Carbon isotopes - (n.)
Both the stable isotope carbon-13 and the unstable carbon-14 are used as tracers of climate-related processes. (see also "radiocarbon." for more on carbon-14) Carbon isotopes are useful in documenting the rate and amount of input of carbon dioxide into the atmosphere from human activities (the burning of fossil fuels and deforestation) and in tracing the path of this added carbon through the carbon cycle. Carbon dioxide in the atmosphere provides the raw material for making the wood in tree trunks and analysis of tree rings shows how the composition of the atmosphere changed. Since 1950, there has been a marked decrease in the ratio of 14C to 12C due to the input of 14C-free carbon from fossil fuels (a phenomenon known as the "Suess Effect"). A similar change is seen in the ratio of 13C to 12C because fossil fuels have less 13C than does the atmosphere (as is true for all carbon compounds derived from photosynthesis). Long-term trends in 14C in tree rings show that the atmospheric concentration of this isotope has changed in the past several thousand years due to natural causes, including changes in production within the atmosphere and changes in the exchange between atmosphere and ocean. On long time-scales, 13C in marine carbonates is useful in reconstructing the productivity history of the ocean, as well as yielding clues to CO2 levels in the geological past.