Ana G. Correa1, Iole B. M. Orselli1, Rodrigo Kerr1

1Laboratório de Estudos dos Oceanos e Clima, Instituto de Oceanografia, Universidade Federal do Rio Grande (FURG), Av. Itália km 8, Rio Grande, 96203-900, RS, Brazil.

Background: Rapid increase of CO2 in the atmosphere is receiving a huge importance due to its effect on global climate, what is related to ocean absorption of this gas. Continental margins play an important role in biogeochemical cycles due to nutrient input and exchanges with atmosphere and open ocean. Studies concerning CO2 net fluxes (FCO2) in the South Atlantic Ocean are limited, mainly concentrated in punctual researches in the Patagonian shelves and one oceanographic cruise in Southeast Atlantic Ocean.

Methods: Continuous measurements of CO2  molar fraction (xCO2) in the seawater and atmosphere were  sampled using a GO-8050/ Li-COR LI-7000, during early spring of 2014. The cruise carried out between Rio de Janeiro and Rio Grande – Brazil (23-32°S, 43-51°W). CO2 partial pressure (pCO2) was calculated using xCO2. FCO2 was determined using wind speed of ECMWF reanalysis project and based on Takahashi et al. (2009) transfer coefficient.

Findings: Average pCO2sw value was (± standard deviation) 367.9 ±11 atm, with a maximum of 423.2 atm and a minimum of 333.1 atm. Mean pCO2atm was 396.7 ±

2.5 atm.  pCO2  average value was -28.75 ±11.1 atm  with a maximum of 23.7

atm and a minimum of -62.5 atm. Wind speed average value was of 34.17 ±7.1 m s-1. FCO2 average value was -87.9 ±41.8. During the cruise a senescent bloom of Trichodesmium spp was observed, with high pCO2sw  values achieving 873 atm (pCO2 of 476 atm), being the only region along the slope where CO2 was released to the atmosphere.

Conclusions: In almost all sampled area pCO2 and FCO2 were negative, defining this region as a CO2  sink during the early spring, agreeing with recent modelling study in the region and contrasting with previous measurements of the southeastern Brazil, that characterized the entire continental shelf as a CO2 source to the atmosphere during spring.