The isotopic ratios 17O/16O and 18O/16O in molecular oxygen and their significance in biogeochemistry

Recently, a new method has been introduced for the estimation of photosynthetic oxygen production from the triple isotope composition (δ17O and δ18O) of dissolved O2 in the ocean and of air O2 in ice cores. This method is based on the deviations (17Δ) from mass dependent respiratory fractionation, the major process affecting the isotopic composition of air O2. To apply this method, the slope in the 17O/16O vs. 18O/16O relationship used for 17Δ calculation must be known with high accuracy. Using numerical simulations and closed system experiments, we show how the respiratory slope is manifested in the 17Δ of O2 in situations where respiration is the only process affecting oxygen isotopic composition (kinetic slope), and in systems in steady state between photosynthesis and respiration (steady state slope). The slopes of the fractionation line in these two cases are different, and the reasons of this phenomenon are discussed. To determine the kinetic respiratory slope for the dominant O2 consumers in aquatic systems, we have conducted new experiments using a wide range of organisms and conditions and obtained one universal value (0.5179 ± 0.0006) in ln(δ17O + 1) vs. ln(δ18O + 1) plots. It was also shown that the respiratory fractionations under light and dark are identical within experimental error. We discuss various marine situations and conclude that the kinetic slope 0.518 should be used for calculating 17Δ of dissolved O2. In contrast, a steady state fractionation slope should be used in global mass balance calculations of triple isotope ratios of O2 in air records of ice cores.