Abstract
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.
Highlights
The Amazon River is a strong source of CO2 to the atmosphere with values of the fugacity of CO2 of the order of 4,000 μatm (Richey et al, 2002; Mayorga et al, 2005)
The transect in May 2014 consisted of 9 stations with CTD casts, sampling for dissolved inorganic nitrogen (DIN), phosphate, dissolved organic carbon (DOC), suspended particulate matter (SPM), fluorescent dissolved organic matter (FDOM, i.e., the DOM fraction with fluorescence properties), and one surface sample for dissolved inorganic carbon (TCO2) and total alkalinity (TA)
The error on predicted TCO2 is 27.1 μmol kg−1. This relationship is in good agreement with the May 2014 observation on the same transect, and the TCO2 of the Camadas Finas cruises 3 and 5 measured on the Amazon continental shelf in boreal autumn (Figure 2b)
Summary
The Amazon River is a strong source of CO2 to the atmosphere with values of the fugacity of CO2 (fCO2) of the order of 4,000 μatm (Richey et al, 2002; Mayorga et al, 2005) It has the strongest freshwater discharge of the world, ranging from a maximum in May–June of ∼250,000 m3/s to a minimum of ∼80,000 m3/s in October–November, preventing saline water intrusion through the river mouth (Geyer and Kineke, 1995). Because of the rather sluggish exchange with the atmosphere, CO2 drawdown driven by the Amazon plume productivity has been observed as far as 25◦W (Lefèvre et al, 1998). This CO2 drawdown persists over a long distance, and has a significant impact on the carbon budget of the tropical Atlantic (Ibánhez et al, 2016)
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