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Biogeosciences An interactive open-access journal of the European Geosciences Union
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Volume 15, issue 14
Biogeosciences, 15, 4387-4403, 2018
https://doi.org/10.5194/bg-15-4387-2018
© Author(s) 2018. This work is distributed under
the Creative Commons Attribution 4.0 License.

Special issue: Assessing environmental impacts of deep-sea mining...

Biogeosciences, 15, 4387-4403, 2018
https://doi.org/10.5194/bg-15-4387-2018
© Author(s) 2018. This work is distributed under
the Creative Commons Attribution 4.0 License.

Research article 19 Jul 2018

Research article | 19 Jul 2018

Abyssal plain hills and internal wave turbulence

Hans van Haren Hans van Haren
  • Royal Netherlands Institute for Sea Research (NIOZ) and Utrecht University, P.O. Box 59, 1790 AB Den Burg, the Netherlands

Abstract. A 400m long array with 201 high-resolution NIOZ temperature sensors was deployed above a north-east equatorial Pacific hilly abyssal plain for 2.5 months. The sensors sampled at a rate of 1Hz. The lowest sensor was at 7m above the bottom (ma.b.). The aim was to study internal waves and turbulent overturning away from large-scale ocean topography. Topography consisted of moderately elevated hills (a few hundred metres), providing a mean bottom slope of one-third of that found at the Mid-Atlantic Ridge (on 2km horizontal scales). In contrast with observations over large-scale topography like guyots, ridges and continental slopes, the present data showed a well-defined near-homogeneous bottom boundary layer. However, its thickness varied strongly with time between  < 7 and 100ma.b. with a mean around 65ma.b. The average thickness exceeded tidal current bottom-frictional heights so that internal wave breaking dominated over bottom friction. Near-bottom fronts also varied in time (and thus space). Occasional coupling was observed between the interior internal wave breaking and the near-bottom overturning, with varying up- and down- phase propagation. In contrast with currents that were dominated by the semidiurnal tide, 200m shear was dominant at (sub-)inertial frequencies. The shear was so large that it provided a background of marginal stability for the straining high-frequency internal wave field in the interior. Daily averaged turbulence dissipation rate estimates were between 10−10 and 10−9m2s−3, increasing with depth, while eddy diffusivities were of the order of 10−4m2s−1. This most intense near-bottom internal-wave-induced turbulence will affect the resuspension of sediments.

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This paper presents high-resolution temperature observations and turbulence estimates from a hilly abyssal "plain" in Pacific nodule areas. Although turbulence levels are considerably lower than over steep topography, a bottom boundary layer, if existent, varies in height over scales far exceeding that of an Ekman layer. This variation is associated with internal wave motions affecting the near-bottom turbulence and thus probably the associated sediment reworking.
This paper presents high-resolution temperature observations and turbulence estimates from a...
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