J. Severino P. Ibánhez

Seasonal and interannual variability of sea‐air CO2 fluxes in the tropical Atlantic affected by the Amazon River plume

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°N, 41–52°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 µmol 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.

The overlooked tropical oceanic CO2 sink

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 × 106 km2 to 0.89 ± 0.06 × 106 km2). 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.

Collapse of the tropical and subtropical North Atlantic CO2 sink in boreal spring of 2010.

Following the 2009 Pacific El Niño, a warm event developed in the tropical and subtropical North Atlantic during boreal spring of 2010 promoted a significant increase in the CO2 fugacity of surface waters. This, together with the relaxation of the prevailing wind fields, resulted in the reversal of the atmospheric CO2 absorption capacity of the tropical and subtropical North Atlantic. In the region 0–30°N, 62–10°W, this climatic event led to the reversal of the climatological CO2 sink of −29.3 Tg C to a source of CO2 to the atmosphere of 1.6 Tg C from February to May. The highest impact of this event is verified in the region of the North Equatorial Current, where the climatological CO2 uptake of −22.4 Tg for that period ceased during 2010 (1.2 Tg C). This estimate is higher than current assessments of the multidecadal variability of the sea-air CO2 exchange for the entire North Atlantic (20 Tg year−1), and highlights the potential impact of the increasing occurrence of extreme climate events over the oceanic CO2 sink and atmospheric CO2 composition.

Net Heterotrophy in the Amazon Continental Shelf Changes Rapidly to a Sink of CO2 in the Outer Amazon Plume

The Amazon continental shelf and adjacent oceanic area were sampled for inorganic and organic carbon parameters in order to improve data coverage and understanding of carbon cycling dynamics within this important region. Seasonal coverage of the Amazon plume on the French Guiana continental shelf further north, was provided by CO2 monitoring using a merchant ship sailing from France to French Guiana (2006-2016). Salinity ranged from 1 to 36 (transects in April 2013, and May 2014). At salinity below 10, strong outgassing was observed with fugacity of CO2 (fCO2) over 2000 atm. This region displayed net heterotrophy, fueled by organic matter with terrestrial origin, as shown by 13C and 15N values of suspended particles. A 13C cross shelf average of -31‰ was measured during May 2014, contrasting with oceanic values in excess of -20‰. The reactivity of this terrestrial material resulted in the local production of dissolved inorganic and organic carbon as well as fluorescent humic compounds. Further offshore, the dilution of freshwater by ocean waters created a sink for CO2, enhanced by biological activity. The strongest CO2 drawdowns, associated with high chlorophyll a concentrations, were observed on the French Guiana continental shelf in the outer Amazon plume, with fCO2 values below 150 uatm. Here, a CO2 sink was present almost throughout the year, with a seasonal maximum of -9.2 mmol CO2 m-2d-1 observed in June 2015. However, both the CO2 and salinity distributions could vary significantly within a few days, confirming the presence of many eddies in this region. The Amazon continental shelf hence behaved as a transition zone between an inshore source of CO2 to the atmosphere and an offshore sink. Some marine phytoplankton production was detected but occurred mainly close to the French Guiana shelf. A mean net CO2 outgassing of 44 ± 43.6 mmol m-2d-1 was estimated for the area. Quantifying the CO2 flux for the entire Amazon shelf area led to a revised annual estimate of the net ocean carbon export of 2.86 1012 mol C from the region.