Oxygen Isotope Fractionation of O2 Consumption Through Abiotic Photochemical Singlet Oxygen Formation Pathways

Oxygen isotope ratios of O2 are important tracers for assessing biological activity in biogeochemical processes in aquatic environments. In fact, changes of 18O/16 O and 17O/16 O ratios of O2 have been successfully implemented as measures for quantifying photosynthetic O2 production and biological O2 respiration. Despite evidence for light-dependent O2 consumption in sunlit surface waters, however, photochemical O2 loss processes have so far been neglected in the stable isotope-based evaluation of oxygen cycling. Here, we established the magnitude of O isotope fractionation for abiotic photochemical O2 elimination through formation of singlet O2, 1O2, and the ensuing oxygenation and oxidation reactions with organic compounds through experiments with rose bengal as 1O2 sensitizer and three different amino acids and furfuryl alcohol as chemical quenchers. Based on the kinetic analysis of lightdependent O2 removal in the presence of different quenchers, we rationalize the observable O isotope fractionation of O2 and the corresponding, apparent 18O kinetic isotope effects (18O-AKIE) with a pre-equilibrium model for the reversible formation of 1O2 and its irreversible oxygenation reactions with organic compounds. While 18O-AKIEs of oxygenation reactions amount to 1.03, the O isotope fractionation of O2 vanishes systematically with increasing ratio of the rates of oxygenation reaction of 1O2 vs. 1O2 decay to ground state oxygen, 3O2 . Our findings imply that O isotope fractionation through photochemical O2 consumption can match contributions from biological respiration at typical dissolved organic matter concentrations of lakes, rivers, and oceans and should therefore be included in future evaluations of biogeochemical O2 cycling.