Reversible scavenging traps hydrothermal iron in the deep ocean

Abstract

Recent studies suggest that seafloor hydrothermal vents could be an important source of iron (Fe) to the surface ocean, stimulating plankton growth and biological carbon export. However, quantifying the supply of hydrothermal Fe to the surface ocean requires accurately modeling its stabilization and removal processes, which are poorly known. Here, we determine the physical speciation of dissolved Fe along an oceanographic transect following a coherent hydrothermal plume that emanates from the East Pacific Rise (EPR) and persists westward over 4,000 km in the Tropical South Pacific. Our observations show that the plume persists horizontally, but descends vertically, and consists primarily of very large Fe colloids. Guided by these observations, we develop a new size-resolved mechanistic model of hydrothermal Fe dispersion in this region, in which the stabilization of hydrothermal Fe is explained by a reversible particulate exchange process. This model accurately captures the lateral dispersion, downward settling and physical speciation of hydrothermal Fe along this transect. An alternate model that uses a hydrothermal source of Fe-binding ligands to facilitate Fe transport within the deep ocean can reproduce the long-range transport of hydrothermal Fe, but does not reproduce the vertical descent of the plume. Our model shows that hydrothermal Fe vented from the EPR is trapped in the deep ocean, and only 1% of this iron ever makes it to the surface where it can stimulate biological productivity. At the global scale, 3-5% of hydrothermal Fe makes it to the surface ocean, the vast majority of which originates from Southern Ocean vents and upwells in the Southern Ocean. Our best estimate of the global supply of hydrothermal Fe to the surface ocean, based on data-constrained estimates of ocean circulation, mantle 3He venting, and the hydrothermal Fe:3He ratio from the EPR, is 0.12±0.07 Gmolyr−1. This is about 60-70 times lower than the supply of Fe from aerosol dust deposition, but could be regionally important in the Antarctic zone of the Southern Ocean.