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A high-resolution nutrient biochemistry and carbonate system surface synoptic data set from the N-NE Brazilian continental shelf was reanalyzed to fill a gap in the time series of the carbonate system in the region and to allow us to perform a historical analysis of its evolution in recent years. We used data collected from 7 oceanographic cruises (n = 852) undertaken between March 1995 and September 2001 during the Brazilian Program "REVIZEE" in the North (N) and Northeast (NE) Economical Exclusive Zones of Brazil. Measured temperature and salinity data, which exhibited strong fluctuations (25.5 degrees C - 29.5 degrees C and 13.2-37.4 units, respectively), showed significant differences between the N and NE campaigns. The concentrations of dissolved inorganic nitrogen (DIN), PO4- and SiO2- were higher in the N region than in the NE region, mainly due to fluvial transport, and nitrogen: phosphorus (N:P) ratios of < 16 and oxygen supersaturation were observed within the Amazon plume. The concentrations of riverine nutrients in the N region support primary production occurring in the offshore plume area. The calculated total alkalinity (1031-2437 mol kg(-1)) values showed strong spatial variations that were mainly associated with the Amazon plume. The calculated pCO(2) values reached 423 mu atm offshore in the NE region during boreal winter. The calculated sea-air CO2 fluxes (average: + 0.3 +/- 1.7 mmol m(-2) d(-1); range: -1.2 to + 2.0 mmol m(-2) d(-1)) showed spatial and temporal variations, with negative values (sink) in the region of the Amazon River plume and positive values (source) offshore in the NE region (4 degrees S to 12 degrees S). The variability in the sea-air CO2 fluxes in the N and NE regions was explained by variations in biological activity and the thermodynamic effect of temperature, respectively. The analysis of available data, complemented with those presented here, indicated that the surface water pCO(2) values showed a positive temporal trend (+ 1.10 +/- 0.2 mu atm yr(-1)) in the NE region during the period of 1987-2010. This rate of increase is lower than that verified to have occurred in the atmosphere (+1.72 +/- 0.01 mu atm yr(-1)) during the same period.

Following the anomalous warming event occurring in the tropical North Atlantic in 2010, higher than usual surface fugacity of CO2 (fCO(2)) was observed. To evaluate the spatial extent of these anomalies and their drivers, and to quantify the sea-air CO2 flux at basin scale, the Mercator-Ocean model is used from 2006 to 2014 within the region 0-30 degrees N, 70-15 degrees W. Model outputs are generally in accordance with underway sea surface temperature, sea surface salinity, and surface fCO(2) recorded by two merchant ships. The anomalous warming of 2010 is well reproduced by the model and is the main driver of fCO(2) anomalies. The first coupled Empirical Orthogonal Function mode, between sea surface temperature and fCO(2), captures more than 70% of the total variance and is characterized by a basin-scale warming associated to positive fCO(2) anomalies. The corresponding principal components are correlated to the Tropical North Atlantic Index and identify 2010 as the year with the highest positive anomaly over 2006-2014. Exceptions to this general pattern are located near the African coast, where the weakening of the coastal upwelling causes negative inorganic carbon anomalies, and close to the Amazon River plume, where fCO(2) anomalies are primarily associated with sea surface salinity anomalies. Although the fCO(2) anomalies of 2010 appear mostly in spring, they affect the annual CO2 budget and lead to an increased CO2 outgassing twice as large (46.2 Tg C per year) as the mean annual flux over the 2006-2014 period (23.3 Tg C per year).

To reduce uncertainty regarding the contribution of continental shelf areas in low latitude regions to the air-sea CO2 exchange, more data are required to understand the carbon turnover in these regions and cover gaps in coverage. For the first time, inorganic carbon and alkalinity were measured along a cross-shelf transect off the coast of Maranhao (North Brazil) in 9 cruises spawning from April 2013 to September 2014. On the last 4 transects, dissolved organic matter and nutrients were also measured. The highest inorganic and organic carbon concentrations are observed close to land. As a result of low productivity and significant remineralization, heterotrophy dominates along the transect throughout the year. Although the temporal variability is significantly reduced at the offshore station with carbon concentrations decreasing seaward, the fugacity of CO2 (fCO(2)) at this station remains significantly higher, especially during the wet season, than the open ocean values measured routinely by a merchant ship further west. Overall, the continental shelf is a weak source of CO2 to the atmosphere throughout the year with an annual mean flux of 1.81 +/- 0.84 mmol m(-2) d(-1). The highest magnitudes of fCO(2) are observed during the wet season when the winds are the weakest. As a result, the CO2 flux does not show a clear seasonal pattern. Further offshore, fCO(2) is significantly lower than on the continental shelf. However, the oceanic CO2 flux, with an annual mean of 2.32 +/- 1.09 mmol m(-2) d(-1), is not statistically different from the CO2 flux at the continental shelf because the wind is stronger in the open ocean.

The intense rainfall in the tropical Atlantic spatially overlaps with the spread of the Amazon plume. Based on remote-sensed sea surface salinity and rainfall, we removed the contribution of rainfall to the apparent Amazon plume area, thus refining the quantification of its extension (0.84 +/- 0.06 x 10(6) km(2) to 0.89 +/- 0.06 x 10(6) km(2)). Despite the previous overestimation of the Amazon plume area due to the influence of rainfall (>16%), our calculated annual CO2 flux based on rainfall-corrected sea surface CO2 fugacity confirms that the Amazon River plume is an atmospheric CO2 sink of global importance (-7.61 +/- 1.01 to -7.85 +/- 1.02 Tg C yr(-1)). Yet we show that current sea-air CO2 flux assessments for the tropical Atlantic could be overestimated in about 10% by neglecting the CO2 sink associated to the Amazon plume. Thus, including the Amazon plume, the sea-air CO2 exchange for the tropical Atlantic is estimated to be 81.1 +/- 1.1 to 81.5 +/- 1.1 Tg C yr(-1).

CO2 fugacities obtained from a merchant ship sailing from France to French Guyana were used to explore the seasonal and interannual variability of the sea-air CO2 exchange in the western tropical North Atlantic (TNA; 5-14 degrees N, 41-52 degrees W). Two distinct oceanic water masses were identified in the area associated to the main surface currents, i.e., the North Brazil Current (NBC) and the North Equatorial Current (NEC). The NBC was characterized by permanent CO2 oversaturation throughout the studied period, contrasting with the seasonal pattern identified in the NEC. The NBC retroflection was the main contributor to the North Equatorial Counter Current (NECC), thus spreading into the central TNA, the Amazon River plume, and the CO2-rich waters probably originated from the equatorial upwelling. Strong CO2 undersaturation was associated to the Amazon River plume. Total inorganic carbon drawdown due to biological activity was estimated to be 154 mu mol kg(-1) within the river plume. As a consequence, the studied area acted as a net sink of atmospheric CO2 (from -72.2 +/- 10.2 mmol m(-2) month(-1) in February to 14.3 +/- 4.5 mmol m(-2) month(-1) in May). This contrasted with the net CO2 efflux estimated by the main global sea-air CO2 flux climatologies. Interannual sea surface temperature changes in the TNA caused by large-scale climatic events could determine the direction and intensity of the sea-air CO2 fluxes in the NEC. Positive temperature anomalies observed in the TNA led to an almost permanent CO2 outgassing in the NEC in 2010.