A multiplexing system for quantifying oxygen fractionation factors in closed chambers

Abstract. The study of isotopic ratios of atmospheric oxygen in fossilized air trapped in ice core bubbles provides information on variations in the hydrological cycle at low latitudes and productivity in the past. However, to refine these interpretations, it is necessary to better quantify the fractionation of oxygen in biological processes such as photosynthesis and respiration. We set up a system of closed biological chambers in which we studied the evolution of the elemental and isotopic composition of O2 due to biological processes. To easily replicate experiments, we developed a multiplexing system which we describe here. We compared measurements of the elemental and isotopic composition of O2 using two different measurement techniques: optical spectrometry (optical-feedback cavity-enhanced absorption spectroscopy, i.e., OF-CEAS technique), which enables higher temporal resolution and continuous data collection, and isotopic ratio mass spectrometry (IRMS) with a flanged air recovery system, thus validating the data analysis conducted through the OF-CEAS technique. As a first application, we investigated isotopic discrimination during respiration and photosynthesis. We conducted a 5 d experiment using maize (Zea mays L.) as the model species. The 18O discrimination value for maize during dark plant respiration was determined as −17.8 ± 0.9 ‰ by IRMS and −16.1 ± 1.1 ‰ by optical spectrometer. We also found a value attributed to the isotopic discrimination of terrestrial photosynthesis equal to +3.2 ± 2.6 ‰ and by IRMS and +6.7 ± 3.8 ‰ by optical spectrometry. These findings were consistent with a previous study by Paul et al. (2023).