Enhance Ocean Carbon Observations: Successful Implementation of a Novel Autonomous Total Alkalinity Analyzer on a Ship of Opportunity

• Published in: Frontiers in Marine Science
• Main Authors: Seelmann, Katharina, Steinhoff, Tobias, Aßmann, Steffen, Körtzinger, Arne
• Format: Article in Journal/Newspaper
• Language: English
• Published: 2020
• Subjects: article; ScholarlyArticle; ddc:333.7; Published Version; total alkalinity (TA); ocean carbon observations; autonomous analyzer; North Atlantic; Ships of Opportunity; SOOP
• Online Access: https://doi.org/10.3389/fmars.2020.571301; https://nbn-resolving.org/urn:nbn:de:gbv:8:3-2021-00040-9; https://macau.uni-kiel.de/receive/macau_mods_00000968; https://macau.uni-kiel.de/servlets/MCRFileNodeServlet/macau_derivate_00002019/fmars-07-571301.pdf

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 ...