The chemistry of iron in hydrothermal plumes

Abstract

This thesis investigates the role of submarine hydrothermal vents in the global marine Fe
budget. While debate continues over the sources of dissolved Fe to the global deep-ocean
dissolved Fe budget, it had been presumed, until recently, that all the Fe emitted from
hydrothermal vents precipitates and sinks to the seafloor close to the vent source.
However, in the open ocean, dissolved Fe exists at concentrations greater than the
predicted solubility because of the presence of organically complexed Fe. If similar
complexes were formed in the hydrothermal systems then there would be the potential for
dissolved Fe export via hydrothermal plumes to the deep-ocean.
To investigate the fate of hydrothemally sourced Fe, samples were collected from hightemperature
hydrothermal vent-field plumes at 9°N on the East Pacific Rise and at 5°S on
the Mid-Atlantic Ridge. The samples from the East Pacific Rise were analysed for Fe and
dissolved and particulate organic carbon. Although hydrothermal systems are presumed to
be inorganically dominated, elevated concentrations of dissolved organic carbon compared
to background seawater were detected in near-field buoyant plumes and the concentration
of organic carbon appeared to relate to the total Fe concentration, consistent with the
presence of some organic-Fe interaction.
Non-buoyant plume samples from the Mid-Atlantic Ridge were analysed for total
dissolvable and dissolved Fe and Mn as well as speciation studies on a subset of the
dissolved Fe samples using Competitive Ligand Exchange – Cathodic Stripping
Voltammetry. The dissolved Fe concentrations in the dispersing plume were higher than
predicted from dissolved Fe(II) oxidation rates alone. Further investigation into the
speciation of the dissolved Fe revealed the presence of stable Fe-ligand complexes, similar
to those detected in the open ocean, but with higher concentrations. If these Fe-ligand
complexes were representative of all hydrothermal systems, submarine venting could
potentially provide between 11 to 22% of the global deep-ocean dissolved Fe budget.
Buoyant plume samples from the same vent site were analysed for total dissolvable and
dissolved Fe and Mn as well as particulate Fe, Mn, P, V, Cu, Zn and the rare earth
elements. Fe isotopes were also analysed in the particulate fraction, as a potential tool for
tracing the biogeochemical cycle of Fe in the ocean. The forms of particulate Fe were
elucidated using the particulate trace element data, enabling the isotope fractionation
caused by Fe sulfide precipitation to be determined. A diagnostic isotope signature for a
potential stabilised dissolved Fe fraction was predicted to be isotopically heavier than the
original vent fluid, potentially enabling Fe inputs from hydrothermal vents to be traced
throughout the ocean.

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