Ocean Acidification and Long‐Term Changes in the Carbonate System Properties of the South Atlantic Ocean

The wind-driven part of the South Atlantic Ocean is primarily ventilated through central and intermediate water formation. Through the water mass formation processes, anthropogenic carbon (C-ant) is introduced into the ocean's interior which in turn makes the South Atlantic region vulnerable to ocean acidification. C-ant and the accompanying acidification effects have been estimated for individual sections in the region since the 1980s but a comprehensive synthesis for the entire basin is still lacking. Here, we quantified the C-ant accumulation rates and examined the changes in the carbonate system properties for the South Atlantic using a modified extended multiple linear regression method applied to five hydrographic sections and data from the GLODAPv2.2021 product. From 1989 to 2019, a mean C-ant column inventory change of 0.94 +/- 0.39 mol C m(-2) yr(-1) was found. C-ant accumulation rates of 0.89 +/- 0.33 mu mol kg(-1) yr(-1) and 0.30 +/- 0.29 mu mol kg(-1) yr(-1) were observed in central and intermediate waters, accompanied by acidification rates of -0.0020 +/- 0.0007 pH units yr(-1) and -0.0009 +/- 0.0009 pH units yr(-1), respectively. Furthermore, increased remineralization was observed in intermediate waters, amplifying the acidification of this water mass, especially at the African coast along 25 degrees S. This increase in remineralization is likely related to circulation changes and increased biological activity nearshore. Assuming no changes in the observed trends, South Atlantic intermediate waters will become unsaturated with respect to aragonite in similar to 30 years, while the central water of the eastern margins will become unsaturated in similar to 10 years

Sur le rôle des turbillons des Aiguilles dans l'absorption du carbone anthropique et l'état de l'acidification des océans dans l'océan Atlantique Sud

Depuis la révolution industrielle, les activités humaines libèrent de grandes quantités de dioxyde de carbone (CO2) dans l'atmosphère. Une partie de cet excès de CO2 est captée par les océans et a provoqué des perturbations et des changements dans le cycle du système carbonaté. Ces perturbations du système carbonaté sont désormais connues pour altérer l'état d'acidification des océans. Dans l'océan Atlantique Sud, les grands tourbillons des Aiguilles, sont parmi les plus grandes structures à méso-échelle des océans. Parce qu'ils sont des structures anticycloniques, ces tourbillons sont associés à des régions où l'océan perd de la chaleur vers l'atmosphère, mais leurs rôles par rapport au système carbonaté sont encore mal connus. Ainsi, l'objectif principal de cette recherche doctorale était d'étudier la relation entre les tourbillons des Aiguilles et la capture et le transport du CO2/Cant tout au long de leur vie ainsi que le rôle que ces structures jouent dans l'état d'acidification de l'océan Atlantique Sud.Lors de cette thèse, nous avons pu démontrer que les tourbillons des Aiguilles sont capables non seulement de capter plus de CO2 que les eaux environnantes, mais aussi de transférer ce carbone en profondeur dans la colonne d'eau et peuvent transporter plus de Cant le long de leur trajectoires. Etant donné que les observations indiquent que 30% de ces structures libérées dans le courant des Aiguilles atteignent la côte ouest de l'océan Atlantique Sud et interagissent même avec le courant du Brésil, nous pouvons les désigner comme l'un des déclencheurs susceptibles d'intensifier l'acidification observée pour les couches centrales de cette région.​Human activities have been releasing large amounts of carbon dioxide (CO2) into the atmosphere since the Industrial Revolution. Part of this excess CO2 is captured by the oceans and has been causing perturbations and changes in the carbonate system cycle. These changes in the carbonate system are now known to alter the acidification state of the oceans.In the South Atlantic Ocean are observed the Agulhas eddies, which are among the largest mesoscale structures in the oceans. Because they are anticyclonic structures, these eddies are associated with regions where the ocean loses heat to the atmosphere, but its role in relation to the carbonate system is still poorly studied. Thus, the main objective of this doctoral research was to investigate the relationship between the Agulhas eddies and CO2/Cant uptake and transport throughout their lives and which role these structures play in the acidification state in the South Atlantic Ocean.As a main conclusion of this thesis, we have been able to demonstrate that the Agulhas eddies are able not only to capture more CO2 than the surrounding waters, but also to transfer into the water column and can carry more Cant along their trajectories. As studies show that 30% of these structures released in the Agulhas leakage reach the west coast of the South Atlantic Ocean and even interact with the Brazilian Current, we can indicate them as one of the triggers that may be intensifying the acidification observed for the central layers of this region.Atividades humanas vêm liberando grandes quantidades de dióxido de carbono (CO 2 ) na atmosfera desde a Revolução Industrial. Parte desse excesso de CO 2 é capturado pelos oceanos (carbono antropogênico, C ant ) e vêm causando perturbações e alterações no ciclo do sistema carbonato. Essas alterações no sistema carbonato alteram o estado de acidificação dos oceanos.No oceano Atlântico Sul observa-se os vórtices das Agulhas, que estão entre as maiores estruturas de mesoescala dos oceanos. Por serem estruturas anticiclônicas, estes vórtices estão associados às regiões em que o oceano perde calor para a atmosfera, porém seu papel em relação ao sistema carbonato ainda é pouco estudado. Dessa forma, o objetivo principal dessa pesquisa de doutorado foi investigar a relação dos vórtices das Agulhas com a captura e transporte de CO 2 /C ant ao longo de suas vidas e qual o papel dessas estruturas no estado de acidificação no oceano Atlântico Sul. Como conclusão principal dessa tese, pudemos demonstrar que os vórtices das Agulhas são capazes não só de capturar mais CO 2 do que as águas ao seu redor, como também de transferir para o interior da coluna d’água, podendo carregar mais C ant ao longo de suas trajetórias. Como estudos mostram que 30% dessas estruturas liberadas no vazamento das Agulhas atingem a costa Oeste do Oceano Atlântico Sul e chegam a interagir com a Corrente do Brasil, podemos indicá-los como um dos gatilhos que podem estar intensificando a acidificação observada para as camadas centrais dessa região

