Intercomparison of Instrumentation for Settling Velocity and Settling Flux Measurements in the Deep Sea and Open Ocean

The settling velocity of suspended particles is of importance for sediment transport modelling, e.g. in coastal zones or the bottom boundary layer, as well as for carbon and particle flux export calculations in the open ocean. In this presentation we compare two new instruments that have recently been commercialized by Sequoia for these purposes. One, the LISST-RTSSV is a camera system designed for measuring suspended sediment size, concentration and settling velocity in the deep sea down to 6,000 m depth. The other, LISST-OST is an optical sediment trap designed to measure particle flux on Argo floats.

Here, we present the first results from a laboratory intercomparison between the two instruments. In a controlled environment, the two sensors were installed together, and a range of different particle types were allowed to settle through their sensing zones. We show that the particle flux from a camera system like the LISST-RTSSV is highly sensitive to the appearance (or not!) of a few large particles in the field of view. Because the particle concentration in the open ocean is very low, camera methods will not necessarily work well for settling velocity or settling flux measurements. The LISST-OST measures the diffuse attenuation flux from which we derive equations for the mass settling flux. The diffuse attenuation can be measured with high accuracy and precision and is thus suitable for settling flux measurements in low concentration environment like the open ocean. Because our laboratory experiments concerned settling of dense, dis-aggregated sediment grains, and because particles in the open ocean are of organic origin and/or aggregated into marine snow, the equations we present constrain upper limits to the settling flux from the LISST-OST.