Particle production by mesopelagic fauna enhances deep-ocean carbon sequestration

Oceanic carbon sequestration via the biological pump operates through two main export pathways: gravitational sinking of particulate organic carbon (POC) from the surface and injection at depth by migrating fauna. Where the exported POC remineralizes in the water column critically determines the longevity of carbon storage. However, an incomplete view of the organic particle dynamics over depth hinders our understanding of POC flux attenuation and its role in carbon sequestration. Here, we combine observations from in situ particle camera observations (Underwater Vision Profilers; UVP5) with machine learning and empirical algorithms to reconstruct three-dimensional particle size distributions and sinking POC fluxes. We find increased particle biovolume and size in the central mesopelagic zone (400-800 m depth), coinciding with stabilized or even enhanced POC flux, consistent with particle injection by mesopelagic fauna. Assessment of the two export pathways indicates that animal-mediated injection contributes nearly as much to carbon sequestration as the gravitational pathway, despite representing only a small fraction of the total global carbon export. This apparent paradox can be explained by deeper, more efficient sequestration via animal-mediated export, particularly in low- latitude regions with shallower gravitational flux attenuation. These findings point to underappreciated sensitivities of oceanic carbon sequestration to climate and human pressures on mesopelagic fauna.