How are isotopes used to understand the global carbon cycle?

Carbon dioxide is a natural greenhouse gas, which absorbs heat and radiates it back to the surface.  This greenhouse effect operated long before humans ever occupied the earth; without it, our climate would be about 30oC cooler than at present. CO2 is not the only greenhouse gas - methane and nitrous oxide are two other naturally occurring gases that contribute to the greenhouse effect; in fact, they are more potent greenhouse gases than CO2 on a molecular basis. However, CO2 is present at much higher concentrations, so that it has the greatest total radiative effect.

We have now increased the concentration of CO2 in the atmosphere by over 30%, from 280 parts per million (ppm) before the beginning of the industrial revolution to almost 370 ppm.  This increase has been well documented, and its source is the burning of coal, oil, and natural gas for fuel, as well as the clearing and burning of forested land for agriculture and other land uses.  The CO2 produced by these activities can be estimated by studying the amount of fuel used by the world's population and the change in forested land area.  For the years 1980-89, this amounted to about 7.2 gigatons (Gt) of carbon per year (yr).  Of this, only about 3.3/yr Gt was stored in the atmosphere, so that more than half of the CO2 emitted by human activities was absorbed by carbon sinks.  If we wish to slow the accumulation of CO2 in the atmosphere to avoid the changes in climate that could result, we must understand how carbon is transferred from the various components of the global carbon cycle and how humans have affected the processes that control it.

The vast majority of carbon on earth is locked in inorganic rock and mineral material.  A tiny fraction cycles through the "active" pools of carbon: the atmosphere, the ocean, and the land surface. The transfer of carbon between these reservoirs is known as the global carbon cycle . You can see that the ocean contains the most carbon of any active pool.  Carbon is transferred into the ocean by the transport of organic and inorganic material in rivers, and by the dissolution of CO2 into surface waters. CO2 dissolves into water to form carbonic acid (H2CO3), which dissociates into bicarbonate (HCO3-) or carbonate (CO32-) ions:

CO2 + H2O -> H2CO3 -> H+ + HCO3-  -> 2H+ + CO32-

This reaction is controlled by the CO2 concentration in the atmosphere and the water temperature, in that cold water can hold more dissolved CO2.  As the CO2 concentration in the atmosphere increases, more CO2 dissolves into surface waters.  The first 50 meters or so of ocean water is well mixed because of the action of the wind. This mixed layer is a small portion of the total volume of the ocean that exchanges with deeper water only on the order of centuries. However, the mixed layer is not saturated with CO2 because carbon is removed from the surface by phytoplankton, or marine plants, that convert bicarbonate to organic material in photosynthesis.  In addition some types of zooplankton, or marine animals, use carbonate and calcium ions to create calcium carbonate shells. When zooplankton and phytoplankton die, the calcium carbonate and organic material that escapes decomposition at the surface sinks into the deep ocean.

We now understand the transport and mixing patterns in the ocean because the release of radioactive isotopes during aboveground testing of atomic bombs provided tracers of ocean circulation. Using transport models and estimates of sea surface temperature and wind speed, ocean modelers have calculated that the ocean sink is about 2 Gt/yr. Since humans released about 7 Gt/yr during 1980-89 and only 3.2 Gt/yr are found in the atmosphere, there must be an additional carbon sink on land. This is surprising because 1.6 Gt/yr of the carbon released by human activities came from the destruction of forested lands. Therefore the remainder of the biosphere must have absorbed 1.8 Gt/yr.

The terrestrial biosphere is a very active component of the carbon cycle that exchanges on the order of 50 Gt/yr of carbon with atmosphere via Net Primary Production (NPP) and respiration .  NPP is the sum of the carbon fixed by photosynthesis subtracted by plant respiration, or autotrophic respiration. Net Ecosystem Production (NEP) is NPP subtracted by the decomposition of organic material, or heterotrophic respiration.  Over large areas, we can calculate Net Biome Production (NBP) as NEP subtracted by the loss of carbon through large-scale disturbances such as fire and clearcutting.  Mathematically:

GPP = NPP + RA = NEP + RH = NBP + RD

where GPP is Gross Primary Production, RA is autotrophic respiration, RH is heterotrophic (non-plant) respiration, and RD is the loss of carbon due to large-scale disturbance.

