Preprints
https://doi.org/10.5194/egusphere-2025-3588
https://doi.org/10.5194/egusphere-2025-3588
08 Aug 2025
 | 08 Aug 2025
Status: this preprint is open for discussion and under review for Ocean Science (OS).

Metrological concepts applied to Total Alkalinity measurements in seawater: reference materials, inter-laboratory comparison and uncertainty budget

Gaëlle Capitaine, Samir Alliouane, Thierry Cariou, Jonathan Fin, Paola Fisicaro, and Thibaut Wagener

Abstract. Total alkalinity (TA) measurements in seawater are crucial for characterizing and monitoring the oceanic carbonate system. While international best practices and guidelines exist, the field still lacks widely available traceable reference materials and a well-established uncertainty budget of the measurement method. In this study, we applied key metrological principles – development of reference materials, inter-laboratory comparison and uncertainty quantification – to TA measurements. We developed two reference materials, including an artificial material with a rigorously characterized reference value and an associated uncertainty budget, being potentially traceable to the International System of units (SI). These materials were tested in an inter-laboratory comparison involving five laboratories and demonstrated the applicability and interest of the reference materials developed for quality control. Additionally, we established an uncertainty budget for the TA measurement method using two metrological approaches. The resulting expanded uncertainty was 5 µmol kg⁻¹ (k = 2) in TA, approaching the 4 µmol kg⁻¹ target set by the Global Ocean Acidification Observing Network for climate monitoring. These findings mark a significant step toward improving the quality and comparability of TA measurements, thereby strengthening long-term ocean carbonate system monitoring.

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Gaëlle Capitaine, Samir Alliouane, Thierry Cariou, Jonathan Fin, Paola Fisicaro, and Thibaut Wagener

Status: open (until 11 Oct 2025)

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Gaëlle Capitaine, Samir Alliouane, Thierry Cariou, Jonathan Fin, Paola Fisicaro, and Thibaut Wagener
Gaëlle Capitaine, Samir Alliouane, Thierry Cariou, Jonathan Fin, Paola Fisicaro, and Thibaut Wagener

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Short summary
Measuring total alkalinity in seawater is essential for understanding and monitoring the ocean carbonate system. To improve the reliability of these measurements, we developed reference materials and tested them in an inter-laboratory comparison. We also thoroughly quantified, for the first time, the uncertainty of the standard measurement method. These results, as well as the key metrological tools developed, support more accurate long-term monitoring of the ocean carbonate system.
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