Anthropogenic CO2, air–sea CO2 fluxes, and acidification in the Southern Ocean: results from a time-series analysis at station OISO-KERFIX (51° S–68° E)

• Published in: Ocean Science
• Main Authors: Metzl, Nicolas, Monaco, Claire, Leseurre, Coraline, Ridame, Céline, Reverdin, Gilles, Chau, Thi Tuyet Trang, Chevallier, Frédéric, Gehlen, Marion
• Format: Text
• Language: English
• Published: 2024
• Subjects: Southern Ocean
• Online Access: https://doi.org/10.5194/os-20-725-2024; https://os.copernicus.org/articles/20/725/2024/

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 ( f CO 2 ) and fluxes at monthly scale. Anthropogenic CO 2 (C ant ) is estimated in the water column and is 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 . At the subsurface, the trend of C ant is estimated at + 0.53 ± 0.01 <svg:svg xmlns:svg="http://www.w3.org/2000/svg" width="64pt" height="10pt" class="svg-formula" dspmath="mathimg" md5hash="0335676cdfde01c2427480e63a96dffe"><svg:image xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="os-20-725-2024-ie00001.svg" width="64pt" height="10pt" src="os-20-725-2024-ie00001.png"/> </svg:svg> µ mol kg −1 yr −1 with a detectable increase in the trend in recent years. At the surface during austral winter the oceanic f CO 2 increased at a rate close to or slightly lower than in the atmosphere. To the contrary, in summer, we observed contrasting f CO 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 regional air–sea CO 2 fluxes evolved from an annual source to the atmosphere of 0.8 molC m −2 yr −1 in 1985 to a sink of −0.5 molC m −2 yr −1 in 2020. Over 1985–2020, the annual pH trend in surface waters of - 0.0165 ± 0.0040 <svg:svg xmlns:svg="http://www.w3.org/2000/svg" width="88pt" height="10pt" class="svg-formula" dspmath="mathimg" md5hash="f78ffdaaa34caa0f5eaa9c9ca35bfe50"><svg:image xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="os-20-725-2024-ie00002.svg" width="88pt" height="10pt" src="os-20-725-2024-ie00002.png"/> </svg:svg> per decade was mainly controlled by the ...