Assessment of Sinking Particle Size Spectra from Marine Snow Catcher Deployments during EXPORTS

Sinking particles are one of the main export pathways that allow organic carbon produced in the surface ocean to be transported to depth where it is removed from the atmosphere. Particle size influences particle sinking velocity and thus plays a critical role in controlling the distribution of carbon within the water column. Moreover, understanding how sinking velocity links to particle size will allow us to use satellite-derived particle size distributions (PSDs) for a mechanistic understanding of the biological carbon pump. It is often assumed that particle sinking velocities increase as a function of particle size following Stokes' law and that small particles either do not sink or are remineralized within the upper mesopelagic. However, the presence of small particles has been observed at greater depths (>1000m) challenging this understanding. Here we show how size and sinking velocity relate by comparing the size distributions of non-, slow- and fast-sinking particles. Particles were collected as part of the first NASA-led EXPORTS field campaign conducted in Aug/Sep 2018 in the NE Pacific. Non-, slow- and fast-sinking particles were collected with the Marine Snow Catcher (11 stations with 3 depths between 20 and 500 m). Particles were analyzed using FlowCAM and data processed using a specific Python code. We computed PSDs using normalized number spectra and normalize volume spectra. Our data suggests that the non-sinking fractions contain fewer large particles (steeper PSD slope) than the fast-sinking fraction as assumed by Stokes' law. Furthermore, our data indicate a decrease of particle size with depth suggesting the presence of processes destroying particles.