Measurement and modelling of deep sea sediment plumes and implications for deep sea mining

Abstract

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...

Numerical Simulation of Deep-Sea Sediment Transport Induced by a Dredge Experiment in the Northeastern Pacific Ocean

“…However, the amount of re-mobilised sediment that does not go into resuspension was found to be notably larger. A recent plume study by Spearman et al (2020) carried out on a seamount near the Canary Islands, showed that a release rate of 0.2 kg s −1 of sediment into the water led to a maximum SSC similar to our measurements. Using a different release rate results in a weaker correlation between modelled and sensor data.…”

“…To date, knowledge of deep-sea sediment transport from numerical modelling is limited to a few studies that were carried out based on accompanied observations e.g., in Pacific Ocean (Jankowski et al, 1996;Nakata et al, 1997;Jankowski and Zielke, 2001;Roliniski et al, 2001;Schriever and Thiel, 2013;Aleynik et al, 2017) or in the vicinity of Canary Islands (Spearman et al, 2020). Many modelling efforts have been associated with a particular resuspension experiment and were developed commercially with a restricted access to the underlying data (personal communication with A.J.…”

“…Many modelling efforts have been associated with a particular resuspension experiment and were developed commercially with a restricted access to the underlying data (personal communication with A.J. Jankowski and also see Spearman et al, 2020). In addition, the role of flocculation during plume dispersion and its direct impact on the settling velocity of particles has not been considered in the past.…”

Extent of impact of deep-sea nodule mining midwater plumes is influenced by sediment loading, turbulence and thresholds

“…Building on this understanding, we use the validated DP1 dynamic plume model to provide realistic source conditions for a regional-scale numerical simulation of an ambient plume from a commercial nodule mining operation in the CCFZ. We used the TELEMAC model (see "TELEMAC numerical model" section), which is an established model that has previously been used for case-studies of deep-sea mining sediment plumes 45,46 , to hindcast ocean conditions and the associated transport and mixing. The goal of the TELEMAC model is not to predict the exact ocean conditions at a given date, but to provide physically reasonable ocean dynamics as the basis for a plausible model of a midwater ambient plume.…”

Simulated mining-related sedimentation impacts on the deep-sea macrofauna of the Chatham Rise, New Zealand

“…As the fate of the plume is subject to local environmental conditions, effects are likely much farther-reaching and could cover more than twice the area disturbed (Gjerde et al 2016). This limitation has been recognised and, in response, a number of recent studies have modelled sediment plume dynamics from ship discharges (Rzeznik et al 2019), laboratory experiments (Gillard et al 2019) and in situ seabed disturbances (Kulkarni et al 2018, Spearman et al 2020. Whilst this modelling is a step in the right direction, these recent experiments do not assess biological responses.…”