Rationale NBS19 carbonate, a primary reference material (RM) for the Vienna Pee Dee Belemnite (VPDB) scale realisation introduced in 1987, was exhausted in 2009, and no primary RM was available for several years. This study describes the preparation and characterisation of a new RM, IAEA‐603 (Ca‐carbonate, calcite of marble origin), which shall serve as a new primary RM (replacement for NBS19) or primary calibrator aimed at the highest realisation of the VPDB scale for δ13C and δ18O values, including the VPDB‐CO2 δ18O scale. Methods IAEA‐603 preparation and characterisation (value transfer) against NBS19 were performed by addressing the major modern technical requirements for the production and characterisation of RMs (ISO Guide 35). IAEA‐603 was produced in a large quantity, and the first batch was sealed into ampoules (0.5 g) to ensure RM integrity during storage; four other batches were sealed for long‐term storage. The most accurate method of CO2 preparation for isotope mass spectrometry was used, namely carbonate–H3PO4 reaction under controlled conditions. Results The assigned values of δ13C = +2.460 ± 0.010‰ and δ18O = −2.370 ± 0.040‰ (k = 1) are based on a large number of analyses (~10 mg aliquots) performed at IAEA and address all the known uncertainty components. For aliquots down to 120 μg, the δ18O uncertainty remains unchanged but shall be doubled for δ13C. The uncertainty components considered are as follows: (a) material homogeneity (within and between the 5200 ampoules produced), (b) value assignment against NBS19, (c) storage effects and (d) effect of the 17O correction. Conclusions The new primary RM IAEA‐603 replaces NBS19 in its use as the highest calibrator for the VPDB δ13C and δ18O scale, including the VPDB‐CO2 δ18O scale. The use of IAEA‐603 will allow laboratories worldwide to establish consistent realisation of the scales for δ13C and δ18O values and metrological comparability of measurement results for decades. The VPDB scale definition based on NBS19 stays valid.

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Submarine groundwater discharge (SGD) into a shallow lagoon on the west coast of Mauritius Island (Flic-en-Flac) was investigated using radioactive ( 3 H, 222 Rn, 223 Ra, 224 Ra, 226 Ra, 228 Ra) and stable ( 2 H, 18 O) isotopes and nutrients. SGD intercomparison exercises were carried out to validate the various approaches used to measure SGD including radium and radon measurements, seepage-rate measurements using manual and automated meters, sediment bulk conductivity and salinity surveys. SGD measurements using benthic chambers placed on the floor of the Flic-en-Flac Lagoon showed discharge rates up to 500 cm/day. Large variability in SGD was observed over distances of a few meters, which were attributed to different geomorphological features. Deployments of automated seepage meters captured the spatial and temporal variability of SGD with a mean seepage rate of 10 cm/day. The stable isotopic composition of submarine waters was characterized by significant variability and heavy isotope enrichment and was used to predict the contribution of fresh terrestrially derived groundwater to SGD (range from a few % to almost 100 %). The integrated SGD flux, estimated from seepage meters placed parallel to the shoreline, was 35 m 3 /m day, which was in a reasonable agreement with results obtained from hydrologic water balance calculation (26 m 3 /m day). SGD calculated from the radon inventory method using in situ radon measurements were between 5 and 56 m 3 /m per day. Low concentrations of radium isotopes observed in the lagoon water reflected the low abundance of U and Th in the basalt that makes up the island. High SGD rates contribute to high nutrients loading to the lagoon, potentially leading to eutrophication. Each of the applied methods yielded unique information about the character and magnitude of SGD. The results of the intercomparison studies have resulted a better understanding of groundwater-seawater interactions in coastal regions. Such information is an important pre-requisite for the protection management of coastal freshwater resources.

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