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Impact of physical and biological processes on temporal variations of the ocean carbon sink in the mid-latitude North Atlantic (2002–2016)

Impact of physical and biological processes on temporal variations of the ocean carbon sink in the mid-latitude North Atlantic (2002–2016)
Impact of physical and biological processes on temporal variations of the ocean carbon sink in the mid-latitude North Atlantic (2002–2016)
The ocean is currently a significant net sink for anthropogenically remobilised CO2, taking up around 24% of global emissions. Numerical models predict a diversity of responses of the ocean carbon sink to increased atmospheric concentrations in a warmer world. Here, we tested the hypothesis that increased atmospheric forcing is causing a change in the ocean carbon sink using a high frequency observational dataset derived from underway pCO2 (carbon dioxide partial pressure) instruments on ships of opportunity (SOO) and a fixed-point mooring between 2002 and 2016. We calculated an average carbon flux of 0.013 Pg yr−1 into the ocean at the Porcupine Abyssal Plain (PAP) site, consistent with past estimates. In spite of the increase in atmospheric pCO2, monthly average seawater pCO2 did not show a statistically significant increasing trend, but a higher annual variability, likely due to the decreasing buffer capacity of the system. The increasing





ΔpCO2
led to an increasing trend in the estimated CO2 flux into the ocean of 0.19 ± 0.03 mmol m−2 day−1 per year across the entire 15 year time series, making the study area a stronger carbon sink. Seawater pCO2 variability is mostly influenced by temperature, alkalinity and dissolved inorganic carbon (DIC) changes, with 77% of the annual seawater pCO2 changes explained by these terms. DIC is in turn influenced by gas exchange and biological production. In an average year, the DIC drawdown by biological production, as determined from nitrate uptake, was higher than the DIC increase due to atmospheric CO2 dissolution into the surface ocean. This effect was enhanced in years with high nutrient input or shallow mixed layers. Using the rate of change of DIC and nitrate, we observed Redfieldian carbon consumption during the spring bloom at a C:N ratio of 6.2 ± 1.6. A comparison between SOO and PAP sustained observatory data revealed a strong agreement for pCO2 and DIC. This work demonstrates that the study area has continued to absorb atmospheric CO2 in recent years with this sink enhancing over time. Furthermore, the change in pCO2 per unit nitrate became larger as surface buffer capacity changed.
Biological production, CO flux, North Atlantic, pCO variability
0079-6611
1-14
Macovei, Vlad A.
107afbdf-5458-4226-85ed-eda3aee63ae5
Hartman, Susan E.
951bae25-3ce8-4305-8739-3e564631ab34
Schuster, Ute
3ba01afd-9a84-46e5-ae83-2e3ae708250f
Torres-Valdés, Sinhué
d66ec9fe-9bb3-48cd-97e0-246950f5eca1
Moore, C. Mark
7ec80b7b-bedc-4dd5-8924-0f5d01927b12
Sanders, Richard J.
02c163c1-8f5e-49ad-857c-d28f7da66c65
Macovei, Vlad A.
107afbdf-5458-4226-85ed-eda3aee63ae5
Hartman, Susan E.
951bae25-3ce8-4305-8739-3e564631ab34
Schuster, Ute
3ba01afd-9a84-46e5-ae83-2e3ae708250f
Torres-Valdés, Sinhué
d66ec9fe-9bb3-48cd-97e0-246950f5eca1
Moore, C. Mark
7ec80b7b-bedc-4dd5-8924-0f5d01927b12
Sanders, Richard J.
02c163c1-8f5e-49ad-857c-d28f7da66c65

Macovei, Vlad A., Hartman, Susan E., Schuster, Ute, Torres-Valdés, Sinhué, Moore, C. Mark and Sanders, Richard J. (2020) Impact of physical and biological processes on temporal variations of the ocean carbon sink in the mid-latitude North Atlantic (2002–2016). Progress in Oceanography, 180, 1-14, [102223]. (doi:10.1016/j.pocean.2019.102223).

Record type: Article

Abstract

The ocean is currently a significant net sink for anthropogenically remobilised CO2, taking up around 24% of global emissions. Numerical models predict a diversity of responses of the ocean carbon sink to increased atmospheric concentrations in a warmer world. Here, we tested the hypothesis that increased atmospheric forcing is causing a change in the ocean carbon sink using a high frequency observational dataset derived from underway pCO2 (carbon dioxide partial pressure) instruments on ships of opportunity (SOO) and a fixed-point mooring between 2002 and 2016. We calculated an average carbon flux of 0.013 Pg yr−1 into the ocean at the Porcupine Abyssal Plain (PAP) site, consistent with past estimates. In spite of the increase in atmospheric pCO2, monthly average seawater pCO2 did not show a statistically significant increasing trend, but a higher annual variability, likely due to the decreasing buffer capacity of the system. The increasing





ΔpCO2
led to an increasing trend in the estimated CO2 flux into the ocean of 0.19 ± 0.03 mmol m−2 day−1 per year across the entire 15 year time series, making the study area a stronger carbon sink. Seawater pCO2 variability is mostly influenced by temperature, alkalinity and dissolved inorganic carbon (DIC) changes, with 77% of the annual seawater pCO2 changes explained by these terms. DIC is in turn influenced by gas exchange and biological production. In an average year, the DIC drawdown by biological production, as determined from nitrate uptake, was higher than the DIC increase due to atmospheric CO2 dissolution into the surface ocean. This effect was enhanced in years with high nutrient input or shallow mixed layers. Using the rate of change of DIC and nitrate, we observed Redfieldian carbon consumption during the spring bloom at a C:N ratio of 6.2 ± 1.6. A comparison between SOO and PAP sustained observatory data revealed a strong agreement for pCO2 and DIC. This work demonstrates that the study area has continued to absorb atmospheric CO2 in recent years with this sink enhancing over time. Furthermore, the change in pCO2 per unit nitrate became larger as surface buffer capacity changed.

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Accepted/In Press date: 13 November 2019
e-pub ahead of print date: 20 November 2019
Published date: 1 January 2020
Keywords: Biological production, CO flux, North Atlantic, pCO variability

Identifiers

Local EPrints ID: 437910
URI: http://eprints.soton.ac.uk/id/eprint/437910
ISSN: 0079-6611
PURE UUID: abc38f0a-2a7d-423f-ba3e-4b61c3a3497b
ORCID for C. Mark Moore: ORCID iD orcid.org/0000-0002-9541-6046

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Date deposited: 24 Feb 2020 17:30
Last modified: 26 Nov 2021 02:42

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Contributors

Author: Vlad A. Macovei
Author: Susan E. Hartman
Author: Ute Schuster
Author: Sinhué Torres-Valdés
Author: C. Mark Moore ORCID iD
Author: Richard J. Sanders

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