Benthic environmental baseline investigations in the manganese nodule area of the central Indian Basin

Abstract

In order to exploit the manganese nodule deposits in future, an assessment of the environmental impact due to potential mining activity, has been undertaken in the Central Indian Basin Under this programme, seabed surveys in five selected areas have been conducted to study the bathymetry, nodule distribution and benthic fauna Each of the areas, having dimensions of 10 x 10 nautical miles, has a total relief of 160-200 m with variable undulations (10-60 m) and few elevated features (50-100 m) The sites with relatively flat topography have higher sediment thickness (50-100 m) than those with uneven topography (50-100 m) Nodules in areas of thicker sediment cover have low abundances (32 out of 19 locations; < 3 kg/sq.m, average abundance: 1-2 kg/sq-m), as compared to those with relatively thin sediment cover, (11 out of 25 locations: 5-12 kg/s.q.m, and average abundance: 3.5-5.0 kg/sq.m) Almost all nodules appear to be buried under these sediments Nodule exposure ranges between 0 and 2%, except in one station with 50% coverage The sediment underlying the nodules is predominantly radiolarian ooze with occasional burrows (mainly thalassinoid type) Out of the 12 cores studied for benthic activity, five displayed pronounced layered structure in the lop (0-15 cm) layer and a comparatively coherent sub-layer (below 15 cm) In three cores, evidence of partial induration/lithification has been noticed, leading to hard ground formation Quantitative estimates of dap-sea benthos recorded from core samples indicate moderately rich fauna The meiofauna density ranges from 3-45/10 cm sup(2) dominated by Nematode Similarly, the macrofaunal density ranges from 2-16/0.25 m sup(2) and dominated by Polychaeta The ratio of macro-to meiofauna in the total population was 1:500