Fig. 1: Ocean alkalinity enhancement through benthic carbonate dissolution.

Photosynthetically-derived particulate organic carbon (POC) sinks from surface waters to the seafloor, whereupon it is respired by microorganisms to CO₂, leading to undersaturation of sediment porewaters with respect to carbonate minerals^28,56. A fraction of the metabolic CO₂ is consumed during the dissolution of calcium carbonate artificially added to the seafloor (white circles). This results in a flux of alkalinity and DIC to the water column in a 2:1 ratio, thereby raising the pH and lowering the partial pressure of CO₂¹⁵. Equilibration with the atmosphere will produce the envisioned CO₂ uptake from the atmosphere to the water column (dashed arrow).