Carbonate pump feedbacks on alkalinity and the carbon cycle in the 21<sup>st</sup> century and beyond

Planchat, Alban; Bopp, Laurent; Kwiatkowski, Lester; Torres, Olivier

doi:https://doi.org/10.5194/egusphere-2023-1218

Preprints

https://doi.org/10.5194/egusphere-2023-1218

Preprints

11 Jul 2023

| 11 Jul 2023

Carbonate pump feedbacks on alkalinity and the carbon cycle in the 21^st century and beyond

Alban Planchat, Laurent Bopp, Lester Kwiatkowski, and Olivier Torres

Abstract. Ocean acidification is likely to impact all stages of the ocean carbonate pump, i.e. the production, export, dissolution and burial of biogenic CaCO₃. However, the associated feedbacks on anthropogenic carbon uptake and ocean acidification have received little attention. It has previously been shown that Earth system model (ESM) carbonate pump parameterizations can affect and drive biases in the representation of ocean alkalinity, which is critical to the uptake of atmospheric carbon and provides buffering capacity towards associated acidification. In the sixth phase of the Coupled Model Intercomparison Project (CMIP6), we show divergent responses of CaCO₃ export at 100 m this century, with anomalies by 2100 ranging from -74 % to +23 % under a high-emissions scenario. The greatest export declines are projected by ESMs that consider pelagic CaCO₃ production to depend on the local calcite/aragonite saturation state. Despite the potential effects of other processes on alkalinity, there is a robust negative correlation between anomalies in CaCO₃ export and salinity-normalized surface alkalinity across the CMIP6 ensemble. Motivated by this relationship and the uncertainty in CaCO₃ export projections across ESMs, we perform idealized simulations with an ocean biogeochemical model and confirm a limited impact of carbonate pump anomalies on twenty-first century ocean carbon uptake and acidification. However between 2100 and 2300, we highlight a potentially abrupt shift in the dissolution of CaCO₃ from deep to subsurface waters when the global scale mean calcite saturation state reaches about 1.23 at 500 m (likely when atmospheric CO₂ reaches 900 to 1100 ppm). During this shift, upper ocean acidification due to anthropogenic carbon uptake induces deep ocean acidification driven by a substantial reduction in CaCO₃ deep dissolution following its decreased export at depth. Although the effect of a diminished carbonate pump on global ocean carbon uptake and surface ocean acidification remains limited until 2300, it can have a large impact on regional air-sea carbon fluxes, particularly in the Southern Ocean.

Received: 05 Jun 2023 – Discussion started: 11 Jul 2023

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The requested preprint has a corresponding peer-reviewed final revised paper. You are encouraged to refer to the final revised version.

Journal article(s) based on this preprint

03 May 2024

The carbonate pump feedback on alkalinity and the carbon cycle in the 21st century and beyond

Alban Planchat, Laurent Bopp, Lester Kwiatkowski, and Olivier Torres

Earth Syst. Dynam., 15, 565–588, https://doi.org/10.5194/esd-15-565-2024,https://doi.org/10.5194/esd-15-565-2024, 2024

