Rapid dispersal of a hydrothermal plume by turbulent mixing

Abstract

The water column imprint of the hydrothermal plume observed at the Nibelungen field (8°18' S 13°30' W) is highly variable in space and time. The off-axis location of the site, along the southern boundary of a non-transform ridge offset at the joint between two segments of the southern Mid-Atlantic Ridge, is characterized by complex, rugged topography, and thus favorable for the generation of internal tides, subsequent internal wave breaking, and associated vertical mixing in the water column. We have used towed transects and vertical profiles of stratification, turbidity, and direct current measurements to investigate the strength of turbulent mixing in the vicinity of the vent site and the adjacent rift valley, and its temporal and spatial variability in relation to the plume dispersal. Turbulent diffusivities Kp were calculated from temperature inversions via Thorpe scales. Heightened mixing (compared to open ocean values) was observed in the whole rift valley within an order of Kp around 10-3 m2 s-1. The mixing close to the vent site was even more elevated, with an average of Kp = 4 x 10-2 m2 s-1. The mixing, as well as the flow field, exhibited a strong tidal cycle, with strong currents and mixing at the non-buoyant plume level during ebb flow. Periods of strong mixing were associated with increased internal wave activity and frequent occurrence of turbulent overturns. Additional effects of mixing on plume dispersal include bifurcation of the particle plume, likely as a result of the interplay between the modulated mixing strength and current speed, as well as high frequency internal waves in the effluent plume layer, possibly triggered by the buoyant plume via nonlinear interaction with the elevated background turbulence or penetrative convection.