Carbon dioxide removal via macroalgae open-ocean mariculture and sinking: an Earth system modeling study

Abstract. In this study, we investigate the maximum physical and biogeochemical potential of macroalgae open-ocean mariculture sinking (MOS) as an ocean-based carbon dioxide removal (CDR) method. Embedding a model into Earth system model, simulate in surface layer followed by fast carbon-rich macroalgal biomass to deep seafloor (depth>3000 m), which assumes no remineralization harvested during quick sinking. We also test combination MOS with artificial upwelling (AU), fertilizes pumping nutrient-rich deeper water surface. The simulations are done under RCP 4.5, moderate-emissions pathway. When deployed globally between years 2020 2100, captured exported is 270 PgC, further boosted AU 447 PgC. Because feedbacks system, oceanic inventory only increases 171.8 PgC (283.9 AU) idealized simulations. More than half remains ocean after cessation at year 2100 until 3000. major side effect on pelagic ecosystems reduction phytoplankton net primary production (PNPP) due competition for nutrients canopy shading. shrinks mid-layer oxygen-minimum zones (OMZs) reducing organic matter export to, in, subsurface intermediate waters, while it creates new OMZs oxygen consumption from sunken biomass. impacts global cycle atmospheric terrestrial reservoirs when enhancing reservoir. enriches dissolved inorganic ocean. Effects mostly reversible MOS, though recovery not complete sensitivity experiment without biomass, whole MOS-captured permanently stored ocean, but lack remineralized causes long-term nutrient decline layers thus reduces PNPP. Our results suggest that has, theoretically, considerable CDR However, our such large-scale deployment would have substantial effects marine biogeochemistry, up reorganization food webs over large parts