From source to sink: complex dual role of the Kairei hydrothermal plume in deep ocean trace metal cycling at the Central Indian Ridge

Hydrothermal venting represents a significant source of trace metals to the deep ocean, but key aspects of these fluxes remain poorly constrained. Whether high-temperature hydrothermal venting at a particular site represents a net source for fluid-sourced trace metals to the open ocean is determined by complex processes during plume mixing, which can also make them sinks for (seawater-derived) elements. We present a comprehensive dataset of dissolved trace metals (Cd, Co, Cu, Fe, Ga, Mn, Mo, Ni, Pb, U, V, and REY) for endmember fluids and from water column profiles covering the buoyant plume and the non-buoyant plume up to 30 km away from the mixed ultramafic/mafic-hosted Kairei Hydrothermal Field (KHF) at the Central Indian Ridge and a background station at 56 km distance. The non-buoyant plume was sampled with a newly developed autonomous trace metal rosette (TMR) for contamination-free hydrocasts. The KHF acts as a significant source of Fe, Mn, Co, and to a lesser degree of Ga to the deep Indian Ocean, with size fractionation revealing Fe being present predominantly as small particles (53 % at 0.2–0.8 µm) and large colloids (28 % at 0.015–0.2 µm) which are removed quickly near the source, while the soluble fraction (<0.015 µm) is more stable and exported to the deep ocean. Similarly, Pb was found to be largely present in the small particulate fraction (95 %) close to the source and being rapidly removed as particulate sulfides with little effect on dissolved concentrations, while all other investigated elements were found largely in the soluble fraction (<0.015 µm). Conversely, the plume effectively scavenges REY, V, and notably Cd from the water column as well as the fluid-sourced contributions, with near-quantitative Cd removal observed in the plume despite ∼100-fold endmember enrichment. Copper and Ni exhibit complex non-conservative behavior, both being depleted relative to background seawater in the near-field, likely reflecting initial sulfide precipitation; Cu is subsequently re-enriched in the far-field via oxidative release during plume aging, while any comparable Ni release likely falls below our detection limit given its lower endmember concentration, making the plume an apparent net sink for Ni. The relatively low H 2 S/Fe ratio of 0.90 at Kairei compared to other hydrothermal systems likely favors greater metal transport to the water column. These findings demonstrate that hydrothermal contributions to marine trace metal budgets are highly system-specific, controlled by endmember chemistry and plume processes, highlighting the need for detailed multi-element investigations across diverse hydrothermal settings.