Anthropogenic CO ₂, air-sea CO ₂ fluxes and acidification in the Southern Ocean: results from a time-series analysis at station OISO-KERFIX (51°S-68°E)

The temporal variation of the carbonate system, air-sea CO ₂ fluxes and pH is analyzed in the Southern Indian Ocean, south of the Polar Front, based on in-situ data obtained from 1985 to 2021 at a fixed station (50°40’S–68°25’E) and results from a neural network model that reconstructs the fugacity of CO ₂ (fCO ₂) and fluxes at monthly scale. Anthropogenic CO ₂ (C _ant) was estimated in the water column and detected down to the bottom (1600 m) in 1985 resulting in an aragonite saturation horizon at 600 m that migrated up to 400 m in 2021 due to the accumulation of C _ant. In subsurface, the trend of C _ant is estimated at +0.53 (±0.01) µmol.kg ^-1.yr ^-1 with a detectable increase in recent years. At the surface during austral winter the oceanic fCO ₂ increased at a rate close or slightly lower than in the atmosphere. To the contrary, in summer, we observed contrasting fCO ₂ and dissolved inorganic carbon (C _T) trends depending on the decade and emphasizing the role of biological drivers on air-sea CO ₂ fluxes and pH inter-annual variability. The region moved from an annual source of 0.8 molC.m ^-2.yr ^-1 in 1985 to a sink of -0.5 molC.m ^-2.yr ^-1 in 2020. In 1985–2020, the annual pH trend in surface of -0.0165 (± 0.0040).decade ^-1 was mainly controlled by anthropogenic CO ₂ but the trend was modulated by natural processes. Using historical data from November 1962 we estimated the long-term trend for fCO ₂, C _T and pH confirming that the progressive acidification was driven by atmospheric CO ₂ increase. In 59 years this leads to a diminution of 11 % for both aragonite and calcite saturation state. As atmospheric CO ₂ will desperately continue rising in the future, the pH and carbonate saturation state will decrease at a faster rate than observed in recent years. A projection of future C _T concentrations for a high emission scenario (SSP5-8.5) indicates that the surface pH in 2100 would decrease to 7.32 in winter. This is up to -0.86 lower than pre-industrial pH and -0.71 lower than pH observed in 2020. The aragonite under-saturation in surface waters would be reached as soon as 2050 (scenario SSP5-8.5) and 20 years later for a stabilization scenario (SSP2-4.5) with potential impacts on phytoplankton species and higher trophic levels in the rich ecosystems of the Kerguelen Island area.