Summer trends and drivers of sea surface fCO<sub>2</sub> and pH changes observed in the southern Indian Ocean over the last two decades (1998–2019)
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Published:2022-05-25
Issue:10
Volume:19
Page:2599-2625
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ISSN:1726-4189
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Container-title:Biogeosciences
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language:en
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Short-container-title:Biogeosciences
Author:
Leseurre CoralineORCID, Lo Monaco Claire, Reverdin GillesORCID, Metzl Nicolas, Fin Jonathan, Mignon Claude, Benito Léa
Abstract
Abstract. The decadal changes in the fugacity of CO2 (fCO2) and pH in surface waters are investigated in the southern Indian Ocean
(45–57∘ S) using repeated summer observations, including measurements of fCO2, total alkalinity (AT) and total
carbon (CT) collected over the period 1998–2019 in the frame of the French monitoring programme OISO (Océan Indien Service d'Observation). We used three datasets (underway fCO2, underway AT–CT and station AT–CT) to evaluate the trends of fCO2 and pH and their
drivers, including the accumulation of anthropogenic CO2 (Cant). The study region is separated into six domains based on the
frontal system and biogeochemical characteristics: (i) high-nutrient low-chlorophyll (HNLC) waters in the polar front zone (PFZ) and (ii) north part
and (iii) south part of HNLC waters south of the polar front (PF), as well as the highly productive zones in fertilised waters near (iv) Crozet Island and (v) north and (vi) south of Kerguelen Island. Almost everywhere, we obtained similar trends in
surface fCO2 and pH using the fCO2 or AT–CT datasets. Over the period 1998–2019, we observed an increase in
surface fCO2 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 fCO2 trend is close to the
atmospheric CO2 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 CT mainly due to the accumulation of Cant (around +0.6 µmol kg−1 yr−1). In the PFZ, our data
show slower fCO2 and pH trends (around +1.3 µatm yr−1 and −0.0013 yr−1, respectively) associated with an
increase in AT (around +0.4 µmol kg−1 yr−1) that limited the impact of a more rapid accumulation of Cant
north of the PF (up to +1.1 µmol kg−1 yr−1). In the fertilised waters near Crozet and Kerguelen islands, fCO2
increased and pH decreased faster than in the other domains, between +2.2 and +4.0 µatm yr−1 and between −0.0023 and
−0.0043 yr−1. The fastest trends of fCO2 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 CT (up to
+1.4 µmol kg−1 yr−1) mainly explained by the uptake of Cant. Our data also show rapid changes in short periods and
a relative stability of both fCO2 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 in the long-term in order to explore how the carbonate system will evolve in this region in the next decades.
Publisher
Copernicus GmbH
Subject
Earth-Surface Processes,Ecology, Evolution, Behavior and Systematics
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