The chemistry of iron in hydrothermal plumes

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

Name dc:type.qualificationname

Ph.D.

Level dc:type.qualificationlevel

doctoral

Grantor dc:publisher.institution

University of Southampton

Year dc:date.issued

2008

Author dc:creator

• Bennett, Sarah Anne

Advisors dc:contributor.advisor

• German, Christopher
• Statham, Peter