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

The decadal changes in the fugacity of CO2 (fCO2) and pH in surface waters are investigated in the southern Indian Ocean (45–57&l...

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Main Authors:	C. Leseurre, C. Lo Monaco, G. Reverdin, N. Metzl, J. Fin, C. Mignon, L. Benito
Format:	Article
Language:	English
Published:	Copernicus Publications 2022-05-01
Series:	Biogeosciences
Online Access:	https://bg.copernicus.org/articles/19/2599/2022/bg-19-2599-2022.pdf

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author	C. Leseurre C. Lo Monaco G. Reverdin N. Metzl J. Fin C. Mignon L. Benito
author_facet	C. Leseurre C. Lo Monaco G. Reverdin N. Metzl J. Fin C. Mignon L. Benito
author_sort	C. Leseurre
collection	DOAJ
description	<p>The decadal changes in the fugacity of <span class="inline-formula">CO<sub>2</sub></span> (<span class="inline-formula">fCO<sub>2</sub></span>) and pH in surface waters are investigated in the southern Indian Ocean (45–57<span class="inline-formula"><sup>∘</sup></span> S) using repeated summer observations, including measurements of <span class="inline-formula">fCO<sub>2</sub></span>, total alkalinity (<span class="inline-formula"><i>A</i><sub>T</sub></span>) and total carbon (<span class="inline-formula"><i>C</i><sub>T</sub></span>) collected over the period 1998–2019 in the frame of the French monitoring programme OISO (<i>Océan Indien Service d'Observation</i>). We used three datasets (underway <span class="inline-formula">fCO<sub>2</sub></span>, underway <span class="inline-formula"><i>A</i><sub>T</sub></span>–<span class="inline-formula"><i>C</i><sub>T</sub></span> and station <span class="inline-formula"><i>A</i><sub>T</sub></span>–<span class="inline-formula"><i>C</i><sub>T</sub></span>) to evaluate the trends of <span class="inline-formula">fCO<sub>2</sub></span> and pH and their drivers, including the accumulation of anthropogenic <span class="inline-formula">CO<sub>2</sub></span> (<span class="inline-formula"><i>C</i><sub>ant</sub></span>). 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 <span class="inline-formula">fCO<sub>2</sub></span> and pH using the <span class="inline-formula">fCO<sub>2</sub></span> or <span class="inline-formula"><i>A</i><sub>T</sub></span>–<span class="inline-formula"><i>C</i><sub>T</sub></span> datasets. Over the period 1998–2019, we observed an increase in surface <span class="inline-formula">fCO<sub>2</sub></span> and a decrease in pH ranging from <span class="inline-formula">+</span>1.0 to <span class="inline-formula">+</span>4.0 <span class="inline-formula">µ</span>atm yr<span class="inline-formula"><sup>−1</sup></span> and from <span class="inline-formula">−</span>0.0015 to <span class="inline-formula">−</span>0.0043 <span class="inline-formula">yr<sup>−1</sup></span>, respectively. South of the PF, the <span class="inline-formula">fCO<sub>2</sub></span> trend is close to the atmospheric <span class="inline-formula">CO<sub>2</sub></span> rise (<span class="inline-formula">+</span>2.0 <span class="inline-formula">µ</span>atm yr<span class="inline-formula"><sup>−1</sup></span>), and the decrease in pH is in the range of the mean trend for the global ocean (around <span class="inline-formula">−</span>0.0020 <span class="inline-formula">yr<sup>−1</sup></span>); these trends are driven by the warming of surface waters (up to <span class="inline-formula">+</span>0.04 <span class="inline-formula"><sup>∘</sup></span>C yr<span class="inline-formula"><sup>−1</sup></span>) and the increase in <span class="inline-formula"><i>C</i><sub>T</sub></span> mainly due to the accumulation of <span class="inline-formula"><i>C</i><sub>ant</sub></span> (around <span class="inline-formula">+</span>0.6 <span class="inline-formula">µ</span>mol kg<span class="inline-formula"><sup>−1</sup></span> yr<span class="inline-formula"><sup>−1</sup></span>). In the PFZ, our data show slower <span class="inline-formula">fCO<sub>2</sub></span> and pH trends (around <span class="inline-formula">+</span>1.3 <span class="inline-formula">µ</span>atm yr<span class="inline-formula"><sup>−1</sup></span> and <span class="inline-formula">−</span>0.0013 <span class="inline-formula">yr<sup>−1</sup></span>, respectively) associated with an increase in <span class="inline-formula"><i>A</i><sub>T</sub></span> (around <span class="inline-formula">+</span>0.4 <span