Simulations of settling marine aggregates in a stratified fluid*

Settling marine aggregates is essential in transporting dissolved carbon dioxide from the ocean surface to the deep sea. While sinking, they accumulate in thin layers where density stratifications are present, becoming nutrient hotspots for bacterial and animal activity. Here, we simulate settling aggregates in a density-stratified fluid. We assemble fractal aggregates as a collection of cubes to model a marine aggregate. In the absence of stratification, the flow around the aggregate is computed at a limit of zero Reynolds number using a boundary integral method. A term involving a volume integral is added to the boundary integral formulation to allow variable density in the ambient fluid. We couple the velocity with the advection-diffusion equation to track the density over time. We use this method to quantify how stratification affects the aggregate settling speed and residence time in a sharp stratification.