Beware of effects on isotopes of dissolved oxygen during storage of natural iron-rich water samples: A technical note.

RATIONALE: Investigations of the isotope ratios of dissolved oxygen (I´18 ODO values) provide valuable information about the oxygen cycle in aquatic systems. However, oxidation of Fe (II) may change the pristine I´18 ODO values during storage and can lead to a misinterpretation. We sampled an Fe (II)-rich spring system and measured I´18 ODO values afterwards to determine influences of Fe-oxidation. METHODS: Water samples were collected from a spring and related stream and the I´18 ODO values were measured in fresh, 4- and 13-day old samples with an isotope ratio mass spectrometer. 3 replicates were measured for each sample with a 1σ of ± 0.2 ‰. On-site parameters and Fe (II) contents were also measured over the course of the spring by multi parameter probes and spectrophotometry. RESULTS: The I´18 ODO values over the course of the spring system in fresh, 4- and 13-day old samples revealed differences of up to 8 ‰. We explain this increase by the consumption of DO by Fe (II) oxidation. After 85 meters' flow length the differences in I´18 ODO values between fresh and older samples decreased due to consumption of Fe (II). CONCLUSION: False interpretations of I´18 ODO values are possible if the values of Fe-rich water samples are measured after too long storage, and we recommend measurement immediately after sampling.