Role of Colloids in Controlling Mobilization and Transport of Trace Metals during the Formation of Deep Sea Sediment Plumes

Katja Schmidt, German Federal Institute for Geosciences and Natural Resources, Hannover, Germany, Timo Brengelmann, University of Oldenburg, Oldenburg, Germany, Sophie Paul, Jacobs University Bremen, Department of Physics and Earth Sciences, Bremen, Germany and Eric P Achterberg, GEOMAR Helmholtz Centre for Ocean Research Kiel, Kiel, Germany
Industrial mining activities at the seafloor will induce benthic metal fluxes from the surface sediment layers into bottom seawater, either in dissolved or in (nano)particulate form. However, we presently lack detailed information on the mobilisation, transport and fate of trace metals during such activities. When discussing the geochemical behaviour of these elements during sediment suspension, it is necessary to address their distribution between different physical and chemical species, in comparison to natural background seawater conditions.

To study the mobilization of Fe, Mn, Cu, Co, Ni, and Pb from the sediment into the dissolved phase, we conducted batch experiments with suspensions of deep sea sediments from the Clarion-Clipperton Zone (CCZ) in the Northern Central Pacific and deep ocean seawater (1g/l). After 10 min of sediment suspension, we observe a strong increase of all trace metals in 0.2 µm filtered solution relative to the original seawater concentrations. However, only a small fraction of these “dissolved” trace metals occur in the size pool smaller than 0.02 µm (less than 0.1% of Fe, 5% of Mn, 20% of Cu, 12% of Co, 10% of Ni). Instead, they are mainly present as or bound to nanoparticles and colloids (NPCs) in the size fraction of 0.02 µm to 0.2 µm. This is in contrast to natural bottom seawater in the CCZ, where 70-90% of Mn, Ni, Cu, Cd, and Co are present in the smaller 0.02 µm pool. The dissolved (< 0.2 µm) Fe concentrations in the suspension experiments are more than three orders of magnitude higher compared to the original background seawater (up to 10 µM), and likely occur in form of nanoparticulate Fe oxyhydroxide, mobilized from Fe-rich clay (smectite) in the sediment. Total dissolved metal concentrations in our experiments are very similar to pore water concentration peaks we observe in some of the CCZ sediment cores, were they can be related to the presence of Fe oxyhydroxide nanoparticles.