L'objet de l'étude était la réalisation et l'analyse de mesures de pH et d'alcalinité lors de la mission Ovide 2004 en une centaine de station réparties entre le Groenland et le Portugal. Ces mesures donnent des indications sur le cycle du carbone dans l'océan et permettent en particulier d'estimer la contribution du carbone d'origine anthropique. Le carbone d'origine anthropique est pour partie stocké dans les océans, il est donc crucial d'en faire l'inventaire et de déterminer son devenir.

The carbon system is defined by four variables: pH, Total Alkalinity (AT), partial pressure of carbon dioxide (pCO2) and Total Inorganic Carbon (CT). The knowledge of two of these variables allows the calculation of the other two by means of a set of equations deduced from thermodynamic equilibrium. During the OVIDE 2004 cruise carried out between 5th June and 6th July on board the RN THALASSA pH and TA measurements were sampled fiom bottle depths at selected stations (Table 1) and analysed on board. Moreover, pCOz has been continuously detennined in surface waters along the vesse1 track. In this cruise, unlike OVIDE 2002, samples for CT were also taken, but will be analyzed at lab. CT is also calculated from pH and AT. In this report we resume the activities, methods and results obtained during the OVIDE 2004 cruise. Besides, at the end of the report, two more reports from A. Dickson (Scripps) and from Fiz F. Pérez are included. During the cruise 80 samples for intercalibration were collected and sent to A. Dickson. In the Dickson's report, very important deviations were detected from the measured obtained on board and those obtained in the Dickson's lab one year later. Fiz F. Pérez showed that these important and very significant differences were not due to the analytical work made on board. We are including here the last email form A. Dickson because it is very conclusive.