Carbon Isotope Discrimination in C3 Land Plants is Independent of Atmospheric PCO2

Abstract:

The δ¹³C of terrestrial C3 plant tissues and soil organic matter is important for understanding the carbon cycle, inferring past climatic and ecological conditions, and predicting responses of vegetation to future climate change. Plant δ¹³C depends on the δ¹³C of atmospheric CO₂ and mean annual precipitation (MAP), but an unresolved decades-long debate centers on whether terrestrial C3 plant δ¹³C responds to p_CO2. Here, the p_CO2-dependence of C3 land plant δ¹³C was tested using isotopic records from low- and high-p_CO2 times spanning historical through Eocene data. Historical data do not resolve a clear p_CO2-effect (-1.2±1.0 to 0.59±0.34‰/100 ppmv), and organic carbon records of the Pleistocene-Holocene transition implicate changes in MAP and ecosystems, rather than p_CO2, as the major driver of δ¹³C changes. Fossil collagen and tooth enamel data constrain p_CO2-effects most tightly to -0.03±0.13 and -0.03±0.24‰/100 ppmv between 200 and 700 ppmv. Combining all constraints yields a preferred value of 0.0±0.2‰/100 ppmv (2 s.e.), i.e. there is effectively no p_CO2 effect. Recent models of p_CO2-dependence imply unrealistic MAP for Cenozoic records.