class="inline-formula">µ</span>mol kg<span class="inline-formula"><sup>−1</sup></span> yr<span class="inline-formula"><sup>−1</sup></span>) that limited the impact of a more rapid accumulation of <span class="inline-formula"><i>C</i><sub>ant</sub></span> north of the PF (up to <span class="inline-formula">+</span>1.1 <span class="inline-formula">µ</span>mol kg<span class="inline-formula"><sup>−1</sup></span> yr<span class="inline-formula"><sup>−1</sup></span>). In the fertilised waters near Crozet and Kerguelen islands, <span class="inline-formula">fCO<sub>2</sub></span> increased and pH decreased faster than in the other domains, between <span class="inline-formula">+</span>2.2 and <span class="inline-formula">+</span>4.0 <span class="inline-formula">µ</span>atm yr<span class="inline-formula"><sup>−1</sup></span> and between <span class="inline-formula">−</span>0.0023 and <span class="inline-formula">−</span>0.0043 <span class="inline-formula">yr<sup>−1</sup></span>. The fastest trends of <span class="inline-formula">fCO<sub>2</sub></span> and pH are found around Kerguelen Island north and south of the PF. These trends result from both a significant warming (up to <span class="inline-formula">+</span>0.07 <span class="inline-formula"><sup>∘</sup></span>C yr<span class="inline-formula"><sup>−1</sup></span>) and a rapid increase in <span class="inline-formula"><i>C</i><sub>T</sub></span> (up to <span class="inline-formula">+</span>1.4 <span class="inline-formula">µ</span>mol kg<span class="inline-formula"><sup>−1</sup></span> yr<span class="inline-formula"><sup>−1</sup></span>) mainly explained by the uptake of <span class="inline-formula"><i>C</i><sub>ant</sub></span>. Our data also show rapid changes in short periods and a relative stability of both <span class="inline-formula">fCO<sub>2</sub></span> 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.</p>
first_indexed	2024-04-12T17:44:43Z
format	Article
id	doaj.art-e435d3c27c0043de886de1c5dcb9e3ec
institution	Directory Open Access Journal
issn	1726-4170 1726-4189
language	English
last_indexed	2024-04-12T17:44:43Z
publishDate	2022-05-01
publisher	Copernicus Publications
record_format	Article
series	Biogeosciences
spelling	doaj.art-e435d3c27c0043de886de1c5dcb9e3ec2022-12-22T03:22:43ZengCopernicus PublicationsBiogeosciences1726-41701726-41892022-05-01192599262510.5194/bg-19-2599-2022Summer 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)C. LeseurreC. Lo MonacoG. ReverdinN. MetzlJ. FinC. MignonL. Benito<p>The decadal changes in the fugacity of <span class="inline-formula">CO<sub>2</sub></span> (<span class="inline-formula">fCO<sub>2</sub></span>) and pH in surface waters are investigated in the southern Indian Ocean (45–57<span class="inline-formula"><sup>∘</sup></span> S) using repeated summer observations, including measurements of <span class="inline-formula">fCO<sub>2</sub></span>, total alkalinity (<span class="inline-formula"><i>A</i><sub>T</sub></span>) and total carbon (<span class="inline-formula"><i>C</i><sub>T</sub></span>) collected over the period 1998–2019 in the frame of the French monitoring programme OISO (<i>Océan Indien Service d'Observation</i>). We used three datasets (underway <span class="inline-formula">fCO<sub>2</sub></span>, underway <span class="inline-formula"><i>A</i><sub>T</sub></span>–<span class="inline-formula"><i>C</i><sub>T</sub></span> and station <span class="inline-formula"><i>A</i><sub>T</sub></span>–<span class="inline-formula"><i>C</i><sub>T</sub></span>) to evaluate the trends of <span class="inline-formula">fCO<sub>2</sub></span> and pH and their drivers, including the accumulation of anthropogenic <span class="inline-formula">CO<sub>2</sub></span> (<span class="inline-formula"><i>C</i><sub>ant</sub></span>). 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 <span class="inline-formula">fCO<sub>2</sub></span> and pH using the <span class="inline-formula">fCO<sub>2</sub></span> or <span class="inline-formula"><i>A</i><sub>T</sub></span>–<span class="inline-formula"><i>C</i><sub>T</sub></span> datasets. Over the period 1998–2019, we observed an increase in surface <span class="inline-formula">fCO<sub>2</sub></span> and a decrease in pH ranging from <span class="inline-formula">+</span>1.0 to <span class="inline-formula">+</span>4.0 <span class="inline-formula">µ</span>atm yr<span class="inline-formula"><sup>−1</sup></span> and from <span class="inline-formula">−</span>0.0015 