Oxygen Isotope Fractionation of O Consumption through Abiotic Photochemical Singlet Oxygen Formation Pathways.

Oxygen isotope ratios of O are important tracers for assessing biological activity in biogeochemical processes in aquatic environments. In fact, changes in the O/O and O/O ratios of O have been successfully implemented as measures for quantifying photosynthetic O production and biological O respiration. Despite evidence for light-dependent O consumption in sunlit surface waters, however, photochemical O loss processes have so far been neglected in the stable isotope-based evaluation of oxygen cycling. Here, we established the magnitude of the O isotope fractionation for abiotic photochemical O elimination through formation of singlet O, O, and the ensuing oxygenation and oxidation reactions with organic compounds through experiments with rose bengal as the O sensitizer and three different amino acids and furfuryl alcohol as chemical quenchers. Based on the kinetic analysis of light-dependent O removal in the presence of different quenchers, we rationalize the observable O isotope fractionation of O and the corresponding, apparent O kinetic isotope effects (O-AKIE) with a pre-equilibrium model for the reversible formation of O and its irreversible oxygenation reactions with organic compounds. While O-AKIEs of oxygenation reactions amount to 1.03, the O isotope fractionation of O decreased to unity with increasing ratio of the rates of oxygenation reaction of O vs O decay to ground state oxygen, O. Our findings imply that O isotope fractionation through photochemical O consumption with isotope enrichment factors, O-ϵ, of up to -30‰ 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 O cycling.