Measurements of CO 2 parameters ( i.e. Total Alkalinity (TA) and Dissolved Inorganic Carbon (DIC)) were made from June 2005 to September 2007 in six EGEE (“Etude de la circulation océanique et de savariabilitédans le Golfe de GuinEE”) cruises to better assess air-sea CO 2 fluxes in the Gulf of Guinea (6°N - 10°S, 10°E - 10°W). Two empirical relationships TA-Salinity and DIC-Salinity-Temperature were established. These relationships were then used to estimate the monthly fugacity of CO 2 (fCO 2 ) and air-sea CO 2 fluxes. The monthly mean flux of CO 2 reaches 1.76 ± 0.82 mmol·m -2 ·d -1 (resp. 2.90 ± 1.45 mmol·m -2 ·d -1 ) at the north of the Equator (resp. at the South). The north-south gradient observed as the patterns of the air-sea CO 2 fluxes was mainly driven by the oceanic fCO 2 . This gradient was due to the low values of the CO 2 parameters flowing by the Guinea Current (6°N - 0°) from the west to the east while the air-sea CO 2 fluxes increased in the south (10°S - 0). In the north, the climatology of Takahashi underestimated the CO 2 fluxes in the Gulf of Guinea when comparing to the estimated fluxes. This was due to the north-south gradient, which did not well reproduce by the climatology of Takahashi.
During the 1980-2000 period, the fugacity of CO2 (fCO2) measurements has been carried out to follow the evolution of oceanic CO2 in the eastern equatorial Atlantic [
Different methods have been used to assess these air-sea CO2 fluxes [
The region of the tropical Atlantic belt represented also a source of CO2 with a low seasonal variability [
Six oceanographic cruises (
A total of 195 samples surface seawater was collected for Dissolved Inorganic Carbon (DIC) and TA Total Alkalinity (TA) analyses. Samples were poisoned with a saturated HgCl2 solution to stop biological activities. DIC and TA were measured using potentiometric titration that derived from the method developed by Edmond [
Cruises | Dates |
---|---|
EGEE 1 | 7th June - 6th July 2005 |
EGEE 2 | 29th August - 30th September 2005 |
EGEE 3 | 27th May - 7th July 2006 |
EGEE 4 | 19th November - 1st December 2006 |
EGEE 5 | 6th June - 3rd July 2007 |
EGEE 6 | 1st - 30th September 2007 |
The fCO2 measured during EGEE 3 was used to determine the best set of dissociation constants for the calculation of fCO2 at stations where DIC and TA were recorded. It was also used to validate fCO2 derived from our extrapolated DIC and TA along the EGEE 3 cruise track. Then, EGEE data were supplemented by data from FOCAL 4, 6 and 8, CITHER 1, and EQUALANT 99 cruises provided by several authors [
The precipitation dataset was extracted from the Global Precipitation Climatology Project (GPCP) [
The air-sea CO2 fluxes (F) is calculated using Equation (1):
Whereas
In addition, the fluorescence was measured using the CTD sensor during the EGEE cruises while chlorophyll a was sampled only during EGEE 3 cruise and analyzed according to the HPLC standard technique [
Cruises | FOCAL (4, 6, 8) | CITHER 1 | EQUALANT 99 |
---|---|---|---|
Dates | July - August 1983 (F4) January - February 1984 (F6) July - August 1984 (F8) | January - March 1993 | July - August 1999 |
fluorescence data will be used in this study instead of chlorophyll a since it is available in all cruises.
The following paragraphs outline the chemistry of carbon dioxide in the ocean. When it is dissolved in the seawater, the carbonate system can be described by the Equations (2), (3), (4) and (5).
