A 3-D Model of Lagrangian Marine Particles

Marine particles lock atmospheric carbon into organic structures and pump it into the deep ocean, a process known as the ocean’s biological carbon pump (BCP). Part of this rain of organic carbon into the deep waters is attenuated as respiration, dissolution, fragmentation and solubilisation attack the particulate carbon and return it back into its inorganic form. Sediment traps and imaging systems have revealed the fundamental importance that particle material composition and shape play in the structure and dynamics of the particulate organic carbon (POC) flux, but these data are still too scarce and scattered to provide us with a mechanistic picture on what controls the attenuation of POC flux. Here, we present the first global model of dynamic marine particles using a Lagrangian and mechanistic description of BCP state variables. A Lagrangian model tracks individual particles as they move throughout the model space. This framework allows us to resolve the radius, density, stickiness and settling speed of particles as they are transported by the 3-D ocean circulation, sink and interact with their ecosystem through primary production, aggregation, grazing by zooplankton, bacteria attachment, dissolution below the lysocline and solubilisation. We use the model to calculate the spectra of particle size, velocity, density and porosity at different depths and basins, and quantify spatial variations in the vertical attenuation of POC flux and its dependence on factors such as seawater temperature and faecal pellet abundance.