Preprints
https://doi.org/10.5194/esd-2021-104
https://doi.org/10.5194/esd-2021-104

  14 Jan 2022

14 Jan 2022

Review status: this preprint is currently under review for the journal ESD.

Carbon Dioxide Removal via Macroalgae Open-ocean Mariculture and Sinking: An Earth System Modeling Study

Jiajun Wu1, David P. Keller1, and Andreas Oschlies1,2 Jiajun Wu et al.
  • 1GEOMAR Helmholtz Centre for Ocean Research Kiel, Kiel, Germany
  • 2Kiel University, Germany

Abstract. In this study we investigate open-ocean macroalgae mariculture and sinking (MOS) as ocean-based carbon dioxide removal (CDR) method. Embedding a macroalgae model into an Earth system model, we simulate macroalgae mariculture in the open-ocean surface layer followed by fast sinking of the carbon-rich macroalgal biomass to the deep seafloor (depth > 3,000 m). We also test the combination of MOS with artificial upwelling (AU), which fertilizes the macroalgae by pumping nutrient-rich deeper water to the surface. The simulations are done under RCP4.5 a moderate emission pathway. When deployed globally between years 2020 and 2100, the simulated CDR potential of MOS is 270 PgC, which is further boosted by AU to 447 PgC. More than half of MOS-sequestered carbon retains in the ocean after cessation at year 2100 until year 3000.

The major side effect of MOS on pelagic ecosystems is the reduction of phytoplankton net primary production (PNPP) due to the nutrient competition and canopy shading by macroalgae. MOS shrinks the mid layer oxygen minimum zones (OMZs) by reducing the organic matter export to, and remineralization in, subsurface and intermediate waters, while it creates new OMZs on the seafloor by oxygen consumption from remineralization of sunken biomass. MOS also impacts the global carbon cycle, reduces the atmospheric and terrestrial carbon reservoir when enhancing the ocean carbon reservoir. MOS also enriches the dissolved inorganic carbon in the deep ocean. Effects are mostly reversible after cessation of MOS, though recovery is not complete by year 3000. In a sensitivity experiment without remineralization of sunk MOS biomass, the entire MOS-captured carbon is permanently stored in the ocean, but the lack of remineralized nutrients causes a long-term nutrient decline in the surface layers and thus reduces PNPP.

Our results suggest that MOS has a considerable potential as an ocean-based CDR method. However, MOS has inherent side effects on marine ecosystems and biogeochemistry, which will require a careful evaluation beyond this first idealized modeling study.

Jiajun Wu et al.

Status: open (until 25 Feb 2022)

Comment types: AC – author | RC – referee | CC – community | EC – editor | CEC – chief editor | : Report abuse
  • CC1: 'Comment on esd-2021-104', K. Caldeira, 15 Jan 2022 reply
  • AC1: 'Comment on esd-2021-104: correction of molar C:P ratio of macroalgae', Jiajun Wu, 17 Jan 2022 reply
  • RC1: 'Comment on esd-2021-104', Lennart Bach, 18 Jan 2022 reply

Jiajun Wu et al.

Jiajun Wu et al.

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Short summary
In this study we investigate an ocean-based carbon dioxide removal method: macroalgae open-ocean mariculture and sinking (MOS), which aims to cultivate seaweeds in the open ocean surface and sink matured biomass quickly to the deep seafloor. Our results suggest that MOS has a considerable potential as an ocean-based CDR method. However, MOS has inherent side effects on marine ecosystems and biogeochemistry, which will require a careful evaluation beyond this first idealized modeling study.
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