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. Plain Language Summary: Cobalt (Co) plays a significant role as a rare and vital bioactive trace metal essential for phytoplankton in the global ocean. The present study deals with the intricate biogeochemical processes of total dissolved Cobalt (dCo) in the Indian Ocean, comprising the Arabian Sea, the Bay of Bengal (BoB), and the southern sector of the Indian Ocean (30°S) and sheds light on the crucial sources of dCo as riverine input, atmospheric dust, continental shelf and Submarine Groundwater Discharge to the surface waters whereas the Java‐ Sumatra subduction zone acts as a supplier of dCo to the deep waters. Though Arabian Sea OMZ waters have similar dCo cycling to the global ocean OMZ, the BoB shows a peculiar dCo distribution with surface maxima that is attributed to its large riverine, atmospheric and shelf sediment input to the surface water. Due to the intricate internal cycling of dCo with dissolved oxygen, the changing global climate and ocean deoxygenation will critically affect the dCo reservoir in the water column. Key Points: Rivers, atmospheric dust, continental shelf, and Submarine Groundwater Discharge supply dCo to surface watersHigh dCo in deeper waters near the Java‐Sumatra Subduction Zone could be due to release from the subduction zoneContrasting internal cycling of dCo is observed in the Arabian Sea, the Bay of Bengal and the southern sector of the Indian Ocean [ABSTRACT FROM AUTHOR]