Using Models of Carbon Isotope Fractionation during Photosynthesis to Understand the Natural Fractionation Ratio

Abstract

The fractionation correction b is used to correct for the fractionation of 14C by using information from 13C in samples. This value is assumed to have a value of 2, where the 14C/12C ratio is double that of the 13C/12C ratio. While natural and laboratory fractionation are usually not considered separately, this article explores the differential fractionation of 14C and 13C during the process of photosynthesis. Values of δ13Cp can be used to calculate Δ13Cp values, which in turn can be used to calculate Δ14Cp, the discrimination against 14CO2 during photosynthesis. Models can then be built of Δ14Cp/Δ13Cp, an approximation for the natural fractionation ratio. This approximation suggests that for C3 plants the ratio is ∼1.90 and for C4 plants the ratio is more variable. While error introduced by the natural fractionation is small, it is also possibly systematic, as b = 2.0 does not seem physiologically possible following these models of carbon fractionation during photosynthesis. The central aim of this study is to illustrate that b derives not from a natural constant, but rather from a variable natural process.