Spatial Variability of Dissolved Cobalt in the Indian Ocean Waters: Contrasting Behavior in the Arabian Sea, the Bay of Bengal and the Southern Sector of the Indian Ocean

The present study explored the dynamics of total dissolved Cobalt (dCo) in the Indian Ocean, revealing different distribution patterns in the different sub-basins, nutrient-type in the southern sector, hybrid-type in the Arabian Sea to scavenged-type in the Bay of Bengal (BoB). The dCo in the coastal water of the Arabian Sea displays elevated (0.12–0.13 nmol L⁻¹) abundance and diminishes gradually toward the central Arabian Sea. Similarly, in the BoB, dCo concentrations are notably higher in the northern region (0.11 nmol L⁻¹) and gradually decrease toward the south (0.03 nmol L⁻¹ at 5°N). The Arabian Sea with higher biological uptake and remineralization in the oxycline supports a higher abundance of dCo in the intermediate oxygen minimum zone (OMZ), much a like the OMZs of the Atlantic and the Pacific Oceans. The influence of the phytoplankton community shift and uptake on the dCo distribution in the Indian Ocean could be inferred from the association between Co and phosphate in the photic waters. Our observation demonstrates a scavenging type dCo profile in the BoB due to its higher riverine as well as dust inputs in addition to its supply from continental shelf sediments. Such a higher concentration of dCo in the surface waters of the northern BoB masks the dCo signal associated with nitrite maxima. dCo gets removed by its scavenging with Mn oxides at deeper depths, as reflected by higher particulate Co in the BoB. Subduction fluids contribute significantly to the dCo inventory of the deep water in the Indian Ocean near the Java-Sumatra subduction zone.