The CISE-LOCEAN seawater isotopic database (1998-2021)

2022

Article référencé dans le Web of Science WOS:000808540000001

Reverdin G., Waelbroeck C., Pierre C., Akhoudas C., Aloisi G., Benetti M., Bourlès Bernard, Danielsen M., Demange J., Diverrès Denis, Gascard J. C., Houssais M. N., Le Goff H., Lherminier P., Lo Monaco C., Mercier H., Metzl N., Morisset S., Naamar A., Reynaud T., Sallee J. B., Thierry V., Hartman S. E., Mawji E. W., Olafsdottir S., Kanzow T., Velo A., Voelker A., Yashayaev I., Haumann F. A., Leng M. J., Arrowsmith C., Meredith M.

Earth System Science Data, 2022, 14 (6), p. 2721-2735 ISSN 1866-3508

The characteristics of the CISE-LOCEAN seawater isotope dataset (delta O-18, delta H-2, referred to as delta D) are presented (https://doi.org/10.17882/71186; Waterisotopes-CISE-LOCEAN, 2021). This dataset covers the time period from 1998 to 2021 and currently includes close to 8000 data entries, all with delta O-18, three-quarters of them also with delta D, associated with a date stamp, space stamp, and usually a salinity measurement. Until 2010, samples were analyzed by isotopic ratio mass spectrometry and since then mostly by cavity ring-down spectroscopy (CRDS). Instrumental uncertainty in this dataset is usually as low as 0.03 parts per thousand for delta O-18 and 0.15 parts per thousand for delta D. An additional uncertainty is related to the isotopic composition of the in-house standards that are used to convert data to the Vienna Standard Mean Ocean Water (VSMOW) scale. Different comparisons suggest that since 2010 the latter have remained within at most 0.03 parts per thousand for delta O-18 and 0.20 parts per thousand for delta D. Therefore, combining the two uncertainties suggests a standard deviation of at most 0.05 parts per thousand for delta O-18 and 0.25 parts per thousand for delta D. For some samples, we find that there has been evaporation during collection and storage, requiring adjustment of the isotopic data produced by CRDS, based on d-excess (delta D - 8x delta O-18). This adjustment adds an uncertainty in the respective data of roughly 0.05 parts per thousand for delta O-18 and 0.10 parts per thousand for delta D. This issue of conservation of samples is certainly a strong source of quality loss for parts of the database, and "small" effects may have remained undetected. The internal consistency of the database can be tested for subsets of the dataset when time series can be obtained (such as in the southern Indian Ocean or North Atlantic subpolar gyre). These comparisons suggest that the overall uncertainty of the spatially (for a cruise) or temporally (over a year) averaged data is less than 0.03 parts per thousand for delta O-18 and 0.15 parts per thousand for delta D. However, 18 comparisons with duplicate seawater data analyzed in other laboratories or with other datasets in the intermediate and deep ocean suggest a larger scatter. When averaging the 18 comparisons done for delta O-18, we find a difference of 0.082 parts per thousand with a standard error of 0.016 parts per thousand. Such an average difference is expected due to the adjustments applied at LOCEAN to saline water data produced either by CRDS or isotope ratio mass spectrometry (IRMS), but the scatter found suggests that care is needed when merging datasets from different laboratories. Examples of time series in the surface North Atlantic subpolar gyre illustrate the temporal changes in water isotope composition that can be detected with a carefully validated dataset.

Sciences fondamentales / Techniques d'analyse et de recherche [020] ; Limnologie physique / Océanographie physique [032]

Fonds IRD [F B010085237]

fdi:010085237