On the triple oxygen isotope composition of carbon dioxide from some combustion processes

Abstract

The triple oxygen isotope composition (Δ17O) of CO2 from different sources is gaining in importance as possible tracer of gross carbon exchanges between major reservoirs. Here we present the Δ17O of CO2 from natural gas and propane–butane combustion, wood chips burning, car exhaust and human breath. All investigated CO2 samples had negative Δ17O value compared to the CO2–water equilibration line (slope θ=0.522; zero intercept), which was inherited from the oxidant air O2. However, for all combustion experiments, the Δ17O value of CO2 was significantly higher than the Δ17O value of air O2. The oxygen isotope composition of CO2 from natural gas (δ18O=21.6±0.7‰; Δ17O=−0.30±0.02‰) and from propane–butane (δ18O=22.5±0.8‰; Δ17O=−0.32±0.02‰) combustion is explained by kinetic fractionation of ambient air O2. In case of wood chips burning (δ18O=19.4±1.0‰; Δ17O=−0.21±0.02‰) the wood inherent O also affected the triple oxygen isotope composition of the CO2. Car exhaust CO2 (δ18O=32.6±3.0‰; Δ17O=−0.32±0.03‰) might have likely equilibrated with the condensed water in the exhaust line. The isotope composition of breath CO2 (δ18O=35±1‰; Δ17O=−0.03±0.03‰) was controlled by equilibration with body water. We assess the Δ17O value of CO2 as a potential tracer for anthropogenic CO2 emission.