The Central Indian Ridge (CIR) hosts numerous hydrothermal vent fields now targeted for seafloor massive sulfide (SMS) mining, yet its water-column hydrography remains poorly characterized relative to the Atlantic and Pacific ridges. We present a full-depth hydrographic analysis based on nine CTD stations (2,068–3,837 m) occupied between 31 March and 9 April 2023 during the MIRAE expedition aboard R/V ISABU, spanning 9.8–12.9°S at ~66°E. Temperature–salinity relationships, potential-density (σθ) structure and dissolved-oxygen distributions were used to identify resident water masses and quantify meridional variability. Five water masses were resolved: Tropical Surface Water (T > 26 °C, S ≈ 34.9–35.1, z < 200 m), South Indian Central Water (T ≈ 8–26 °C, 100–700 m), Antarctic Intermediate Water (AAIW; salinity minimum S ≈ 34.68–34.75 with an oxygen minimum of 70.9–87.5 µmol kg⁻¹ at 555–1,200 m), Circumpolar Deep Water (CDW; T ≈ 1.5–5 °C, S ≈ 34.70–34.78, 1,200–3,500 m) and Antarctic Bottom Water (T < 2 °C, σθ > 27.75 kg m⁻³, z > 2,800 m). AAIW was detected at all nine stations using a combined salinity–oxygen-minimum criterion, confirming its progressive equatorward erosion; CDW dominated the deep layer, and near-bottom oxygen (163–168 µmol kg⁻¹) indicated well-ventilated deepest basins. The intermediate oxygen-minimum layer combined with a sluggish, weakly stratified deep environment implies limited potential for rapid dilution of a mining sediment plume, with direct implications for the exposure of chemosynthetic vent fauna. This dataset provides an essential hydrographic baseline for environmental impact assessment of hydrothermal-vent ecosystems and prospective SMS mining on the CIR.

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