OneArgo’s bridging role in ocean CO2 observations – The Baltic Sea pilot case

The Ocean currently takes up ca. 25 % of anthropogenic CO₂ emissions and hosts the largest carbon pool between atmospheric, terrestrial and oceanic biospheres. To properly observe and document changes in ocean CO₂, a combined approach between observation and modelling, but also the combination of different observation approaches is needed.

The OneArgo programme and its autonomous float measurements can provide the link between research vessel-based observations of the whole water column such as organized in the GO-SHIP programme and surface CO₂ observations by Ships of Opportunity (SOOP), organized in the SOCONET (Surface Ocean CO₂ Reference Observing Network) programme or its European pillar ICOS (Integrated Carbon Observation System). Autonomous float measurements have the additional benefit to outperform these observation programmes in timeliness for operational applications thanks to its fully near real-time data availability. While the required raise in funding for a global OneArgo implementation slowly helps to build up the array, we present work from the Baltic Sea that can be seen as a regional pilot.

Here, (1) surface carbon measurements by a SOOP, (2) vertical profiling float data of pCO₂, and (3) research-vessel based water sampling of the water column build the foundation for a comprehensive CO₂ observation network. By cross-validating data across the different research infrastructures, we ensure that data are interoperable. Next steps are to integrate data from the different sources into a comprehensive 4D BGC product of Baltic Sea CO₂. While its global counterpart will be based on profiling float pH observations instead of pCO₂, the approach and procedures can be mimicked, e.g., in the subpolar North Atlantic, a key region of the oceanic carbon cycle.

Combined, the three different research infrastructures provide highly complementary information to quantify CO₂ uptake, on the cycling and fate of CO₂ in the water column, and to inform on timescales of CO₂ sequestration. Future float deployments in the Baltic Sea will involve sensors for nitrate, oxygen debt, and hyperspectral radiometry to expand OneArgo’s bridging role and scope, e.g., to link up with satellite remote sensing products.