Mass transfer to and from small particles in the sea1,2

Slowly sinking particles exchange mass with their liquid environment if dissolution processes within the particles maintain a surface concentration excess or deficiency χ 0 of a soluble substance above background. If the exchange is controlled by diffusion in the liquid phase, the total mass loss is proportional to χ 0 , diffusivity D, and particle diameter d. If advection associated with the sinking of the particle dominates over diffusion, the exchange rate varies as χ 0 D ⅔ d 2 and is weakly affected by particle density and fluid viscosity. This is the case for spherical particles of a diameter 70 µ m. The resulting dissolution rate per unit depth of sinking is constant for large particles, but increases sharply once the particles are small and slow enough for mass exchange to be controlled by diffusion. With oxygen supply supposed the rate‐controlling process of particle decomposition, a realistic depth of decomposition of order 1 km is calculated for 200‐ µ m‐diameter spherical particles. The generation of an oxygen minimum at such depths then requires the production near the sea surface of sufficiently large organic particles in sufficient numbers.