Short summary

Alban Planchat, Laurent Bopp, Lester Kwiatkowski, and Olivier Torres

Interactive discussion

Status: closed

RC1:
'Comment on egusphere-2023-1218', Anonymous Referee #1, 11 Oct 2023
Review of “Carbonate pump feedbacks on alkalinity and the carbon cycle in the 21^st century and beyond” by Planchat et al.
Using CMIP6 models, the authors explored how the biogenic CaCO3 export at a 100 m depth responds to projected increases in atmospheric CO2 and how the different responses impact ocean surface alkalinity, the saturation state with respect to calcite, and oceanic CO2 uptake during the 21^st century. Motivated by the CMIP6 model spread in the projected CaCO3 export, the authors further explored the oceanic responses to imposed CaCO3 export changes for the extended time-period of 2100-2300 using an offline ocean biogeochemistry model. The authors nicely showed that the carbonate pump is one of the least constrained processes for the oceanic carbon pump and its projected uncertainty is very large. By assessing how the projected uncertainty can propagate into uncertainties in simulated oceanic carbon cycle on decadal to multi-centennial timescales, this study elucidates potential feedbacks from the carbonate pump on future carbon cycles. It is especially interesting to see that CaCO3 dissolution could respond abruptly to ocean acidification and that the sudden shift in CaCO3 dissolution could impact the regional patterns of oceanic CO2 uptake within the next century. The relatively minor effects of changing carbonate pumps on the oceanic CO2 uptake, compared to changing ocean physical dynamics, seem also novel and insightful. Overall, I have minor points.
The long-term response of CaCO3 dissolution to ocean acidification and its feedback on the Southern Ocean CO2 uptake seem very interesting. A question is how realistic this could be. Although a detailed model description is already presented in a previous publication by the same author, it might be worth highlighting here how the CaCO3 dissolution is parameterized in the PISCES model and how this parameterization compares with some observational constraints and/or models (e.g., Subhas et al. (2017); Liang et al. (2023) ). Related with this, the authors might say something about how CaCO3 dissolution is parameterized among the CMIP models in Introduction, as the effects of CaCO3 dissolution would become increasingly more important for the oceanic carbon cycle on longer timescales than the effects of CaCO3 export.

Please revise figures. In many figures (e.g., Figure 3), X- and Y-axes labels and units seem incomplete.

Line #288-289: From Fig. 4b, I don’t see that the indirect acidification rises from the deep to the surface. Instead, I see that the depth of CaCO3 dissolution (therefore basification) rises from the deep ocean towards the surface.
Line #299-300: Increasing subsurface POC remineralization itself would cause subsurface ocean acidification, opposing the effects of reduced CO2 uptake.
Data availability: Perhaps, the authors can make the NEMO-PISCES sensitivity experiments available to the public?

References:
Liang, H., Lunstrum, A. M., Dong, S., Berelson, W. M., & John, S. G. (2023). Constraining CaCO₃ export and dissolution with an ocean alkalinity inverse model. Global Bigeochem. Cycles, 37, e2022GB007535. https://doi.org/10.1029/2022GB007535
Subhas, A. V., Adkins, J. F., Rollins, N. E., Naviaux, J., Erez, J., & Berelson, W. M. (2017). Catalysis and chemical mechanisms of calcite dissolution in seawater. Proceedings of the National Academy of Sciences, 114(31), 8175-8180. https://doi.org/doi:10.1073/pnas.1703604114
Citation: https://doi.org/10.5194/egusphere-2023-1218-RC1
- AC1: 'Reply on RC1', Alban Planchat, 22 Dec 2023
  
  The comment was uploaded in the form of a supplement: https://egusphere.copernicus.org/preprints/2023/egusphere-2023-1218/egusphere-2023-1218-AC1-supplement.pdf
  
  Citation: https://doi.org/10.5194/egusphere-2023-1218-AC1
RC2:
'Comment on egusphere-2023-1218', John Dunne, 13 Nov 2023

