Abstract
Oxygen isotope ratios of O(2) are important tracers for assessing biological activity in biogeochemical processes in aquatic environments. In fact, changes in the (18)O/(16)O and (17)O/(16)O 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), (1)O(2), and the ensuing oxygenation and oxidation reactions with organic compounds through experiments with rose bengal as the (1)O(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 (18)O kinetic isotope effects ((18)O-AKIE) with a pre-equilibrium model for the reversible formation of (1)O(2) and its irreversible oxygenation reactions with organic compounds. While (18)O-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 (1)O(2) vs (1)O(2) decay to ground state oxygen, (3)O(2). Our findings imply that O isotope fractionation through photochemical O(2) consumption with isotope enrichment factors, (18)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(2) cycling.