source of stratospheric water vapor.

wavelengths near 0.43 (blue) and 0.66 pm (orange), and the respiration and decay reaction releases an equivalent amount of energy in the form of heat. By comparing the intensity of reflected radiation at various wavelengths in the visible part of the spectrum, it is possible to estimate the rate of photosynthesis in (2.5) by phytoplankton and land plants, which is referred to as net primary productivity (NPP).

Figure 2.24 shows the global distribution of net primary productivity for June 2002. Enhanced marine productivity is clearly evident in the bands of equatorial and coastal upwelling, but NPP is generally higher over land areas with growing vegetation than anywhere over the oceans. Rates are particularly high in the boreal forests. The "greening" of the large northern hemisphere continents in spring and summer draws a substantial amount of CO2 out of the atmosphere and stores it in plant biomass, which is subject to decay at a more uniform rate throughout the year. These exchanges are responsible for the pronounced annual cycle in the Mauna Loa CO2 time series seen in Fig. 1.3. The annual cycle is even more pronounced at high-latitude northern hemisphere stations. In contrast, CO2 concentrations at the South Pole exhibit a much weaker annual cycle (Fig. 1.3).

The rate of exchange of carbon between the atmosphere and the biosphere is estimated to be —0.1-0.2 kg C m-2 year-1. Hence, the time that a typical molecule of CO2 resides in the atmosphere is 1.6 kg C m-2 (from Table 2.3) divided by —0.15 kg C m-2 year-1,

Net Primary Productivity (kgC/m2/year)

Fig. 2.24 Rate of carbon uptake by photosynthesis, commonly referred to as net primary productivity, averaged over June 2002, in units of kg m-2 year-1. Values are low over high latitudes of the southern hemisphere because of the lack of sunlight. [Based on NASA Sea WiFS imagery.]

or —10 years. The green part of the biosphere responsible for this large exchange rate is capable of storing only —10% of the atmospheric carbon at any given time. Hence, if a large quantity of CO2 were injected into the atmosphere instantaneously, the concentration would remain elevated for a time interval much longer than 10 years. The relaxation time would be determined by the rate of exchange of carbon between the atmosphere and the larger reservoirs in the Earth system listed in Table 2.3. The timescale for the growth of tree trunks and root systems is on the order of decades, and the corresponding timescale for the burial of organic matter is much longer than that, because only —0.1% of the plant biomass that is photosynthesized each year is eventually buried and incorporated into sedimentary rocks within the Earth's crust (the organic carbon reservoir in Table 2.3). Most of the organic carbon generated by photosynthesis undergoes oxidation by (2.6) when plants decay, when soils weather, or when forests and peat deposits burn. In anoxic (i.e., oxygen deficient) environments, the carbon in decaying organic matter is returned to the atmosphere in the form of methane.

The marine biosphere absorbs dissolved CO2 within the euphotic zone and releases it throughout the deeper layer of the ocean in which plants, animals, and detritus decay as they sink toward the ocean floor. The sinking of organic matter has the effect of transporting CO2 downward, reducing its concentration in the topmost few tens of meters of the ocean. Were it not for the action of this gravity-driven biological pump, atmospheric CO2 concentrations, which are in equilibrium with concentrations in water at the ocean surface, would be —1000 ppmv, roughly 2.6 times greater than observed, and the acidity of the water in the euphotic zone would be high enough to quickly dissolve the world's coral reefs.

Within anoxic regions of the oceans [i.e., the regions in which the ventilation of dissolved O2 into the waters below the euphotic zone is insufficient to keep pace with the rate of oxygen consumption in the decay reaction (2.6)], the organic debris that settles out of the euphotic zone reaches the ocean floor and forms layers of sediment, some of which are eventually incorporated into the organic carbon reservoir in the Earth's crust. Cores from sediments containing organic carbon are among the principal sources of proxy data on the climate of the past few million years. Shells and skeletons of sea animals

44 The Earth System that settle to the ocean floor are converted into limestone (CaCO3) rocks. This inorganic carbon reservoir of the Earth's crust is the largest of the carbon reservoirs in the Earth system.

2.3.3 Carbon in the Oceans

The carbon in the oceanic reservoir exists in three forms: (1) dissolved CO2 or H2CO3, also known as carbonic acid, (2) carbonate (CO3 ) ions paired with Ca2+ and Mg2+ and other metallic cations, and (3) bicarbonate (HCO3- ) ions. The third form is by far the largest of the oceanic carbon reservoirs (Table 2.3). The dissolved carbon dioxide concentration equilibrates with atmospheric concentrations through the reaction

An increase in the atmospheric concentration of CO2 thus tends to raise the equilibrium concentration of dissolved CO2. Carbonic acid, in turn, dissociates to form bicarbonate ions and hydrogen ions

H2CO3

HCO-

thereby causing the water to become more acidic. The increasing concentration of H+ ions shifts the equilibrium between carbonate and bicarbonate ions

HCO-

toward the left. The net effect, obtained by adding (2.7), (2.8), and the reverse of (2.9) is

which incorporates the added carbon into the bicarbonate reservoir without any net increase in the acidity of the ocean. The ability of the ocean to take up and buffer CO2 in this manner is limited by the availability of ions in the carbonate reservoir.