Continuous pCO2 and Sea-Air CO2 Net Fluxes from the Following Ocean Rings in the South Atlantic (FORSA) cruise

Here we provide CO2-system properties that were continuously measured in a southeast-northwest transect in the South Atlantic Ocean in which six Agulhas eddies were sampled. The Following Ocean Rings in the South Atlantic (FORSA) cruise occurred between 27th June and 15th July 2015, from Cape Town – South Africa to Arraial do Cabo – Brazil, on board the first research cruise of the Brazilian Navy RV Vital de Oliveira, as part of an effort of the Brazilian High Latitude Oceanography Group (GOAL). Finally, it contributed to the activities developed by the following Brazilian networks: GOAL, Brazilian Ocean Acidification Network (BrOA), Brazilian Research Network on Global Climate Change (Rede CLIMA). The focus of the first study using this dataset (Orselli et al. 2019a) was on investigate the role played by the Agulhas eddies on the sea-air CO2 net flux along their trajectories through the South Atlantic Ocean and model the seawater CO2–related properties as function of environmental parameters. This data has been used to contribute to the scientific discussion about the Agulhas eddies impact on the changes of the marine carbonate system, which is an expanding oceanographic subject (Carvalho et al. 2019; Orselli et al. 2019b; Ford et al. 2023). Seawater and atmospheric CO2 molar fraction (xCO2sw and xCO2atm, respectively) were continuously measured during the cruise track, as well as the sea surface temperature (T) and salinity (S). The following sampling methodology is fully described in Orselli et al. (2019a). The underway xCO2 sampling was taken using an autonomous system GO–8050, General Oceanic®, equipped with a non-dispersive infrared gas analyzer (LI–7000, LI–COR®). The underway T and S were sampled using a Sea-Bird® Thermosalinograph SBE21. Seawater intake to feed the continuous systems of the GO-8050 and the SBE21 was set at ~5 m below the sea surface. The xCO2 system was calibrated with four standard gases (CO2 concentrations of 0, 202.10, 403.20, and 595.50 uatm) within a 12 h interval along the entire cruise. Every 3 h the system underwent a standard reading, to check the derivation and allow the xCO2 corrections. The xCO2 measurements were taken within 90 seconds interval. After a hundred of xCO2sw readings, the system was changed to atmosphere and five xCO2atm readings were taken (Pierrot et al., 2009). xCO2 (umol mol–1) inputs were corrected by the CO2 standards (Pierrot et al., 2009). Thermosalinograph data were corrected using the CTD surface data. Then, together with the pressure data, these data were used to calculate the pCO2 of the equilibrator and atmosphere (pCO2eq and pCO2atm, respectively, uatm), following Weiss & Price (1980). Using the pCO2eq, which is calculated at the equilibrator temperature, it is possible to calculate the pCO2 at the in situ temperature (pCO2sw, uatm), according to Takahashi et al. (2009). Another common calculation regarding pCO2sw data, is the temperature-normalized pCO2sw (NpCO2sw, uatm). This means that the temperature effect is removed when one calculates the NpCO2sw for the mean cruise temperature. The procedure followed the Takahashi et al. (2009) and considered the mean cruise temperature of 20.39°C. The results obtained allow one to investigate the exchanges of CO2 at the ocean-atmosphere interface by calculating the pCO2 difference between these two reservoirs (DeltapCO2, DpCO2=pCO2sw–pCO2atm, uatm). Negative (positive) DpCO2 results indicate that the ocean acts as a CO2 sink (source) for the atmosphere. To determine the FCO2, the monthly mean wind speed data of July 2015 (at 10 m height) were extracted from the ERA-Interim atmospheric reanalysis product of the European Centre for Medium Range Weather Forecast (http://apps.ecmwf.int/datasets/data/interim-full-moda/levtype=sfc/) since the use of long-term mean is usual (e.g., Takahashi et al., 2009). The average wind speed for the period and whole area was 6.8 ± 0.6 m s−1, ranging from 5.6 to 8.3 m s−1. The CO2 transfer coefficients proposed by Takahashi et al. (2009) and Wanninkhof (2014) were used. With all these data together, the FCO2 was determined according to Broecker & Peng (1982), where FCO2 is the sea-air CO2 net flux (mmol m–2 d–1; FT09 and FW14 are the Sea-air CO2 flux calculated using the coefficients described in Takahashi et al. (2009) and Wanninkhof (2014), respectively)