Incorporating the effects of photorespiration into terrestrial paleoclimate reconstruction

Paleoclimate reconstruction from the carbon stable isotopes in fossilized terrestrial organic matter relies upon a simple relationship between isotope discrimination and photosynthetic rate or leaf stomatal conductance (ci/ca). The carbon in plant tissues, however, represents the net carbon gained during photosynthesis, i.e., the balance of carbon gained through the stomata minus the carbon lost via photorespiration and respiration. While carbon isotope discrimination during respiration is negligible, 13C discrimination during photorespiration can change plant isotope composition by several per mil. Our analysis of 123 Arabidopsis thaliana plants grown under a large range of subambient-to-highly-elevated pCO2 revealed a positive response between carbon isotope discrimination and pCO2 that was both consistent with the theoretical estimate for photorespiration and mathematically independent of ci/ca. Studies that seek to reconstruct climate records from plant-derived substrates must account for changes in atmospheric pCO2 when attributing isotopic shifts to changes in photosynthetic rate or stomatal conductance driven by environmental conditions.