Constraining the oceanic hydrothermal iron flux using iron isotopes
Iron isotopes are an emerging tools for assessing the provenance of metal inputs to the ocean, and for exploring the effects of biogeochemical cycling and redox processes. Here we present results of the analysis of soluble, dissolved and total dissolvable Fe and dissolved Fe isotope (δ56Fe) distributions, in seawater samples collected from above hydrothermal sites on the Mid-Atlantic Ridge during the UK GEOTRACES GA13 cruise. Our Fe isotope data show that δ56Fe values are as low as −1.91‰ in hydrothermal plumes above the basalt-hosted TAG site, and as low as −6.95‰ above the ultramafic-hosted Rainbow site. Differences in δ56Fe values between the two sites reflect differences in the proportion of dissolved Fe that precipitates as Fe-sulfide vs Fe-oxides. At both locations, the δ56Fe signal of dissolved Fe evolves to heavier values in the distal part of the hydrothermal plume, likely due to exchange of iron with the particulate fraction and mixing with background seawater. This is most evident in the Rainbow plume where addition of Fe from the particulate to the dissolved phase is required to close the plume δ56Fe budget. Our data also show a relationship between δ56Fe and apparent oxygen utilisation which contrasts with previous work showing the N. Atlantic δ56Fe is dominated by boundary source signatures.
Our study shows that the isotopic ratio of dissolved Fe can be used to distinguish Fe inputs from different ocean sources but also the exchange of iron between dissolved and particle phases. Improving our understanding of how the Fe cycle fuels primary productivity.