Investigation of hydrothermal activity in the South West Indian ridge region using Ra isotopes and 227Ac as tracers

Hydrothermal vents have been shown to be important vectors for various chemical elements into the ocean. However, both the intensity of the chemical fluxes associated with these systems and the fate of the chemical elements along the plume are still largely overlooked. At two stations located above the South West Indian Ridge (SWIR), we investigate the distributions of the Ra quartet (223Ra, 224Ra, 226Ra and 228Ra) and 227Ac that have been used as tracers of hydrothermal activity. While the vertical distributions of 226Ra, 228Ra and 227Ac do not show a clear enrichment at depth, unusual signatures of excess 223Ra and 224Ra near the seafloor are attributed to the presence of a hydrothermal activity. The discrepancy observed between the different isotopes is attributed to different chemical reactivity when seawater circulates within the crust and/or to different regeneration rates within the fluid. A 1D diffusion model applied to the vertical profiles of short-lived Ra isotopes provided an estimation of the vertical eddy diffusivity coefficients (KZ) between 38 cm2 s-1 and 149 cm2 s- 1. These high values suggest strong mixing likely favored by the complex bathymetry in the region. By combining these KZ with the vertical gradient of dissolved Fe (dFe), we estimate a vertical flux of dFe that ranges from 139 to 1173 nmol m- 2 d-1. These results confirm that low -expansion -rate ridges could be significant sources of dFe to the deep ocean.