The effect of plume processes on the Fe isotope composition of hydrothermally derived Fe in the deep ocean as inferred from the Rainbow vent site, Mid-Atlantic Ridge, 36°14′N

The Rainbow hydrothermal vent site, which is the largest known point source for dissolved Fe delivered to the deep North Atlantic ocean, has remained invariant in its Fe isotope composition over at least the past 16,000 years, based on analysis of metalliferous sediments beneath the plume. Because of the conservative behavior of Fe in the Rainbow plume, δ56Fe values of particles in the neutrally buoyant plume (−0.18±0.05‰) and underlying sediments (−0.19±0.05‰) are indistinguishable from the δ56Fe values of the high-temperature fluid sources (−0.23±0.04‰). Particles from the near-vent, buoyant stage of the plume, however, have higher δ56Fe values (+0.15‰ to +1.20‰) relative to the original vent fluid, consistent with fractionation during oxidation of Fe(II)aq to Fe(III)aq. Isotope compositions become invariant in the plume once all Fe(II)aq is fully oxidized, preserving the original composition of the vent fluid. The constant Fe isotope compositions of the vent fluids over time implies that changes in seawater Fe isotope composition of the North Atlantic ocean, as they are recorded in Fe–Mn crusts, requires changes in the relative fluxes of Fe to the ocean.