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Abstract:
The isotope composition of seawater is an efficient method for detecting mixing between water masses. To measure long term or large scale hydrological processes at the ocean surface, it is necessary to be able to precisely compare datasets produced by different laboratories. The oxygen and hydrogen isotope (δ18O and δ2H) composition of marine waters can be measured using isotope ratio mass spectrometry (IRMS) and near-infrared laser absorption spectroscopy (LS) techniques. The IRMS and equilibration method is thought to provide results on the activity scale, while LS provides results on the concentration scale. However, the effect of dissolved seawater salts on the measurement is not sufficiently assessed and seems sometimes contradictory in the literature. For this purpose, we made artificial seawater and a pure NaCl solution from a freshwater of known isotope composition. The solutions were measured by four different laboratories allowing us to compare the two techniques. We show that minor corrections are necessary to correct seawater measurements for the salt effect and report them on the concentration scale. Interestingly, seawater measurements using LS (type Picarro) coupled to a liner are not on the concentration scale and require a correction of ~ 0.09‰ for δ18O, while the correction is relatively less significant for δ2H (~ 0.13‰). Moreover, we found for IRMS measurements that the salt effect can differ between different laboratories but seems reproducible for a given laboratory. A natural sea water sample was then analyzed by the different laboratories participating in the study. We found that applying the corrections increases the reproducibility of the isotope measurement significantly, with inter-laboratory standard deviation decreasing from 0.06 to 0.02‰ and 0.55 to 0.23‰ for δ18O and δ2H, respectively. Thus, comparing sea water datasets produced in different laboratories requires that each laboratory carries out its own calibration with artificial seawater and presents measurements on the concentration scale.
