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

Oxygen isotope ratios of O-2 are important tracers for assessing biological activity in biogeochemical processes in aquatic environments. In fact, changes in the O-18/O-16 and O-17/O-16 ratios of O-2 have been successfully implemented as measures for quantifying photosynthetic O-2 production and biological O-2 respiration. Despite evidence for light-dependent O-2 consumption in sunlit surface waters, however, photochemical O-2 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-2 elimination through formation of singlet O-2, O-1(2), and the ensuing oxygenation and oxidation reactions with organic compounds through experiments with rose bengal as the O-1(2) sensitizer and three different amino acids and furfuryl alcohol as chemical quenchers. Based on the kinetic analysis of light-dependent O-2 removal in the presence of different quenchers, we rationalize the observable O isotope fractionation of O-2 and the corresponding, apparent O-18 kinetic isotope effects (O-18-AKIE) with a pre-equilibrium model for the reversible formation of O-1(2) and its irreversible oxygenation reactions with organic compounds. While O-18-AKIEs of oxygenation reactions amount to 1.03, the O isotope fractionation of O-2 decreased to unity with increasing ratio of the rates of oxygenation reaction of O-1(2) vs O-1(2) decay to ground state oxygen, O-3(2). Our findings imply that O isotope fractionation through photochemical O-2 consumption with isotope enrichment factors, O-18-& varepsilon;, of up to -30 parts per thousand 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-2 cycling.