Abstract
Over recent decades, observations based on merchant vessels (Ships of Opportunity—SOOP) equipped with sensors measuring the CO2 partial pressure (pCO2) in the surface seawater formed the backbone of the global ocean carbon observation system. However, the restriction to pCO2 measurements alone is one severe shortcoming of the current SOOP observatory. Full insight into the marine inorganic carbon system requires the measurement of at least two of the four measurable variables which are pCO2, total alkalinity (TA), dissolved inorganic carbon (DIC), and pH. One workaround is to estimate TA values based on established temperature-salinity parameterizations, but this leads to higher uncertainties and the possibility of regional and/or seasonal biases. Therefore, autonomous SOOP-based TA measurements are of great interest. Our study describes the implementation of a novel autonomous analyzer for seawater TA, the CONTROS HydroFIAⓇ TA system (-4H-JENA engineering GmbH, Germany) for unattended routine TA measurements on a SOOP line operating in the North Atlantic. We present the installation in detail and address major issues encountered with autonomous measurements using this analyzer, e.g., automated cleaning and stabilization routines, and waste handling. Another issue during long-term deployments is the provision of reference seawater in large-volume containers for quality assurance measurements and drift correction. Hence, a stable large-volume seawater storage had to be found. We tested several container types with respect to their suitability to store seawater over a time period of 30 days without significant changes in TA. Only one gas sampling bag made of polyvinylidene fluoride (PVDF) satisfied the high stability requirement. In order to prove the performance of the entire setup, we compared the autonomous TA measurements with TA from discrete samples taken during the first two trans-Atlantic crossings. Although the measurement accuracy in unattended mode (about ± 5 μmol kg^-1) slightly deteriorated compared to our previous system characterization, its overall uncertainty fulfilled requirements for autonomous TA measurements on SOOP lines. A comparison with predicted TA values based on an established and often used parameterization pointed at regional and seasonal limitations of such TA predictions. Consequently, TA observations with better coverage of spatiotemporal variability are needed, which is now possible with the method described here.
Highlights
The world ocean so far has taken up about 25% of the cumulative anthropogenic CO2 emissions since 1750 (Friedlingstein et al, 2019, 2020), which is a direct consequence of the enormous buffer capacity of the marine inorganic carbon system
Before we started with the actual testing, we set certain stability requirements, which needed to be fulfilled by the container: After 30 days of storage, each TA value after x days (TAx) need to fall within the ± 10 μmol kg−1 uncertainty range required by Integrated Carbon Observation System” (ICOS) for ship-based total alkalinity (TA) measurements
This value is based on the systems typical relative uncertainty of 0.1 % at a reference seawater TA of ≈ 2,215 μmol kg−1 demonstrated under field conditions (Seelmann et al, 2019)
Summary
The world ocean so far has taken up about 25% of the cumulative anthropogenic CO2 emissions since 1750 (Friedlingstein et al, 2019, 2020), which is a direct consequence of the enormous buffer capacity of the marine inorganic carbon system. In view of the central role of the ocean’s CO2 sink and its vulnerability to global change (Doney et al, 2009), we need to better observe and document the changing marine carbon cycle. This requires a globally concerted observational effort that makes use of the existing observation networks. A strong provider of high-quality ocean CO2 data from waters around Europe—with participation of our working group—is the ocean component of the European research infrastructure “Integrated Carbon Observation System” (ICOS). All pCO2 data acquired from this operational observation platform are quality-controlled according to international standards and protocols and delivered at regular intervals to SOCAT
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