Deep-sea foraminifera from the Cassidaigne Canyon (NW Mediterranean): assessing the environmental impact of bauxite red mud disposal.

Benthic foraminiferal assemblages were investigated from two sites along the axis of the Cassidaigne Canyon (NW Mediterranean Sea). Both areas are contaminated by bauxite red mud enriched in iron, titanium, vanadium and chromium. These elemental enrichments are related to bauxite-derived minerals and various amorphous phases. At the shallowest station located very close to the pipe outlet, the benthic living foraminiferal community is characterised by a very low diversity and by an unusual dominance of Gyroidina umbonata and Bulimina marginata. The mechanical stress related to downslope transport of red mud is a likely source of hydro-sedimentary pollution precluding the settlement of diverse fauna. The living and dead foraminiferal faunas from the deepest site are typical of oligo-mesotrophic conditions prevailing in natural environments. There, bauxite residues have obviously no environmental impact on foraminiferal faunas. The bioavailability of trace metals is likely low as elemental enrichments were not observed in foraminiferal test chemistry.