Carbon is made up of three isotopes, mainly 12C, around 1% 13Cand a tiny proportion of radioactive 14C. The effect of thermodynamic equilibria and the kinetics of chemical reactions leads to isotopic fractionations of varying intensity. Δ13C, the deviation of the 13C/12Cratio from a standard, is used to characterize the various reservoirs of the carbon cycle, including the atmosphere, terrestrial and marine biospheres, carbonates and sedimentary organic matter.
Fossil fuels are originally derived from the sedimentation and transformation over time of photosynthetic organic matter. The δ13Cof an oil, coal or natural gas depends on its origin, making it possible to establish a spatial and temporal inventory of the δ13CofCO2 emissions over recent decades.
Since the late 1970s, measurements of atmospheric pCO2 have been accompanied by measurements of δ13C, which indicate marked seasonality, as well as a slow decline approaching 1 ‰ over 40 years. It is possible to confirm this trend and go back beyond the industrial era by measuring the δ13Cof trees showing annual wood growth rings. Since the end of the 18th century atmospheric δ13Chas fallen by around 2 ‰, in parallel with the increase in pCO2.
Quantitative interpretation of atmospheric δ13Cmust take into account emissions from fossil fuel combustion, but also other terms such as deforestation with its isotopic signature of photosynthetic carbon. In addition, the temporal evolution of pCO2 and δ13Cis also affected by the diffusion and partial sequestration of atmosphericCO2 by the ocean and terrestrial biosphere.
