Seasonal copepod lipid pump promotes carbon sequestration in the deep North Atlantic

Physical, chemical and biological processes can mediate carbon transfer from surface ocean waters to below the permanent pycnocline and so promote ocean carbon sequestration. Passive sinking of organic and carbonate-rich biogenic particles - the ‘biological pump’ -has been estimated to account for a sequestration flux of 2 - 8 gC m-2 yr-1 at around 1000m depth. Here we identify a comparably important mechanism for sequestering carbon in the North Atlantic and other sub-polar seas. We estimate that as a result of the annual vertical migration of overwintering copepods, between 2 and 6 gC m-2 yr-1 are actively transported to below the permanent pycnocline as lipids. Only 25 - 50% of these lipids are carried back to the surface in spring with the surviving copepods, resulting in a sequestration flux of 1 to 4 gC m-2 yr-1. This ’lipid pump’ has gone largely un-recorded in either direct measurements of carbon sequestration, or estimates based on surface production and export flux. In addition, elemental ratios of nitrogen, phosphorus, silicon and iron to carbon are extremely low or zero in lipids, so the lipid pump does not strip the surface ocean of limiting nutrients, and decouples the carbon sink from nutrient replenishment rates.