In situ measurements of marine particle settling velocity and size distributions using the remote optical settling tube

Abstract

A remote optical settling tube was deployed in the deep ocean to obtain the first ever in situ distributions of particle settling velocity. These distributions were extrapolated to particle size distributions assuming several possible fixed and variable (with size) particle densities. Cumulative slopes of the particle size distribution (with fixed particle densities of 1.1–2.0 g cm−3) were consistent with earlier measurements made shipboard in the same region. Cumulative slopes obtained using a more realistic variable density (as would be the case for populations including zero‐order and first‐order aggregates) were considerably lower, suggesting that the remote settling tube is detecting aggregated material more accurately than can be done with impedance‐based particle size analyzers. This is supported by the fact that the particulate relative real refractive index was also computed from the specific beam attenuation coefficient and yielded expected results only when the variable density size distribution was applied. These data and analyses demonstrate that the remote optical settling tube can be used effectively (even at low concentrations of suspended material) for in situ determinations of the particle size distribution, especially where relatively fragile aggregates may be present.