Stable isotopes are atoms that contain the same number of protons, but a different number of neutrons. For example, 12C contains 6 protons and 6 neutrons, whereas 13C contains 6 protons and 7 neutrons. This slight difference in mass between 12C and 13C produces small differences in the reaction rates and binding energies of molecules containing the isotopically substituted atoms. Stable isotopes are distinct from unstable (radioactive) isotopes such as 14C in that they are very slow to decay. The most common stable isotopes used in Earth system science are those of carbon (13C and 12C) and oxygen (16O and 18O). The stable isotope ratio (13C/12C or 18O/16O) of CO2 contains information about where the CO2 has been, and how it has been transformed.
Plant photosynthesis discriminates against 13C. In other words, plant carbon tends to have less 13C than the CO2 from which it is formed. This discrimination provides a powerful tool for interpreting changes in d13C of atmospheric CO2 which is generally applied in one of three ways. First, it can be used to partition CO2 fluxes between the land and oceans, and thus help to constrain the location and processes involved in creating the so called ‘missing carbon sink’. The reason we can do this is because there little carbon isotope discrimination associated with exchange of CO2 between the ocean and atmosphere. Consequently, if the sink is in the ‘land’, changes in atmospheric CO2 concentrations will be accompanied by large changes in d13C, whereas if the sink is due to absorption of CO2 by the ocean, changes in CO2 concentrations will have little effect on d13C. Second, discrimination by C3 plants is influenced by environmental factors such as availability of light, water and nutrients, and consequently, provides a means of interpreting changes in d13C of atmospheric CO2 in terms of environmental changes, e.g. drought, El Nino and global warming. And third, since d13C of atmospheric CO2 has changed over time due to addition of 13C-depleted fossil fuels, carbon isotope ratios of respired CO2 differ slightly from those of photosynthesis. This is termed the ‘isotope disequilibrium effect’ and may provide a means for determining the relative importance of respiration and photosynthesis on changes in atmospheric CO2.
The d18O of CO2 generally reflects the last water in which the CO2 was dissolved and has been used to distinguish plant photosynthesis from ground respiration. This is because high rates of transpiration cause plant water to become enriched in 18O relative ground water. Consequently, respiration from soils emits CO2 which is depleted in 18O relative to the ‘retroflux’ of CO2 from plants.
Documents:
• Draft manuscript on simulating carbon isotopes in SiB (2002)
• Presentation on Simulating Stable Carbon Isotopes at Many Scales
• Report to NSF on the development of the C13 model in SiB
• Proposal to NOAA Carbon Cycle Program (2000)
• NOAA Report (2001)
• Proposal (2002) to NSF which supports our studies of stable isotopes in the Community Climate System Model
This research has been supported by the US National Science Foundation (NSF-ATM-9896261) and by the National Oceanic and Atmospheric Administration -- Office of Global Programs.
