The impact of hydrothermally-sourced Fe on ocean biogeochemistry: new investigations from the East Pacific Rise, 9-10°N.

We have initiated a new study as part of NSF's Ridge 2000 program to investigate the fate of dissolved Fe(II) released from high-temperature hydrothermal venting on the East Pacific Rise and its potential impact on local deep-ocean biogeochemical budgets. Our principal focus is on the biogeochemical cycling of Fe and the role of microbial activity in that cycle: Fe is released in abundance from high-temperature vents, it plays a major role in modifying the gross flux of other elements from hydrothermal vents to the oceans and hydrothermal sources have the potential to dominate the global deep ocean budget of dissolved Fe: an essential micronutrient. To achieve our goals we have collected samples from close to vent-sites at 9-10°N on the East Pacific Rise over a period extending from late 2004 to late 2007 that brackets an episode of recent volcanism (Winter 2005-2006). Thus, we are in a position to investigate both the "steady state" input from a deep-ocean vent-system and also how that system has been perturbed following volcanic activity out to a period of 1-2 years post-eruption. Sampling has been conducted using time-series sediment traps deployed on long-term moorings that have sampled particles settling from buoyant and non-buoyant plumes at 9°50`N and at a contrasting (sulfide- free) vent-site at 9°30`N. Our project is nested within a larger study (LADDER) that spans across disciplines from segment-scale ocean circulation in the 9-10°N area to larval transport between adjacent vent sites. Our specific on-going work includes: i) analyses for major and trace element flux variations; ii) investigation for Fe isotope fractionation within our hydrothermal plume samples; and iii) a study of the role of Fe- oxidising bacteria in catalysing Fe oxidation and/or Fe isotope fractionation within these hydrothermal plumes. For example, the mineralogical structure and character of Fe oxyhydroxide minerals may, themselves, result from the microbial catalysis of Fe oxidation.