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

Investigations of the isotope ratios of dissolved oxygen (δ¹⁸ O_DO ) provide valuable information about the oxygen cycle in aquatic systems. However, oxidation of Fe(II) may change pristine δ¹⁸ O_DO values during storage and can lead to a misinterpretation. We sampled an Fe(II)-rich spring system and measured δ¹⁸ O_DO values at various time intervals in order to determine influences of Fe-oxidation.

Water samples were collected from an Fe-rich spring and related stream and the δ¹⁸ O_DO values were measured in fresh, 4- and 13-day-old samples with an isotope ratio mass spectrometer. Three 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 system by multi-parameter probes and spectrophotometry.

The δ¹⁸ O_DO 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 a flow length of 85 m the differences in δ¹⁸ O_DO values between fresh and older samples decreased because most of the Fe(II) was consumed.

False interpretations of δ¹⁸ O_DO values are possible if Fe-rich water samples are measured after too long storage, and we recommend measurement immediately after sampling.