Biominerals and the vertical flux of particulate organic carbon from the surface ocean

Particulate inorganic carbon (PIC; calcium carbonate) is thought to be a significant source of light scattering in the sea. It also provides ballast for particulate matter, driving the ocean's biological carbon pump. During three trans‐Atlantic cruises, we measured particle optical properties plus concentrations of the three major components of sinking aggregates [particulate organic carbon (POC), PIC and biogenic silica (BSi)]. PIC contributed 15–23% of particle backscattering in oligotrophic subtropical gyres and temperate waters. Light scattering properties allowed quantification of the surface PIC:POC ratio. The ratio of the two ballast minerals (PIC:BSi) was significantly, inversely, correlated to POC concentration, allowing robust modeling of the density of sinking aggregates. Results showed greater PIC:POC ratios and sinking rates in oligotrophic regions due to greater relative abundance of PIC.