The manuscript “Carbonate pump feedbacks on alkalinity and the carbon cycle in the 21st century and beyond” by Planchat et al analyzes ocean carbonate cycling in CMIP6 ESMs and performs an additional idealized analysis with NEMO-PISCES simulations to explore the underlying mechanisms. The manuscript is an important addition quantifying trends and uncertainties from the CMIP6 generation ESMs on ocean carbonate cycling changes out to 2100 where the impact on carbon uptake is shown to be small, as well as post 2100 in which internal ocean changes to carbonate saturation states lead to fundamental shifts in carbonate cycling and impacts on surface alkalinity. I recommend publication with minor technical revision as outlined below.
Technical points:
20 – I think “basis” should be “base”
35 – “towards” should be “to” and “, which explains why it” should be “in what”
43-44 – I am not sure if this statement is appropriate as a blanket statement or only in the context of the steady state/preindustrial ocean. I also do not think the current Zeebe reference about Boron is relevant to the argument… perhaps the authors intended this one instead? Zeebe, R. E., & Wolf-Gladrow, D. (2001). CO2 in seawater: equilibrium, kinetics, isotopes (No. 65). Gulf Professional Publishing.
132 and Equation 1 – The text says that alpha is multiplied by the PIC production, but inspection of the equation for alpha looks like it is zero at preindustrial CO2 of 285 ppm which would mean zero PIC production… should “multiplied by” be “added to” – in which case alpha has units of PIC production, or perhaps the equation goes to 1 at preindustrial? Also, the authors should specify how they derived the value of 0.15.
145 – add “, respectively” after “sDIC”
146-147 – The authors should specify what tolerance for “drift” was used to exclude these models but include others with less drift.
198 – The “effect” should be more specific – do the authors mean chemistry “acidification”, or fluxes “carbon uptake” or both?
Figure 2 – yellow line “crab_low” should be “carb_low”
249 – “that” should be “those”
250 – I think “affected” should be “driven”
Figure 3 – Why no symbols for NEMO-PISCES simulations on panel A? Unclear why a few symbols in A and C have a black box around them, “7” in panel C and “yr” in axis labels are not showing up, and Y-axis legend for panel D also seems to have formatting issues in my pdf version.
278 – remove second “is”
281 – This sentence does not make sense to me and should be reworked “Although a calcite saturation state threshold can robustly be pointed out, the shift itself should be reversible regarding this environmental control parameter, and should remain dependent to it.”
282-284 – “If subsurface Ωcalc is back at higher values than the threshold, then the vertical PIC dissolution should shift back at depth, probably without hysteresis.” I should be made clear that this is resented as a hypothesis untested in he present study and thus speculation. As such, the next sentence shouldn’t begin with “Therefore” as if the previous assertion had been proven. Also, it is not clear whether the Chen study was evidence for or against a tipping point. This should be clarified.
288 – add comma after “uptake”
293 – remove comma after “uptake”
316-317 “As there is a robust negative correlation between CaCO3 export anomalies and salinity-normalized surface alkalinity in the
CMIP6 ensemble, salinity-normalized surface alkalinity observations could be used to identify historical trends in PIC export.” Are the authors sure about that? Carter et al (https://agupubs.onlinelibrary.wiley.com/doi/10.1002/2015GB005308) argue that the earliest historical signals only emerge in 2030.

Citation: https://doi.org/10.5194/egusphere-2023-1218-RC2
- AC2: 'Reply on RC2', Alban Planchat, 22 Dec 2023
  
  The comment was uploaded in the form of a supplement: https://egusphere.copernicus.org/preprints/2023/egusphere-2023-1218/egusphere-2023-1218-AC2-supplement.pdf
  
  Citation: https://doi.org/10.5194/egusphere-2023-1218-AC2

Interactive discussion

Status: closed

RC1:
'Comment on egusphere-2023-1218', Anonymous Referee #1, 11 Oct 2023
Review of “Carbonate pump feedbacks on alkalinity and the carbon cycle in the 21^st century and beyond” by Planchat et al.
Using CMIP6 models, the authors explored how the biogenic CaCO3 export at a 100 m depth responds to projected increases in atmospheric CO2 and how the different responses impact ocean surface alkalinity, the saturation state with respect to calcite, and oceanic CO2 uptake during the 21^st century. Motivated by the CMIP6 model spread in the projected CaCO3 export, the authors further explored the oceanic responses to imposed CaCO3 export changes for the extended time-period of 2100-2300 using an offline ocean biogeochemistry model. The authors nicely showed that the carbonate pump is one of the least constrained processes for the oceanic carbon pump and its projected uncertainty is very large. By assessing how the projected uncertainty can propagate into uncertainties in simulated oceanic carbon cycle on decadal to multi-centennial timescales, this study elucidates potential feedbacks from the carbonate pump on future carbon cycles. It is especially interesting to see that CaCO3 dissolution could respond abruptly to ocean acidification and that the sudden shift in CaCO3 dissolution could impact the regional patterns of oceanic CO2 uptake within the next century. The relatively minor effects of changing carbonate pumps on the oceanic CO2 uptake, compared to changing ocean physical dynamics, seem also novel and insightful. Overall, I have minor points.
The long-term response of CaCO3 dissolution to ocean acidification and its feedback on the Southern Ocean CO2 uptake seem very interesting. A question is how realistic this could be. Although a detailed model description is already presented in a previous publication by the same author, it might be worth highlighting here how the CaCO3 dissolution is parameterized in the PISCES model and how this parameterization compares with some observational constraints and/or models (e.g., Subhas et al. (2017); Liang et al. (2023) ). Related with this, the authors might say something about how CaCO3 dissolution is parameterized among the CMIP models in Introduction, as the effects of CaCO3 dissolution would become increasingly more important for the oceanic carbon cycle on longer timescales than the effects of CaCO3 export.

