Iron persistence in a distal hydrothermal plume supported by dissolved–particulate exchange

Abstract

Hydrothermally-sourced dissolved metals have been recorded in all ocean basins. In the oceans’ largest known hydrothermal plume, extending westward across the Pacific from the Southern East Pacific Rise, dissolved iron and manganese were shown by the GEOTRACES program to be transported halfway across the Pacific. Here, we report that particulate iron and manganese in the same plume also exceed background concentrations, even 4000 km from the source. Both dissolved and particulate iron deepen by more than 350 m relative to 3He – a non-reactive tracer of hydrothermal input – crossing isopycnals. Manganese shows no similar descent. Individual plume particle analyses indicate that particulate iron occurs within low-density organic matrices, consistent with its slow sinking rate of 5-10 m year-1. Chemical speciation and isotopic composition analyses reveal that particulate iron consists of Fe(III) oxyhydroxides, while dissolved iron consists of nanoparticulate Fe(III) oxyhydroxides and an organically-complexed iron phase. The descent of plume dissolved iron is best explained by reversible exchange onto slowly sinking particles, likely mediated by organic compounds binding iron. We suggest that in ocean regimes with high particulate iron loadings, dissolved iron fluxes may depend on the balance between stabilization in the dissolved phase and the reversibility of exchange onto sinking particles.