High precision δ¹⁸O measurements of atmospheric dioxygen using optical-feedback cavity-enhanced absorption spectroscopy (OF-CEAS)

Abstract. Atmospheric dioxygen concentration and isotopic composition are closely linked to the carbon cycle through anthropic CO₂ emissions and biological processes such as photosynthesis and respiration. Measurement of isotopic ratio of atmospheric dioxygen, trapped in ice core bubbles, bring information about past variation in the hydrological cycle at low latitudes, as well as past productivity. Currently, the interpretation of those variations could be drastically improved with a better (i.e. quantitative) knowledge of the oxygen fractionation that occur during photosynthesis and respiration processes. This could be achieved, for example, during experiment using closed- biological chambers. In order to estimate the fractionation coefficient with a good precision, one of the principal limitations is the need of high frequency on-line measurements of δ¹⁸O of O₂ and O₂ concentration. To address this issue, we developed a new instrument, based on the OF-CEAS (Optical-Feedback Cavity-Enhanced Absorption Spectroscopy) technique, enabling high temporal resolution and continuous measurements of dioxygen concentration as well as δ¹⁸O of O₂, both simultaneously. Minimum of Allan deviation occurred between 10 and 20 minutes while precision reached 0.002 % for O₂ concentration and 0.06 ‰ for δ¹⁸O of O₂, which correspond to the optimal integration time and analytical precision before drift start degrading the measurements. While humidity did not affect much the measured values, O₂ concentration had an influence on δ¹⁸O of O₂, which should hence be quantified. To ensure good quality of O₂ concentration and δ¹⁸O of O₂ measurements we eventually proposed to measure calibration standard every 20 minutes.