Technical note: Large offsets between different datasets of seawater isotopic composition – an illustration of the need to reinforce intercalibration efforts

Abstract. We illustrate offsets in surface seawater isotopic composition between recent public datasets from the Atlantic Ocean and the subtropical southeastern Indian Ocean. The observed offsets between datasets often exceed 0.10 ‰ in δ18O and 0.50 ‰ in δ2H. They might in part originate from different sampling of seasonal, interannual, or spatial variability. However, they likely mostly originate from different instrumentations and protocols used to measure the water samples. Estimation of the systematic offsets is required before merging the different datasets in order to investigate the spatiotemporal variability of isotopic composition in the world ocean surface waters. This highlights the need to actively share seawater isotopic composition samples dedicated to specific intercomparison of data produced in different laboratories and to promote best practices, a task to be addressed by the new Scientific Committee of Oceanic Research (SCOR) working group 171.