Distributions of carbonate properties along the water column in the Mediterranean Sea: Spatial and temporal variations

Surface and water column measurements of the total alkalinity and the pH were recorded in the Mediterranean Sea and in the Southern Tyrrhenian Sea from May–June 2007 and from November 2006 and February 2008, respectively. The measurements were conducted within the framework of the Italian VECTOR Research Project activities.The total alkalinity was measured using a potentiometric titration with an open cell system. The pH was also determined using a potentiometric method, with a combination of glass/reference electrode with an NTC temperature sensor. The total alkalinity varied linearly with the salinity in the Mediterranean Sea and the measure ranged between 2431 and 2638 μmol kg−1, with the lowest values in the upper layer of the westernmost data stations (2431 – 2439 μmol kg−1). This pattern resulted from the influence of less salty water with low-alkalinity intruding from the nearby Atlantic waters. With respect to the seasonal variation, the data collected in the Southern Tyrrhenian Sea did not have any statistically significant differences for the alkalinity parameter in the water column layers. The measured pH exhibited high and variable values in the surface layer and an increasing gradient from the western to the eastern Mediterranean data collection stations, particularly at intermediate depths around 300–500 m. The minimum pH and the maximum of the total alkalinity were commonly found at mid-depth in correspondence of the core of Levantine Intermediate Water.The pCO2 values were calculated from the direct measurements of the alkalinity and the pH. During the summer of 2007, the pCO2 in the surface water of the Mediterranean was, on average, above the equilibrium with the atmospheric pCO2, thus implying that the CO2 was escaping the sea. Seasonal measurements at the VTM station in the Southern Tyrrhenian Sea indicated that the pCO2 surface values ranged from 323 to 430 ppm, with the lowest values in February 2007 and 2008, when the VTM station represented a sink for the atmospheric CO2. The Tracer combining Oxygen, inorganic Carbon and total Alkalinity (TrOCA) was used to estimate the distribution of the anthropogenic CO2. The results show that the entire water column has already been invaded by the anthropogenic CO2 throughout the Mediterranean Sea. Generally, the concentrations of the anthropogenic CO2 in the intermediate and deep layers were higher than those measured in the Atlantic waters.