where, K1 and K2 represent stoichiometric equilibrium constants for the description of the carbonate system in the seawater. The different sums of the dissolved forms (i.e. CO2,
In Equation (5), the carbonate ion
Unfortunately, the concentrations of the individual species of the carbon dioxide system in solution cannot be measured directly. The Equations (2), (3), (4) and (5) have six unknown variables (i.e. CO2,
Moreover, oceanic fCO2 was estimated from TA and DIC using the different dissociation constants. In order to choose the best dissociation constants, measured fCO2 [
For each year, the relationships established by [
The annual and monthly fluxes of CO2 in GG (6˚N - 10˚S; 10˚E - 10˚W) derived from the [
Cruises | |||
---|---|---|---|
fCO2 (μatm) | FOCAL 6 (January - February 84) | CITHER 1 (January - March 93) | EQUALANT 99 (July - August 99) |
Measured | 373 ± 23 | 370 ± 30 | 398 ± 30 |
Calculated | 377 ± 8 | 373 ± 5 | 403 ± 11 |
by dividing its weekly (resp. monthly) mean value by seven (resp. by 30 days). [
The study area (6˚N - 10˚S; 10˚E - 10˚W) was divided in two regions (see
According to [
Before computing TA and DIC, it is useful to remind and understand the processes, which impact the distribution of these carbon components in the ocean. Air-sea exchange of CO2 changes the content of the inorganic carbon species in seawater but leaves TA unaltered. TA is the equivalent of all bases that can accept a proton to the
carbonic acid endpoint. Bicarbonate and carbonate are roughly 98% of TA at pH = 8.1 [
The
To remove the seasonal and interannual variability, the anomalies of the air-sea CO2 fluxes were computed
CO2 fluxes (mmol・m−2・d−1) | ||
---|---|---|
Year | January - June | July - December |
2005 | 1.58 ± 1.16 | 4.08 ± 1.61 |
2006 | 1.39 ± 1.06 | 3.75 ± 1.47 |
2007 | 1.52 ± 1.17 | 4.01 ± 1.54 |
and standardized for 2005, 2006 and 2007. During these periods, no relationship was found between ENSO index and the air-sea CO2 fluxes (not shown). The Hovmöller diagram (
This sub-section highlights the differences between the CO2 fluxes estimated during EGEE 3 in 2006, the climatology of [
tended to smooth the difference between northern and southern waters. In the case of F4 and F8, the comparison is made by averaging the CO2 fluxes between 5˚N - 5˚S and along 4˚W where data were available.
From June to December, the climatology of [
During F4 and F8 cruises, the fluxes reached −0.25 ± 0.79 mmol・m−2・d−1 and −0.16 ± 1.27 mmol・m−2・d−1 (resp. 1.28 ± 1.32 mmol・m−2・d−1 and 1.71 ± 2.09 mmol・m−2・d−1) in the north (resp. in the south). The weak values in the north implied an equilibrium state with the atmosphere, while the high values in the south indicated that the region was a source. The climatology of [
The purpose of this paper is to 1) assess the best annual air-sea CO2 fluxes estimates and 2) quantify its seasonal and interannual variability in the Gulf of Guinea. EGEE data from June 2005 to September 2007 were used to realize this work.
The relationships established by [
since the biological activity had a weak impact on the variability of the oceanic CO2.
The ocean CO2 source was higher in the south of the Equator than in the north. This was due to the upwelling system that transports DIC rich water at the surface. A north-south gradient was also observed in the distribution of the air-sea CO2 fluxes in the Gulf of Guinea. The Guinea Current that allowed transporting eastward-unsalted waters due to precipitations and contributed to the dilution and the decrease of CO2 in the north induces this gradient. In both regions, the air-sea CO2 fluxes presented a clear seasonality with low values in January - May and high values in July - October. When using the same gas transfer coefficient [
We thank the crew members of the N/O Antéa for their help during the cruise and particularly Bernard Bourles (IRD member), the coordinator of the EGEE/AMMA project. Grateful thank to Fidel Yoroba and Michel Agba for their advices. TMI data are available at www.remss.com. QuikScat (or SeaWinds) data are produced by Remote Sensing Systems and sponsored by the NASA Ocean Vector Winds Science Team. Data are available at www.remss.com. DIC and TA analyses have been performed by the service national d’analyses des paramètres du CO2 (SNAPO-CO2) at the LOCEAN laboratory.
UrbainKoffi,GeorgesKouadio,Yves K.Kouadio, (2016) Estimates and Variability of the Air-Sea CO2 Fluxes in the Gulf of Guinea during the 2005-2007 Period. Open Journal of Marine Science,06,11-22. doi: 10.4236/ojms.2016.61002