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

Production of Fe(III) particles in hydrothermal plumes is of fundamental im portance to the long-term effect of hydrothermal circulation on seawater co mposition. To elucidate the fundamental controls on Fe redox kinetics and s olution/particle partitioning in neutrally buoyant plumes, we sampled near- field (<3 km) plume particles at 9 degrees 45'N on the East Pacific Rise in 1996, returning in 1997 to sample both particulate and dissolved phases (0 .40 mu m filter). Concentrations of dissolved Fe varied from 320 to 20 nM i n proximal (<0.3 km from vent site) to distal samples (1-3 km downfield), c onstituting similar to 85-50% of total Fe, respectively. Based on vent flui d dilution factors calculated from dissolved Mn, a mass balance for vent fl uid Fe at this site indicates that similar to 65% of Fe is lost to particul ate sulfide settling in the buoyant plume, and that particulate Fe in dista l (1-3 km) samples is twice as concentrated as predicted from dilution of p articles 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) co lloid production and aggregation occurs primarily in the neutrally buoyant plume at relatively high dilutions, preventing generation of high particula te Fe concentrations (11-56 nM observed). A general investigation of Fe(II) oxidation rates in plumes worldwide gives Fe(II) oxidation half-lives as s hort as 17 min at some Atlantic sites, and as long as 6 h at some Pacific s ites. The calculations indicate that the distribution of Fe particles in pl umes depends chiefly on inter-basin differences in ambient deep water chemi stry (primarily pH and dissolved O-2) and on local currents driving plume d ilution, and to a much lesser extent on variations in primary vent fluid co mposition. Long-term changes in thermohaline circulation or ocean biogeoche mistry may therefore alter Fe dynamics and minor element fluxes associated with global hydrothermal activity, independent of variations in crustal pro duction rates. Copyright (C) 1999 Elsevier Science Ltd.