Controls on sinking velocities and mass fluxes of size-fractionated
marine particles in recent U.S. GEOTRACES cruises
Abstract
Particle composition is an important parameter that influences sinking
velocity of marine particles. Most current studies, however, are limited
by either a lack of routine measurements of particle composition or low
sampling resolution in the water column. Here, we compile full
ocean-depth size-fractionated (1-51 and >51 μm) particle
concentration and composition of suspended particulate matter from three
recent U.S. GEOTRACES cruises to calculate their corresponding sinking
velocity and mass flux. Our model is based on Stokes’ Law and
incorporates a newly updated power-law relationship between particle
size and porosity. The integration of the porosity-size relationship
decreases the power applied to size in Stokes’ Law to 0.8. The medians
of average sinking velocity in total particles are 15.4, 15.2, and 7.4
m/d, in the North Atlantic, Southeast Pacific, and western Arctic Ocean,
respectively. We examine the relative importance of particle
concentration, composition, size, and hydrography on sinking fluxes.
Particle concentration is the major control of the variability and
magnitude of mass flux, while particle composition is the second most
important term. Increasing porosity with aggregate size and a dominance
of smaller particles diminishes the importance of the size dependence in
mass flux, elevating the relative importance of composition and thus
density. Viscosity of seawater can result in up to a factor of two
difference in mass flux between polar and tropical oceans. This work
serves as one of the first studies to offer quantitative perspectives
for the contribution from different factors to mass flux in field
observations of marine particles.