Long-term propagation of tailings from deep-sea mining under variable conditions by means of numerical simulations

Numerical experiments that simulate the dispersion and resettling of particulate matter in a potential deep-sea mining area are used to estimate the possible long-term effects from deep-sea mining on the benthic ecosystem. The mining of manganese nodules is estimated to stir up 50,000 tonne of sediment per day, an estimated 4000 tonne of which is transported to the surface together with the nodules. The potential mining site is located in the eastern equatorial Pacific, an area where hydrographic conditions close to the surface are highly variable. In order to determine the variations of the transport of tailings, the simulations were run for El Nino, and La Nina conditions. Resettlement of stirred-up sediments is determined by the grain-size distribution (and hence settling velocity) of the particulate matter and scavenging processes. Two different grain-size distributions, both derived from measurements, are applied, which are characterised by finer and coarser grains. The flux of biogenic matter obtained from a model is used to simulate the additional downflux of particles caused by scavenging. Results differ strongly depending on the properties of the released sediments. Resettling of 90-95% of the total mass of the relatively fine grain-size distribution takes 3-14 years depending on the water depth of the release, whereas it is deposited shortly after release for the coarser distribution. (C) 2001 Elsevier Science Ltd. All rights reserved.