Applicability of CO2-related concentration indicators by classifying various seawaters in the world

●It is necessary to monitor CO2 in seawater to ensure its safe offshore storage.●CO2 concentration in seawater varies greatly with biological activity.●To avoid false positive, using multiple indicators is necessary and effective.●By classifying seawaters, the indicators can have acceptably small variations.●The applicability of the proposed indicators to world seawaters was proven. Carbon dioxide capture and storage is expected to reduce large amount of CO2 emissions from large sources, such as coal power plants. For offshore CO2 storage, it is necessary to monitor CO2 concentration in seawater to ensure safe storage. However, such concentrations have large regional and seasonal variations due to biological activity. In our previous study, to avoid the misidentification of large natural fluctuations as abnormally high CO2 concentrations (false positive), a method was proposed using multiple indicators of CO2-related concentration in seawater: namely, anomalies are judged only if there are data that exceed statistical criteria, e.g. 3-sigma, of all the multiple correlations of different types. This is because any single indicator can have data exceeding such criteria in theory. In this study, we examined the universality of applying multiple indicators of different types to seawaters around the world. The results show that the world seawaters can be classified into three groups using the relationships between alkalinity and salinity and the vertical profiles of dissolved oxygen, and that at least one of the four indicators has a clear correlation in each group, suggesting that the applicability of the proposed method using multiple indicators has been proved.