Physical Geography seminar: Quantifying iron fluxes to the ocean
Professor Rob Raiswell, University of Leeds
Estimates of iron fluxes to the surface ocean are usually based on the product of a mass flux and a dissolved concentration term. Mass fluxes are often only crudely quantified and concentration data are typically derived for dissolved species on the assumption that solid phases are essentially unreactive in seawater. Herein lies two of the biggest problems in studying the iron cycle; firstly it is difficult to isolate truly dissolved (i.e. aqueous) iron species from the smallest solid phases (e.g. nanoparticles) and secondly, nanoparticles are sufficiently reactive to act as both sources and sinks for aqueous and particulate iron species in seawater. Thus synthetic nanoparticulate ferrhydrite dissolves to form aqueous species, aggregates into particles and ages to form goethite but the rates of these processes (and their influence on bioavailability) may be considerably modified by organic associations.
Unfortunately ‘dissolved’ iron measurements have commonly been made on filtrates < 0.45 μm which comprise an unknown mixture of aqueous and nanoparticulate iron species. A better approach to estimating fluxes is to define size fractions that optimise the separation of aqueous species (A, defined as <0.02 μm), nanoparticles (N, 0.02 to 1 μm) and particles (P> 1 um) in line with current best analytical practice. These A, N and P size definitions are somewhat arbitrary and do not clearly discriminate between aqueous and nanoparticulate species (but do allow some account to be taken for their different behaviour in seawater). The A fraction in seawater contains mainly organically-bound iron species (such as siderophores or polysaccharides) that are associated with aqueous iron (and also with the smallest iron (oxyhydr)oxide nanoparticles). The N fraction comprises iron (oxyhydr)oxide minerals, probably ferrihydrite and goethite, also closely associated with organic species. The N fraction is at least comparable in size to the A fraction in the global riverine and atmospheric fluxes to the surface ocean and vastly predominates in the glacial flux. Hence more attention needs to be directed to characterising the mineralogy, bioavailability, and organic associations of the N fraction, and the kinetics of its reactions, transformations and aggregation.