INCREASED CARBON ISOTOPE FRACTIONATION BY TERRESTRIAL PLANTS FOLLOWING THE LAST GLACIAL MAXIMUM DRIVEN BY AN INCREASE IN ATMOSPHERIC pCO2

Carbon isotope measurements of terrestrial organic matter (δ¹³C) have been used to document changes water availability, plant community composition, the δ¹³C value of atmospheric CO₂ (δ¹³C_CO2), and atmospheric pCO₂ concentration. Here we document a global 2.0‰ increase in carbon isotope fractionation [Δδ¹³C = (δ¹³C_CO2 - δ¹³C)/(1 + δ¹³C/1000)] across Termination 1 (18,000 to 11,500 yrs BP) measured in both fossil leaves and bulk terrestrial organic matter (TOM) of C₃ plant species from 21 sites worldwide. We use our recently quantified relationship between Δδ¹³C value and pCO₂ level to reconstruct an increase in pCO₂ from 192 to 279 ppm from the Late Glacial through the Holocene, which is nearly identical in trend and absolute value to the ice core record (193 to 270 ppm). We contend that local or regional changes in environmental conditions, substrate heterogeneity, and plant community shifts exert secondary control over plant carbon isotope fractionation relative to the primary pCO₂ effect. Because of the robustness of the proxy relationship to diverse C₃ plants growing under multiple levels of water availability, error from the model relationship is small; the greatest source of error is instead caused by variability in the Δδ¹³C measurements. For this reason, we conclude that pCO₂ level can be accurately reconstructed from the terrestrial plant record by compiling data from a large number of diverse sites. We contend that there is a strong potential for terrestrial δ¹³C measurements to be an excellent proxy for reconstructing pCO₂ levels across the history of C₃ plant plants.