Distribution, deposition and impact of submarine mine tailings disposal on the fjord bottom in Frænfjorden, western Norway

Frænfjorden, a fjord on the west coast of Norway, has been studied to increase our knowledge of the environmental effects of submarine tailings placement (STP). Fine-grained tailings consisting primarily of calcite are disposed of by Omya Hustadmarmor. The dataset, including multibeam echosounder data, shallow seismic, video data, grab samples and sediment cores, demonstrates that tailings are primarily deposited in an up to 2 km-wide area with a tailings thickness of up to 20 m. Minor quantities of tailings are spread outside the STP by tidal currents. Sediment cores show the difference in colour, mineralogy and grain size between the natural sediments and the tailings. The tailings have a white/grey colour, are very fine grained, contain high amounts of calcite and have a lower water content than the natural sediments. A series of multibeam echosounder data from 2013 to 2017 show that deposition of tailings has triggered several small gravity flows and slides. The tailings within the STP have a low stability compared to the natural sediments outside the STP because of their fine grain size and higher sensitivity, high slope angles, high sediment accumulation rates preventing normal consolidation, and loading of tailings causing overpressure in the underlying sediments.