Deep-sea hydrothermal vents and the waters in the reactive mixing zone above vent orifices appear to be an important source of fine material that can pass through 200 and 400 nm filters. In this work, nanoparticle size is defined operationally as that material, which can pass through a 200 nm filter. We investigated three vent sites (Lau Basin, Middle Atlantic Ridge and 9 N East Pacific Rise), which have fluids that range from sulfide rich to metal rich. Using chemical analyses along with SEM-EDS, TEM, XRD and EELS, we identified materials found in these (nano)particulate phases including pyrite, metal sulfides, silicate and aluminosilicate material. Nanoparticulate pyrite can comprise up to 38% of the total iron passing through a 200 nm filter, and can form by two pathways (FeS with H2S or FeS with polysulfides). Previous work determined that nanoparticulate pyrite is kinetically stable against oxidation and has a calculated Stokes’ settling velocity of ~1.5 m/yr. Enrichment of Mg, K and Fe in (alumino)silicates suggests that reverse weathering or authigenic clay formation occurs in the waters within 1-2 meters of the vent orifice where hot vent waters mix with cold oxygenated bottom waters. Thus, both nanoparticulate pyrite and metal-bearing (alumino)silicate phases have the potential for transport away from vent sites and to act as a source of Fe and other metals to the global ocean. Moreover, we present data on in situ hydrogen peroxide formation when hot anoxic vent and diffuse flow fluids enriched in dissolved Fe(II) entrain and react with O2 in bottom waters. In this dynamic reaction zone, we hypothesize that a catalytic Fe(II)-O2-H2S cycle occurs where Fe(II) reacts with O2 to form the oxidants, Fe(III) and ROS, which in turn oxidize H2S to elemental sulfur [reforming Fe(II)] and Mn(II) to Mn(III), respectively. The production of ROS may exert a significant control on the abiotic oxidation of both metal and organic carbon species in vent plumes.

Solution and (nano)particulate chemistry within the dynamic mixing zone of the first 2 meters of hydrothermal vent orifices

255th National Meeting (2018)

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