Particle settling rates increase with depth in the ocean

Time and depth-scales of particulate organic carbon degradation and CaCO3 and biogenic opal dissolution are critical to understanding the depth distribution of CO2 and dissolved nutrients in the ocean. The speed at which particles sink and the factors that control sinking speed are of primary importance to the distribution of oceanic nutrient concentrations and to the preservation of biogenic material in the sediment record. Sequencing sediment trap collectors deployed at US JGOFS sites in the equatorial Pacific and Arabian Sea provided a time series of particle fluxes from which particle settling rates were estimated. A comparison of settling velocities obtained from 100–500 m (Pilskaln et al., Deep Sea Research 45 (1998) 1803) to settling velocities obtained for depths between 1000 and 3500 m indicate an increase of a factor of 2–10 between 100 and 2000 m and an increase of 15–60% between 2000 and 3500 m. The increase in settling velocity in the deep ocean is generally correlated with the loss of Corg with depth. Lithogenic content does not appear to impact particle settling rate. Variability in particle settling rate is systematically related to physical forcing of the surface ocean in the Equatorial Pacific, but not in the Arabian Sea. The increase in particle settling rate with depth likely influences the delivery of CaCO3 to the sea floor.