The active deep overturning circulation of the Mediterranean is emerging as one of the most effective mechanisms in transporting the atmospheric imprint on the carbon cycle to the interior of the basin. There is growing evidence that sites of dense water formation over the continental shelf, such as the Northern Adriatic Sea, play a key role in this process. Nevertheless, little is known about the inorganic carbon chemistry of the Adriatic sea, and CO2 absorption and its fate.
The winter of 2012 experienced peculiar meteorological conditions with an extended period of cold weather with strong winds that triggered, in February, a massive formation of an extremely cold and dense (potential density anomaly > 30.00 kg m− 3) Northern Adriatic Dense Water (NAdDW) water mass. This event provided a unique opportunity to study this process at sub-basin scale taking into account CO2 adsorption within the NAdDW source area (the Gulf of Trieste in the northern Adriatic), and its spreading over the shelf and into the Southern Adriatic Pit.
The northern Adriatic and the Gulf of Trieste, during winter, act as a CO2 sink. The average air-sea CO2 flux of 60 mmol m− 2 d− 1 estimated during the exceptional 2012 event was at least 3 times higher than the flux measured in winter 2008 when dense water was produced through the same mechanism but under less extreme conditions. In winter 2012, absorbed CO2 resulted in the decrease of pHT25 down to 7.907 (− 0.034 pHT units) and the strong evaporation induced by wind-increased total alkalinity (TA) to 2673 (+ 16 μmol kg− 1).
Following its formation in the North, the NAdDW plume entering the Southern Adriatic, observed in March 2012, exhibited significantly modified values. The plume was characterized by colder temperature (~ 10 °C), lower pHT25 (7.947 pHT units) and higher alkalinity (2635 μmol kg− 1) than the surrounding water masses along the western Adriatic shelf. However, the signal of atmospheric CO2 enrichment was weaker than in the northern Adriatic source region, as well as positive apparent oxygen utilization (AOU) values (~ 20 μmol kg− 1) were recorded. This is suggestive of oxygen consumption in the water mass.
Observed changes in both physical and biogeochemical properties were similar to those observed in 2008, suggesting that mixing with Levantine Intermediate Waters (LIW) was the main driver modulating the changes of AOU and inorganic carbon chemistry in both winters. The rising of pHT25 and AOU due to the mixing indicates that NAdDW, at its origin, was richer in atmospheric CO2 than the LIW was, thus confirming the relevance of the Northern Adriatic Sea for CO2 adsorption.
The study provides the first characterization of inorganic carbon chemistry, including carbonate minerals saturation states (ΩAr and ΩCa), in the bottom waters both on the slope and along the expected pathways of dense water cascading in the Adriatic Sea. Therefore, it can represent a baseline to improve the knowledge on the acidification process and impacts as well as being useful for comparison with other benthic environments.
Cantoni C., Luchetta A., Chiggiato J., Cozzi S., Schroeder K. & Langone L., in press. Dense water flow and carbonate system in the southern Adriatic: A focus on the 2012 event. Marine Geology. Article (subscription required).
