Dispersion of Helium from the TAG hydrothermal vent field: perspectives from coupled physics-geochemistry model experiments

Hydrothermal vents are oceanic sources of biogeochemical constituents. Some of these constituents, such as iron, significantly contribute to global biogeochemical cycles. Yet, their fate, i.e., transport and mixing through physical processes, and modification of their concentration through bio-geochemical processes, remains poorly quantified. Using state-of-the-art physical-biogeochemical (CROCO-PISCES) model simulations that resolve submesoscale processes, internal gravity waves and parameterized mixing processes, we analyse the physical processes involved in the dispersion of passive tracers (i.e. Helium) released at the Trans-Atlantic Geotraces (TAG) hydrothermal site.

A reference simulation features a horizontal grid spacing of 1 km, 150 terrain-following vertical levels, and includes high-frequency atmospheric and tidal forcing. Helium is initialized and continuously released at TAG, following a distribution that is constrained by observations. We also ran sensitivity experiments, without tides and with a smooth bathymetry designed to investigate the effects of CMIP (Coupled Models Intercomparison Project) model coarse bathymetries on the circulation.

At short spatial and time scales (~20 km, ~10 days), we find that tidal processes are instrumental in the tracer dispersion. Through comparisons between the reference and the no-tides simulations, we show that tidal currents and internal tides drive the dispersion within the TAG surrounding valley, and tidally-induced mixing drives the vertical dispersion of tracers, especially on the flanks of the valley walls and within fracture zones. At longer and larger scales (>20 km, >10 days), submesoscale and mesoscale instabilities catalyzed by the interaction of currents with the ridge topography lead to the formation of eddies that trap tracers and escape from the ridge valley to wander at depth preferentially westward of the ridge. Small-scale topographic structures such as fracture zones and abyssal hills control the dispersion and notably slows down the dispersion of tracers outside of the ridge valley. Simulation with smooth bathymetry hence shows a more isotropic and rapid dispersion. This could lead to biases in the inferred pathways of tracers in global models. Next, we will investigate the fate of active tracers, such as iron, which is impacted by biogeochemical processes, such as scavenging and complexation by ligands.