In-situ measurements of particle settling velocity in the deep sea

Abstract

Two instruments, the remote optical settling tube and the benthic autonomous settling tube, have been deployed on the Nova Scotian continental rise at a depth of 4820 m. The few results obtained suggest that optical and gravimetric methods give comparable results. Assumptions about particle density little affect the settling velocity distribution, but do scale the total magnitude. By using a distribution of declining density with increasing size indicating increasing water content, the total concentration inferred from the light attenuation by using Mie theory is quite close to the total found from gravimetric data. The coarser end of the size distribution changes more with overall change in concentration than the fine end. The settling velocity distribution shows the presence of several modes. The two principal ones can be matched with modal sizes of ∼3.3 μm and ∼60 μm in Coulter Counter data allowing inference of Stokes density contrasts of 1.5 and 0.061 g cm −3, respectively. At concentrations of about 200 mg m −3, more than 50% of the material has a settling velocity less than 2 × 10 −4 cm s −1 (probably < 3 μm in size), but this percentage appears to decrease with an increase in total concentration.