Origin of CO₂ undersaturation in the western tropical Atlantic

Abstract

A B S T R A C T Underway fCO 2 has been measured from two merchant ships sailing from France to French Guyana and France to Brazil, and during two zonal cruises from Africa to French Guyana. In the western Tropical Atlantic, the strongest undersaturation is associated with the Amazon discharge near 55 • W. In the 5 • S-10 • N, 65-35 • W region, the carbon system is strongly correlated to salinity and robust empirical relationships could be determined. This region is a sink of CO 2 in May-June during the high-flow period of the Amazon river. The eastward propagation of Amazon waters is observed when the retroflection of the North Brazil Current takes place. In August 2008, freshwater is observed as far as 40 • W when the North Equatorial Counter Current is quite strong. The Amazon plume, defined as salinities less than 34.9, is a sink of CO 2 of 0.96 mmol m −2 d −1 . Further east, near 27 • W, CO 2 undersaturation is recorded thoughout the year between 5 • N and 8 • N. This is caused by the high precipitation associated with the presence of the intertropical convergence zone (ITCZ). Removing the temperature effect leads to low (high) fCO 2 associated with low (high) salinities in boreal summer (winter), which is consistent with the seasonal migration of the ITCZ.

Carbon dioxide dynamics and fluxes in coastal waters influenced by river plumes

“…7.6). The relationship between pCO 2 and sea surface salinity (SSS) is remarkably similar among the different studies, with the lowest pCO 2 value of 125 µatm reported at an SSS value of 18 by Lefèvre et al (2010). Here, pCO 2 is nearly linearly related to SSS, in great contrast to the complex relationships observed in subtropical and middle-latitude coastal oceans impacted by nutrient-rich large rivers such as in the case of the Mississippi (Fig.…”

“…CO 2 data in the Amazon River plume have been reported by Ternon et al (2000), Körtzinger (2003), Cooley et al (2007), and Lefèvre et al (2010). DIC and TAlk showed quasi-conservative mixing behavior while pCO 2 is below atmospheric equilibrium, in contrast with surrounding waters of the tropical Atlantic Ocean that are above atmospheric equilibrium (Fig.…”

“…The river plumes they had studied were, however, limited to rivers of relatively low discharge and had data with little seasonal coverage. On the other end of the spectrum, several studies have provided an estimate of the CO 2 sink of the Amazon River plume and estimated the overall sink in the plume from an extrapolation based on the relationship between the partial pressure of CO 2 (pCO 2 ) and salinity (Ternon et al 2000;Körtzinger 2003;Cooley et al 2007;Lefèvre et al 2010). Additional data have shown that the relationship between pCO 2 and salinity in the Amazon is remarkably stable on a seasonal basis (Lefèvre et al 2010).…”

“…On the other end of the spectrum, several studies have provided an estimate of the CO 2 sink of the Amazon River plume and estimated the overall sink in the plume from an extrapolation based on the relationship between the partial pressure of CO 2 (pCO 2 ) and salinity (Ternon et al 2000;Körtzinger 2003;Cooley et al 2007;Lefèvre et al 2010). Additional data have shown that the relationship between pCO 2 and salinity in the Amazon is remarkably stable on a seasonal basis (Lefèvre et al 2010). Seasonal CO 2 distributions and air-water fluxes in inner estuaries and river plumes have also been reported for the Changjiang River (Wang et al 2000;Chen et al 2008;Chou et al 2009b;Zhai and Dai 2009) and preliminarily for the Mississippi River (Cai 2003;Dagg et al 2005; Lohrenz et al 2010;Huang 2013).…”

“…TAlk, DIC, and the pCO 2 vs. salinity distributions from the Amazon River plume (reproduced based on data supplied by N. Lefèvre). See Lefèvre et al (2010) for site and method description and for further discussion.…”