Regional relationship between total alkalinity and salinity in the surface waters of the western South Atlantic margin

We examined the surface total alkalinity (AT) and salinity (SSS) distributions and their relationship along the southwestern South Atlantic Ocean margin off the Brazilian coast. In situ hydrographic data from shelf and slope stations were measured during three oceanographic cruises in October 2014 (austral spring), April 2018 (austral autumn) and June 2019 (austral winter). The northern portion of the study area lies within the Brazil Current domain, transporting warm and salty Tropical Water, while further south the Subtropical Shelf Water and the freshwater plume derived from the Patos Lagoon and Plata River influence the temperature, salinity, and AT distribution. The present study aimed to determine the relationship between AT and SSS to establish a model for reconstructing a surface time series for the marine carbonate system in the undersampled region of the subtropical southwestern South Atlantic Ocean margin. The AT – SSS relationship showed a significant positive linear relationship (r2=0.76), reflecting both current-dominated (i.e., outer and northernmost stations) and freshwater mixing processes (i.e., inner and southernmost stations and continental inputs). The observed AT values were well correlated with the reported ranges for the tropical and subtropical South Atlantic Ocean, and the predicted AT obtained from the Global Ocean Data Analysis Project and the World Ocean Database 2013 SSS data confirmed lower AT in shelf waters under the influence of continental inputs, which increased towards the outer shelf and slope. Finally, the newly proposed regional AT – SSS relationship improves previous linear regressions for the southwestern Atlantic Ocean margin. •Surface AT and SSS patterns show a decreasing pattern from north to south along the southwestern South Atlantic margin.•The relationship found between surface AT and SSS reveals that salinity explains ∼ 80 % of AT variability.•The estimated AT values reflect both current-dominated and freshwater mixing processes along the southwestern SAO margin.