For the years 1980-89 NBP appears to have increased. There has been a great deal of debate over the mechanisms by which the loss of carbon to the atmosphere in forest destruction was approximately balanced by increased carbon uptake in the rest of the terrestrial biosphere. Possible explanations that have been discussed are 1) forest growth, 2) CO2 fertilization, 3) nitrogen deposition. Forest inventories in North America suggest that while forested areas have diminished in the tropics, some abandoned farm land in North America has returned to forest and absorbed carbon in re-growing trees.  In addition, CO2 is the substrate for plant growth, so that higher concentrations of CO2 have a "fertilizing" effect and induce greater growth. This is an active area of research in ecology that is discussed in detail in a separate section.  Finally, air pollution releases nitrogen into the atmosphere that is deposited on natural ecosystems.  This may also have had a "fertilizing" effect.

The evidence that the terrestrial biosphere has acted as a carbon sink is growing. Formerly, the remaining CO2 unaccounted for by a balancing of fossil fuel and terrestrial emissions, storage in the atmosphere, and oceanic uptake was called the "missing sink" of carbon.  It is was difficult to prove the existence of terrestrial carbon uptake by any direct means due to the complexity of terrestrial ecosystems and the many factors that can affect them. However, there is now additional evidence that this missing sink resides in the terrestrial biosphere. The distribution of CO2 in the atmosphere is fairly uniform, but there are some variations over space and time. You may have noticed that the record of CO2 concentration in the atmosphere is not a straight trajectory, but shows yearly oscillations, particularly in the high latitudes.  This is because the majority of the land surface occurs in the Northern Hemisphere, where ecosystems have a distinctly seasonal exchange with the atmosphere. During the growing season when photosynthetic rates are high, temperate ecosystems draw down atmospheric CO2. In winter when many species are dormant or have low photosynthetic rates, ecosystem fluxes are dominated by respiration, and global atmospheric CO2 concentrations slightly increase. Since we began measuring CO2 concentrations in the atmosphere, there has been a detectable increase in the magnitude of the seasonal oscillation; that is, the distance between the peak and the trough of the curve has increased. This is an indication of an increased activity in the biosphere, either as a result of a change in the length of the growing season or a change in biological activity during the growing season or both.  However, we cannot distinguish between an increased peak (higher respiration) or a decreased trough (higher photosynthesis) by looking at the CO2 concentration record alone.

The spatial distribution of CO2 has helped to resolve this problem. Atmospheric CO2 concentrations are slightly higher where CO2 is emitted and slightly lower where CO2 is absorbed. By examining the CO2 concentration gradient and taking into account models of atmospheric transport, scientists have found certain areas to be CO2 sinks.  Atmospheric measurements represent very large areas, so that it's difficult to separate ocean and terrestrial sinks based on gradient studies alone.  However, stable isotopes provide a means of partitioning the absorption of CO2 into oceanic and photosynthetic uptake. We have discussed that photosynthesis of terrestrial plants has a unique isotopic signature.  Photosynthetic discrimination of carbon by C3 plants varies from 22 to 35‰, so that the CO2 respired from ecosystems dominated by C3 plants carries this signature. In contrast, the bulk d of the atmosphere is -8‰.  It is influenced by the isotopically light signature of fossil fuel CO2, which originated from plant and animal material, but this can be accounted for since these sources are well quantified. Since there is little discrimination during dissolution of CO2 into the ocean (about 1‰), gradients of 13C/12C in the atmosphere may be used to distinguish between the terrestrial and oceanic sinks if values for photosynthetic discrimination and the isotopic composition of respiration can be estimated on a global scale.

Atmospheric gradients of 18O/16O also contain important information about the role of the terrestrial biosphere in the carbon cycle. We have discussed that CO2 leaving ecosystems carries the 18O signature of water in leaves and soil, as oxygen in CO2 exchanges with oxygen in water. This signature is very different from 18O/16O of ocean water, so that measurements of oxygen isotopes in atmospheric CO2 provide an additional, independent method for partitioning oceanic and terrestrial sinks of carbon. To utilize oxygen isotopes on a global scale, modelers must estimate both photosynthetic discrimination of 18O and the oxygen isotopic composition of plant and soil water globally.

While the physiological and physical mechanisms that control isotopic composition are well understood at small scales such as leaf scale, much work remains to be done at the whole ecosystem and regional scales. Ecologists are studying the processes that influence the carbon and oxygen isotope composition of the CO2 leaving the terrestrial land surface in a variety of ecosystems around the world. This can improve the global calculations of CO2 sources and sinks on land and in the oceans. If we understand the fate of the carbon that has already been released by human activities, we can improve predictions about the future trajectory of atmospheric CO2 and climate. By integrating ecosystem-scale measurements of 13CO2 and 18CO2 in studies around the world, BASIN aims to improve our understanding of the processes that control the isotopic composition of CO2 in the atmosphere for better quantification of carbon sources and sinks and their fate during the next century.


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