The CISE-LOCEAN seawater isotopic database (1998–2021)
Authors: Reverdin Gilles , Waelbroeck Claire , Pierre Catherine , Akhoudas Camille , Aloisi Giovanni , Benetti Marion , Bourlès Bernard , Danielsen Magnus , Demange Jérôme , Diverrès Denis , Gascard Jean-Claude , Houssais Marie-Noëlle , Le Goff Hervé , Lherminier Pascale , Lo Monaco Claire , Mercier Herlé , Metzl Nicolas , Morisset Simon , Naamar Aïcha , Reynaud Thierry , Sallée Jean-Baptiste , Thierry Virginie , Hartman Susan E. , Mawji Edward W. , Olafsdottir Solveig , Kanzow Torsten , Velo Anton , Voelker Antje , Yashayaev Igor , Haumann F. Alexander , Leng Melanie J. , Arrowsmith Carol , Meredith Michael 
Published: June 2022

Abstract. The characteristics of the CISE-LOCEAN seawater isotope dataset (δ18O, δ2H, referred to as δD) 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 δ18O, three-quarters of
them also with δD, 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 ‰ for δ18O and
0.15 ‰ for δD. 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 ‰ for δ18O and
0.20 ‰ for δD. Therefore, combining the two
uncertainties suggests a standard deviation of at most
0.05 ‰ for δ18O and
0.25 ‰ for δD. 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 (δD − 8×δ18O). This adjustment
adds an uncertainty in the respective data of roughly
0.05 ‰ for δ18O and
0.10 ‰ for δD. 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 ‰ for δ18O and 0.15 ‰ for δD. 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 δ18O, we
find a difference of 0.082 ‰ with a standard error of
0.016 ‰. 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.