LAUSR

Rainbow Ridge, a 1,950 m deep upthrusted ultramafic block along the axis of the Mid-Atlantic Ridge has an active hydrothermal vent system at 2,400 m on its western slope. However, within one kilometer from the vent excessive temperatures are barely measurable, probably due to strong turbulent mixing. This mixing is studied here using a 400-m long high-resolution temperature sensor array moored with a 600-m ranging 75 kHz acoustic Doppler current profiler. Rich internal wave turbulence was recorded, characterized by 100-200 m upshoots and >200 m large overturning in particular near the end of the warming phase of the up and down moving tide. These highly nonlinear internal waves of tides interacting with buoyancy frequency waves extend up to 400 m above the sloping bottom of the ridge. Whilst a turbulent ‘bottom boundary layer’ could barely be defined, the more intense turbulence higher up in the water column is suggested to lead to the strong dispersion of the hydrothermal plume.