Dissolved and particulate Fe in a hydrothermal plume at 9°45′N, East Pacific Rise:: Slow Fe (II) oxidation kinetics in Pacific plumes

Abstract

Production of Fe(III) particles in hydrothermal plumes is of fundamental importance to the long-term effect of hydrothermal circulation on seawater composition. To elucidate the fundamental controls on Fe redox kinetics and solution/particle partitioning in neutrally buoyant plumes, we sampled near-field (<3 km) plume particles at 9°45′N on the East Pacific Rise in 1996, returning in 1997 to sample both particulate and dissolved phases (0.40 μm filter). Concentrations of dissolved Fe varied from 320 to 20 nM in proximal (<0.3 km from vent site) to distal samples (1–3 km downfield), constituting ∼85–50% of total Fe, respectively. Based on vent fluid dilution factors calculated from dissolved Mn, a mass balance for vent fluid Fe at this site indicates that ∼65% of Fe is lost to particulate sulfide settling in the buoyant plume, and that particulate Fe in distal (1–3 km) samples is twice as concentrated as predicted from dilution of particles in proximal plume water. These observations are consistent with a calculated Fe(II) oxidation half-time of 3.3 h, long enough that Fe(III) colloid production and aggregation occurs primarily in the neutrally buoyant plume at relatively high dilutions, preventing generation of high particulate Fe concentrations (11–56 nM observed). A general investigation of Fe(II) oxidation rates in plumes worldwide gives Fe(II) oxidation half-lives as short as 17 min at some Atlantic sites, and as long as 6 h at some Pacific sites. The calculations indicate that the distribution of Fe particles in plumes depends chiefly on inter-basin differences in ambient deep water chemistry (primarily pH and dissolved O 2) and on local currents driving plume dilution, and to a much lesser extent on variations in primary vent fluid composition. Long-term changes in thermohaline circulation or ocean biogeochemistry may therefore alter Fe dynamics and minor element fluxes associated with global hydrothermal activity, independent of variations in crustal production rates.