Experimental determination of isotopic fractionation of 13C and 14C during natural processes

Theoretical calculations by Bigeleisen and Mayer (1947) and Craig (1954) have shown that the relative isotopic enrichment 14C/12C in the thermodynamic process is approximately twice that of the ratio 13C/12C. This relationship is used especially for the normalization of 14C dates. We recalculated this theoretical coefficient with the isotope enrichment factor partition functions of various isotopic species of CO2, using the homogeneous set of spectroscopic data of Jobard and Chedin (1975). It appears that the ratio of epsilon 14C/epsilon 13C should be slightly higher than 2. We experimentally evaluated this relationship in a natural environment. We investigated the isotope fractionation which occurs during photosynthesis between atmospheric CO2 and plants with different (C3 and C4) photosynthetic cycles. Measured values: 2.3 +- 0.2 (C3/C4 plants) and 2.65 +- 0.2 (atmospheric CO2/C3 plant) can be reconciliated at 2.3 assuming a slight (< 1%) 14C depletion of CO2 absorbed by the plant. (author)