Distributions of carbonate properties and oxygen along the water column (0–2000 m) in the central part of the NW Mediterranean Sea (Dyfamed site): influence of winter vertical mixing on air–sea CO2 and O2 exchanges

doi:10.1016/S0967-0645(02)00027-9

Deep Sea Research Part II: Topical Studies in Oceanography

Volume 49, Issue 11, 2002, Pages 2049-2066

https://doi.org/10.1016/S0967-0645(02)00027-9 Get rights and content

Abstract

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–Provençal Basin of the Mediterranean made it possible to assess the vertical distributions (5–2000 m) 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°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 150–200 m 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 μmol kg⁻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°C. The fugacity of CO₂, (fCO₂) varies between 320 and 430 μatm in surface water, according to the season. Below the seasonal thermocline, the maximum fCO₂ (about 410 μatm) is located around 150–200 m. 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, fCO₂ in surface water was on average below equilibrium with atmospheric fCO₂, thus implying that CO₂ was entering the sea. However, on this time scale, even with high exchange coefficients, the estimated CO₂ uptake had no significant influence on the inorganic carbon content in the water column.

Introduction

Since the Mediterranean Sea is considered as a vulnerable sea due to the high population density surrounding it, it is essential to have a better knowledge of the present state of the carbonate system. Few measurements on the carbonate system in the Mediterranean Sea, however, have been published (Millero et al., 1979; Brunet et al., 1984; Perez et al., 1986; Frankignoulle et al., 1990; Hecq et al., 1986; Copin-Montégut, 1993; Delgado and Estrada, 1994). Some of these measurements are relatively old, and their precision and accuracy are not as good as the present ones. For two years, monthly measurements were carried out on the carbonate system at the Dyfamed Station (43°25′N, 07°52′E) located in the central part of the northwestern basin of the Mediterranean Sea (Fig. 1) in order to assess the seasonal variations of the system and the relationships between the distributions of the inorganic carbon species and other properties such as temperature, salinity, oxygen and nutrients. As the central part of the northwestern basin of the Mediterranean Sea is the site of deep convection processes in winter, it is important to assess what effects these processes have on the air–sea CO₂ exchanges. The comparison of data from these two years, the first with low-mixing processes and the second with strong-mixing processes, indicates that CO₂ uptake from the atmosphere occurs during vertical mixing. This uptake appears to be very low compared with the O₂ import.

Section snippets

Methodology

Water samples for pH, alkalinity and oxygen were collected approximately every month at the Dyfamed Station at 22 depths between 5 and 2000 m (bottom depth: 2350 m) using a CTD rosette system (Seabird SBE 911+ with fluorescence and oxygen probes) from February 1998 to February 2000. Oxygen samples were titrated using an automated Winkler method with electrochemical endpoint detection. The addition of reagents for the reduction of oxygen was done immediately after sample collection, and the

Hydrological structure

The hydrological structure of the northwestern Mediterranean Sea is well known. A synthesis of the physical oceanography of the western Mediterranean has been recently published (Send et al., 1999), and a detailed study of the water mass circulation in the Ligurian–Provençal Basin is provided by Sournia et al. (1990) and Astraldi et al. (1995). In the central part of the northwestern Mediterranean Sea, where the Dyfamed Station is located, the hydrological structure is characterized by surface

Conclusion

In the central part of the northwestern Mediterranean Sea the same relationship was found between alkalinity and salinity as in the Alboran Sea. Nevertheless, more precise measurements for the Dyfamed samples showed that the alkalinity of the western Mediterranean deep-water is slightly higher than the alkalinity of the water above the LIW for the same salinity. This difference may be due to the redissolution of the calcium carbonate, but also to a slightly different alkalinity of the surface

Acknowledgements

This work was undertaken as part of the JGOFS-France program at the Dyfamed time series station. Financial support was provided by the PROOF program. We thank J.-C. Marty, responsible for the Dyfamed program, who facilitated this research, the captains and crews of the R.V. Tethys II, as well as Agnès Stock and Jacques Chiaverini for their work at sea. We acknowledge Liliane Merlivat for the Carbon Buoy data and Meteo France for the wind data.

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Citation Excerpt :
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Distributions of carbonate properties and oxygen along the water column (0–2000 m) in the central part of the NW Mediterranean Sea (Dyfamed site): influence of winter vertical mixing on air–sea CO2 and O2 exchanges

Abstract

Introduction

Section snippets

Methodology

Hydrological structure

Conclusion

Acknowledgements

Deep Sea Research I

Deep-Sea Research II

Deep-Sea Research I

Deep-Sea Research I

Deep-Sea Research I

Marine Chemistry

Marine Chemistry

Progress in Oceanography

Redfield ratios of remineralization determined by nutrient data analysis

Global Biogeochemical Cycles

The seasonal and interannual variability in the Ligurian–Provençal Basin

Alkalinity and carbon budgets in the Mediterranean

Global Biogeochemical Cycles

CO2 system in a Mediterranean frontal zone

Scientia Marina

Distributions of carbonate properties and oxygen along the water column (0–2000 m) in the central part of the NW Mediterranean Sea (Dyfamed site): influence of winter vertical mixing on air–sea CO₂ and O₂ exchanges

CO₂ system in a Mediterranean frontal zone