A comparative study of total alkalinity and total inorganic carbon near tropical Atlantic coastal regions

Abstract

This paper is based on a comparison of the carbon parameters at the western and eastern borders of the tropical Atlantic using data collected from 55 cruises. Oceanic and coastal data, mainly total alkalinity (TA), total dissolved inorganic carbon (CT), sea surface salinity (SSS) and sea surface temperature (SST), were compiled from different sources. These data were subdivided into three subsets: oceanic data, coastal data and adjacent to the Brazilian (western) and African coastal areas (eastern) data. Significant differences between the TA data (2099.4±286.4 µmol kg−1) at the western and eastern edges (2198±141.9 µmol kg−1) were observed. Differences in the CT values between the western edge (1779.6±236.4 µmol kg−1) and eastern edge (1892.2±94.2 µmol kg−1) were also noted. This pattern was due to the different variabilities in the carbon parameters between the eastern and western border coastal areas and to the biogeochemistry that drives these parameters. In the western coastal area, the physical features of the continental carbon and oceanic waters mixing with the freshwater that flows from the Amazon and Orinoco Rivers to the South American coast are different than the physical features of the water that flows from the Congo, Volta and Niger Rivers in the eastern region. Applying the TA empirical relationship to TA with values of SSS<35 in the western and eastern regions leads to a higher root mean square error (rmse) in the eastern and western regions. Therefore, most of the existing TA empirical relationships are most useful at the regional scale due to the difference in the water properties of each region. The relationships of TA and CT determined in the western and eastern regions do not reproduce in situ data well, especially at the adjacent edges. This difference is explained by the difference between the African and Brazilian coasts in terms of their carbon parameter characteristics and processes responsible for their variation. Based on the mixing model, it has been shown that the primary productivity in the eastern region is higher than that in the western region. This is one of the reasons why the carbon parameters are higher in the eastern region. For each region studied, an equation for TA is introduced in this study.