The long-range transport of hydrothermal Fe from the SEPR

Dissolved Fe is important because it is often limiting nutrient for primary production in the ocean. Nevertheless, it has been underestimated that most of the hydrothermal Fe is lost from solution in the vicinity of vents owing to precipitation, aggregation, and scavenging. However, this thought was challenged by a study of Resing et al, which demonstrated that the hydrothermal dissolved Fe can be transported for a long distance. Their data were collected from 35 stations between Papeete, Tahiti, and Manta, Ecuador in the Pacific during the US GEOTRACES. The most salient aspect is an extensive, mid-depth plume of high Dissolved Fe and Dissolved Mn that reaches over a distance of more than 4,000 km to the west of the SEPR(South Eastern Pacific Rise). This aspect is larger than that observed plumes across the Atlantic, Indian, Arctic, and Southern oceans. Moreover, the lateral extent of the hydrothermal dissolved Fe anomaly exceeds that of Mn despite dissolved Mn is oxidized normally more slowly than dissolved Fe in sea water. Hydrothermal dissolved Fe is rapidly removed from the dissolved phase at directly over the axis in accordance with previous studies from the SEPR. On the contrary, after the 21 station, hydrothermal Fed behaves more conservatively and decreases its concentration linearly correlated with 3Hexs within the plume, indicating that Fed is somehow stabilized against loss from solution. Perhaps, it is outcome of complexation by dissolved organic ligands or by incorporation into inorganic or organic colloids that reside within the dissolved (<0.2 mm) size fraction. The PISCES model is a relatively complex ocean general circulation and biogeochemistry model that includes planktons, nutrients, carbon particles etc. The model applied the input of 3He as a function of ridge spreading rate and simulates hydrothermal Fe efflux via a fixed Fe: 3He ratio from a global hydrothermal fluid data. In the model, when dissolved Fe-stabilizing ‘ligands’ are added in an equimolar ratio with hydrothermal dissolved Fe, it is able to reproduce both the westward dissolved Fe plume extent and relationship between dissolved Fe and 3Hexs. These model experiments indicate that the stabilization of dissolved Fe against loss from solution governs its persistence and transport in the deep ocean. Moreover, if the linear relationship between dissolved Fe and 3Hexs concentrations in the SEPR is representative of steady state mid ocean ridge hydrothermal inputs to the ocean, global effective hydrothermal dissolved Fe input is at least fourfold higher than previous estimates. Therefore, stabilization of hydrothermal dissolved iron is important to carbon export as much as magnitude of hydrothermal Fe emissions.