Preprints
https://doi.org/10.5194/egusphere-2023-1218
https://doi.org/10.5194/egusphere-2023-1218
11 Jul 2023
 | 11 Jul 2023

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

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

Status: closed

Comment types: AC – author | RC – referee | CC – community | EC – editor | CEC – chief editor | : Report abuse
  • RC1: 'Comment on egusphere-2023-1218', Anonymous Referee #1, 11 Oct 2023
  • RC2: 'Comment on egusphere-2023-1218', John Dunne, 13 Nov 2023

Status: closed

Comment types: AC – author | RC – referee | CC – community | EC – editor | CEC – chief editor | : Report abuse
  • RC1: 'Comment on egusphere-2023-1218', Anonymous Referee #1, 11 Oct 2023
  • RC2: 'Comment on egusphere-2023-1218', John Dunne, 13 Nov 2023
Alban Planchat, Laurent Bopp, Lester Kwiatkowski, and Olivier Torres
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.