Long-range transport of hydrothermal iron facilitated by dissolved-particulate exchange

Deep-sea hydrothermal venting has a strong impact on ocean chemistry close to vent sites; however, effects of these systems on basin- and global-scale ocean biogeochemistry have only started to be observed and modeled. The U.S. GEOTRACES Eastern Pacific Zonal Transect investigated potential long-range hydrothermal influences by sampling the length of the 15°S Southern East Pacific Rise (EPR) plume as far as 150°W, following a plume extending >4000 km from the EPR based on He-3 distributions and the distribution of metal-rich sediments. It was recently reported that dissolved iron (dFe) in this plume is apparently transported conservatively >4000km across the South Pacific, with potential for ultimate upwelling in the Fe-limited Southern Ocean. The mechanism by which this dissolved Fe is maintained in solution, however, remains a topic of active debate. Here we show that hydrothermal Fe in the particulate phase, which exceeds the dissolved Fe concentration over much of the plume, is also enriched relative to background across the same 4300 km transect but that maximum pFe concentrations both decrease non-conservatively and deepen progressively relative to dissolved 3He anomalies across the section. Stokesian settling would require an FeOOH particle size of ~0.5 µm in order to explain the ~500 m “sag” observed over the ~50 y of plume transport. A striking feature of the dissolved Fe plume is that it, too, descends in the water column over the length of the plume, following the particulate Fe plume. This suggests an active exchange between dissolved and particulate Fe species on timescales that are short relative to plume advection across the South Pacific. Thus, the large-scale distribution of particulate Fe appears to influence the preservation of the dFe pool, suggesting that the basin-scale impact of hydrothermal vent systems needs to consider both Fe phases and their long-term interactions.