Reconstrução do sistema carbonato marinho no Atlântico Tropical Oeste: tendências e variabilidades em 20 anos de projeto PIRATA (8°N 38°W)

Abstract

The Western Tropical Atlantic is a crucial region when it comes to understanding the CO2 dynamics in the tropics, as it is subject to large inputs of freshwater from the Amazon River and the ITCZ rainfall, as well as the input of CO2-rich waters from upwelling of subsurface water. This study aims to reconstruct the surface marine carbonate system from 1998 to 2018 using sea surface temperature (SST) and sea surface salinity (SSS) data from the PIRATA buoy at 8°N 38°W and describe its variability in time. Two empirical models were used to calculate total alkalinity (TA) and dissolved inorganic carbon (DIC) from SSS. From these two parameters, as well as SSS and SST data, calculate pH, in total scale, and CO2 fugacity (fCO2) values were then estimated. Only DIC, pH and fCO2 showed a statistically significant trend in time, where DIC showed an increase of 0.717 ± 0,108 μmol kg-1 year-1, pH decreased .0014 ± 0,0001 pH units year-1, and fCO2 had an increase of 1.539 ± 0,028 μatm year-1. Two different seasons were observed when data were analyzed: a dry season from January to June, when SSTs were lower (27.00 ± .61°C) and SSS was higher (35.94 ± 0.34), matching the period when the ITCZ is over the South American continent, Amazon river plume is restricted to western shelf areas and Equatorial upwelling is less intense, and a rainy season from July to December, when SSTs were higher (28.35 ± 0.7°C) and SSS was lower (35.01 ± 0.8), matching the period when the ITCZ is at its northern range, the Amazon plume is spread eastwards through the North Brazil Current’s retroflection and the Equatorial upwelling is more active. Along with that, TA, DIC and pH varied positively with SSS, with higher values (TA: 2352.88 ± 19.74μmol kg-1; DIC: 2013.82 ± 18.6μmol kg-1; pH: 8.0604 ± 0.0121) during dry season and lower values (TA: 2299.3 ± 46.32μmol kg-1; DIC: 1967.61 ± 40.0μmol kg-1; pH: 8.0485 ± 0.0119) during rainy season. On the other hand, fCO2 varied positively with SST, with higher values (391.13 ± 13.76μatm) during rainy, upwelling season and lower values (385.85 ± 12.86μatm) during dry season, showing that both SSS and SST variability play an important role in the CO2 solubility in the region. Besides seasonality, the El Niño Southern Oscilation probably played an important role on the carbonate system’s variability, as variations in this study with Fourier period from 1.4 to 4.7 years matched with observed impacts of the El Niño at the tropical Atlantic and the Amazon basin.