Baseline studies and evaluation of effects of surface discharge of deep-sea mining - INDEX area
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Hydrochemical properties of the water column were measured at the Indian Experiment (INDEX) site in the Central Indian Ocean Basin, as a part of baseline studies for the environmental impact assessment of benthic disturbance. The surface mixed layer was very thin (approx 40m) and was characterized by high oxygen, high pH and low nutrients with nitrate limiting productivity. Below this layer, three distinct layer could be indentified, based on their hydrochemical properties: The Salinity Maximum layer which appeared at 125-175 m depth was associated with shallow oxygen minimum (1.25 ml/l) and weak maxima in nutrients; The Oxygen Maximum Layer in the depth range 250-500 m was associated with high oxygen concentrations (4.5 ml/l), high pH (> 7.8) and weak minima in nutrients; The Salinity Minimum Layer corresponding to the Antarctic Intermediate Water was seen as a narrow band, about 400 m thick, in the depth range 800-1200 m. The bottom water was characterized by high oxygen and high nutrients. An evaluation of possible effects of surface discharge of mining fines - a slurry consisting of fine nodule fragments, bottom water and sediments - suggests that the discharge, with an expected solid content of 50 g/l, will induce increased primary production, through nitrate enrichment in nitrogen limited surface waters, of the order of 1.343 x 10 sup(-2) g fixed carbon per litre of discharge, while the organic C associated with the sediment will result in a net oxygen demand of 306 ml per litre of discharge. Based on the above figures and the baseline characteristics of the water column, a dilution factor of 0.98 x 10 sup(-4) has been calcutated for the surface discharge to have non-discernible effects.
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