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 (delta O-18(DO)) provide valuable information about the oxygen cycle in aquatic systems. However, oxidation of Fe(II) may change pristine delta O-18(DO) values during storage and can lead to a misinterpretation. We sampled an Fe(II)-rich spring system and measured delta O-18(DO) values at various time intervals in order to determine influences of Fe-oxidation.Methods Water samples were collected from an Fe-rich spring and related stream and the delta O-18(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 1 sigma of +/- 0.2 parts per thousand. On-site parameters and Fe(II) contents were also measured over the course of the spring system by multi-parameter probes and spectrophotometry.Results The delta O-18(DO) values over the course of the spring system in fresh, 4- and 13-day-old samples revealed differences of up to 8 parts per thousand. We explain this increase by the consumption of DO by Fe(II)-oxidation. After a flow length of 85 m the differences in delta O-18(DO) values between fresh and older samples decreased because most of the Fe(II) was consumed.Conclusions False interpretations of delta O-18(DO) values are possible if Fe-rich water samples are measured after too long storage, and we recommend measurement immediately after sampling.