Distribution of dissolved iron in sediment pore waters at submillimetre resolution

Abstract

MUCH effort has been directed at measuring concentration gradients at the sediment/water interface of aquatic systems, where the biogeochemical cycling of natural and pollutant species is particularly active1. Precise measurements of oxygen gradients using microelectrodes2 and estimates from independently determined fluxes3 suggest that concentration gradients in this region often extend only to depths of ˜1 mm, much less than the resolution (˜1 centimetre) of conventional techniques4–7. We have developed a new method for measuring pore-water composition in which diffusive equilibrium is established rapidly (within minutes) in a thin film of gel inserted in the sediment. On removal, the dissolved components are fixed, allowing chemical measurements to be made at high spatial resolution (<1 mm) on a stable solid phase. Using MeV-proton-induced X-ray emission (PIXE) to analyse the dried gel, we have measured iron concentrations in lacustrine pore waters at submillimetre resolution, revealing steep concentration gradients and sub-surface maxima consistent with a hypothesis of localized, reductive dissolution of fresh material.