[1] The ocean’s “biological pump” is the ensemble of processes by which carbon fixed by phytoplankton at the sunlit surface is transported to depth, where it may be sequestered for centuries and thus withdrawn from exchange with the atmosphere. [2] The pump’s efficiency turns on a single quantity: the depth at which sinking organic particles are consumed by bacteria and zooplankton and their carbon returned to dissolved form. [3] Carbon remineralized in the shallow “twilight zone,” just below the sunlit layer, is readily returned to the surface by mixing within years; carbon that survives to greater depths is effectively locked away. [4] For decades models represented this decline in particle flux with a single global curve, as though the rate at which sinking matter is stripped away were everywhere the same.
[5] Recent observations have unsettled that convenience. [6] The depth of remineralization, it now appears, varies markedly from region to region, governed by the temperature of the mid-water column, the size and mineral ballasting of the sinking particles, and the composition of the animal communities that intercept them. [7] In cold, productive waters particles tend to sink faster and deeper before being consumed, whereas in warm, sluggish waters much of the flux is respired near the surface and never reaches sequestering depths. [8] The consequence is that two regions of identical surface productivity may differ substantially in how much carbon they actually export to the deep sea. [9] A single global curve, in short, obscures precisely the variation that determines the pump’s contribution to the planet’s carbon balance.
