Exploring the CO₂ fugacity along the east coast of South America aboard the schooner Tara

Abstract. The air-sea CO₂ flux in the coastal ocean is a key component of the global carbon budget. However, due to the scarcity of data, the many sources and sinks of carbon and their complex interactions in these waters remain poorly understood. In 2021, the Tara schooner collected 14,000 km of CO₂ fugacity (fCO₂) measurements along the coast of South America, including in the Amazon River-Ocean continuum (https://doi.org/10.5281/zenodo.13790065, Olivier et al., 2024a). The interactions between the Amazon River and its oceanic plume are complex, and under a combined influence of many processes such as tides and bathymetry. Downstream of the Amazon River plume, the fCO₂ is low compared with that of the atmosphere, reaching a minimum of 42 μatm. In the river, fCO₂ reaches up to 3000 μatm. South of the estuary, the waters of the North Brazil Current have a fCO₂ exceeding 400 μatm. Along the Brazil Current, fCO₂ is around 400 μatm and decreases, as does temperature, as the schooner sails away from the equator. Nevertheless, in all the data collected in this coastal environment, salinity varies greatly, and therefore describes best the variability of fCO₂. Despite the strong variability and uncertainties in the data, comparison with discrete samples of other carbonate parameters shows that the mean differences (2 µatm) are within the range of uncertainties related to the chemical formula used for the comparison. This data set helps to fill the gap in our knowledge of the behavior of fCO₂ in the under-sampled region of the Brazilian coast.