Rethinking sinking: Imaging the flow fields of natural marine aggregates to derive sinking velocity

Abstract

AbstractThe marine biological carbon pump is driven by the interplay between particle sinking velocity and remineralisation. Despite its importance, sinking velocity of natural marine aggregates is not routinely measured, but often calculated from aggregate size and density using Stokes’ law. Yet, comparing calculated and experimentally measured sinking velocities has shown that Stokes’ law does not accurately predict the size-to-sinking relationship of marine aggregates because size-dependent changes in porosity are not well considered. We analysed the flow fields around 81in situcollected aggregates using Particle Image Velocimetry (PIV) in order to determine the controlling factors for aggregate settling and provide a better scaling for their porosity. The flow fields around all types and sizes of aggregates strongly resembled those of impermeable, porous spheres. Using an independently derived scaling of porosity with size, we could accurately predict the sinking velocity of laboratory- and field-collected aggregates with known density. Our improved sinking velocity calculations provide carbon flux predictions that matched measured flux well, while flux calculated with sinking velocity calculations using Stokes’ law underestimated flux on average by 2-fold. When applying our improved estimates for settling velocity to a global data set of vertical particle abundance and size-distribution, our findings reveal that small particles (< 500 μm) contribute 40% to total carbon flux in high latitudes and 70% in equatorial regions. This substantial contribution of small particles to total carbon fluxes across various oceans has previously been attributed to their high numerical abundances. Here, we provide a mechanistic basis for this phenomenon, i.e., their relatively low porosities render them compact and dense particles with high size-specific settling velocities and thus a high carbon-to-volume ratio.