72. Surface total alkalinity, salinity and temperature: a study case in the South-western Atlantic Ocean

Leticia Cotrim Da Cunha (1)*, Cintia Albuquerque (1)*, Rodrigo Kerr (2), Iole Orselli (2)

1 Faculdade de Oceanografia, Rio de Janeiro State University (UERJ), Rio de Janeiro – RJ, 20550-900, Brazil
2 Instituto de Oceanografia, Federal University of Rio Grande (FURG), Rio Grande – RS, 96203-900, Brazil

Background
Total alkalinity in open ocean surface waters is mainly controlled by salinity, although processes such as calcification, primary production and respiration may contribute to changes in it. Here we analysed surface total alkalinity samples (AT) from two cruises along the Brazilian shelf break in the South-western Atlantic, from 23°S to 32°S, from 2014 and (planned) 2015. This area of the ocean still lacks CO2-system data, and only recently more oceanographic sampling initiatives for this purpose were taken. Our main objectives are: a) correlate the AT values to surface salinity (SSS) and temperature (SST) in order to assess the role of shelf biogeochemistry; b) check how this area relates to established AT x salinity relationships, and c) provide a simple, robust tool to reconstruct historical data for AT and other CO2-system parameters to this area of the ocean.

Methods
Surface samples (circa 5 metres) were collected and analysed according to Dickson et al. (2007). Total alkalinity analyses were conducted independently in both open and closed cell titration, surface temperature and salinity data were taken from ship CTD measurements. Samples were analysed at both FURG and UERJ laboratories.

Findings
AT data from 2014 was positively related to surface salinity (TA = 50.81SSS + 546.26, R² = 0.90, n = 48) and to SST (TA = 95.34SST + 305.88, R² = 0.47, n= 48), very close to the previous equation proposed by Millero et al (1998) for the Atlantic Ocean.

Conclusions
Preliminary results point to a control on AT by the balance between evaporation and precipitation in the open ocean region, despite Trichodesmium blooms in 2014. There are neither significant alkalinity riverine inputs in this portion of the Brazilian coast nor influence of the Patos Lagoon plume in the southernmost AT samples.