Please revise figures. In many figures (e.g., Figure 3), X- and Y-axes labels and units seem incomplete.

Line #288-289: From Fig. 4b, I don’t see that the indirect acidification rises from the deep to the surface. Instead, I see that the depth of CaCO3 dissolution (therefore basification) rises from the deep ocean towards the surface.
Line #299-300: Increasing subsurface POC remineralization itself would cause subsurface ocean acidification, opposing the effects of reduced CO2 uptake.
Data availability: Perhaps, the authors can make the NEMO-PISCES sensitivity experiments available to the public?

References:
Liang, H., Lunstrum, A. M., Dong, S., Berelson, W. M., & John, S. G. (2023). Constraining CaCO₃ export and dissolution with an ocean alkalinity inverse model. Global Bigeochem. Cycles, 37, e2022GB007535. https://doi.org/10.1029/2022GB007535
Subhas, A. V., Adkins, J. F., Rollins, N. E., Naviaux, J., Erez, J., & Berelson, W. M. (2017). Catalysis and chemical mechanisms of calcite dissolution in seawater. Proceedings of the National Academy of Sciences, 114(31), 8175-8180. https://doi.org/doi:10.1073/pnas.1703604114
Citation: https://doi.org/10.5194/egusphere-2023-1218-RC1
- AC1: 'Reply on RC1', Alban Planchat, 22 Dec 2023
  
  The comment was uploaded in the form of a supplement: https://egusphere.copernicus.org/preprints/2023/egusphere-2023-1218/egusphere-2023-1218-AC1-supplement.pdf
  
  Citation: https://doi.org/10.5194/egusphere-2023-1218-AC1
RC2:
'Comment on egusphere-2023-1218', John Dunne, 13 Nov 2023

