Trends and drivers of sea surface fCO2 and pH changes observed in the Southern Indian Ocean over the last two decades (1998–2019)

• Main Authors: Leseurre, Coraline, Monaco, Claire, Reverdin, Gilles, Metzl, Nicolas, Fin, Jonathan, Mignon, Claude, Benito, Léa
• Format: Text
• Language: English
• Published: 2022
• Subjects: Indian; Kerguelen; Kerguelen Island; Kerguelen Islands
• Online Access: https://doi.org/10.5194/bg-2022-22; https://bg.copernicus.org/preprints/bg-2022-22/

The decadal changes of the fugacity of CO 2 (fCO 2 ) and pH in surface waters are investigated in the Southern Indian Ocean (45° S–57° S) using repeated summer observations, including measurements of fCO 2 , total alkalinity (A T ) and total carbon (C T ) collected over the period 1998–2019 in the frame of the French monitoring program OISO. We used three datasets (underway fCO 2 , underway A T -C T and station A T -C T ) to evaluate the trends of fCO 2 and pH and their drivers, including the accumulation of anthropogenic CO 2 (C ant ). The study region is separated into three domains based on the frontal system and biogeochemical characteristics: (i) High Nutrients Low Chlorophyll (HNLC) waters in the Polar Front Zone (PFZ), (ii) HNLC waters south of the Polar Front (PF) and (iii) the highly productive zones in fertilized waters near Crozet and Kerguelen Islands. Almost everywhere, we obtained similar trends in surface fCO 2 and pH using the fCO 2 or A T -C T datasets. Over the period 1998–2019, we observed an increase in surface fCO 2 and a decrease in pH ranging from +1.0 to +4.0 µatm yr −1 and from −0.0015 to −0.0043 yr −1 , respectively. South of the PF, the fCO 2 trend is close to the atmospheric CO 2 rise (+2.0 µatm yr −1 ) and the decrease in pH is in the range of the mean trend for the global ocean (around −0.0020 yr −1 ); these trends are driven by the warming of surface waters (up to +0.04 °C yr −1 ) and the increase in C T , mainly due to the accumulation of C ant (around +0.6 µmol kg −1 yr −1 ). In the PFZ, our data show slower fCO 2 and pH trends (around +1.3 µatm yr −1 and −0.0013 yr −1 , respectively) associated with an increase in A T (around +0.4 µmol kg −1 yr −1 ) that limited the impact of a more rapid accumulation of C ant north of the PF (up to +1.1 µmol kg −1 yr −1 ). In the fertilized waters near Crozet and Kerguelen Islands, fCO 2 increased and pH decreased faster than in the other domains, between +2.2 and +4.0 µatm yr −1 and between −0.0023 yr −1 and −0.0043 yr −1 . The fastest trends of fCO 2 and pH are found around Kerguelen Island north and south of the PF. These trends result from both a significant warming (up to +0.07 °C yr −1 ) and a rapid increase in C T (up to +1.4 µmol kg −1 yr −1 ), mainly explained by the uptake of C ant . Our data also show rapid changes on short periods and a relative stability of both fCO 2 and pH in recent years at several locations both north and south of the PF, which leaves many open questions, notably the tipping point for the saturation state of carbonate minerals that remains highly uncertain. This highlights the need to maintain observations on the long-term in order to explore how the carbonate system will evolve in this region in the next decades.