Sea-air CO2 fluxes along the Brazilian continental margin

Abstract

Measurements of the marine carbonate system on tropical and subtropical continental margins are poorly distributed in space and time, with many uncertainties persisting regarding the role of carbon exchanges at the ocean-atmosphere interface in these areas. To calculate sea-to-air CO2 fluxes in Marine Ecoregions along the Brazilian continental margin (4°N to 34°S), we used data from the Surface Ocean CO2 Atlas (SOCAT v2020), collected up to 400 km from the coast, at the surface (5 m), between 1991 and 2018, with the aim of investigating the role of ecoregions as potential sinks or sources of atmospheric CO2. The temperature and salinity of seawater presented variability in the north-south direction mainly because of the broad latitudinal range, reflecting typical patterns of tropical (T = 27.4°C ±1.49; S = 36.4 ±1.91) and subtropical waters (T = 22.8°C ±3.41; S = 35 ±2.91), in addition to the greater or lesser influence of river inputs in each ecoregion. The pCO2 values in the surface waters varied from 121.81 (Amazon) to 478.92 µatm (Eastern), differing significantly between ecoregions and showing an expected decadal increasing trend, both in the atmosphere and in the seawater. The calculated values of CO2 fluxes showed non-homogeneous spatio-temporal variations, from-24.37 mmol m-2 d-1 (Rio Grande) to 9.87 mmol m-2 d-1 (Southeastern). Throughout the analyzed time series, we observed that the Northeast, Amazon and Eastern ecoregions acted predominantly as sources of CO2 and the Southeastern ecoregions and, mainly, Rio Grande, acted predominantly as sinks of atmospheric CO2.