Utilizing 234 Th/ 238 U disequilibrium to constrain particle dynamics in hydrothermal plumes in the Southwest Indian Ocean

Metal-enriched minerals have been widely observed near hydrothermal vent fields. However, the dynamics of particulate metals influenced by hydrothermal activities is poorly constrained. Here, radioactive 234 Th in both dissolved and particulate phases were used to examine the kinetics of particle-reactive metal adsorption, removal, and residence in a newly found hydrothermal plume over the Southwest Indian Ridge. The results showed a relatively low value on 234 Th/ 238 U ratios (i.e., 0.73–0.88) compared to the deep oceans, indicating an enhanced adsorption of particle-reactive metals onto particulate matter in the plume. Based on the 234 Th- 238 U disequilibria, the adsorption and sinking rate constants of 234 Th averaged (0.009±0.001) d −1 and (0.113±0.024) d −1 in the hydrothermal plume, corresponding to the residence times of (115±19) d and (16±5) d for dissolved and particulate 234 Th, respectively. This timescale allows vent-discharged particle-reactive metals to disperse hundreds to thousands of miles away. Thus, hydrothermal activities might influence the metal distribution in deep ocean over a very large scope. Also, a high sinking flux of (36.2±5.4) Bq/(m 2 ·d) for 234 Th was observed for the plume, suggesting an enrichment of metal in particles deposited close to the vent. The enhancement of particle sinking could also benefit the transport of organic carbon and nitrogen and fuel the benthic ecosystems under the plume regimes. Thus, hydrothermal plumes may have an impact on both the elemental geochemistry and/or ecosystem to the deep oceans interior than previous expectation.