Trace Element Distributions and Size Fractionation in the Edmond Hydrothermal Plume, Central Indian Ridge

Because cycling of the entire ocean volume through hydrothermal plumes is rapid relative to thermohaline circulation (order 10³ years), understanding the processes active within these plumes is crucial if we are to assess their impact on global geochemical cycles. Preliminary results from hydrothermal plume particle analyses at the Kairei and Edmond systems (Indian Ocean) have indicated that key processes, previously identified to be important in the Atlantic Ocean, also appear broadly applicable to the Indian Ocean. This was not immediately expected because parallel work has shown that the rate of dissolved iron (II) oxidation in hydrothermal plumes decreases systematically from the Atlantic to the Indian and Pacific Oceans. Here, we examine dissolved-particulate trace metal distributions in greater detail within one Indian Ocean plume (Edmond), together with the complementary vent-fluid data, to investigate these processes further. Upon oxidation, dissolved iron (II) initially forms colloidal iron (III) which then aggregates to form particulate iron (III) - the oxyhydroxide particles which apparently co-precipitate and adsorb dissolved metals from the surrounding seawater. What has remained unstudied, however, is the role that colloidal rather than aggregated particulate Fe may play in these systems. To investigate this we have combined studies of large-volume plume-particle samples (1.0μ m filters) with a series of dissolved, colloidal and finer-grained particles collected using a CTD-rosette. Here, we will discuss the distributions of Fe and the relative fractionations of Mn, Cu (representative of the chalcophile elements) and P (representative of the oxyanions) within and between different hydrothermal "pools": dissolved, colloidal, fine particles and coarse particles as determined from filtration through 0.1, 0.4 and 1.0μ m filters.