Deep sea mining concerns the extraction of poly-metallic nodules, cobalt-rich crusts and sulphide deposits from the ocean floor. The exploitation of these resources will result in adverse ecological effects arising from the direct removal of the substrate and, potentially, from the formation of sediment plumes that could result in deposition of fine sediment on sensitive species or entrainment of sediment, chemicals and nutrients into over-lying waters. Hence, identifying the behaviour of deep-sea sediment plumes is important in designing mining operations that are ecologically acceptable. Here, we present the results of novel in situ deep sea plume experiments undertaken on the Tropic seamount, 300 nautical miles SSW of the Canary Islands. These plume experiments were accompanied by hydrographic and oceanographic field surveys and supported by detailed numerical modelling and high resolution video settling velocity measurements of the in situ sediment undertaken in the laboratory. The plume experiments involved the controlled formation of benthic sediment plumes and measurement of the plume sediment concentration at a specially designed lander placed at set distances from the plume origin. The experiments were used as the basis for validation of a numerical dispersion model, which was then used to predict the dispersion of plumes generated by full-scale mining. The results highlight that the extent of dispersion of benthic sediment plumes, resulting from mining operations, is significantly reduced by the effects of flocculation, background turbidity and internal tides. These considerations must be taken into account when evaluating the impact and extent of benthic sediment plumes.Background. There is increasing global concern over the long-term availability of secure and adequate supplies of critical raw materials, known as E-tech elements, which have an essential contribution to emerging 'green' technologies 1 . These E-tech elements are present (in different concentrations and combinations) in poly-metallic nodules on the ocean floor and cobalt-rich crusts on seamounts 2 . Additionally, seafloor massive sulphide deposits contain economically attractive concentrations of a variety of minerals including copper, gold, silver and zinc 3 . Plans to extract these minerals involve a seafloor harvester, creating sediment disturbance through its motion across the sea bed, cutting of the substrate, collection of the minerals, discharge of uneconomic sediment after processing and, potentially, discharge of over-burden covering buried deposits 4-7 . Such operations will remove substrate (and any ecology bound to, or buried within, the substrate), and also generate turbidity plumes that may result in deposition of sediment onto sensitive species in the near to far-field and/or entrainment of sediment, nutrients and chemicals into over-lying waters.A review of previous sediment resuspension experiments 8 describes the various deep sea plume experiments undertaken to date 9-13 . These experiments mainly consisted of towed ha...

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