Benthic nepheloid layers and the Ekman thermal pump

Near-bottom layers of high and homogeneous turbidity capped by high-turbidity gradients are found on the continental rise and the Blake-Bahama outer ridge off the coast of the eastern United States. These layers average about 200 m in thickness and are characterized by suspended sediment concentrations of about 100 μg/1. The layers are highly time variable in their thickness and intensity. They can be explained as being frictional boundary layers if a mechanism is invoked for the addition of heat near the bottom to destabilize the water column. Temperature profiles often show these bottom layers to be generally well mixed and not bounded at their tops by strong temperature gradients. Such ‘uncapped’ isothermal layers suggest growth by near-bottom heating. Bottom current observations show a down-slope Ekman veering of approximately 10° which, at velocities between 20 and 30 cm/s, would produce significant downslope pumping of heat, given the prevailing upslope increase in bottom water temperature. For comparison this mechanism would heat a 10-m-thick bottom layer 55 times greater than would the upward geothermal flux. Such downslope advection of heat in the bottommost several meters could produce the time variable thick boundary layers observed.

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