Marine organisms incorporate bicarbonate ions into their shells and skeletons through the reaction

A fraction of the calcium carbonate created in Eq. (2.11) settles on the sea floor and forms limestone deposits, while the remainder dissolves through the reverse reaction.

2HCO

Limestone deposits tend to be concentrated in continental shelves beneath shallow tropical seas that provide an environment hospitable to coral. At these levels in the ocean, the acidity of the water is low enough that shells and skeletons deposited on the ocean floor do not dissolve.

The Ca2+ ions that marine organisms incorporate into their shells enter the ocean by way of the weathering of rocks in rain water that is carried to the oceans in rivers. Some of these ions are derived from the weathering of calcium-silicate rocks in the reaction

2HCO-SiO2 + H2O

The net effect of Eqs. (2.11) and (2.13), in combination with Eq. (2.7), is

CaSiO

CO2 : CaCO3

which has the effect of taking up CO2 from the atmospheric and oceanic reservoirs and incorporating it into the much larger reservoir of inorganic carbon sedimentary rocks in the Earth's crust.

From a climate perspective, the chemical reactions (2.7)-(2.14) are virtually instantaneous. In contrast, the timescale over which the oceanic reservoirs adjust to changes in atmospheric CO2 is governed by the ventilation time for the deeper layers of the ocean, which is on the order of centuries. Calcium carbonate formation is limited by the availability of calcium ions, which is determined by the rate of weathering of calcium silicate rocks, as described in the following subsection.

2.3.4 Carbon in the Earth's Crust

The organic and inorganic carbon reservoirs in the Earth's crust are both very large, the exchange rates in and out of them (apart from the burning of fossil fuels) are very slow, and residence times are on the order of many millions of years. Carbon enters both of these reservoirs by way of the biosphere, as described earlier. Most deposits of the organic carbon in natural gas, oil, coal, and shales and other sedimentary rocks were formed in anoxic ocean basins. The even larger inorganic carbon reservoir, consisting mostly of calcium carbonate

(CaCO3), is almost exclusively a product of the marine biosphere.

Weathering exposes organic carbon in sedimentary rock to the atmosphere, allowing it to be oxidized, thereby completing the loop in what is sometimes referred to as the long term inorganic carbon cycle. Currently the burning of fossil fuels is returning as much carbon to the atmosphere in a single year as weathering would return in hundreds of thousands of years! The mass of carbon that exists in a form concentrated enough to be classified as "fossil fuels" represents only a small fraction of the organic carbon stored in the Earth's crust, but it is nearly an order of magnitude larger than the mass of carbon currently residing in the atmosphere.

On timescales of tens to hundreds of millions of years, plate tectonics and volcanism play an essential role in renewing atmospheric CO2. This "inorganic carbon cycle," summarized in Fig. 2.25, involves subduction, metamorphism, and weathering. Limestone sediments on the sea floor are subducted into the Earth's mantle along plate boundaries where continental plates are overriding denser oceanic plates. At the high temperatures within the mantle, limestone is transformed into metamorphic rocks by the reaction

The CO2 released in this reaction eventually returns to the atmosphere by way of volcanic eruptions. The metamorphic rocks containing calcium in chemical combination with silicate are recycled in the form of newly formed crust that emerges in the mid-ocean ridges. The metamorphism reaction (2.15), in combination with weathering, and the carbonate formation reaction (2.14) form a closed loop in which carbon atoms cycle back and forth between the atmospheric

Carbon

Calcium

Carbon

atmosphere

W

oceans

(S

metamorphic rocks

limestone

quartz

Fig. 2.25 Schematic of the long-term inorganic carbon cycle, also referred to as the carbonate-silicate cycle. The symbol S denotes sedimentation, M denotes metamorphosis, and W denotes weathering.

CO2 reservoir and the inorganic carbon reservoir in the Earth's crust on a timescale of tens to hundreds of millions of years.

At times when the rate at which CO2 is injected into the atmosphere by volcanic eruptions exceeds the rate at which calcium ions are made available by weathering, atmospheric CO2 concentrations increase and vice versa. The injection rate is determined by rate of metamorphism of carbonate rocks, which, in turn, depends on the rate of plate movement along convergent boundaries where subduction is occurring. The rate of weathering, however, is proportional to the rate of cycling of water in the atmospheric branch of the hydrologic cycle, which increases with increasing temperature. The fact that weathering involves the chemical reaction (2.13) makes the temperature dependence even stronger. Hence, high ambient temperatures and slow plate movements are conducive to a draw-down of atmospheric CO2 and vice versa. The changes in atmospheric CO2 in response to imbalances between (2.14) and (2.15) on timescales of tens of millions of years are believed to have been quite substantial.

Guide to Alternative Fuels

Guide to Alternative Fuels

Your Alternative Fuel Solution for Saving Money, Reducing Oil Dependency, and Helping the Planet. Ethanol is an alternative to gasoline. The use of ethanol has been demonstrated to reduce greenhouse emissions slightly as compared to gasoline. Through this ebook, you are going to learn what you will need to know why choosing an alternative fuel may benefit you and your future.

Get My Free Ebook


Responses

  • d brandagamba
    What are the two primary reservoirs of CARBON?
    8 years ago
  • Rayyan
    What is smallest carbon reservoir in the earth systems?
    8 years ago

Post a comment