The manuscript “Carbonate pump feedbacks on alkalinity and the carbon cycle in the 21st century and beyond” by Planchat et al analyzes ocean carbonate cycling in CMIP6 ESMs and performs an additional idealized analysis with NEMO-PISCES simulations to explore the underlying mechanisms. The manuscript is an important addition quantifying trends and uncertainties from the CMIP6 generation ESMs on ocean carbonate cycling changes out to 2100 where the impact on carbon uptake is shown to be small, as well as post 2100 in which internal ocean changes to carbonate saturation states lead to fundamental shifts in carbonate cycling and impacts on surface alkalinity. I recommend publication with minor technical revision as outlined below.
Technical points:
20 – I think “basis” should be “base”
35 – “towards” should be “to” and “, which explains why it” should be “in what”
43-44 – I am not sure if this statement is appropriate as a blanket statement or only in the context of the steady state/preindustrial ocean. I also do not think the current Zeebe reference about Boron is relevant to the argument… perhaps the authors intended this one instead? Zeebe, R. E., & Wolf-Gladrow, D. (2001). CO2 in seawater: equilibrium, kinetics, isotopes (No. 65). Gulf Professional Publishing.
132 and Equation 1 – The text says that alpha is multiplied by the PIC production, but inspection of the equation for alpha looks like it is zero at preindustrial CO2 of 285 ppm which would mean zero PIC production… should “multiplied by” be “added to” – in which case alpha has units of PIC production, or perhaps the equation goes to 1 at preindustrial? Also, the authors should specify how they derived the value of 0.15.
145 – add “, respectively” after “sDIC”
146-147 – The authors should specify what tolerance for “drift” was used to exclude these models but include others with less drift.
198 – The “effect” should be more specific – do the authors mean chemistry “acidification”, or fluxes “carbon uptake” or both?
Figure 2 – yellow line “crab_low” should be “carb_low”
249 – “that” should be “those”
250 – I think “affected” should be “driven”
Figure 3 – Why no symbols for NEMO-PISCES simulations on panel A? Unclear why a few symbols in A and C have a black box around them, “7” in panel C and “yr” in axis labels are not showing up, and Y-axis legend for panel D also seems to have formatting issues in my pdf version.
278 – remove second “is”
281 – This sentence does not make sense to me and should be reworked “Although a calcite saturation state threshold can robustly be pointed out, the shift itself should be reversible regarding this environmental control parameter, and should remain dependent to it.”
282-284 – “If subsurface Ωcalc is back at higher values than the threshold, then the vertical PIC dissolution should shift back at depth, probably without hysteresis.” I should be made clear that this is resented as a hypothesis untested in he present study and thus speculation. As such, the next sentence shouldn’t begin with “Therefore” as if the previous assertion had been proven. Also, it is not clear whether the Chen study was evidence for or against a tipping point. This should be clarified.
288 – add comma after “uptake”
293 – remove comma after “uptake”
316-317 “As there is a robust negative correlation between CaCO3 export anomalies and salinity-normalized surface alkalinity in the
CMIP6 ensemble, salinity-normalized surface alkalinity observations could be used to identify historical trends in PIC export.” Are the authors sure about that? Carter et al (https://agupubs.onlinelibrary.wiley.com/doi/10.1002/2015GB005308) argue that the earliest historical signals only emerge in 2030.

Citation: https://doi.org/10.5194/egusphere-2023-1218-RC2
- AC2: 'Reply on RC2', Alban Planchat, 22 Dec 2023
  
  The comment was uploaded in the form of a supplement: https://egusphere.copernicus.org/preprints/2023/egusphere-2023-1218/egusphere-2023-1218-AC2-supplement.pdf
  
  Citation: https://doi.org/10.5194/egusphere-2023-1218-AC2

Peer review completion

AR: Author's response | RR: Referee report | ED: Editor decision | EF: Editorial file upload

ED: Publish subject to minor revisions (review by editor) (03 Jan 2024) by Sebastian G. Mutz

AR by Alban Planchat on behalf of the Authors (12 Jan 2024) Author's response Author's tracked changes Manuscript

ED: Publish as is (30 Jan 2024) by Sebastian G. Mutz

AR by Alban Planchat on behalf of the Authors (08 Feb 2024)

Journal article(s) based on this preprint

03 May 2024

The carbonate pump feedback on alkalinity and the carbon cycle in the 21st century and beyond

Alban Planchat, Laurent Bopp, Lester Kwiatkowski, and Olivier Torres

Earth Syst. Dynam., 15, 565–588, https://doi.org/10.5194/esd-15-565-2024,https://doi.org/10.5194/esd-15-565-2024, 2024

Short summary

Alban Planchat, Laurent Bopp, Lester Kwiatkowski, and Olivier Torres

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Short summary

Ocean acidification is likely to impact all stages of the ocean carbonate pump. We show divergent responses of CaCO₃ export over this century in Earth system models, with anomalies by 2100 ranging from -74 % to +23 % under a high-emissions scenario. While we confirm the limited impact of carbonate pump anomalies on twenty-first century ocean carbon uptake and acidification, we highlight a potentially abrupt shift in CaCO₃ dissolution from deep to subsurface waters beyond 2100.


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Carbonate pump feedbacks on alkalinity and the carbon cycle in the 21st century and beyond

Journal article(s) based on this preprint

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Interactive discussion

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Carbonate pump feedbacks on alkalinity and the carbon cycle in the 21^st century and beyond