Distributions of carbonate properties and oxygen along the water column (0-2000m) in the central part of the NW Mediterranean Sea (Dyfamed site): influence of winter vertical mixing on air-sea CO sub(2) and O sub(2) exchanges

• Published in: Deep-sea research. Part II, Topical studies in oceanography Topical studies in oceanography, 2002-01, Vol.49 (11), p.2049-2066
• Main Authors: Copin-Montegut, C, Begovic, M
• Format: Article
• Language: English
• Subjects: Marine
• ISSN: 0967-0645

Monthly measurements of pH, alkalinity and oxygen over two years (February 1998-February 2000) at the Dyfamed site in the central zone of the Ligurian-Provencal Basin of the Mediterranean made it possible to assess the vertical distributions (5-2000m) and the seasonal variations of these properties. Alkalinity varies linearly with salinity between surface water and the Levantine Intermediate Water (marked by a maximum of temperature and salinity). In deep water, total alkalinity is also correlated linearly to salinity, but the slope of the regression line is 15% less. In surface water, the pH at 25 degree C varies between 7.91 and 8.06 on the total proton scale depending upon the season. The lowest values are observed in winter, the highest in spring and in summer. These variations are primarily due to biological production. The pH goes through a minimum around 150-200m and a small maximum below the intermediate water. The total dissolved inorganic carbon content (deduced from pH and alkalinity) is variable in surface water (2205-2310 mu molkg super(-1)) and has a maximum in intermediate water, which is related to the salinity maximum. Normalized total inorganic carbon at a constant salinity is strongly negatively correlated with pH at 25 degree C. The fugacity of CO sub(2), (fCO sub(2)) varies between 320 and 430 mu atm in surface water, according to the season. Below the seasonal thermocline, the maximum fCO sub(2) (about 410 mu atm) is located around 150-200m. The presence of a minimum of oxygen in the intermediate water of this area has been observed for several years, but our measurements made it possible to specify the relationship between oxygen and salinity in deep water. Data from the intense vertical mixing during the winters of 1999 and 2000 were used to calculate the oxygen quantity exchanged with the atmosphere during these periods. The estimated quantity of oxygen entering the Mediterranean Sea exceeds that deduced from exchange coefficients calculated with the formula of Wanninkhof and McGillis. During the vertical mixing in the 1999 winter, fCO sub(2) in surface water was on average below equilibrium with atmospheric fCO sub(2), thus implying that CO sub(2) was entering the sea. However, on this time scale, even with high exchange coefficients, the estimated CO sub(2) uptake had no significant influence on the inorganic carbon content in the water column.