Gaining insights into the seawater carbonate system using discrete fCO2 measurements

Understanding the ocean carbon sink and its future acidification-derived changes requires accurate and precise measurements with good spatiotemporal coverage. In addition, a deep knowledge of the thermodynamics of the seawater carbonate system is key to interconverting between measured and calculated variables. To gain insights into the remaining inconsistencies in the seawater carbonate system, we assess discrete water column mea-surements of carbon dioxide fugacity (fCO(2)), dissolved inorganic carbon (DIC), total alkalinity (TA), and pH measured with unpurified indicators, from hydrographic cruises in the Atlantic, Pacific, and Southern Oceans included in GLODAPv2.2020 (19,013 samples). An agreement of better than +/- 3% between fCO(2) measured and calculated from DIC and pH is obtained for 94% of the compiled dataset, while when considering fCO(2) measured and calculated from DIC and TA, the agreement is better than +/- 4% for 88% of the compiled dataset, with a poorer internal consistency for high-CO2 waters. Inspecting all likely sources of uncertainty from measured and calculated variables, we conclude that the seawater carbonate system community needs to (i) further refine the thermodynamic model of the seawater carbonate system, especially K-2, including the impact of organic com-pounds and other acid-base systems on TA; (ii) update the standard operating procedures for the seawater carbonate system measurements following current technological and analytical advances, paying particular attention to the pH methodology that is the one that evolved the most; (iii) encourage measuring discrete water column fCO(2) to further check the internal consistency of the seawater carbonate system, especially given the new era of sensor-based seawater measurements; and (iv) develop seawater Certified Reference Materials (CRMs) for fCO(2) and pH together with seawater CRMs for TA and DIC over the range of values encountered in the global ocean. Our conclusions also suggest the need for a re-evaluation of the adjustments applied by GLODAPv2 to pH, which were based on DIC and TA consistency checks but not supported by fCO(2) and DIC consistency.

Automated summary

Accurate and precise measurements, as well as a deep knowledge of the thermodynamics of the seawater carbonate system, are necessary to understand the ocean carbon sink and its future acidification-derived changes. Inconsistencies in the seawater carbonate system call for the refinement of the thermodynamic model, updates to measurement procedures, encouragement of further measurements, and the development of seawater Certified Reference Materials (CRMs).