Constraining the oceanic hydrothermal iron flux using iron isotopes

One of the most prominent findings of the GEOTRACES programme is that hydrothermal inputs of iron to the ocean can be detected many thousands of kilometers away from mid-ocean ridges. However, the chemical processes in hydrothermal plumes that regulate the dispersal of ridge-derived metals are poorly constrained, making it difficult to assess the far field impacts of hydrothermal sources on the deep ocean iron inventory and primary productivity in surface waters.

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.