| Summary: | Chemical properties of the water column were examined at the Indian Deep-sea Environment Experiment (INDEX) site in the Central Indian Basin (CIB), as a part of baseline studies prior to the benthic disturbance experiment for the environmental impact assessment of mining of polymetallic nodules. The study shows three equatorward moving water masses. (a) The Subsurface Salinity Maximum in the depth range 125–200 m, characterized by high salinity (34.74–34.77 psu) and oxygen minimum associated with weak maxima in nutrients. (b) The Deep Oxygen Maximum (234–245 mM) in the depth range 250–750 m, associated with minima in nutrients and relatively high pH. (c) The Salinity Minimum Water (34.714–34.718 psu) corresponding to the Antarctic Intermediate Water (AAIW) at depths 800–1200m in the density (sy) range 27.2–27.5. Progressive changes in these characteristics are attributed to mixing with waters above and below, and to oxidation of organic detritus en route. Among the three water masses, the oxygen maximum water shows the lowest changes in its properties, which may suggest that this water mass is moving the fastest.
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