to <span class="inline-formula">−</span>0.0043 <span class="inline-formula">yr<sup>−1</sup></span>, respectively. South of the PF, the <span class="inline-formula">fCO<sub>2</sub></span> trend is close to the atmospheric <span class="inline-formula">CO<sub>2</sub></span> rise (<span class="inline-formula">+</span>2.0 <span class="inline-formula">µ</span>atm yr<span class="inline-formula"><sup>−1</sup></span>), and the decrease in pH is in the range of the mean trend for the global ocean (around <span class="inline-formula">−</span>0.0020 <span class="inline-formula">yr<sup>−1</sup></span>); these trends are driven by the warming of surface waters (up to <span class="inline-formula">+</span>0.04 <span class="inline-formula"><sup>∘</sup></span>C yr<span class="inline-formula"><sup>−1</sup></span>) and the increase in <span class="inline-formula"><i>C</i><sub>T</sub></span> mainly due to the accumulation of <span class="inline-formula"><i>C</i><sub>ant</sub></span> (around <span class="inline-formula">+</span>0.6 <span class="inline-formula">µ</span>mol kg<span class="inline-formula"><sup>−1</sup></span> yr<span class="inline-formula"><sup>−1</sup></span>). In the PFZ, our data show slower <span class="inline-formula">fCO<sub>2</sub></span> and pH trends (around <span class="inline-formula">+</span>1.3 <span class="inline-formula">µ</span>atm yr<span class="inline-formula"><sup>−1</sup></span> and <span class="inline-formula">−</span>0.0013 <span class="inline-formula">yr<sup>−1</sup></span>, respectively) associated with an increase in <span class="inline-formula"><i>A</i><sub>T</sub></span> (around <span class="inline-formula">+</span>0.4 <span class="inline-formula">µ</span>mol kg<span class="inline-formula"><sup>−1</sup></span> yr<span class="inline-formula"><sup>−1</sup></span>) that limited the impact of a more rapid accumulation of <span class="inline-formula"><i>C</i><sub>ant</sub></span> north of the PF (up to <span class="inline-formula">+</span>1.1 <span class="inline-formula">µ</span>mol kg<span class="inline-formula"><sup>−1</sup></span> yr<span class="inline-formula"><sup>−1</sup></span>). In the fertilised waters near Crozet and Kerguelen islands, <span class="inline-formula">fCO<sub>2</sub></span> increased and pH decreased faster than in the other domains, between <span class="inline-formula">+</span>2.2 and <span class="inline-formula">+</span>4.0 <span class="inline-formula">µ</span>atm yr<span class="inline-formula"><sup>−1</sup></span> and between <span class="inline-formula">−</span>0.0023 and <span class="inline-formula">−</span>0.0043 <span class="inline-formula">yr<sup>−1</sup></span>. The fastest trends of <span class="inline-formula">fCO<sub>2</sub></span> and pH are found around Kerguelen Island north and south of the PF. These trends result from both a significant warming (up to <span class="inline-formula">+</span>0.07 <span class="inline-formula"><sup>∘</sup></span>C yr<span class="inline-formula"><sup>−1</sup></span>) and a rapid increase in <span class="inline-formula"><i>C</i><sub>T</sub></span> (up to <span class="inline-formula">+</span>1.4 <span class="inline-formula">µ</span>mol kg<span class="inline-formula"><sup>−1</sup></span> yr<span class="inline-formula"><sup>−1</sup></span>) mainly explained by the uptake of <span class="inline-formula"><i>C</i><sub>ant</sub></span>. Our data also show rapid changes in short periods and a relative stability of both <span class="inline-formula">fCO<sub>2</sub></span> 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.</p>https://bg.copernicus.org/articles/19/2599/2022/bg-19-2599-2022.pdf
spellingShingle	C. Leseurre C. Lo Monaco G. Reverdin N. Metzl J. Fin C. Mignon L. Benito 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) Biogeosciences
title	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)
title_full	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)
title_fullStr	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)
title_full_unstemmed	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)
title_short	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)
title_sort	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
url	https://bg.copernicus.org/articles/19/2599/2022/bg-19-2599-2022.pdf
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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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