Revisiting the biological pump using the new continuous vertical sequestration approach

The ocean contains about 40 times more carbon than the atmosphere, i.e. it stores 38,000 Pg C of dissolved inorganic carbon (DIC) vs. 900 Pg C of carbon dioxide in the present atmosphere. The biological carbon pump contributes to ocean carbon storage by moving DIC out of the surface ocean into deeper waters. Many studies have assumed that storage of biogenic DIC (DIC-bio) on climate-relevant timescales (typically 100 years or more) only occurs in the deep ocean. Here we show, in contrast, that this storage can occur at all depths in the water column, mostly above 2,000 and even 1,000 m. To illustrate this continuous vertical sequestration, we use the fraction of water that remains in the ocean interior for 100 years or more, computed with a data-assimilated circulation model, to estimate the DIC-bio sequestration fluxes resulting from the combined action of different biological pump pathways. With this new approach, global carbon sequestration for 100 years or more driven by the biological pump is 0.9-2.6 Pg C/y, which is up to 6 times larger than usual estimates that ignore the contribution of upper-ocean sequestration. Biogeochemical models should thus compute sequestration fluxes over all water-column depths. In evaluations of future ocean deliberate carbon dioxide removal (CDR) strategies, biologically-driven carbon sequestration fluxes should also be estimated over the whole water column.