Spreading of a Deep‐Ocean Tracer in a Suite of Eulerian and Lagrangian Simulations

Abstract Hydrothermal vents are important sources of biogeochemical constituents with a global relevance, like iron. The fate of these elements is poorly constrained, both by the physical mechanisms of transport and mixing, and by the geochemical speciation processes. Here we set up a regional simulation of the ocean circulation that resolves submesoscale and tidal processes around the Trans‐Atlantic Geotraverse (TAG) hydrothermal vent to investigate how the small scales of the dynamics impact the dispersal of a hydrothermal plume. A passive Eulerian tracer is continuously released at TAG and spreads over 260 days. We design sensitivity experiments to isolate key forcing mechanisms: a simulation without tides, a simulation with a bathymetry that is smoothed to remove kilometer‐scale features, and a simulation with Lagrangian particles to be compared to the Eulerian framework. We find that the kilometer‐scale seafloor topography is essential to drive submesoscale instabilities and generate submesoscale eddies that capture seafloor material and transport it off the ridge, increasing dramatically the horizontal spreading as compared to the smooth‐bathymetry simulation. Tides are found to increase vertical diffusivity above the ridge, but their impact on the horizontal dispersal is negligible. Eulerian and Lagrangian experiments lead to very similar horizontal dispersal pathways, although the vertical spreading is much reduced in the Lagrangian simulation, which is explained by the lack of an explicit representation of mixing processes.