PhD Defense

Six cruises have been carried out in the eastern equatorial Atlantic between June 2005 and September 2007 which main aim is to study the distribution of the carbon parameters, the process induced their variability and to estimate the air- sea CO2 flux.

195 surface seawater samples were taken for Total Alkalinity (TA) and Dissolved Inorganic Carbon (DIC) analyses. During these cruises a strong north–south gradient is observed in sea surface temperature (SST), sea surface salinity (SSS), DIC and TA. Low (or high) surface salinity waters are associated with low (or high) TA and low (high) DIC. They are observed north of the equator (0°–6°N) in the Guinea current, whereas higher concentrations are observed south of the equator in the South Equatorial current. As no significant temporal variability has been observed for TA and DIC, the whole dataset has been used to determine robust relationships between TA and SSS, and between DIC and SSS and Sea Surface Temperature (SST).TA–SSS and DIC–SST–SSS relationships are determined and used to calculate surface fCO2. Monthly maps of the mean fCO2 are obtained, on a one degree grid, using SST and SSS fields for the region 10°W–10°E, 6°N–10°S from June to November during the period of 2005 to 2007. The monthly mean flux of CO2 ranges from 1.69 ±1.94 mmol m−2 d−1 in November to 2.78 ±1.65 mmol m−2 d−1 in August over the region.

These results allowed the comparison of our air –sea CO2 flux and those of climatology by using the same gas exchange coefficient. Our air –sea CO2 flux estimated show a stronger gradient between northern and southern waters than the climatology, probably due to a lack of data in the climatology as well as its 4° latitude by 5° longitude resolution. However, the two methods of estimates the air –sea CO2 flux converge to the eastern equatorial Atlantic is a CO2 source for the atmosphere.