Diurnal, seasonal, and interannual variations in δ(¹⁸O) of atmospheric O₂ and its application to evaluate changes in oxygen, carbon, and water cycles

Variations in the δ(18O) of atmospheric O2, δatm(18O), is an indicator of biological and water processes associated with the Dole-Morita effect (DME). The DME and its variations have been observed in ice cores for paleoclimate studies, however, variations in present-day’s δatm(18O) have never been detected so far. Here, we present diurnal, seasonal, and interannual variations of δatm(18O) based on observations at a surface site in central Japan. The average diurnal δatm(18O) cycle reached a minimum during the daytime, and its amplitude was larger in summer than in winter. We found that use of δatm(18O) enabled separation of variations of atmospheric δ(O2/N2) into contributions from biological activities and fossil fuel combustion. The average seasonal δatm(18O) cycle reached at a minimum in summer, and the peak-to-peak amplitude was about 2 per meg. A box model that incorporated biological and water processes reproduced the general characteristics of the observed diurnal and seasonal cycles. A slight but significant secular increase of δatm(18O) by (0.22 ± 0.14) per meg a−1 occurred during 2013–2022. The box model could reproduce the secular trend if consideration was given to long-term changes of terrestrial gross primary production (GPP), photorespiration, and δ(18O) of leaf water (δLW(18O)). We calculated changes of δLW(18O) using a state-of-the-art, three-dimensional model, MIROC5-iso. A comparison between the observed and simulated δatm(18O) values suggested that there had been a recent increase of global GPP, a slight decrease of photorespiration, and an increase of carboxylation (total carbon fixation).