Simulating large scale transport of suspended matter

In this paper a Lagrangian numerical transport model is presented that simulates suspended matter concentrations on large scales. The model is based on the velocity fields of a 22-layer version of the 3.5 degrees x 3.5 degrees Hamburg Large Scale Geostrophic OGCM. Here, the model is applied to the transport of resuspended sediment from the hypothetical source of deep sea mining in the eastern equatorial Pacific. The advection and dispersion of an initially concentrated cloud are simulated for a time range of five decades. Three-dimensional time dependent concentration fields, sedimentation rates at the ocean bottom, the residence time of the particles in the water column and the length of their transport paths are presented. The computed sedimentation rates are compared to the natural background values and estimates of possible consequences for the benthic ecosystem are made. Three experiments are described in this paper. The first one simulates the drift of a particle in the conveyor belt over 1850 years. This experiment is performed to test the advection scheme of the transport model and the currents that are simulated with the underlying circulation model. The second and the third experiment simulate the dispersion of resuspended sediment close to the ocean's bottom and the release of tailings from ocean mining close to the ocean's surface, respectively. In the last two experiments the suspended matter cloud is represented by Lagrangian tracers which possess a mass and diameter distribution according to observations. A main result of experiment two and three is that for the near-bottom source of suspended matter, the drift of resuspended sediment is confined to less than 1000 km, whereas the release of tailings into the surface layer may result in basin wide transport of the fine-grained fraction of the material. The residence time (which here is the time between the release of a Lagrangian tracer and its touch-down at the ocean bottom) of the medium-sized particles is 2 to 3 years for the near-bottom source and up to 20 years for the surface release. The computed sedimentation rates are up to five orders larger than the natural background. Thus, the additional particle flux caused by deep sea mining might easily bury the thin layer of food on which the benthic ecosystem feeds. We also compare the computed residence times of the particles within the water column with residence times derived from U-238 to Th-230 observations. We estimate, that the settling velocity of the particles doubles by scavenging through biogenic particle fluxes and physical particle interactions (which are not included in the model yet). (C) 1998 Elsevier Science B.V.