The carbonate system in the Western Black Sea Shelf: new observations reveal drivers of carbon source/sink variability

The Black Sea is a large marginal European sea with a unique biogeochemical feature of complete anoxia below 200 m depth. Past studies have also found surface dissolved inorganic carbon (DIC) concentrations and total alkalinity (TA) much above typical global ocean values. Despite the acknowledged importance of shelf areas in global carbon cycle assessments and the uniqueness of this environment, the Black Sea remains a poorly observed region for surface measurements of the partial pressure of carbon dioxide (pCO₂). Here, we close this observational gap and provide an updated assessment of the Western Black Sea shelf carbonate system by using two years of data from a new coastal observing station, established in 2022 within the Helmholtz Association European Partnership SEA-ReCap project, and equipped with a FerryBox. Additional measurements with small boat and research cruise surveys provide a comprehensive analysis of the biogeochemical variability in the Western Black Sea region, with some significant anomalous events. We find the typical annual cycle of seawater pCO₂ is largely temperature-driven, with monthly averages ranging between 340 and 500 µatm. The region alternates between a carbon source/sink status, but overall there is an efflux to the atmosphere of 2.2 ± 6.7 mmol C m^-2 day^-1. Superposed on the typical cycle, we observed strong non-thermal anomalous events. Examples are low-salinity water influx driving new biological production, with an associated drop in seawater pCO₂ to below 200 µatm, or coastal upwelling bringing bottom shelf water low in oxygen and high in DIC to the surface, raising the pCO₂ to above 800 µatm. Using the similarities between the observations at the coastal station and those we performed on the moving vessels, we were able to calculate a regionally integrated air-sea exchange component of the carbon budget. The restricted Western Black Sea shelf emitted 0.012 Tg C to the atmosphere during a regular year, while during an anomalous year, this amount increased to 0.020 Tg C. Finally, we used a robust relationship between laboratory measured samples of DIC and TA from the research cruise and calculated values from underway pCO₂ and pH measurements to create quasi-continuous time series of all carbonate system parameters at the coastal station. These results and the sustained monitoring at the coastal station will help us assess the region’s resilience to climate and anthropogenic forcings, as we have been able to demonstrate in parallel studies on extreme storms in this region we captured in 2023.