Using Carbon Isotope Equilibrium to Screen Pedogenic Carbonate Oxygen Isotopes: Implications for Paleoaltimetry and Paleotectonic Studies

Figure 3

Carbon isotope interpretation framework. This figure (design modified from Koch (1998)) shows the hypothetical C isotopic fractionation pathways for plants using both the C₃ and C₄ photosynthetic pathways in a preindustrial world. The burning of fossil fuels has shifted δ¹³C_atm by > −1.5‰ since 1850. Distributions of typical modern C₃ () and C₄ () δ¹³C_plant values are from Cerling and Harris [48] and have been modified from the publication by +1.5‰ to reflect a preindustrial δ¹³C_atm value of −6.5‰. While there is some minor temperature dependence in the C isotope fraction factor between CO₂ and calcite [41], based upon the prevailing theoretical groundwork, δ¹³C_carb values should be ~ +14.9‰ relative to their coexisting soil organic matter (SOM). This relationship is empirically verified in Figure 5.