Estimating Mass Flux From Size-Fractionated Filtered Particles: Insights Into Controls on Sinking Velocities and Mass Fluxes in Recent US GEOTRACES Cruises

We compile full ocean-depth size-fractionated (1-51 and >51 mu m) particle concentration and composition of suspended particulate matter from three recent U.S. GEOTRACES cruises, and exploit detailed information of particle characteristics measured to give insights into controls on sinking velocity and mass flux. Our model integrates the concept of fractal scaling into Stokes' Law by incorporating one of two porosity-size power law relationships that result in fractal dimensions of 1.4 and 2.1. The medians of pump-derived total (>1 mu m) mass flux in the upper 100 m of gyre stations are 285.1, 609.2, and 99.3 mg/m(2)/d in the North Atlantic, Eastern Tropical South Pacific, and Western Arctic Ocean cruises, respectively. In this data set, variations in particle concentration were generally more important than sinking velocity in controlling variations in mass flux. We examine different terms in a Stokes' Law model to explore how variations in particle and water column characteristics from these three cruises affect mass flux. The decomposition of different aspects of the Stokes' relationship sheds light on the lowest total mass flux of the three cruises in the Western Arctic, which could be explained by the Arctic having the lowest particle concentrations as well as the lowest sinking velocities due to having the smallest particle sizes and the most viscous water. This work shows the importance of both particle characteristics and size distribution for mass fluxes, and similar methods can be applied to existing and future size-fractionated filtered particulate measurements to improve our understanding of the biological pump elsewhere.